Civil and Environmental Engineering and Geodetic Science Part IV TYPES OF GPS OBSERVABLE AND METHODS...

Civil and Environmental Engineering and Geodetic Science

Part IV

TYPES OF GPS OBSERVABLE AND METHODS OF THEIR PROCESSING

GS608


Basic GPSGPS Observables

• Pseudoranges

• precise/protected P1, P2 codes (Y-code under AS) - available only to the military users• clear/acquisition C/A code

- available to the civilian users

• Carrier phases

• L1, L2 phases, used mainly in geodesy and surveying

• Range-rate (Doppler)


• Pseudoranges - geometric range between the transmitter and the receiver, distorted by the lack of synchronization between satellite and receiver clocks, and the propagation media

• recovered from the measured time differencetime difference between the instant of transmission and the epoch of reception.

• P-code pseudoranges can be as good as 20 cmgood as 20 cm or less, while the L1 C/A code range noise level reaches even a meter or more

Basic GPS ObservablesBasic GPS Observables


• Carrier phase - a difference between the phases of a carrier signal received from a spacecraft and a reference signal generated by the receiver’s internal oscillator

• contains the unknown integer ambiguity, Nunknown integer ambiguity, N, i.e., the number of phase cycles at the starting epoch that remains constant as long as the tracking is continuous

• phase cycle slipcycle slip or loss of lockloss of lock introduces a new ambiguity unknown.

• typical noisenoise of phase measurements is generally of the order of a few millimeters or less

Basic GPS observablesBasic GPS observables


Ambiguity: the initial bias in a carrier-phase observation of an arbitrary number of cycles between the satellite and the receiver; the uncertainty of the number of complete cycles a receiver is attempting to count.

• The initial phase measurement made when a GPS receiver first locks onto a satellite signal is ambiguous by an integer number of cycles since the receiver has no way of knowing when the carrier wave left the satellite.

• This ambiguity remains constant as long as the receiver remains locked onto the satellite signal and is resolved when the carrier-phase data are processed.

• If wavelength is known, the distance to a satellite can be computed once the total number of cycles is established via carrier-phase processing.


Doppler Effect on GPS observableDoppler Effect on GPS observable

• The Doppler equation for electromagnetic wave, where fr and fs are received and transmitted frequencies

• In case of moving emitter or moving receiver the receiver frequency is Doppler shifted

• The difference between the receiver and emitted frequencies is proportional to the radial velocity vr of the emitter with respect to the receiver

c

rff

vdt

drrv

sr

r

1

cos

srsr fvc

fff 1


Doppler Effect on GPS observableDoppler Effect on GPS observable

• For GPS satellites orbiting with the mean velocity of 3.9 km/s, assuming stationary receiver, neglecting Earth rotation,

• the maximum radial velocity 0.9 km/s is at horizon

• and is zero at the epoch of closest approach

• For 1.5 GHz frequency the Doppler shift is 4.5·103 Hz we get:

• 4.5 cycles phase change after 1 millisecond, or change in the range by 90 cm


Phase ObservablePhase Observable

• Instantaneous circular frequency f is a derivative of the phase with respect to time

• By integrating frequency between two time epochs the signal’s phase results

• Assuming constant frequency, setting the initial phase (t0) to zero, and taking into account the signal travel time ttr corresponding to the satellite-receiver distance , we get

dt

df

t

t

dtf0

ctfttf tr


Pseudorange ObservablePseudorange Observable

edtdtcP

TTc

dtTdtTcP

ttcP

ki

sr

sr

sr

ssrr

sr

)(

)(

)(

)(

sr - geometric range to the satellite

tr, ts – time of signal reception at the receiver and the signal transmit at by the satellite (both are subject to time errors, i.e., offsets from the true GPS time)

dtr,dts – receiver and transmitter (satellite) clock corrections (errors)

c – speed of light

e – random errors (white noise)


Taking into account all error sources (and also simplifying some terms), we arrive at the final observation equations of the following form (for pseudorange and phase observable)


PI

fT c dt dt b M e

PI

fT c dt dt b M e

i

k

i

k i

k

i

k

i

k

i i

k

i

k

i

k

i

k i

k

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k

i i

k

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k

, , , ,

, , , ,

( )

( )

1 2 1 1

2 3 2 2

1

2

2

2

i

k

i

k i

k

i

k

i

k

i

k k

i i

k

i

k

i

k

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k i

k

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k

i

k

i

k

i

k

i i

k

i

k

I

fT N c dt dt m

I

fT N c dt dt b m

, , , ,

, , , , ,

( )

( )

1 1 1 1 0 1 1

2 2 2 1 2 0 2 2

1

2 0

2

2 0

Basic GPS Observable 1/4Basic GPS Observable 1/4

222

0, ik

ik

ikk

i ZZYYXXsqrt and

The primary unknowns are Xi, Yi, Zi – coordinates of the user (receiver)

1,2 stand for frequency on L1 and L2, respectively

i –denotes the receiver, while k denotes the satellite


Basic GPS Observable 2/4Basic GPS Observable 2/4

P Pi

k

i

k

, ,,1 2 pseudoranges measured between station i and satellite k on L1 and L2

i

k

i

k

, ,,1 2 phase ranges measured between station i and satellite k on L1 and L2

0 0

k

i, initial fractional phases at the transmitter and the receiver, respectively

N Ni

k

i

k

, ,,1 2 ambiguities associated with L and L , respectively1 2

1 19 cm and 2 24 cm are wavelengths of L1 and L2 phases

i

k - geometric distance between the satellite k and receiver i,

I

f

I

fi

k

i

k

1

2

2

2, - ionospheric refraction on L1 and L2, respectively

Ti

k - the tropospheric refraction term

2,12

jf

I ionoph

kiiono

grj

Using our earlier notation for the ionospheric correction we have:

tropi.e., in our earlier notation


Basic GPS Observables 3/4Basic GPS Observables 3/4

dti - the i-th receiver clock errordtk - the k-th transmitter (satellite) clock errorf1, f2 - carrier frequenciesc - the vacuum speed of light

e ei

k

i

k

i

k

i

k

, , , ,1 2 1 2, , , - measurement noise for pseudoranges and phases on L1 and L2

bi,1, bi,2 , bi,3 - interchannel bias terms for receiver i that represent the possible time non-synchronization of the four measurements

bi i

k

i

k

, , ,1 1 2 - interchannel bias between and

b b P Pi i i

k

i

k

i

k

i

k

, , , , , ,,2 3 1 1 1 2 biases between and , and

M M m mi

k

i

k

i

k

i

k

, , , ,, , ,1 2 1 2 multipath on phases and ranges


• The above equations are non-linear and require linearization (Taylor series expansion) in order to be solved for the unknown receiver positions and (possibly) for other nuisance unknowns, such as receiver clock correction

• Since we normally have more observations than the unknowns, we have a redundancy in the observation system, which must consequently be solved by the Least Squares Adjustment technique

• Secondary (nuisance) parameters, or unknowns in the above equations are satellite and clock errors, troposperic and ionospheric errors, multipath, interchannel biases and integer ambiguities. These are usually removed by differential GPS processing or by a proper empirical model (for example troposphere), and processing of a dual frequency signal (ionosphere).


Basic GPS Observable 4/4Basic GPS Observable 4/4ki

kii

ki

ki

kik

ik

i eMbdtdtcTf

IP 2,)(

2

1

• Assume that ionospheric effect is removed from the equation by applying the model provided by the navigation message

• Assume that tropospheric effect is removed from the equation by estimating the dry+wet effect based on the tropospheric model (e.g., by Saastamoinen, Goad and Goodman, Chao, Lanyi)

• Satellite clock correction is also applied based on the navigation message

• Multipath and interchannel bias are neglected

• The resulting range equation :

kii

ki

ki

kii

ki

kkik

ik

i

ecdtP

ecdtcdtf

ITP

0,

2

1

corrected observable

Four unknowns: 3 receiver coordinates and receiver clock correction


Instantaneous DopplerInstantaneous Doppler

• Observed Doppler shift scaled to range rate; time derivative of the phase or pseudorange observation equation

satellitejreceiveridtdt

t

tc

ji

ji

ji

ji

ji

,),(error clock

combined theof derivative a is

cosvji

Instantaneous radial velocity between the satellite j and the receiver i, and v is satellite tangential velocity, see a slide “Doppler effect on GPS observable” (corresponds to in the notation used in figure 6.3)

r


Instantaneous DopplerInstantaneous Doppler

• Used primarily to support velocity estimation

• Can be used for point positioning

satellitejreceiveridtdt

t

tc

ji

ji

ji

ji

ji

,),(error clock

combined theof derivative a is

j

ij

ij

ji

ji t

t

tt )(

)(

)()(

ij andt )( Are instantaneous position vector of the satellite, and the

unknown receiver position vector; correspond to rs and rp in the notation used in Figure 6.3

• dot denotes time derivative


Integrated Doppler ObservableIntegrated Doppler Observable• The frequency difference between the nominal (sent) signal and the locally generated replica fg can be used to recover pseudorange difference through so-called integrated Doppler count (more accurate than instantaneous Doppler):

Observed: Njk

Where ik and ij are the distances from the receiver i to the position of the satellite at epochs k and j.

sg

ijikg

jksgjk

ff

c

fttffN

differencefrequency and

ZY, X, scoordinatestation

being unknownswith



R1 = cdt +f12 + T + eR1

R2 = cdtf22 + T + eR2

1 = f12 + T +

2 = f22 + T +

- integer ambiguities R pseudorange

I / f2 - ionospheric effect phase

T - tropospheric effect geometric range

eR1, eR2, white noise wavelength

Basic GPS observablesBasic GPS observables(simplified form)(simplified form)


GPS Positioning(point positioning with pseudoranges)

t

signal transmittedsignal received

range, = ct



Point Positioning with PseudorangesPoint Positioning with Pseudorangeski

kii

ki

ki

kik

ik

i eMbdtdtcTf

IP 2,)(

2

1

• Assume that ionospheric effect is removed from the equation by applying the model provided by the navigation message

• Assume that tropospheric effect is removed from the equation by estimating the dry+wet effect based on the tropospheric model (e.g., by Saastamoinen, Goad and Goodman, Chao, Lanyi)

• Satellite clock correction is also applied based on the navigation message

• Multipath and interchannel bias are neglected

• The resulting equation :

kii

ki

ki

kii

ki

kkik

ik

i

ecdtP

ecdtcdtf

ITP

0,

2

1

corrected observable


Point Positioning with PseudorangesPoint Positioning with Pseudoranges

• Linearized observation equation

5.0222

0,

00,

ik

ik

ikk

i

iii

ki

ii

ki

ii

kik

ik

i

ZZYYXX

cdtZZ

YY

XX

P

• Geometric distance obtained from known satellite coordinates (broadcast ephemeris) and approximated station coordinates

• Objective: drive (“observed – computed” term) to zero by iterating the solution from the sufficient number of satellites (see next slide)

ki

kiP 0,0,


Point Positioning with PseudorangesPoint Positioning with Pseudoranges

unknowns of vector theis x and matrix)nt (coefficiematrix design a called isA

, vector side handleft given theisy while,,

)1(

0,0,

0,0,

0,0,

0,0,

Axythus

dt

Z

Y

X

cZYX

cZYX

cZYX

cZYX

P

P

P

P

i

i

i

i

i

ni

i

ni

i

ni

i

mi

i

mi

i

mi

i

li

i

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ki

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ni

ni

mi

mi

li

li

ki

ki

• Minimum of four independent observations to four satellites k, l, m, n is needed to solve for station i coordinates and the receiver clock correction

• Iterations: reset station coordinates, compute better approximation of the geometric range

• Solve again until left hand side of the above system is driven to zero

ji 0,

i

i

i

edapproximati

i

i

updatedi

i

i

Z

Y

X

Z

Y

X

Z

Y

X


• In the case of multiple epochs of observation (or more than 4 satellites) Least Squares Adjustment problem!

• Number of unknowns: 3 coordinates + n receiver clock error terms, each corresponding to a separate epoch of observation 1 to n


Dilution of Precision (DOP)Dilution of Precision (DOP)

Accuracy of GPS positioning depends on:

• the accuracy of the range observables

• the geometric configuration of the satellites used (reflected in the design matrix A)

• the relation between the measurement error, obs, and the positioning error: pos = DOP• obs

• DOP is called dilution of precision

• for 3D positioning, PDOP (position dilution of precision), is defined as a square root of a sum of the diagonal elements of the normal matrix (ATA)-1 (corresponding to x, y and z unknowns)

• In differential GPS we use RDOP (relative DOP) term


Dilution of PrecisionDilution of Precision

PDOP is interpreted as the reciprocal value of the volume of tetrahedron that is formed from the satellite and user positions

ReceiverReceiver

Good PDOP (usually < 7) Bad PDOPPosition error p= r PDOP, where r is the observation error (or standard deviation)


Dilution of PrecisionDilution of Precision

• The observation standard deviation, denoted as r or obs is the number that best describes the quality of the pseudorange (or phase) observation, thus is is about 0.2 – 1.0 m for P-code range and reaches a few meters for the C/A-code pseudorange.

• Thus, DOP is a geometric factor that amplifies the single range observation error to show the factual positioning accuracy obtained from multiple observations

• It is very important to use the right numbers for r to properly describe the factual quality of of your measurements.

• However, most of the time, these values are pre-defined within the GPS processing software (remember that Geomatics Office never prompted you about the observation error (or standard deviation)) and user has no way to manipulate that. This values are derived as average for a particular class of receivers (and it works well for most applications!)


Dilution of PrecisionDilution of Precision• DOP concept is of most interest to navigation. If a four channel receiver is used, the best four-satellite configuration will be used automatically based on the lowest DOP (however, most of modern receivers have more than 4 channels)

• This is also an important issue for differential GPS, as both stations must use the same satellites (actually with the current full constellation the common observability is not a problematic issue, even for very long baselines)

• DOP is not that crucial for surveying results, where multiple (redundant) satellites are used, and where the Least Squares Adjustment is used to arrive at the most optimal solution

• However, DOP is very important in the surveying planning and control (especially for kinematic and fast static modes), where the best observability window can be selected based on the highest number of satellites and the best geometry (lowest DOP); check the Quick Plan option under Utilities menu in Geomatics Office


Differential GPS (DGPS)

• DGPS is applied in geodesy and surveying (for the highest accuracy, cm-level) as well as in GIS-type of data collection (sub meter or less accuracy required)

• Data collected simultaneously by two stations (one with known location) can be processed in a differential mode, by differing respective observables from both stations

• The user can set up his own base (reference) station for DGPS or use differential services provided by, for example, Coast Guard, which provides differential correction to reduce the pseudorange error in the user’s observable


Differential GPS (DGPS)

• So, DGPS can be performed by collecting data (phase and/or range) by two simultaneously tracking receivers, where one of them is placed on the known location

• These data are then processed together in a single adjustment to provide high-accuracy positioning information

• Or, one can use DGPS services that provide correction terms, which account for error sources due to atmosphere and SA (when activated) in pseudorange measurement; this correction is applied by the receiver to the observed pseudorange, which is subsequently used for navigation/positioning


By differencing observables with respect to simultaneously tracking receivers, satellites and time epochs, a significant reduction of errors affecting the observables due to:

• satellite and receiver clock biases,

• atmospheric as well as SA effects (for short baselines),

• inter-channel biases

is achieved

DGPS: Objectives and Benefits


Differential GPSDifferential GPS

• Selective Availability (SA), if it is on• Satellite clock and orbit errors• Atmospheric effects (for short baselines)

Using data from two receivers observing the same satellite simultaneously removes (or significantly decreases) common errors, including:

Base station with known location

Unknown positionSingle difference

mode


Differential GPS

• Receiver clock errors• Atmospheric effects

(ionosphere, troposphere)• Receiver interchannel bias

Using two satellites in the differencing process, further removes common errors such as:

Base station with known location

Unknown position

Double difference mode



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ljj

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kj

kjj

kj

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kjk

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li

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lil

il

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ki

kii

ki

ki

kik

ik

i

eMbdtdtcTf

IP

eMbdtdtcTf

IP

eMbdtdtcTf

IP

eMbdtdtcTf

IP

1,1,2,1,

1,1,2,1,

1,1,2,1,

1,1,2,1,

)(

)(

)(

)(

2

1

2

1

2

1

2

1

Consider two stations i and j observing L1 pseudorange to the same two GPS satellites k and l:


DGPS Concept• The single-differenced (SD) measurement is obtained by differencing two observables of the satellite k , tracked simultaneously by two stations i and j:

kij

eMij

bij

dtckij

Tf

kij

Ikij

kij

P kji 1,2,2

11,

1,

• It significantly reduces the atmospheric errors and removes the satellite clock and orbital errors; differential effects are still there (like iono, tropo and multipath, and the difference between the clock errors between the receivers)

• In the actual data processing some differential errors (tropo) can be neglected for short baselines, while remaining differential ionospheric, differential clock error, and interchannel biases might be estimated (if possible)


DGPS Concept• By differencing one-way observables from two receivers, i and j, observing two satellites, k and l, or simply by differencing two single differences to satellites k and l, one arrives at the double-differenced (DD) measurement:

klij

eMklij

Tf

klij

Iklij

klij

P klji 1,2

11, 1,

lij

eMij

bij

dtclij

Tf

lij

Ilij

lij

P

kij

eMij

bij

dtckij

Tf

kij

Ikij

kij

P

lji

kji

1,2,21

1,

1,2,21

1,

1,

1,

• In the actual data processing the differential tropospheric, ionospheric and multipath errors are neglected; the only unknowns are the station coordinates

Double difference

Two single differences


Note: the SD and DD equations were derived here for pseudorange observable, only as an example, because pseudorange equation is simpler (and shorter) than phase equation. SD and DD are most often used with phase observations

• Pseudorange observations are most often (but not only) used in navigation and point-positioning mode

• Or DGPS services are used to obtain the pseudorange correction (see the future notes for more info on DGPS services) in order to achieve sub-meter accuracy from pseudorange observations (which is otherwise in the order of a few meters)


Differential Phase Observations

1,

*1,2

11,

1,

*1,2

11,

1,1

1,1

lij

mij

dtclij

Nlij

Tf

lij

Ilij

lij

kij

mij

dtckij

Nkij

Tf

kij

Ikij

kij

lji

kji

1,1,2

11, 1,1

klij

mklij

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Tf

klij

Iklij

klij

klji

),,(1,

*1, 11 00 jtit

kij

Nkij

N

Double difference

Two single differences

Single difference ambiguity


Differential Phase Observations

• Double differenced (DD) mode is the most popular for phase data processing

• In DD the unknowns are station coordinates and the integer ambiguities

• In DD the differential atmospheric and multipath effects are very small and are neglected

• The achievable accuracy is cm-level for short baselines (below 10-15 km); for longer distances, DD ionospheric-free combination is used (see the future notes for reference!)

• Single differencing is also frequently used, however, the problem there is non-integer ambiguity term (see previous slide), which does not provide such strong constraints into the solution as the integer ambiguity for DD


Triple Difference Observable

Differencing two double differences, separated by the time interval dt provides triple-differenced measurement, that in case of phase observables effectively cancels the phase ambiguity biases, N1 and N2

ij dtkl

ij dtkl

Iij dtkl

fTij dtkl m

ji dtkl

ij dtkl

Pij dtkl

ij dtkl

Iij dtkl

fTij dtkl M

ji dtkl e

ij dtkl

, , ,,

, , , , ,

, , ,,

, , , , ,

112 1 1

112 1 1

In both equations, for short baselines, the differential effects are neglected and the station coordinates are the only unknowns


Note: Observed phases (in cycles) are converted to so-called phase ranges (in meters) by multiplying the raw phase by the respective wavelength of L1 or L2 signals

Thus, the units in the above equations are meters!

Positioning with phase ranges is much more accurate as compared to pseudoranges, but more complicated since integer ambiguities (such as DD ambiguities) must be fixed before the preciase positioning can be achieved

So called float solution (with ambiguities approximated by real numbers) is less accurate that the fixed solution

Triple difference (TD) equation does not contain ambiguities, but its noise level is higher as compared to SD or DD, so it is not recommended if the highest accuracy is expected


St. 1

St. 2

2 (base)3 4

1

11

12 2

1

22

31

32

42

41

Positioning with phase observations: A ConceptPositioning with phase observations: A Concept


41

42

412

31

32

312

21

22

212

11

12

112

:sdifference singlefour

SD

SD

SD

SD

41

42

21

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31

32

21

22

3212

11

12

21

22

1212

DD

DD

DD

:sdifference double three

Positioning with phase observations: A ConceptPositioning with phase observations: A Concept

• Three double difference (based on four satellites) is a minimum to do DGPS with phase ranges after ambiguities have been fixed to their integer values

• Minimum of five simultaneously observed satellites is needed to resolve ambiguities

• Thus, ambiguities must be resolved first, then positioning step can be performed

• Ambiguities stay fixed and unchanged until cycle slip (CS) happens


Cycle Slips

• Sudden jump in the carrier phase observable by an integer number of cycles

• All observations after CS are shifted by the same integer amount

• Due to signal blockage (trees, buildings, bridges)

• Receiver malfunction (due to severe ionospheric distortion, multipath or high dynamics that pushes the signal beyond the receiver’s bandwidth)

• Interference

• Jamming (intentional interference)

• Consequently, the new ambiguities must be found



Some useful linear combinationsSome useful linear combinations

• Created usually from double-differenced (DD) phase observations, derived as a linear combination of the phase observations on L1 and L2 frequencies

• Ion-free combination - eliminates ionospheric effects

• Widelane – its long wavelength of 86.2 cm supports fast ambiguity resolution


Useful linear combinationsUseful linear combinations

• Ion-free combination

• The conditions applied to derive this linear combination are:

• sum of ionospheric effects on both frequencies multiplied by constants (to be determined) must be zero

• sum of the constants is 1, or one constant is set to 1

• Used over long baselines (over 15 km), where DD differential ionospheric effect becomes significant

022

21

21 f

klij

I

f

klij

I


Ionosphere-free combinationIonosphere-free combination

• ionosphere-free phase measurement

2211222111

22112,1

NNT 1 and 21

22

21

22

2

22

21

21

1

ff

f

ff

f

• complication: ambiguity term is non-integer !

• similarly, ionosphere-free pseudorange can be obtained

222

21

12,1 Rf

fRR

222111 NN


Useful linear combinationsUseful linear combinations

• widelanewidelane where is in cycles

the corresponding wavelength

ij w

kl

ij

kl ij

kl

ij

kl

w

kl

ij

kl

ij w

klI

f

f

fT N N

ij, , ,,

1

2 1

1

2

2

w cm 1 2

2 1

86 2.

w 1 2

[meter]

Simplifies ambiguity resolution, as for the long wavelength it is much easier as opposed to L1 or L2 phase observations

Complication: ionospheric effects are amplified by a factor of 77/60 (i.e., f1/f2),

higher noise


• Differential Global Positioning System (DGPS) services provide differential corrections to a GPS receiver in order to improve the accuracy of the navigation solution.

• DGPS corrections originate from a reference station at a known location. The receivers in these reference stations can estimate errors in the GPS because, unlike the general population of GPS receivers, they have an accurate knowledge of their position.

• As a result of applying DGPS corrections, the horizontal accuracy of the system can be improved from 10-15 m (100m under SA) (95% of the time) to better than 1m (95% of the time).

Differential GPS (DGPS) ServicesDifferential GPS (DGPS) Services


DGPS Services: A ConceptDGPS Services: A Concept

• There exists a reference station (or a network of stations) with a known location that can determine the range corrections (due to atmospheric, orbital and clock errors), and transmit them to the users equipped with proper radio modem.

• The DGPS reference station transmits pseudorange correction information for each satellite in view on a separate radio frequency carrier in real time.

• DGPS is normally limited to about 100 km separation between stations.

• Improves positioning with ranges by 100 times (to sub-meter level)


DGPS Services

• Starfix II OMNI-STAR (John E. Chance & Assoc, Inc.)

• U.S. Coast Guard• Federal Aviation Administration• GLOBAL SURVEYOR™ II NATIONAL,

Natural Resources Canada• Differential Global Positioning

System (DGPS) Service, AMSA, Australia


Wide Area Differential GPS (WADGPS)Wide Area Differential GPS (WADGPS)

• Differential GPS operation over a wider region that employs a set of monitor stations spread out geographically, with a central control or monitor station.

• WADGPS uses geostationary satellites to transmit the corrections in real time (5-10 sec delay) to the remote users.

• For example: OMNISTAR, Differential Corrections Inc., WAAS (FAA-developed Wide Area Augmentation System)


Atmospheric layer

A Schematic of the WAASA Schematic of the WAAS


• The WAAS improves the accuracy, integrity, and availability of the basic GPS signals

• A WAAS-capable receiver can give you a position accuracy of better than three meters, 95 percent of the time

• This system should allow GPS to be used as a primary means of navigation for enroute travel and non-precision approaches in the U.S., as well as for Category I approaches to selected airports throughout the nation

• The wide area of coverage for this system includes the entire United States and some outlying areas such as Canada and Mexico.

• The Wide Area Augmentation System is currently under development and test prior to FAA certification for safety-of-flight applications.

WAASWAAS


• Total correction estimation is accomplished by the use of one or more GPS "Base Stations" that measure the errors in the GPS pseudo-ranges and generate corrections.

• A "real-time" DGPS involves some type of wireless transmission system.

• VHF systems for short ranges (FM Broadcast)

• low frequency transmitters for medium ranges (Beacons)

• geostationary satellites (OmniSTAR) for coverage of entire continents.

• A GPS base station tracks all GPS satellites that are in view at its location. Given the precise surveyed location of the base station antenna, and the location in space of all GPS satellites at any time from the ephemeris data that is broadcast from all GPS satellites an expected range to each satellite can be computed for any time

• The difference between that computed range and the measured range is the range error.

WADGPSWADGPS


• If that information can quickly be transmitted to other nearby users, they can use those values as corrections to their own measured GPS ranges to the same satellites.

• The range and range rate correction are generated

• The range correction is an absolute value, in meters, for a given satellite at a given time of day.

• The range-rate term is the rate that correction is changing, in meters per second. That allows GPS users to continue to use the "correction, plus the rate-of-change" for some period of time while waiting for a new message.

• In practice, OmniSTARTM would allow about 12 seconds in the "age of correction" before the error from that term would cause a one-meter position error.

• OmniSTARTM transmits a new correction message every two and a half seconds, so even if an occasional message is missed, the user's "age of data" is still well below 12 seconds.

WADGPSWADGPS



OmniSTAR's unique "Virtual Base Station" technology generates corrections optimized for the user's location. OmniSTAR receivers output both high quality RTCM-SC104 (Radio Technical Commission for Maritime Services) Version 2 corrections and differentially corrected Lat/Long in NMEA format (National Marine Electronics Association).



OmniSTAR receiver


Radio Modems

Date post:	18-Jan-2016
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Civil and Environmental Engineering and Geodetic Science Part IV TYPES OF GPS OBSERVABLE AND METHODS...

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