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SBAS L5 ICD DRAFT V1

JASMIN-GMV-5100-D-5110-PU-1.1 21.06.2013 1/65 JASMIN project is funded by the European Commission. The results are the property of the European Commission. No distribution or copy is permitted unless prior authorization is given by the European Commission

JASMIN Service Contract no ENTR/93/PP/ENT/SAT/11/5476


Name (Company) Date

Written by: GMV team 21.06.2013

Approved by: A. Cezón (GMV) 21.06.2013

Project Company WP Doc. Ref. Classification* Version. Issue

JASMIN GMV 5100 D-5110 PU 1.1

Partner’s reference (optional)

GMV 21207/13 V2/13

* PU Unclassified / PP Programme Participant / RE Restricted / CO Confidential



Table of Contents 1 INTRODUCTION .............................................................................................................. 6

2 REFERENCES................................................................................................................. 8 2.1.1 Applicable Documents ........................................................................... 8 2.1.2 Reference Documents ........................................................................... 8

3 SBAS L5 RF CHARACTERISTICS ...................................................................................10

4 SBAS L5 DATA STRUCTURE .........................................................................................11

5 SBAS L5 DATA CONTENT .........................................................................................12

5.1 ICD Model A .......................................................................................................12 5.1.1 ICD Model A changes with respect to L1-SBAS MOPS ........................13 5.1.2 Summary of the different MTs in ICD Model A ......................................18

5.2 ICD Model B .......................................................................................................19 5.2.1 Introduction ...........................................................................................19 5.2.2 ICD Model B message definition ...........................................................20 5.2.3 Summary of the different MTs in ICD Model B ......................................32 5.2.4 ICD Model B Alternative .......................................................................32

6 SBAS L5 MESSAGE TABLES .........................................................................................36

6.1 Summary of the different MTs in ICD Model A ...................................................36

6.2 Summary of the different MTs in ICD Model B ...................................................44

7 ICD CONSOLIDATION ....................................................................................................48

7.1 SBAS L5 RF Consolidation ................................................................................48

7.2 SBAS L5 Data Structure Consolidation ..............................................................48

7.3 SBAS L5 Data Content Consolidation ................................................................48 7.3.1 ICD Model A .........................................................................................48 7.3.2 ICD Model B .........................................................................................50

8 ANNEX A: ICD MODEL A DETAILED MESSAGE DEFINITION .............................................51

8.1 MT0: Don’t Use for Safety Applications ..............................................................51 8.1.1 MT1: PRN Mask ...................................................................................51 8.1.2 MTs 2: Modified Messages: Fast Corrections .......................................52 8.1.3 MT6_1: Integrity Information .................................................................53 8.1.4 MT6_2: Integrity and Fast Corrections ..................................................53 8.1.5 MT7+10: Degradation Parameters ........................................................54 8.1.6 MT9: SBAS Satellite Navigation Message ............................................55 8.1.7 MT12: SBAS Network Time/UTC offset Parameters .............................56 8.1.8 MT17: GEO Satellite Almanacs ............................................................58



8.1.9 MT24: Mixed Fast Corrections/Long Term Corrections .........................59 8.1.10 MT25: Long Term Satellite Corrections ...............................................59 8.1.11 MT18: IGP Mask...................................................................................60 8.1.12 MT26: Ionospheric Delay Corrections ...................................................60 8.1.13 MT27: SBAS Service Message .............................................................60 8.1.14 MT28: Clock-Ephemeris Covariance Matrix Message ..........................60 8.1.15 MT30: Inter-Signal Corrections and Integrity Message .........................61 8.1.16 MT34: Satellite and IGP unified alert ....................................................61 8.1.17 MT35: UDRE and ISC alerts .................................................................62 8.1.18 IODF Removal ......................................................................................63

Table of Annexes

List of Tables Table 2-1: Applicable Documents .......................................................................................... 8

Table 2-2: Reference Documents .......................................................................................... 9

Table 5-1: ICD Model A MTs an changes with respect SBAS L1 ICD ...................................16

Table 5-2: Proposed L1/L5 ICD Model A slots. .................................................................18

Table 5-3: Example of second MT Mask Message ...............................................................18

Table 5-5: Proposed ICD Model A MT6_1 ............................................................................18

Table 5-6: ICD Model A modified MT6_2 & MT2 messages. ................................................18

Table 5-7: Proposed ICD Model A modified MT7+10 message ............................................18

Table 5-8: Proposed MT9 in ICD Model A ............................................................................18

Table 5-9: Modified part of MT12 ..........................................................................................18

Table 5-10: Proposed MT17 in ICD Model A ........................................................................18

Table 5-11: Inter-Signal Corrections and Integrity Message Type 30 ....................................18

Table 5-12: Satellite and IGP unified alert MT34 from SBAS L1/L5 ICD [RD5]. ....................18

Table 5-13: ICD Model A modified MT35 ..............................................................................18

Table 5-15: Proposed L5 satellite mask in ICD Model B .......................................................20

Table 5-16: Proposed L5 satellite alert message structure to support L1/L5 users. ..............22

Table 5-17: content of ICD Model B satellite correction message. ......................................24

Table 5-18: content of ICD Model B satellite ephemeris messages. .....................................27

Table 5-19: ICD Model B content of degradation parameters message. ...............................30

Table 5-20: ICD Model B MT6 message: Update Intervals. ..................................................34

Table 5-21: Proposed ICD Model B Alternative modified MT6 message. .............................35



Table 5-22: ICD Model B Alternative modified MT6 message: Update Intervals. ..................35

Table 6-1: ICD Model A Message Structure summary. .........................................................37

Table 6-2: ICD Model B Message Structure summary. .........................................................44

Table 7-1: ICD Model A MTs and Consolidation ...................................................................48

Table 7-2: ICD Model B MTs and Consolidation ...................................................................50

Table 8-1: Current MT26 content ..........................................................................................50

List of Figures Figure 5-1: Example of ICD Model A modified MT6 message. ..............................................14



Change Records Version.

Issue Date Description Name (Company)

1.0Draft 29.04.2013 First version of the document J. Fidalgo and Ana Cezón

(GMV)

1.0 10.05.2013 Document updated with internal GMV review

J. Fidalgo and Ana Cezón (GMV)

1.1 21.06.2013 Document updated with ESA review and PROSBAS RIDs to “SIS Message Definition” Document

J. Fidalgo and Ana Cezón (GMV)



1 INTRODUCTION

This document is the « SBAS L5 ICD V1 » technical note done in the scope of JASMIN project and in particular for Work Packages WP5100.

The purpose of this Technical Note is to provide input material to the standardisation fora (SBAS IWG or other fora organized by EC or considered appropriate as RTCA/EUROCAE).

This is the first version of the SBAS L5 ICD document.

This version of the document (D-5110 V1) is devoted to review current status on L5 SBAS standardisation in terms of ICDs. It provides a draft version of the different SBAS L5 ICDs that are being under discussion at IWG and in the scope of different standardization activities.

The present document is based on activities performed in PROSBAS Project (Prototyping and Support to Standardisation of L1/L5 Multi-Constellation Receiver) PROSBAS is a European Commission project which objective is to support the SBAS L1/L5 multiconstellation standardization. Please, note that according to EC request, internal PROSBAS information, actually under review in the frame of PROSBAS KP1 milestone ,has been used as input to this document.

In particular, the purpose of this document is to review the ICDs with respect to SIS message definition part of the SBAS L1/L5 standard, including a fully new message definition for the L5 signal that can support the augmentation of multiple constellations and multi frequency.

The Interoperability Working Group (IWG) was formed by the different SBAS service providers to ensure that SBAS systems are compatible and to plan future enhancements. Nowadays its main goal is to contribute to the elaboration of a new standard able to augment up to 4 GNSS constellations simultaneously and offering new services for dual frequency (L1/L5) users. For that purpose, two families of SBAS L1/L5 ICD models are currently under consideration in the IWG and also in PROSBAS project:

• ICD Model A, aimed at defining a SBAS L1/L5 ICD model as similar to the current SBAS L1 ICD as feasible. This model proposes a relaxation of the fast correction update intervals and the elimination of the dynamic mask. This model was presented at IWG [RD8].

• ICD Model B, whose main objective is, based upon a similar PRN mask as L1, to simplify the number of messages and structure as much as possible. Their proposal is to eliminate the fast correction, to send a single correction message per satellite



every 120 seconds (orbit, clock and their rates, covariance matrix, DFREI) and a MT6-equivalent message with DFRECI and updates of DFREIs every 6 seconds. This model was presented in IWG22 ([RD1],[RD2]). Additionally an alternative is proposed related with the MT6-equivalent message structure.

It is worth noticing here that once PROSBAS outputs will be more consolidated, the current document will be updated.

This document has been produced following the structure of EUROCAE Draft SBAS L1/L5 ICD ([RD5]).



2 REFERENCES

2.1.1 Applicable Documents

Ref. Document title Document reference Issue Date

[AD1] JASMIN Service Contract Service Contract no ENTR/93/PP/ENT/SAT/11/5476

- 21.12.2011

[AD2]

Table 2-1: Applicable Documents

2.1.2 Reference Documents


[RD1] Possible MOPS Changes for L5. Todd Walter, Stanford University. Presentation for IWG 22

N/A - Jan 2012

[RD2] Walter, T., J. Blanch and P. Enge (2012), “L1/L5 SBAS MOPS to Support Multiple Constellations”, IONGNSS 2012 Proceedings

N/A - Sept 2012

[RD3] Support to EC for analysis of multi-GNSS service performance assessment

MSIL2-GMV-TN-004-SS12_v1.2 - 10/01/2013

[RD4] Minimum Operational Performances standards for global positioning system/wide are augmentation system airbone equipment [SBAS L1 MOPS]

RTCA DO-229D - Feb 2013

[RD5] Signal Specification for EUROCAE-ICD-SBAS-L1-L5-ED-134 - May 2008




SBAS L1/L5 [SBAS L1/L5 ICD]

[RD6] Navstar GPS Space Segment/Navigation User Interfaces

IS-GPS-200 Revision D - Dec 2004

[RD7] Detailed technical specification for the Global Navigation Satellite System – ICAO Annex 10 Volume 1

Amendment 83 July 2007

[RD8] Summary Comparison SBAS L1/L5 ICD Models EC/ESA (EPO)

26 – ICD presentation EC+ESA EPO Jan 2013

[RD9] SIS Message Definition PROSBAS-GMV-TN-D2.2.1 V2.3 June 2013

[RD10] Qualitative Trade-Off on SBAS L1/L5 ICD Models

MSIL2-GMV-TN-004 V1.2 Jan 2013

Table 2-2: Reference Documents



3 SBAS L5 RF CHARACTERISTICS

Note, in general no major modifications are expected with respect to SBAS L1 MOPS ([RD4]).



4 SBAS L5 DATA STRUCTURE It is assumed that SBAS L1/L5 data is always packed in blocks of 250 bps as per current SBAS L1 Signal Specification. Therefore, not an extended explanation is provided here, for more information please refer to SBAS L1 MOPS [RD4] and SBAS L1/L5 ICD [RD5]. All messages shall consist of a message type identifier, a preamble, a data field and a cyclic redundancy check. The structure of a 250-bit message can be broken down into the following structure:

• Preamble: The first 8 bits correspond to part of the preamble. The preamble is a unique 24 bit-word (01010011 10011010 11000110), spread over three successive messages, which enables the initial part of the data to be synchronised during the acquisition phase.

• Message Type Identifier: The next 6 bits identify the message type (0 to 63). The message type identifier shall be a 6-bit value identifying the message type. The message type identifier shall be transmitted MSB first. The message types will depend on the ICD used.

• Data Field: The following 212 bits corresponds to the useful data contained in the message. Each data field parameter shall be transmitted MSB first.

• Parity: The last 24 bits are the parity bits, and are used to check if data is corrupted during the transmission. The SBAS message CRC code shall be calculated in accordance with the SARPS.



5 SBAS L5 DATA CONTENT

The objective of this section is to describe the data content for SBAS L5 data stream. The SBAS L5 Data Content will depend on the ICD model. Two families of SBAS L1/L5 ICD models are currently under consideration in the IWG and also in PROSBAS project [RD9]:

• ICD Model A. SBAS-like ICD, aimed at defining a SBAS L1/L5 ICD model as similar to the current SBAS L1 ICD as feasible. This ICD model proposes a relaxation of the fast correction update intervals and the elimination of the dynamic mask. This model was

presented in IWG [RD8]. It includes several modifications in certain MTs (with respect to SBAS L1 ICD) to allow the augmentation up to 4 constellations. This ICD will be extensively explained in Section 5.1.

• ICD Model B, whose main objective is, based upon a similar PRN mask as L1, to simplify the number of messages and structure as much as possible. Their proposal is to eliminate the fast correction, to send a single correction message per satellite every 120 seconds (orbit, clock and their rates, covariance matrix, DFREI) and a MT6-equivalent message with DFRECI and updates of DFREIs every 6 seconds. This model was presented in IWG22 ([RD1],[RD2]). This ICD will be extensively explained in Section 5.2.

o ICD Model B Alternative: Based on ICD Model B with certain modifications to improve performances for mono/dual constellation case.

As already known, these L1/L5 SBAS ICDs proposed models try to solve the problem of the lack of bandwidth that appears in a multi-constellation scheme. There is a bandwidth limitation due to the fact that the information sent by a SBAS System should fit 250-bits per second (note, Q-channel not considered). Q-channel as a data channel has been discarded and is not current under analysis in PROSBAS project,

It is worth noticing here that once PROSBAS outputs will be more consolidated, the current section will be updated.

5.1 ICD MODEL A

In this section, ICD Mode A will be explained.

A large part of ICD Model A messages are equivalent to those provided in SBAS L1 MOPS [RD4]. The main objective of ICD Model A is to minimize the changes from SBAS L1-like ICD.



In the following section the ICD Model A changes with respect to L1 SBAS MOPS will be explained while in Section 5.1.2 the different ICD Model A MTs will be detailed.

5.1.1 ICD Model A changes with respect to L1-SBAS MOPS

ICD Model A proposes a change in the message structure of former L1-SBAS MOPs to allow a more efficient use of the available bandwidth (specially affecting MT6 and MT2-5, and also MT12), as well as the definition of several new messages (MT7+10, MT35). The objective is to maintain the ICD as similar as SBAS L1 ICD as possible, while allowing more than 51 satellites (up to 102) on mask.

The modifications of ICD Model A with respect to former L1-SBAS MOPs will be explained in the following in the context of an example with 60 satellites in mask.

It is proposed to send the following messages every 6 seconds:

• As many MT6 messages as necessary taking into account the configured satellite mask. We will broadcast 2 MT6: the first one complete (type MT6-1, completed MT6 message), and the last one (type MT6-2, not completed MT6 message) would have the following number of bits free:

o The first MT6 (MT6-1) would include the UDREI information from 51 satellites,

o Therefore the second MT6 (MT6-2) should include the information from the remaining 9 satellites, and thus there would be a number of bits free (according to the new MT6-2 and MT2 structure provided below). This implies that a new message type is defined, MT6-2.

• In case the last broadcast MT6 message (the one which contains the UDREI information from the last satellites in mask) is not complete (this is, there are still free bits in the message, as in the aforementioned example), it could be filled with FCs data for all feasible satellites until the message is completed (let’s say up to x FCs). In this case, the mask position of the first satellite whose FCs are to be broadcast is given before the FCs data are broadcast, in order to avoid the specification of the PRN of each satellite whose FC is broadcast (with the consequent saving of bits).

Next Figure shows the modified MT6 (MT6-2) messages schematic view:



Figure 5-1: Example of ICD Model A modified MT6 message.

• The remaining FCs data for the rest of the satellites (in our case (60-x) FCs) would be sent in modified MT2 messages, the number of which will depend on the remaining satellites whose FCs were not broadcast yet during this 6-minute interval.

• The FCs information sent in these modified MT2 messages and in the last MT6 message (in the above figure MT6-2, if there is room for it, after sending the remaining UDREI values) could be as follows:

o First of all the corresponding IODP is sent (2 bits),

o UDREI values of the remaining (if any) satellites (4 bits x Number of remaining satellites).

o Then the mask position of the first satellite whose FCs are broadcast is given (7 bits, effective range: 1 to 91).

o FCs information from all feasible (taking into account the message free bits) satellites is provided (12 bits per satellite).

Please note that in case a MT6 is complete (type MT6-1) the message structure will be the same as that proposed in MOPS ICD for L1. In order to see the structure of MT6_1, MT6_2 and MT2, please refer to the following sections (sections 5.1.2.3 to 5.1.2.5).

• New MT7+10 comprising the needed information from former SBAS L1 MT7 and MT10. The key point is to consider a single Fast Correction Degradation Factor for each constellation. In order to enhance the ICD Model A Robustness on an



anomalous increase of Integrity parameters, it is proposed to merge the former MT7 and MT10 messages and to define a new MT7+10 message. The message structure of MT7+10 can be checked in Section 5.1.2.6.

• Modification on MT12. The purpose the modification to MT12 is to include, if necessary, the time offset between constellations, when 4 constellations are included. The message structure of MT12 can be checked in Section 5.1.2.8. It is worth noticing here that Galileo time offset integrity bound is not needed to be included since UDRE/DFRE values will be able to cover any feasible integrity issue.

• Concerning IODF Removal, please check Section 5.1.2.19.

• Slight modifications on MT9 and MT17 in order to solve the problem related with the lack of identification of Provider ID identified in L1 SBAS ICD.

• In case Iono is considered in ICD Model A, the information contained in the document SBAS L1/L5 ICD [RD5] can be considered as a starting point for that. In the context of PROSBAS project, the procedure to implement in PROSBAS Prototype the ISCs will be considered in a second phase of the experimentation, once the definitive ICD will be selected.

One preliminary option to implement ISC would be to consider a MT30 similar to MT30-32 proposed in SBAS L1/L5 ICD [RD5]. Nevertheless, in the case of having 91 SVs in mask, it would be needed to have 6 different MT30-MT32 in the case of the approach considered in [RD5]. Then, it is proposed that, instead of having several messages MT30-32, to have a single MT30 but using 3 “Panel” bits used to select for which 17 satellites the information is broadcast. The structure of MT30 can be seen in Section 5.1.2.16.

Concerning the ISC alerts, it has been proposed in PROSBAS not to use a MT33 as in SBAS L1/L5 ICD [RD5]. Instead of that, MT35 can be used for both UDRE Alerts or ISC alerts depending on the bit “ISC alert ld.”. In order to see the MT35 structure, please refer to Section 5.1.2.18.

Nevertheless, current MT34 in SBAS L1/L5 ICD [RD5] should still be maintained for the case UDREs and IGPs simultaneous alerts are present. The problem in that case is that MT34 can only be used for 22 SVs. Then, in the case that more than 22 SVs are in alert simultaneously to IGP alerts, the flag “Global Alarm ld.” should be set to true in order to indicate that the entire L1 service should not be used. In order to see the MT34 structure, please refer to Section 5.1.2.17.



In the following table there is a summary of the different ICD Model A MTs and the changes with respect to L1 SBAS ICD.

Table 5-1: ICD Model A MTs an changes with respect SBAS L1 ICD

Message Type Contents Modifications from SBAS L1 MT

MT0 Don’t use for safety applications (for SBAS testing)

None

MT1 PRN Mask assignments None except adding GALILEO and COMPASS slots

MT2 Fast Corrections Modified in ICD Model A to only contain FCs

MT6_1 Integrity Information None

MT6_2 Integrity and Fast Corrections New message in ICD Model A to allocate the integrity information for the 52th satellite until the last configured in the mask in the case that more than 51 SVs are configured in the mask. The free bits are used to allocate Fast Corrections

MT7+10 Degradation Parameters- Old But Active Data (OBAD)

MT7 and MT10 have been mixed. Only one Fast Correction Degradation Factor for each Constellation.

It is expected modifications in degradation models

MT9 SBAS Satellite Navigation Message

Non-GEO orbits allowed. Change to improve Provider Identification

MT12 SBAS Network Time/UTC offset parameters

Galileo and Compass information has been included. GLONASS already included in SBAS MOPS L1

MT17 SBAS Satellite Almanacs Non-GEO orbits allowed. Change to improve Provider Identification.



Message Type Contents Modifications from SBAS L1 MT

MT18 Ionospheric Grid Points (IGP) Mask

None

MT24 Mixed Fast Corrections and Long Term Corrections

None

MT25 Long Term Corrections None

MT26 Ionospheric Delay Corrections

None

MT27 SBAS Service Message None

MT28 Clock-Ephemeris Covariance Matrix Message

None

MT30 Inter-Signal Corrections and Integrity

New message.

It is proposed to use “Panel” bits to distinguish for which satellites the data is referred instead of having MT30-32

MT34 Satellite and IGP unified alert New message. The same as in SBAS L1/L5 ICD [RD5]

MT35 UDRE and ISC alerts New message.

It will be used for UDRE Alerts and ISC Alerts

A detailed description of the different types of messages is provided in section 9.



5.1.2 Summary of the different MTs in ICD Model A

In the following figure it is provided a high-level schematic structure of the messages and components required to correct a single satellite under the ICD Model A:

Figure 5-2: High-level schematic of messages and components to correct a single satellite under ICD Model A


JASMIN-GMV-5100-D-5110-PU-1.1 21.06.2013

5.2 ICD MODEL B

In this section, ICD Model B [RD2] will be explained.

5.2.1 Introduction

In case of ICD Model B, the approximation is completely different than the one of ICD Model A. The key concept is that there is a single correction message for each satellite, containing all the information to build the corrections for this specific satellite. For that, the separation between fast and slow corrections is completely eliminated.

In this case the strategy is to simplify as much as possible the SBAS-L5 approach independently of the SBAS-L1 ICD. In this case the satellite mask will not be limited to 51 satellites but to 91 satellites as it is the maximum number of satellites that can be alerted in the new integrity message. For doing this it has been proposed to:

o Eliminate fast and slow corrections.

o Broadcast a single correction message per satellite (orbit, clock and their rates, covariance matrix and DFREI). This information will be updated depending on the number of satellites in the mask.

o In order to fulfil the Time To Alert (TTA) requirement, a MT6-equivalent message with DFREI (Integrity Flags) will be sent every 6 seconds to ensure the validity of previous integrity broadcast parameters.

A description of the proposed ICD Model B messages to date is given below (from [RD2]). Note that the messages’ names are still under decision even in [RD2].

In the following table this can be seen, together with the equivalence of ICD Model A MTs with ICD Model B MTs.

Table 5-2: Message type relations between ICD Model A and ICD Model B

L5 MOPS Message Type Equivalent L1 MOPS MT

Max Update Interval [s]

Don’t Use for Safety Applications

6 6

PRN Mask 1 120 Integrity 6 6 SV Corrections 2-5,24,25,28,18,26 30 to 216

(TBC) SBAS Ephemeris 9 30 to 216

(TBC)


JASMIN-GMV-5100-D-5110-PU-1.1 21.06.2013

L5 MOPS Message Type Equivalent L1 MOPS MT

Max Update Interval [s]

Degradation Parameters 7+10 120 SBAS Almanac 17 120

5.2.2 ICD Model B message definition

In this section, the different MTs of ICD Model B will be reviewed from [RD2].

5.2.2.1 MT L5 Satellite Mask Message

MT L5 Satellite Mask Message specifies the satellites used by the system. It will need to also include Galileo and Compass satellites, and 37 PRNs are proposed for each core constellation. In addition, SBAS PRN values should also be considered. Its structure is identical to L1 MT1. This mask is proposed to be used only by the alert message.

In the following table, it is shown the proposed L5 satellite mask and how it aligns with the existing satellite mask. This new mask has been selected in order to not change any previously specified bit. The structure of the new satellite mask message is identical to the existing MT1 in ICD Model A. The only change is that more bits have been defined with PRNs belonging to new constellations.

Note that this proposal has still room to also include IRNSS and QZSS as needed. In addition, it could even be possible to use a second mask in order to include more GNSS constellation as well.

Table 5-3: Proposed L5 satellite mask in ICD Model B

L5 PRN Slot Satellite Assignment Current L1 Slots

1-37 GPS 1-37

38-74 GLONASS 38-61

75-111 Galileo -

112-119 Reserved 62-119

120-158 SBAS 120-138

159-173 Reserved 139-210

174-210 Compass -


JASMIN-GMV-5100-D-5110-PU-1.1 21.06.2013

As in MT1 from ICD Model A, a 0 at a particular location indicates that the satellite is not being used by the service provider while a 1 indicates that the corresponding satellite is being used by the SBAS system.

Under the current proposal, the new mask is only used by the new alert message (described in the following point). That is, the mask allows the user to determine which alert information corresponds to which satellite. No other messages require the satellite mask to determine against which satellite to apply the information.

5.2.2.2 MT Alert Message

MT Alert Message provides the satellite alerts. This message is the equivalent of MT6 in ICD Model A. In order to send the alert information for all satellites within the TTA the alert bits have been reduced with respect to MT6 in ICD Model A, and the new message structure would be as follows:

• IODP: Two initial bits are sent to specify the applicable satellite mask.

• DFRECI: Two bits to alert the satellite status are sent for each of the 91 satellites.

• DFREI: Four bits to provide the Dual Frequency Range Error for up to 7 satellites in each MT6-equivalent message, i.e., every six seconds. However, MT6 may not be used as primary method to follow increases of DFREI because this could generate integrity problems for users who lost the update messages (see [RD2] for further details).

The key point on the alert message is that it is expected to use ICD Model B for multi-constellation SBAS, being able to send corrections for more than 51 SVs within the required TTA. One option would be to completely change the alerting methodology. Nevertheless, ICD Model B proposes not to completely change the alert methodology but just to reduce the required number of alert bits down from four.

In particular, in ICD Model B it is proposed to use only two bits to alert the satellite status. This only requires a not completely but small change on the alert methodology.

With this perspective, instead of being able to update all satellite integrity states to any acceptable value within a single message, in this case it is only possible to alert some satellites to any state and the majority of the satellites have a more limited set of update values.

The two bits are used to indicate one of the following states:


JASMIN-GMV-5100-D-5110-PU-1.1 21.06.2013

• Unchanged (0 0)

• Changed (0 1)

• Not Monitored (NM) (1 0)

• Do Not Use (DNU) (1 1)

The most common state for this message should be unchanged. That is, the full state was provided earlier with the full correction (see MT Satellite correction Message in section 5.2.2.3) and it has not changed since that time.

The next common state would likely be Not Monitored (NM). That is, the satellite is not in view or not sufficiently viewed by the SBAS to provide a valid correction.

Relatively infrequently, the SBAS may elect to change the integrity state from one numerical value to another with this alert message. If it chooses to do so, there is space for seven full four-bit integrity values, called DFREs (Dual Frequency Residual Errors) to distinguish them from the L1-only integrity state values.

Very rarely, the system may need to indicate that one or more satellites have an integrity problem and should not be used for navigation.

Like the L1-only UDREI (UDRE Indicator), the four-bit DFREI (DFRE Indicator) identifies one of sixteen possible states. Fourteen are numerical and will be described later and the last two correspond to Not Monitored and Do Not Use (DNU).

In the following table, it is provided the message format for this alert message. The last seven full DFREI correspond in order to previously identified changed values. That is, the first DFRECI to indicate a change will have its full DFREI in the first full slot at the end of the message. The second DFRECI to indicate a change will have its full DFREI in the second full slot, and so on. Thus, no more than seven DFREIs may be changed from one numerical value to another in between satellite updates. When a DFRECI indicates a change it must continue to do so until the correction message(s) with the invalid DFREI update has timed out (or the confidence value is sufficiently increased by degradation terms).

Table 5-4: Proposed L5 satellite alert message structure to support L1/L5 users.

Parameter No. Of Bits Scale Factor (LSB) Effective Range

IODP 2 1 0 to 3

For each of the 91 satellites

-


JASMIN-GMV-5100-D-5110-PU-1.1 21.06.2013

Parameter No. Of Bits Scale Factor (LSB) Effective Range

DFRECI 2 1 0 to 3

For each of up to 7 SVs

-

DFREI 4 1 0 to 15

Any satellite may be changed to NM or DNU at any time. Such a change does not require a full DFREI update and therefore does not end up filling one of the slots at the end of the message. If there is any danger of needing more than seven slots within the update period, the service provider should send a larger initial value with the full message, increase the message degradation parameters (discussed later), or set some satellites to NM that otherwise could have had valid numerical values.

5.2.2.3 MT Satellite Correction Message

MT Satellite Correction Message proposes to send all the required corrections for an individual satellite in a single message, eliminating the fast corrections. Additionally the clock/ephemeris rate values have reduced their dynamic range, and the quantization values have also been reduced. As far as the equivalent L1 MT28 covariance matrix parameters are concerned, this information is used in the same way as it was for L1. Finally the number of bits to identify IODE has been increased in 2 bits to optimize the data source matching.

The components of the satellite corrections are spread across many messages in the L1 SBAS MOPS. This implies a certain complexity in ICD Model A and different messages should arrive to build the correction of an individual satellite. As shown in Figure 5-2, it requires between 6 and 12 messages to receive the full correction for any individual satellite. The MOPS is designed to be loss tolerant, so the user should be able to operate even if they are missing any individual message, thus there are many possible valid corrections in use depending on which message an aircraft has received or lost. At least three additional messages are required to calculate the integrity bound for the satellite correction. Again this leads to multiple conceivable valid bounds in use. Accounting for all these variations leads to additional complexity both in the aircraft and for the service provider.

ICD Model B proposes to reduce this complexity by combining all of the required corrections for an individual satellite into a single message as shown in Figure 5-3. This message also includes more of the integrity information, so that it may be more closely connected to the


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correction. This approach significantly lowers the number of possible active corrections to two: either the current message, or the one before.

In the next table it is shown the content of the satellite correction message in ICD Model B.

Table 5-5: content of ICD Model B satellite correction message.

Parameter No. Of Bits Scale Factor (LSB) Effective Range Units

PRN 8 1 1 to 210 -

IODE 10 1 0 to 1023 -

δx (ECEF) 11* 0.03 ±30 Meters

δy (ECEF) 11* 0.03 ±30 Meters

δz (ECEF) 11* 0.03 ±30 Meters

δB (ECEF) 12* 0.03 ±60 Meters

δx_dot (ECEF)

8* 2-12 ±0.0313 Meters/sec

δy_dot (ECEF)

8* 2-12 ±0.0313 Meters/sec

δz_dot (ECEF)

8* 2-12 ±0.0313 Meters/sec

δB_dot (ECEF)

9* 2-13 ±0.0313 Meters/sec

Time of day, t0

13 16 0 to 86384 Seconds

Scale exponent

3 1 0 to 7 -

E1,1 9 1 0 to 511 -

E2,2 9 1 0 to 511 -

E3,3 9 1 0 to 511 -

E4,4 9 1 0 to 511 -

E1,2 10* 1 ±512 -

E1,3 10* 1 ±512 -

E1,4 10* 1 ±512 -

E2,3 10* 1 ±512 -

E2,4 10* 1 ±512 -

E3,4 10* 1 ±512 -

DFREI 4 1 0 to 15 -


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The satellite correction message in ICD Model B includes the full PRN value for the satellite. Then, it is not necessary to use the satellite mask to determine to which satellite this information applies. The IODE, which is used to match this correction with the broadcast data from the GNSS satellite, has an extra couple of bits associated with it. This is to allow better matching between these two data sources. Two options are being considered: to use the full IODC for GPS, or to create a 10-bit hash of the broadcast data or some combination of the two approaches.

The clock/ephemeris and clock/ephemeris rate values have reduced dynamic range and quantization level values compared to ICD Model A. As it can be seen in Table 5-17, the dynamic range is reduced to 30 m and the quantization error is reduced to 3 cm. The rate of change corrections have also their quantization levels reduced.

The equivalence of the L1 MT28 clock-ephemeris covariance matrix parameters are in the back half of the satellite correction message in ICD Model B. These will be formed and used in the same manner as they are for the L1-only integrity bounding.

Finally, the full DFREI is included at the end of DFRE satellite correction message. These values combine to provide the basis for the confidence bound. As in the L1 SBAS MOPS, there are additional degradation terms that can increase the overall confidence bound.

In ICD Model B in order to be able to send up 91 satellite correction messages within 150 seconds, it is recommended suppressing the fast correction entirely. For the current WAAS or EGNOS L1 systems, approximately one half of the available bandwidth is taken up by the fast corrections and UDREs in MT 2, 3, and 4. In a multi-constellation context, although ICD Model A has provisions for sending the fast corrections less frequently, there is no longer any need to send separate fast corrections at all. Instead, the full clock and clock rate corrections are described by the δB and δB_dot terms listed in Table 5-17. In fact, everything needed to determine the L1/L5 SBAS correction for an individual satellite is contained within the MT Satellite Correction Message from ICD Model B.

5.2.2.4 MT SBAS Satellite Orbit Message

MT SBAS Satellite Orbit Message provides full orbital information for the broadcast satellites. ICD Model B intends to allow any type of orbit (not only a geostationary orbit as in L1 SBAS MOPS) for L5 messages, and thus Keplerian elements (and some additional information) are proposed to be broadcast in two (instead of one) messages for each SBAS satellite (each satellite will only broadcast its own two messages).


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The GNSS satellite correction message provides a correction on top of the ephemeris messages broadcast from the GNSS satellite themselves. SBAS also needs to provide full orbital information for the satellites from which it broadcasts its messages. In the L1 SBAS MOPS these are referred to as geostationary satellites or GEOs. However, in ICD Model B, it is intended to allow any type of orbit to be used for broadcasting the SBAS L5 messages.

In order to use the SBAS satellite for ranging, in addition to message broadcast, it is necessary to be able to accurately specify its position, for example, at 3 cm level. In order to accommodate a greater variety of orbit types, parameters such as the position, velocity and acceleration as the broadcast elements are no longer used in ICD Model B. It is proposed to use instead Keplerian elements with some additional terms.

Nevertheless, it is not possible to describe all possible orbits with 3 cm using a single message. Instead, it is proposed to send the orbit information in two messages.

In the following table it is presented the content of the ephemeris messages.


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Table 5-6: content of ICD Model B satellite ephemeris messages.

Parameter No. Of Bits Scale Factor (LSB) Effective Range Units

PRN 8 1 1 to 210 -

IODG x 2 8 1 0 to 16 -

Health & Status

3 - - Unitless

Provider ID 4 - - Unitless

Semi-major axis

32 0.01 0 to 4.9 x 107 Meters

Eccentricity 31 2-31 0 to 1 Dimensionless

Inclination 34 π x 2-34 0 to π/2 Radians

ω 34* π x 2-33 ±π Radians

Ω0 34* π x 2-33 ±π Radians

M0 34* π x 2-33 ±π Radians

Cuc 21* 1.3 x 10-9 ±0.0014 Radians

Cus 21* 1.3 x 10-9 ±0.0014 Radians

IDOT 22* 8.5 x 10-14 ±1.78x10-7 Rad/sec

Time of day, t0

13 16 0 to 86384 Seconds

aGf0 12* 0.02 ±40.96 Meters

aGf1 10* 5 x 10-5 ±0.0256 Meters/sec

Scale exponent

3 1 0 to 7 -

E1,1 9 1 0 to 511 -

E2,2 9 1 0 to 511 -

E3,3 9 1 0 to 511 -

E4,4 9 1 0 to 511 -

E1,2 10* 1 ±512 -

E1,3 10* 1 ±512 -

E1,4 10* 1 ±512 -

E2,3 10* 1 ±512 -

E2,4 10* 1 ±512 -

E3,4 10* 1 ±512 -

DFREI 4 1 0 to 15 -


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The full PRN value can be used to ensure that the intended satellite is being tracked. Again, the satellite mask is not needed. A 4-bit IODG is present in each of the messages to link the two halves together. The health and status of the SBAS satellite are indicated with 3 bits as they are defined in the L1 SBAS MOPS.

There are also included the service provider ID bits to match against the approach plate as needed. As already mentioned, the six Keplerian elements are provided, as can be seen in Table 5-18: semi-major axis, eccentricity, inclination, argument of perigee (ω), longitude of ascending node (Ω0), and mean anomaly at reference time (M0). Three more elements are added to improve the orbital accuracy: the amplitudes of the cosine and sine harmonic correction terms to the argument of latitude (Cuc and Cus respectively), and the rate of change of the inclination angle (IDOT). The clock and clock rate correction terms: aGf0 and aGf1, and the time of epoch are also included. Because these terms are a subset of the parameters broadcast for the ephemeris by the GPS satellites, the same process that GPS specifies for converting these elements to the current position may be used.

Finally, the clock-ephemeris covariance matrix parameters and the DFREI for the SBAS satellites are included in the ephemeris messages. Although the bits are spread across two ephemeris messages, the IODG ensures that the two parts are combined to form a unique message. One may not combine two halves with different IODG values.

The SBAS satellite will only send this message pair to describe its own orbit. If other SBAS satellites are used, the user must get the ephemeris messages from those other SBAS satellites. The SBAS satellite supplying the correction stream will broadcast satellite correction messages for the other SBAS satellites even if they come from the same service provider. No satellite correction message is needed for the ephemeris corresponding to the SBAS satellite providing the correction stream, that is, an SBAS satellite will not provide a satellite correction message for itself. It does not need to correct its own ephemeris information.

5.2.2.5 MT SBAS Almanac Message

MT SBAS Almanac Message is currently under definition in the context of ICD Model B.

The objective would be to have it with accuracy within one degree over the longer term (many days or weeks). It will be also intended to fit two SBAS satellites into a single almanac message. Let us remember that the almanac message serve two purposes: to let the user


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know about other SBAS satellites, and to allow the user to determine which SBAS satellites are likely in view at some point in the future.

5.2.2.6 MT Degradation Parameters Message

MT Degradation Parameters contains time-out information and parameters to increase the sigma values over time or when missing messages, taking into account the following approximations:

• In principle, individual values for each satellite are not considered (terms to cover all satellites are proposed).

• The correction degradation term takes into account not only the uncertainty of the acceleration correction, but also the correction rate uncertainty, which is consistent with the results provided in SBAS L1 MOPS [RD4].

In ICD Model A, MT7+10 contain time-out information and parameters to increase the sigma values over time for example when the receiver losses messages. Similar parameters are still needed for the L5 SBAS MOPS, although with the simplified satellite correction message, there are fewer cases that need to be covered. Rather than having individual terms for each satellite as in MT7 SBAS-L1 it is proposed in [RD2] to include terms to cover all satellites. Future modifications may propose different values for different core constellations. This may trade ease of utilization with some level of availability. Currently, just a single value for all satellites could be adequate.

In the following table it is shown the content of ICD Model B degradation parameters message.


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Table 5-7: ICD Model B content of degradation parameters message.

Parameter No. Of Bits Scale Factor (LSB) Effective Range Units Time Out Interval

6 6 60 to 432 Seconds

Update Interval: Icorr

5 6 30 to 216 Seconds

Ccorr 8 0.01 0 to 2.55 Meters Rcorr 8 0.2 0 to 51 mm/sec Acorr 8 0.02 0 to 5.1 mm/sec2

RSSdfc 1 - 0 to 1 Unitless Cer 6 0.5 0 t 31.5 Meters

Ccovariance 7 0.1 0 to 12.7 Unitless DFRE Table 84 σDFRE: DFREI

= 0 4 0.0625 0.125 to 1.0625 Meters

σDFRE: DFREI = 1

4 0.125 0.25 to 2.125 Meters

σDFRE: DFREI = 2

4 0.125 0.375 to 2.375 Meters

σDFRE: DFREI = 3

4 0.125 0.5 to 2.5 Meters

σDFRE: DFREI = 4

4 0.125 0.625 to 2.625 Meters

σDFRE: DFREI = 5

4 0.25 0.75 to 4.75 Meters

σDFRE: DFREI = 6

4 0.25 1 to 5 Meters

σDFRE: DFREI = 7

4 0.25 1.25 to 5.25 Meters

σDFRE: DFREI = 8

4 0.25 1.5 to 5.5 Meters

σDFRE: DFREI = 9

4 0.25 1.75 to 5.75 Meters

σDFRE: DFREI = 10

4 0.5 2 to 10 Meters

σDFRE: DFREI = 11

4 0.5 2.5 to 10.5 Meters

σDFRE: DFREI = 12

4 1 3 to 19 Meters

σDFRE: DFREI = 13

4 3 4 to 52 Meters

Spare - - - -

The DFRE degradation parameters message includes the time-out interval for the satellite correction and SBAS ephemeris messages. This parameter allows for a broad range of options because the number of corrected satellites may vary from below 20 to more than 90. A separate entry specifies the update interval, Icorr. This is a separate entry to allow more flexibility in how many active messages a service provider may choose to allow. This


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parameter specifies the targeted time interval between the satellite correction and ephemeris messages. It also allows for an automatic bump in the confidence value to older messages.

The magnitude of the bump is specified by Ccorr. Parameters to specify correction rate and acceleration uncertainty are also provided (Rcorr and Acorr respectively). These are combined to form a correction degradation term εcorr. For more information on that, please refer to [RD2].

ICD Model B degradation parameters message also includes an RSSdfc bit that determines how the elements of the bounding of the dual frequency corrections are combined. There is an en route degradation parameter that is applied if corrections older than the time-out interval are applied (en route users may continue to use corrections for an additional update interval beyond the time out interval).

Finally there is a covariance term that is used with the MT28 parameters exactly as it is in the L1 SBAS MOPS and therefore becomes part of δDFRE. They are combined to form the dual frequency correction variance overbound. Again, please refer to [RD2] for more information.

ICD Model B degradation parameters message also includes the ability to fully specify the numerical values for σDFRE. This allows the values to be optimized for the service provider’s capability and minimize the quantization penalty inherent in only having 14 numerical values. The L5 MOPS need to be determined before the final achievable DFRE values will be known. This flexibility allows the values to be optimized later and also allows optimization to occur at the service provider level.

Until the user has received this message from the service provider they should assume that the DFRE values are at the maximum value for each index. One should be very careful about changing these values during operation. Either, the system has to assure the lower of the two active values or the system should have a planned interruption in service to allow the old message to time out before generating DFREIs associated with the new message.

5.2.2.7 Ionosphere Information

To date no ionospheric messages are considered to support enhanced L5-only capability.

The messages on the SBAS L5 frequency will specifically correct the ionosphere-free combination of the L1 and L5 signals from the core constellation.

This fact has the advantage that without ionospheric corrections or the need for ionospheric alerts, it becomes easier to broadcast messages according to a rigid pre-planned schedule.


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But let us note that this gain of bandwidth obtained from the fact that no ionosphere corrections are needed, is common to all ICDs under consideration.

Nevertheless, it is possible that some service providers will want to provide a reversionary L5-only service. Such a service may allow for improved operational capability in environments where the L1 frequency is experiencing interference. For the moment, in the context of ICD Model B, ionospheric corrections to support enhanced L5-only capability have not been developed. Receiver Autonomous Integrity Monitoring (RAIM) will support horizontal guidance in regions where L5 is clearly received, but L1 is not. Reversionary L5-only RAIM will allow the aircraft to navigate out of regions with interference.

5.2.3 Summary of the different MTs in ICD Model B

In the following figures it is provided a high-level schematic structure of the messages and components required to correct a single satellite under the proposed ICD Model B respectively.

Figure 5-3: High-level schematic of message structure for the proposed ICD Model B.

5.2.4 ICD Model B Alternative


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The current definition of the ICD Model B proposes an update of up to seven DFREs in each MT6-equivalent message (the so-called “Alert Message”), i.e., every six seconds. The “up to seven” proposal is based on sending the DFRECI values for 91-satellites, but taking into account that a scenario with less than 4 constellations is very feasible, a modification was already proposed by GMV [RD10] to improve performances if less than 91 satellites are configured in the satellite mask.

This modification, called ICD Model B Alternative, is proposed to improve the updating of the DFRE information without changing the ICD Model B messages content, but to improve its use taking into account only the satellites in mask.

ICD Model B Alternative is proposed as an attempt of mitigation of one of the few problems identified so far in ICD Model B proposed definition, improving the robustness of this model against GNSS satellites whose performances are not so good as those already observed for GPS.

Hereafter, a description of the modifications proposed in ICD Model B Alternative is provided. Nevertheless, let us point out that as ICD Model B Alternative is based in ICD Model B, only the modifications will be explained. For all the information of ICD Model B Alternative identical to the one of ICD Model B, please refer to Section 5.2.

ICD Model B Alternative proposes not to systematically send 91 DFRECIs, but only as many DFRECIs as satellites configured in the satellite mask. In this case it is proposed to send not only “up to seven” DFREI values, but all feasible (taken into account the new available free bits in MT6-equivalent message) DFREI values in the corresponding MT6. Considering, for example, a 32-satellite mask (instead of 91 satellites for 4 constellations), this implies the use of 32x2=64 bits, 118 bits less than in DFREC status for the 91 satellites is broadcast (which would imply 91x2=182 bits), which are now free to send other satellites DFREI information if needed.

In the case previously considered it would be thus feasible to send the DFREs values of all satellites in a single message (4 bits per DFREI, 128 bits for 32 satellites).

Therefore, the DFRE information broadcast in each MT-6 equivalent message could be as follows:


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• First of all the corresponding IODP is sent (2 bits),

• Second, the DFRECI values for all satellites in mask (2 bits per satellite),

• Finally, the DFREI values for all feasible (taking into account the message free bits) satellites in mask (4 bits per satellite).

Please note that the proposed alternative is robust upon the loss of message: DFRECI values for all satellites are given every 6 seconds in each MT6-equivalent messages, regardless the number of DFREI values broadcast after the DFRECI information. The DFREI values of all satellites (no matter if their DFREI have changed or not) are broadcast in consecutive alert messages to complete the total satellites in mask.

Taking into account ICD Model B original model, the maximum number of satellites in mask (91), DFREI values for up to 7 satellites could be sent in a single message:

(212 initial bits – 2 IODP bits – 182 DFRECI bits) / 4 (bits for DFREI) = 7 DFREI values,

and therefore the DFREI values for 4 constellations (91 satellites) could be sent using 13 messages (all DFREI values would be updated in 78 seconds if any message is lost). DFREI update intervals for 1-4 constellations are calculated if any message is lost, and are provided in the next table:

Table 5-8: ICD Model B MT6 message: Update Intervals.

#Sats in Mask

# Sats monit

# DFRE per MT6

# ICD Model B MT6

ICD Model B MT6 Update

Interva (secods)l

4C 91 91 7 13 78

3C 91 68 7 10 60

2C 91 45 7 7 42

1C 91 24 7 4 24

The key point of this alternative is to take advantage of the free bits available in case the number of satellites in mask is below 91. In that case DFREI values for more than 7 satellites could be broadcast. Therefore this improvement will positively impact the achieved performances, mainly if 1 or 2 constellations are considered. Next table provides the structure of the ICD Model B Alternative modified MT6 message.


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Table 5-9: Proposed ICD Model B Alternative modified MT6 message.

Parameter # bits Effective Range

IODP 2 0 to 3

For each of the satellites in mask

DFRECI 2 0 to 3

For as many satellites in mask as possible

DFREI 4 0 to 15

Taking into account this new MT6 message, the DFREI update intervals for 1-4 constellations are calculated if any message is lost, and are provided in the following table:

Table 5-10: ICD Model B Alternative modified MT6 message: Update Intervals.

# Sats in Mask

# Sats monit

# DFRE per MT6 #ICD_2 MT6

ICD_2 MT6 Update Interva

(secods)l 4C 91 91 7 13 78

3C 82 68 11 7 42

2C 58 45 23 2 12

1C 31 24 37 1 6

Comparing the MT6 update intervals achieved for the aforementioned option with the equivalent update intervals of the original ICD Model B, it is clear that the decrease in the MT6 update intervals achieved with ICD Model B Alternative will positively impact the achieved performances for 1 to 3 constellations (all DFREIs would be updated in less than 24 seconds in the worst case (3C), decreasing the degradation of these values). For 4 constellations the ICD Model B Alternative does not improve (neither worsens) the figures provided for ICD Model B original model.


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6 SBAS L5 MESSAGE TABLES

The objective of this section is to include a summary table of the proposed message type according to each identified ICD.

6.1 SUMMARY OF THE DIFFERENT MTS IN ICD MODEL A

In the following table there is a summary on the different MTs content in ICD Model A.


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Table 6-1: ICD Model A Message Structure summary.

MT0 Don’t Use For Safety Applications See Section 5.1.2.1

MT1 PRN Mask Parameter Number of bits Effective Range

Mask 210 - IODP 2 0 to 3

MT2 Fast Corrections Parameter Number of bits Effective Range

IODP 2 0 to 3 Mask Position 7 1 to 913

For as many satellites in mask as possible Fast Corrections 12 -256.000-

255.875 (m) MT6_1 Integrity Information

Parameter Number of bits Effective Range

For each of 51 satellites (204 bits) UDREi 4 0 to 15 Spare 8 -

MT6_2 Integrity Information Parameter Number of bits Effective Range

IODP 2 0 to 3

For as many remaining monitored satellites

UDREIs 4 0-15

Mask Position 7 1 to 91


Fast Corrections 12 -256.000-255.875 (m) MT7+10 FCs Degradation Factor


System latency (tlat) 4 0 to 15 For each core constellation (up to 4 constellations, implying 16 bits)

Degradation factor

indicator (ai) 4 0-15

Brrc 10 0-2.046

3 Please note that the size of this parameter is a worst case, and could be reduced in case less satellites are considered.


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Cltc,lsb 10 0-2.046 Cltc,vl 10 0-0.05115 Iltc,vl 9 0-511

Cltc,v0 10 0-2.046 Iltc,v0 9 0-511

Cgeo,lsb 10 0-0.05115 Cgeo,v 10 0-0.05115 Igeo 9 0-511 Cer 6 0-31.5

Ciono_step 10 0-1.023 Iiono 9 0-511

Ciono_ramp 10 0-0.005115 RSSUDRE 1 0-1 RSSiono 1 0-1

Ccovariance 7 0-12.7 Spare 57 -

MT9 GEO ranging functions parameters Parameter Number of bits Effective Range

Spare 4 Provider ID 4 -

t0 13 0 to 86384 URA 4

XG (ECEF) 30 ±42949673 YG (ECEF) 30 ±42949673 ZG (ECEF) 25 ±6710886.4

XG Rate of change 17 ±40.96 YG Rate of change 17 ±40.96 ZG Rate of change 18 ±524.288

XG Acceleration 10 ±0.0064 YG Acceleration 10 ±0.0064 ZG Acceleration 10 ±0.032

aGf0 12 ±0.9537×10−6 aGf1 8 ±1.1642×10−10

MT12 SBAS Network Time / UTC offset parameters Parameter Number of bits Effective Range

A1WNT 24 ± 7.45×10-9 A0WNT 32 ±1 t0t 8 0 to 602112 WNt 8 0 to 255


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ΔtLS 8 ±128 WNLSF 8 0 to 255 DN 8 1 to 7 ΔtLSF 8 ±128 UTC Standard Identifier

3 -

GPS Time-of-Week —TOW

20 0 to 604799

GPS Week Number

10 0 to 1023

GLONASS Indicator

1 0 to 1

GLONASS time offset 24 ±2-8 s

Galileo Indicator 1 0 to 1

Galileo Time Offset 24 ±2-8 s

Compass Indicator 1 0 to 1

Compass Time Offset 24 ±2-8 s


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MT17 GEO satellite almanacs Parameter Number of bits Effective Range

For each of 3 satellites Spare 2

PRN Number 8 0 to 210 Health and Status 8 -

XG (ECEF) 15 ±42595800 YG (ECEF) 15 ±42595800 ZG (ECEF) 9 ±6630000

XG Rate of change 3 ± 40 YG Rate of change 3 ± 40 ZG Rate of change 4 ± 480

to (Time-of-Day)

11 0 to 86336

MT18 Ionospheric Grid Point Masks Parameter Number of bits Effective Range

Number of Bands being broadcast

4 0 to 11

Band Number 4 0 to 10 IODI 2 0 to 3

IGP Mask 201 - Spare 1 -

MT24 Mixed Fast Corrections / Long Term SV Error Corrections Parameter Number of bits Effective Range

Spare 4 - For each of 6 satellites

Fast Corrections 12 For each of 6 satellites

UDREi 4 0 to 15 IODP 2 0 to 3 IODF 2 0 to 3 106-bit long term correction (see MT25 and Section 5.1.2.11)

MT25 Velocity code 0 Long Term SV Error Corrections Parameter Number of bits Effective Range

Velocity Code=0 1 - PRN Mask Number 6 0 to 51

Issue of Data 8 0 to 255 δx (ECEF) 9 ±32 δy (ECEF) 9 ±32


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δz (ECEF) 9 ±32 δaf0 10 ±2-22

PRN Mask Number 6 0 to 51 Issue of Data 8 0 to 255

δx (ECEF) 9 ±32 δy (ECEF) 9 ±32 δz (ECEF) 9 ±32

δaf0 10 ±2-22 IODP 2 0 to 3 Spare 1 -

MT25 Velocity code 1 Long Term SV Error Corrections


Velocity Code=1 1 - PRN Mask Number 6 0 to 51

Issue of Data 8 0 to 255 δx (ECEF) 11 ±128 δy (ECEF) 11 ±128 δz (ECEF) 11 ±128

δaf0 11 ±2-21 δx rate-of-change (ECEF) 8 ±0.0625 δy rate-of-change (ECEF) 8 ±0.0625 δz rate-of-change (ECEF) 8 ±0.0625

δaf1 8 ±2-32 Time-of-Day Applicability

t0 13 0 to 86384

IODP 2 0 to 3 MT26 Ionospheric Delay Corrections


Band Number 4 0 to 10 Block ID 4 0 to 13

For each of 15 Grid Points IGP Vertical Delay

Estimate 9 0 to 63875

GIVEI 4 0 to 15

IODI 2 0 to 3 Spare 7 -

MT27 SBAS Service Message Parameter Number of bits Effective Range


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IODS 3 0 to 7 Number of Service

Messages 3 1 to 8

Service Message Number 3 1 to 8 Number of regions 3 0 to 5

Priority Code 2 0 to 3 δUDRE Indicator – Inside 4 0 to 15

δUDRE Indicator – Outside 4 0 to 15 For each of up to 5 regions:-

Coordinate 1 Latitude 8 ±90 Coordinate 1 Longitude 9 ±180 Coordinate 2 Latitude 8 ±90

Coordinate 2 Longitude 9 ±180 Region Shape 1 -

Spare 15 - MT28 Covariance Matrices


IODP 2 0 to 3 PRN Mask Number 6 0 to 51

Scale exponent 3 0 to 7

• E1,1 9 0 to 511 E2,2 9 0 to 511 E3,3 9 0 to 511 E4,4 9 0 to 511 E1,2 10 ±512 E1,3 10 ±512 E1,4 10 ±512 E2,3 10 ±512 E2,4 10 ±512 E3,4 10 ±512

PRN Mask Number 6 0 t 51 Scale exponent 3 0 to 7

E1,1 9 0 to 511 E2,2 9 0 to 511 E3,3 9 0 to 511 E4,4 9 0 to 511 E1,2 10 ±512 E1,3 10 ±512 E1,4 10 ±512


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E2,3 10 ±512 E2,4 10 ±512 E3,4 10 ±512

MT30 Inter-Signal Corrections and Integrity Message Parameter Number of bits Effective Range

IODJj 2 0 to 3

IODP 2 0 to 3

For 17 slots:

ISC 8 ±2-24 s (TBD)

For 17 slots:

ISC Integrity 4 TBD

Panel 3 1-6

Spare 1 -

MT34 Satellite and IGP unified Alert Message Parameter Number of bits Effective Range

Global Alarm ld. 2 00: Nominal 01: Ionosphere Alarm

10: Global Alarm 11: Spare

IODP 2 0-3

IODI 2 0-3

No. Satellites 5 0-22 (1)

For each alarmed satellite

PRN in mask 8 0-210 (2)

ISC alert ld. 1 0: UDRE alarm 1: ISC alarm only

No. IGPs 5 0-16 (3)

For each alarmed IGP

Band number 4 0-10

Bit in band 8 0-201

Notes 1. Although the possible range of the number of satellites alarmed is 0-31, the

maximum value is limited to 22 due to the number of bits available for the specification of the PRN in mask and the ISC alert Id. of the affected satellites.

2. Mask sequence. The count of 1’s in mask from the first position representing the subject satellite; if set to 0, no satellite is represented.

3. Although the possible range of the number of IGPs alarmed is 0-31, the maximum value is limited to 16 due to the number of bits available for the specification of the band and bit in band of the affected IGPs.

MT35 Parameter Number of bits Effective Range


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IODP 2 0 to 3

For each (up to 102) satellites implying 204 bits

Alert Info 2 0-2

Global Alarm Id. 1 0: Nominal 1: Global Alarm

ISC alert ld. 1 0: UDRE Alarm

Spare 4 -

6.2 SUMMARY OF THE DIFFERENT MTS IN ICD MODEL B

In the following table there is a summary on the different MTs content in ICD Model B. Table 6-2: ICD Model B Message Structure summary.

MT L5 Satellite Mask The structure of the new satellite mask message is identical to the existing MT1 structure.

MT Alert Message Parameter Number of bits Effective Range

IODP 2 0 to 3

For each of 91 satellites

DFRECI 2 0 to 3

For each of up to 7 SVs

DFREI 4 0 to 15

MT Satellite Correction Message Parameter Number of bits Effective Range

PRN 8 1 to 210

IODE 10 0 to 1023

δx (ECEF) 11 ±30

δy (ECEF) 11 ±30

δz (ECEF) 11 ±30

δB (ECEF) 12 ±60

δx_dot (ECEF)

8 ±0.0313

δy_dot (ECEF)

8 ±0.0313

δz_dot (ECEF)

8 ±0.0313

δB_dot (ECEF)

9 ±0.0313

Time of 13 0 to 86384


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day, t0


E1,1 9 0 to 511

E2,2 9 0 to 511

E3,3 9 0 to 511

E4,4 9 0 to 511

E1,2 10 ±512

E1,3 10 ±512

E1,4 10 ±512

E2,3 10 ±512

E2,4 10 ±512

E3,4 10 ±512

DFREI 4 0 to 15

MT SBAS Satellite Orbit Parameter Number of bits Effective Range

PRN 8 1 to 210

IODG x 2 8 0 to 16

Health & Status 3 -

Provider ID 4 -

Semi-major axis 32 0 to 4.9 x 107

Eccentricity 31 0 to 1

Inclination 34 0 to π/2

ω 34 ±π

Ω0 34 ±π

M0 34 ±π

Cuc 21 ±0.0014

Cus 21 ±0.0014

IDOT 22 ±1.78x10-7

Time of day, t0 13 0 to 86384

aGf0 12 ±40.96

aGf1 10 ±0.0256


E1,1 9 0 to 511

E2,2 9 0 to 511

E3,3 9 0 to 511

E4,4 9 0 to 511

E1,2 10 ±512


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E1,3 10 ±512

E1,4 10 ±512

E2,3 10 ±512

E2,4 10 ±512

E3,4 10 ±512

DFREI 4 0 to 15

MT Almanac MT Almanac is still to be defined.L

MT Degradation Parameters Parameter Number of bits Effective Range

Time Out Interval 6 60 to 432

Update Interval: Icorr

5 30 to 216

Ccorr 8 0 to 2.55

Rcorr 8 0 to 51

Acorr 8 0 to 5.1

RSSdfc 1 0 to 1

Cer 6 0 t 31.5

Ccovariance 7 0 to 12.7

DFRE Table 84


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σDFRE: DFREI = 0

4 0.125 to 1.0625

σDFRE: DFREI = 1

4 0.25 to 2.125

σDFRE: DFREI = 2

4 0.375 to 2.375

σDFRE: DFREI = 3

4 0.5 to 2.5

σDFRE: DFREI = 4

4 0.625 to 2.625

σDFRE: DFREI = 5

4 0.75 to 4.75

σDFRE: DFREI = 6

4 1 to 5

σDFRE: DFREI = 7

4 1.25 to 5.25

σDFRE: DFREI = 8

4 1.5 to 5.5

σDFRE: DFREI = 9

4 1.75 to 5.75

σDFRE: DFREI = 10

4 2 to 10

σDFRE: DFREI = 11

4 2.5 to 10.5

σDFRE: DFREI = 12

4 3 to 19

σDFRE: DFREI = 13

4 4 to 52

Spare - -


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7 ICD CONSOLIDATION

The objective of this section is to identify the level of consolidation of the proposed ICDs in relation with RF, Format/structure and Content from PROSBAS point of view.

7.1 SBAS L5 RF CONSOLIDATION

There are no major modifications expected related to RF issues respect to SBAS L1 MOPS [RD4].

The consolidation level can be considered as high.

7.2 SBAS L5 DATA STRUCTURE CONSOLIDATION

There are not modifications expected related to the data structure of L5 SBAS Messages presented in SBAS L1/L5 ICD [RD5] (250 bits per second with 8 preamble bits, 6 MT Identifier bits, 212 data field bits and 24 parity bits).

The consolidation level can be considered as high.

7.3 SBAS L5 DATA CONTENT CONSOLIDATION

In this section the consolidation level of the structure of the different MTs for both ICDs will be presented. In general, the level of ICD consolidation in terms of message content is Medium depending on the ICD.

7.3.1 ICD Model A

Next table shows the degree of consolidation expected for each Message Type in ICD Model A. For messages types labelled as “Low” it is expected some additional information coming from PROSBAS project. The ones labelled as “High” are considered more or less frozen from PROSBAS project perspective:

Table 7-1: ICD Model A MTs and Consolidation

MT ICD MT Consolidation

MT0: Don’t use for safety applications High

MT1: PRN Mask High


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MT2: Fast Corrections Low Modified in ICD Model A to only contain FCs

MT6_1: Integrity Information High

MT6_2: Integrity & FCs Low New message in ICD Model A to allocate the integrity information for the 52th satellite until

the last configured in the mask in the case that more than 51 SVs are configured in the mask. The free bits are used to allocate Fast

Corrections

MT7+10: Degradation Parameters Low MT7 and MT10 have been mixed. Only one Fast Correction Degradation Factor for each Constellation.

MT9: SBAS Satellite Navigation Message High

MT12: SBAS Network Time/UTC offset parameters

Low Galileo and Compass information has been included

MT17: SBAS Satellite Almanacs High

MT18: IGP Mask High

MT24: Mixed Fast Corrections and Long Term Corrections

High

MT25: Long Term Corrections High

MT26: Ionospheric Delay Corrections High

MT27: SBAS Service Message High

MT28: Clock-Ephemeris Covariance Matrix Message

High

MT30: Inter-Signal Corrections and Integrity Low It is proposed to use “Panel” bits to

distinguish for which satellites the data is referred instead of having MT30-32

MT34: Satellite and IGP unified alert Medium. The same as in SBAS L1/L5 ICD [RD5]

MT35: UDRE and ISC alerts Low


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It will be used for UDRE Alerts and ISC Alerts

7.3.2 ICD Model B

Next table shows the degree of consolidation expected for each Message Type in ICD Model B. For messages types labelled as “Low” it is expected some modifications coming from PROSBAS project. The ones labelled as “High” are considered more or less frozen from PROSBAS project perspective:

Table 7-2: ICD Model B MTs and Consolidation

MT Consolidation

MT L5 Satellite Mask Message High

MT Alert Message Low. ICD Model B Alternative is based on the proposal to modify this MT

MT Satellite Correction Message Medium

MT SBAS Satellite Orbit Message Medium

MT SBAS Almanac Message Low

MT Degradation Parameters Message Medium


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8 ANNEX A: ICD MODEL A DETAILED MESSAGE DEFINITION

This section includes the detailed definition of ICD Model A in terms of message structure :

8.1 MT0: DON’T USE FOR SAFETY APPLICATIONS

MT0 is not expected to change with respect L1 SBAS ICD and then we refer to SBAS L1 MOPS [RD4].

8.1.1 MT1: PRN Mask

MT1 is a message that specified the PRN mask.

Let us remember that each GPS, GLONASS, Galileo, Compass or SBAS satellites has a unique pseudo-random noise (PRN) code, which makes it identifiable by the user.

The PRN Mask consists of 210 PRN mask values that correspond to satellite PRN code numbers.

The mask enables the size of the messages to be optimised by showing to which PRNs the data contained in the other subsequent messages are related.

The mask contains 51 bits. A 1 in the nth bit shows that the nth satellite is being monitored by the SBAS system while a 0 means that the corresponding satellite is not being monitored by the SBAS system.

Issue of Data – PRN (IODP) is an indicator that associates the correction data with a PRN mask.

In principle, the only change with respect to L1 SBAS ICD is the fact that one should consider the slots for GALILEO and COMPASS constellations. This is also valid for ICD Model A.

In the following table it is shown the proposed L1/L5 Slots for ICD Model A based on the proposed L1/L5 Slots for ICD Model B from [RD2]:

Table 5-1: Proposed L1/L5 ICD Model A slots. L5 PRN Slot Satellite Assignment Current L1 Slots

1-37 GPS 1-37

38-74 GLONASS 38-61


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L5 PRN Slot Satellite Assignment Current L1 Slots

75-111 Galileo -

112-119 Reserved 62-119

120-158 SBAS 120-138

159-173 Reserved 139-210

174-210 Compass -

In the case that more than 37 GPS satellites are configured in mask (this is also valid for other constellations), as there are not free slots available, it is proposed to send a second MT Satellite Mask. This issue is important for future GPS III with ranges from 1 to 63.

This is also valid for ICD Model A and for ICD Model B Alternative.

In the case of ICD Model B and ICD Model B Alternative, this new MT can be called MT L5 Satellite Mask Message_2. In the case of ICD Model A this new MT can be called MT1_2.

It is recommended to allocate in this second MT Satellite Mask only the SVs from 38 to the number of SVs in the constellation, keeping the first MT Satellite Mask as it has been proposed in order to keep coherent with current MT Satellite Mask.

As an example, let us suppose that only GPS satellites have more than 37 PRNs, for example the future GPS III with PRNs from 1 to 63.

Then, it is proposed to keep the first MT Satellite Mask message as in the previous table and to allocate GPS PRNs from 38 to 63 in the second MT Satellite Mask message that would have the following structure:

Table 5-2: Example of second MT Mask Message

Parameter Number of bits Effective Range Mask 26 -

IODP 2 0 to 3

Spare 184 -

8.1.2 MTs 2: Modified Messages: Fast Corrections

MT2 are several messages with FCs information. For further information, please refer to Section 5.1.1.


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In the following table it is shown the structure of MT2.

Table 5-3: structure of MT2.


IODP 2 0 to 3



FCs 12 -256.000-255.875 (m)

8.1.3 MT6_1: Integrity Information

MT6_1 is a message that contains the integrity information broadcast every 6 seconds by means of the UDREI values for up to 51 satellites. If more than 51 satellites are configured in the mask, the integrity information of the rest of the satellites that cannot be allocated in MT6_1 will be sent in MT6_2.

MT6_1 is used in two instances: to refresh the UDREIs (note that although UDREIs are contained in MT2-5 in SBAS-L1 MOPS, their validity period may require more frequent updating) and to be able to broadcast satellite alerts (Don’t Use) very quickly if necessary (if broadcasting of an alert cannot wait).

In the following table there is the content of MT6_1:

Table 5-4: Proposed ICD Model A MT6_1


For up to 51 satellites 204 -

UDREI 4 0 to 15

Spare 8 -

8.1.4 MT6_2: Integrity and Fast Corrections

MT6_2 is a message that contains Integrity and Fast Corrections information. In the case that more than 51 satellites are configured in the mask, the integrity information of only 51



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satellites can be allocated in MT6_1. Then, the rest of integrity information for satellites from 52th until the last satellite will be allocated in MT6_2. The rest of the free bits (if any, that is, if less than 102 satellites are configured in the mask) is used to broadcast fast corrections. See Section 5.1.1 for further information.

The structure of MT6_2 can be seen in the following table:

Table 5-5: ICD Model A modified MT6_2 & MT2 messages.


IODP 2 0 to 3

For as many remaining monitored satellites (ONLY FOR MT6_2)

UDREIs 4 0-15



FCs 12 -256.000-255.875 (m)

8.1.5 MT7+10: Degradation Parameters

In L1-SBAS MOPS, two different messages MT7 with Fast Corrections degradation factors for each SV and MT10 with degradation factors were defined.

In ICD Model A, these two messages are somewhat unified in a single message MT7+10 comprising the needed information from former SBAS L1 MT7 and MT10.

Next table provides the structure of this message:



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Table 5-6: Proposed ICD Model A modified MT7+10 message


System Latency (tlat) 4 0-15

For each core constellation (up to 4 constellations)

16 -

Degradation factor indicator (aii) 4 0-15

Brrc 10 0-2.046

Cltc,lsb 10 0-2.046

Cltc,vl 10 0-0.05115

Iltc,vl 9 0-511

Cltc,v0 10 0-2.046

Iltc,v0 9 0-511

Cgeo,lsb 10 0-0.05115

Cgeo,v 10 0-0.05115

Igeo 9 0-511

Cer 6 0-31.5

Ciono_step 10 0-1.023

Iiono 9 0-511

Ciono_ramp 10 0-0.005115

RSSUDRE 1 0-1

RSSiono 1 0-1

Ccovariance 7 0-12.7

Spare 57 -

Please, note the in the frame of PROSBAS project is it expected that some modifications will be included for the degradation models.

8.1.6 MT9: SBAS Satellite Navigation Message

MT9 is a message that provides the GEO Navigation information, including the position, velocity and acceleration of the geostationary satellite, in ECEF Coordinates, and its apparent clock time and frequency offsets. It also includes the time of applicability and an accuracy exponent (URA) representing the estimated accuracy of the message.

In L1 SBAS ICD a problem related with the lack of identification of Service Provider ID has been identified since it has a two-step process due to the fact that in MT17, only two bits are used for this purpose.

Then, it has been proposed in the context of PROSBAS project to slightly modify MT9 L1 SBAS using 4 bits for Provider ID. This can be done using 4 of the 8 spare bits present in current MT9. Also MT17 will be modified substituting the 2 Provider ID bits by spare bits.

In the following table it is shown the data content of the proposed MT9:


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Table 5-7: Proposed MT9 in ICD Model A


Spare 4

Provider ID 4

t0 13 0 to 86384

URA 4

XG (ECEF) 30 ±42949673

YG (ECEF) 30 ±42949673

ZG (ECEF) 25 ±6710886.4

XG Rate of Change 17 ±40.96

YG Rate of Change 17 ±40.96

ZG Rate of Change 18 ±524.288

XG Acceleration 10 ±0.0064

YG Acceleration 10 ±0.0064

ZG Acceleration 10 ±0.0032

aGf0 12 ±0.9537 x 10-6

aGf1 8 ±1.1642 x 10-10

In addition, it is proposed to reduce the MT9 Update Interval from 120 to 60 seconds to ensure availability of the GEO ranging function even during maneuvers to have a correct availability rate per day with good performances.

8.1.7 MT12: SBAS Network Time/UTC offset Parameters

In current L1-SBAS MOPS, MT12 is a message that provides the SBAS Network Time/UTC/GLONASS Time Offset parameters. MT12 consisted of the 8-bit preamble, a 6-bit message type identifier by 104 information bits for the UTC parameters, then followed by 3 bits to indicate the UTC time standard from which the offset is determined. The next 20 bits


JASMIN-GMV-5100-D-5110-PU-1.1 21.06.2013

were the Time of Week (TOW) in seconds of the beginning of the message, followed by a 10 bits GPS Week number (WN). The final 75 bits were spare bits reserved to difference between SBAS Network Time and GLONASS time.

In ICD Model A a modification on MT12 has been proposed. The purpose of the modifications to MT12 is to include, if necessary, the time offset between constellations, when 4 constellations are included.

According to SBAS L1 MOPS [RD4], the original MT12 has the following structure:

Therefore, the last 75 bits can be used to send the time offset parameters as follows:


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Table 5-8: Modified part of MT12

Parameter Number of bits

Effective Range

GLONASS Indicator 1 0 or 1

GLONASS Time Offset

24 ±2-8 s

Galileo Indicator 1 0 or 1

Galileo Time Offset 24 ±2-8 s

Compass Indicator 1 0 or 1

Compass Time Offset

24 ±2-8 s

Then, the total bits that are used in MT12 are 212 without any spare bit.

It is worth noticing here that Galileo time offset integrity bound is not needed to be included since UDRE/DFRE values will be able to cover any feasible issue.

8.1.8 MT17: GEO Satellite Almanacs

MT17 is a message that provides the GEO Almanacs.

Almanacs for all GEOs are broadcast periodically to alert the user of their existence, locations, the general service provider and health and status.

As stated for MT9, in order to solve the lack of identification of Provider ID present in L1 SBAS ICD, the two Provider ID bits present in MT17 L1 SBAS ICD are substituted by spare bits (and 4 Provider ID bits are added in MT9).

In the following table the structure of MT17 is provided.


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Table 5-9: Proposed MT17 in ICD Model A


For each of 3 satellites

Spare 2

PRN Number 8 0 to 210

Health and Status 8 -

XG (ECEF) 15 ±42595800

YG (ECEF) 15 ±42595800

ZG (ECEF) 9 ±6630000

XG Rate of change 3 ±40

YG Rate of change 3 ±40

ZG Rate of change 4 ±480

t0 (Time-of-Day) 11 0 to 86336

8.1.9 MT24: Mixed Fast Corrections/Long Term Corrections

MT24 is a message that provides, if needed, the mixed fast corrections and long term satellite error corrections.

MT24 is not expected to change with respect to L1 SBAS ICD and then we refer to SBAS L1 MOPS [RD4].

8.1.10 MT25: Long Term Satellite Corrections

MT25 is a message that includes the long term satellite error corrections.



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8.1.11 MT18: IGP Mask

MT18 is a message that includes the ionospheric grid point mask to be used to apply the ionospheric corrections included in MT26 (in case of L5-only backup).

MT18 is not expected to suffer any modification with respect to L1 SBAS ICD and then we refer to SBAS L1/L5 ICD [RD5] for further information.

8.1.12 MT26: Ionospheric Delay Corrections

MT26 is a message that provides the ionospheric delay corrections for the iono grid points broadcast in MT18 (in case L5-only backup).


8.1.13 MT27: SBAS Service Message

MT27 is a message that broadcast the needed information to increase the σUDRE values in selected areas.


Let us remark that, as stated in Appendix A of SBAS L1 MOPS [RD4], MT27 and MT28 cannot be used both for the same service provider. For example, WAAS only used MT28. There is a good likelihood that MT27 will not be used in Europe. MT28 will be used by MSAS as well.

8.1.14 MT28: Clock-Ephemeris Covariance Matrix Message

MT28 is a message that includes the Clock-Ephemeris Covariance Matrix.


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8.1.15 MT30: Inter-Signal Corrections and Integrity Message

MT30 is a message with inter-signal corrections and integrity. For further information on that, please check Section 5.1.1.


Table 5-10: Inter-Signal Corrections and Integrity Message Type 30

Data Content Bits used Range of values Resolution

IODJj 2 0 to 3 1

IODP 2 0 to 3 1

For 17 slots:

ISCi 8 ±2-24 s (TBD) ±2-31 s (TBD)

For 17 slots:

ISC Integrity 4 TBD TBD

Panel 3 1-6 -

Spare 1 - -

8.1.16 MT34: Satellite and IGP unified alert

MT34 is a message that contains the Satellite and IGP unified alert data. For further information on this MT, please refer to Section 5.1.1.

In the following table the MT34 structure is provided:


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Table 5-11: Satellite and IGP unified alert MT34 from SBAS L1/L5 ICD [RD5].

Data content Bits used Range of values Resolution

Global Alarm Id. 2

00: Nominal; 01: Ionosphere

alarm; 10: Global alarm;

11: Spare

1

IODP 2 0-3 1 IODI 2 0-3 1 No. Satellites 5 0-22 (1) 1 For each alarmed satellite PRN in mask 8 0-210 (2) 1 ISC alert Id. 1 0: UDRE alarm;

1: ISC alarm only 1

No. IGPs 5 0-16 (3) 1

For each alarmed IGP Band number 4 0-10 1 Bit in band 8 0-201 1 Notes.— 1. Although the possible range of the number of satellites alarmed is 0-31, the maximum value is limited to 22 due to the number of bits available for the specification of the PRN in mask and the ISC alert Id. of the affected satellites. 2. Mask sequence. The count of 1's in mask from the first position representing the subject satellite; if set to 0, no satellite is represented. 3. Although the possible range of the number of IGPs alarmed is 0-31, the maximum value is limited to 16 due to the number of bits available for the specification of the band and bit in band of the affected IGPs.

8.1.17 MT35: UDRE and ISC alerts

MT35 is a message with either UDRE Alerts either ISC Alerts prepared to alert up to 102 satellites. For further information on MT35, please check Section 5.1.1.



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Table 5-12: ICD Model A modified MT35


IODP 2 0 to 3

For each (up to 102) satellites 204 -

Alert Info 2 0-2

Global Alarm Id. 1 0: Nominal 1: Global Alarm

ISC alert ld. 1 0: UDRE alarm 1: ISC alarm only

Spare 2 -

Note that MT35 will serve for both UDRE alert and ISC alert depending on the bit “ISC alert ld.”.

8.1.18 IODF Removal

The proposed definition of ICD Model A implies the removal of the Fast Correction Issue of Data (IODF), which were used to prevent erroneous applications of UDREIs. The removal of IODF parameters does not imply any robustness issue whenever the applicable UDREIs will bind all active Fast Corrections. This should be ensured by the system. IODF values were broadcast in messages MT6 and MT2-5 of SBAS L1 ICD model. In SBAS L1 ICD this would be equivalent of having always an IODF=3, so it is not needed to send it. The new definition of MT6_2 and MT2-5 messages already remove the use of IODF. As far as MT6_1 message content is concerned, the removal of IODF bits implies the presence of 8 new spare bits to be available to send other information if needed:

• A feasible partial use of these bits might be to include the corresponding IODP (2 bits) to link the satellites too. This would mitigate the risk in SBAS L1 ICD due to any loss of message or messages changing the mask.


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ANNEX 1 Acronyms

A

AD Applicable Document

D

DFRE Dual Frequency Residual Errors

E

EC European Commission EGNOS European Ground Network Overlay System ESA European Space Agency

F

FC Fast Correction

G

GEO Geostationary Earth Orbit GIVE Grid Ionospheric Vertical Error

I

ICAO International Civil Aviation Organization ICD Interface Control Document IWG Interoperability Working Group

J

JASMIN Joint GNSS Aviation Standardisation for a Multiple constellation Integrated receiver

M

MOPS Minimum Operational Performance Standards MT Message Type

N

NSP Navigation System Panel

P

PROSBAS Prototyping and Support to Standardisation of SBAS L1/L5 Multi-Constellation Receiver

PRN Pseudo Random Noise PU Public

R

RD Reference Document


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RE Restricted RF Rafio-Frequency RTCA Radio Telecommunication Committee for Aviation

S

SBAS Satellite Based Augmentation System SIS Signal In Space

U

UDRE User Differential Range Error

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