-
1
EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN-SL DIVISION
SL-Note-2000-037 (CO)
LONG TERM PRECISION MEASUREMENTSON GPS AND IRIG-B EQUIPMENT
C. Antfolk*), G. Beetham,J-B. Ribes, J. Williamsson*)
Introduction
During November 1999 the progress report of the TimWG was
presented to the LHC PLC. The reportcontained a list of short term
and medium term tests that should be completed during 2000. The
PLCconcluded that ”The TimWG should continue its work with the
objective of producing a final definition of thetiming before the
end of 2000”. The first two items on the list were ”GPS time
synchronisation measurements”and ”IRIG-B data transmission tests”.
This note contains the result of the tests that have so far been
performedand presented to the TimWG.
Geneva, SwitzerlandJuly, 2000
*) Students at ARCADA –polytechnics, Espoo Finland
-
2
Table of contents:
1 Introduction 3
2 Global Positioning System (GPS) 4
2.1 Precise Positioning Service (PPS) 5
2.2 Standard Positioning Service (SPS) 6
2.3 One Pulse Per Second (1PPS) 6
3 System Configuration 7
3.1 The equipment used in the tests 9
3.1.1 GPS engine, TrueTime GPS-VME 9
3.1.2 Synchronised generator, TrueTime VME-SG-2 9
3.1.3 NovaTel (OEM GPS) 10
3.1.4 Time Source 100, Symmetricom 10
3.1.5 System 2000 10
3.1.6 PHILIPS PM6681, timer/counter/analyzer 10
3.1.7 Tektronix TDS420A, digital oscilloscope 11
4 The IRIG Standard 11
4.1 Description of the IRIG-B 12
5 The Tests 13
5.1 TrueTime GPS-VME period time 14
5.2 Two GPS-VME 16
5.3 3 x GPS-VME jitter test with System 2000 as reference 18
5.4 3 x GPS-VME, 3 different antennas, buildings 864 and 866
21
5.5 IRIG-B jitter test 23
5.6 IRIG-B jitter test with 1600m cable 24
5.7 Oscilloscope test, Tektronix TDS 420 A 29
5.8 TrueTime vs NovAtel (OEM GPS) 30
5.9 Different GPS-equipment 31
6 Conclusions 32
-
3
1 INTRODUCTION
The goal with this note was to determine how accurate and how
stable GPS equipments can deliver timeto a system with the aspect
of different locations and equipment. There was also a need to do
measurements onthe equipment using IRIG-B. Normally GPS is used for
positioning, but at CERN the existing equipment isused for timing
and synchronisation purposes i.e. in the Proton Synchrotron (PS)
machine, the Super ProtonSynchrotron (SPS) machine and in the Large
Electron Positron (LEP) machine. In the future it will also beused
in the Large Hadron Collider (LHC) machine and the CERN Neutrino
Gran Sasso (CNGS) project.
The measurements were done at Prevessin, CERN, where GPS
equipment is installed. The GPSequipment available in more than one
unit was those manufactured by TrueTime and used in the
existingmachines. Beside these TrueTime units there was also
available a GPS-engine from NovAtel plus a veryprecise GPS
synchronised reference unit from Datum Inc. and later on a GPS unit
from Symmetricom.
-
4
2 GPS
The Global Positioning System (GPS)* is a space-based radio
navigation system, which is managed forthe Government of the United
States by the U.S. Air Force (USAF), the system operator. GPS was
originallydeveloped as a military force enhancement system and will
continue to play this role. However, GPS has alsodemonstrated a
significant potential to benefit the civil community in an
increasingly large variety ofapplications. In an effort to make
this beneficial service available to the greatest number of users,
whileensuring that the national security interests of the United
States are observed, two GPS services are provided.The Precise
Positioning Service (PPS) is available primarily to the military of
the United States and its alliesfor users properly equipped with
PPS receivers. The Standard Positioning Service (SPS) is designed
toprovide a less accurate positioning capability than PPS for civil
and all other users throughout the world.However on May 1, 2000 the
president of the United States announced that he will end the
practice ofintentionally degrading signals available to the public
from GPS. This means that in peace time civilian usershave the
ability to obtain the same accuracy as the military, but USA can
and will deny GPS signals on aregional basis if their security is
threatened.
The GPS Space Segment consists of 24 Navstar satellites in
semi-synchronous (approximately 12-hour)orbits. The satellites are
arranged in six orbital planes with four satellites in each plane.
The orbital planeshave an inclination angle of 55 degrees relative
to the earth's equator. The satellites have an average
orbitaltitude of 20.200 kilometres (10.900 nautical miles) above
the surface of the earth.
Figure 1: 24 satellites in six orbital planes with four
satellites in each plane
The ranging codes broadcast by the satellites enable a GPS
receiver to measure the transit time of thesignals and thereby
determine the range between a satellite and the user. The
navigation message provides datato calculate the position of each
satellite at the time of signal transmission. From this
information, the userposition and the user clock offset are
calculated using simultaneous equations. Four satellites are
normallyrequired to be simultaneously "in view" of the receiver for
3-D positioning purposes.
*[GPS Signal Specification, 2nd edition, Navstar, June 2
1995]
-
5
Figure 2: A calculated average of satellites insight anywhere on
earth over a 24h period[GPS signal specification, 2nd edition June
2, 1995]
2.1 Precise Positioning Service (PPS)
The PPS (not to be mistaken for one pulse per second also
discussed in this report) is an accuratepositioning velocity and
timing service that is available only to authorized users. The PPS
is primarilyintended for military purposes. Authorization to use
the PPS is determined by the U.S. Department of Defense(DoD), based
on internal U.S. defence requirements or international defense
commitments. Authorized users ofthe PPS include U.S. military
users, NATO military users, and other selected military and
civilian users suchas the Australian Defense Forces and the U.S.
Defense Mapping Agency. The PPS is specified to provide 16metres
Spherical Error Probable (SEP) (3-D) positioning accuracy and 100
nanosecond (one sigma) UniversalCoordinated Time (UTC) time
transfer accuracy to authorized users. This is approximately equal
to 37 metres(3-D) and 197 nanoseconds under typical system
operating conditions.
Access to the PPS is controlled by two features using
cryptographic techniques, Selective Availability(SA) and
Anti-Spoofing (A-S). SA is used to reduce GPS position, velocity,
and time accuracy to theunauthorized users. SA operates by
introducing pseudo random errors into the satellite signals. The
A-Sfeature is activated on all satellites to negate potential
spoofing of the ranging signals. Encryption keys andtechniques are
provided to PPS users, this allows them to remove the effects of SA
and A-S and thereby attainthe maximum accuracy of GPS. PPS
receivers that have not been loaded with a valid cryptographic key
willhave the performance of an SPS receiver.
0
5
10
15
20
25
30
35
40
% o
f ti
me o
ver
24h
Number of satellites visible
Satellites visible over a 24h period
Percentage 0,01 0,04 2,8 23,55 39,13 27,31 6,77 0,41 0,01
4 5 6 7 8 9 10 11 12
-
6
2.2 Standard Positioning Service (SPS)
The SPS is a less accurate positioning and timing service that
is available to all GPS users. At 1 May2000 the SA was set to zero,
which before was set to a level to provide 100 metre (95%)
horizontal accuracywhich was approximately equal to 156 metres 3D
(95%). SPS receivers could achieve approximately 337nanosecond
(95%) UTC time transfer accuracy when the SA was still activated,
compared with 197ns (95%)with SA off. System accuracy degradations
can be increased if necessary to do so, for instance to deny
accuracyto a potential enemy in time of crisis or war. Only the
President of the United States, acting through the U.S.National
Command Authority, has the authority to change the level of SA to
other than peacetime levels. TheSPS is primarily intended for
civilian purposes, although it has a potential peacetime military
use.
2.3 1PPS (one pulse per second)
This signal is synchronised to the internal clock of the
GPS-unit and has a frequency of 1Hz. The TrueTimeunits used in the
tests outputs it as 5V TTL. The positive going edge of the pulse
marks the start of a second.The pulse width is by default 200ms and
the duty cycle will then be 20%. This 1PPS signal was used in
thetests to measure the jitter between different cards by comparing
the rising edges and store the result in a textfile with LabView.
The GPS-VME and VME-SG-2 corrects the time once per second and
rises the 1PPS at thesame time. To have reliable results within a
realistic timeframe for the whole experiment most of the tests
arebased on 50000 measurement data, which corresponds to a time
period of 14 hours, i.e. the tests were ranovernight. It was
noticed that using the computer whilst taking data affected the
results.
Figure 3: 1PPS-signal
-
7
3 System Configuration
All measurements described in this report were made with
instruments connected to a 400MHz PCrunning Windows 95 and LabView
5.1 from National Instruments. First an AT-GPIB/TNT ISA-card was
putinto the PC and configured to work with LabView. GPIB is a
communication standard supported by nearly allinstrument brands and
it was used between the PC and the instruments. Version 5.1 of
LabView was installedand configured to work together with both the
AT-GPIB/TNT card and the instruments used. The
latestLabView-drivers for the instruments were installed and
applications in LabView were made to read and storethe measured
values in a text file.
Figure 4: Applications in LabView for the PM6681 (left) and
TDS420A
A very precise counter, PM6681 from Fluke/Philips, with two
input channels was used if no more thantwo channels were needed.
Since the resolution of the counter was 50ps the counter was by far
good enough.Some of the tests needed more than two channels and
therefore, Tektronix TDS 420A, a 200MHz digitaloscilloscope with
four channels was used. The oscilloscope was able to do four
individual measurements at atime, which LabView through GPIB could
read and save. In all tests where the digital oscilloscope was used
itwas measuring delay from one input to another. In order to check
that the oscilloscope was suitable andaccurate enough for this
purpose a test was set up.
Figure 5: Equipment connected with GPIB
The data stored by LabView was analysed using Microsoft Excel,
where all histograms were created.One can notice that the curve
forms of the histograms are not gaussian and hence the standard
deviation cannot be estimated manually from the histograms.
However, the standard deviation and other statistics can
becalculated by means of Excel (as done in this experiment) or
other mathematical applications. The non-gaussian output of the
GPS-units was disturbing, the first idea was that there was an
iniquity and that theoutput was in fact gaussian. To check whether
this was the case the results from the two instruments used
werecompared to each other. The histograms created were identical
therefore the theory that the non-gaussian
-
8
pattern was created by the instruments was rejected. Next
question that came up was if LabView in some wayaffected the
result, but since it only fetched the data stored in the
instruments and wrote it to a file it was notgenerated by LabView.
This brings to a conclusion that the outputs from the GPS-units are
not gaussian. It ismost likely that the algorithm that calculates
the position and timing creates a non-gaussian output, but sinceno
specification of the engines that do the calculations was available
this theory can not be proven.
-
9
3.1 The equipment used in the tests
The GPS-equipment used in the tests were manufactured by
TrueTime. The cards are designed for aVME-bus based computer and do
not work as stand alone units. Two OEM-GPS units were also
available,one from NovAtel and one from Symmetricom. In addition to
these a reference unit, System 2000manufactured by Datum Inc., was
used as reference in the measurements. Other devices used were
anoscilloscope, Tektronix TDS420A and a PM6681 counter from
Fluke/Philips.
3.1.1 GPS engine, TrueTime GPS-VME
The GPS-engine (GPS-VME) is a precision time source, designed
tosupply precise time to a VME-based computer. The GPS-engine is
capable oflocking on to a maximum of 6 satellites and the
manufacturer (TrueTime) claimsthat the phase accuracy will be less
that 1us and typically less that 500ns to UTC(Universal Time
Coordinated). On the front panel there are five
BNC-connectors,where one can connect other equipments. First there
is the GPS-antenna input,which is used if the card is generating
the time from the antenna, then there is aone code-input that is
used if the time is generated from IRIG-B (described later).Then
there are two output connections, one generator-code where the card
outputsIRIG-B and one where it outputs 1PPS (described later).
Finally there is onefreeze-input, which can be used for time
stamping of events. It can be set to triggon positive and/or
negative going pulse, and when a trigg occurs it saves the time
in a freeze register and makes an interrupt request on the
VME-bus. The computer can then read and processthe time from this
register. The time delivered consists of microseconds through
thousands of years. Althoughthe time differs less than 500ns from
the UTC it can not be more accurate than the least significant bit
that isone microsecond.
3.1.2 Synchronised generator, TrueTime VME-SG-2
The synchronised generator VME-SG-2 is basically the same card
asGPS-VME, but without a GPS-engine. Because of this a VME-SG-2 has
noantenna input and it can therefor only synchronise to an external
IRIG-B signalproduced by another GPS-engine, hence the accuracy to
UTC (Universal TimeCoordinated) will be worse. The manufacturer
claims that with amplitudemodulated IRIG-B signal the phase
accuracy to UTC will be less than 2us andtypically less than 1us.
In addition to the ability to synchronise to a amplitudemodulated
IRIG-B signal it can also synchronise itself to a DC-level
modulatedIRIG-B signal, TrueTime claims that it then keeps its time
within onemicrosecond to UTC. Like the VME-GPS it has 1PPS output,
code-output foroutput of IRIG-B, code-input for amplitude modulated
IRIG-B input and finallya freeze input with a resolution of
1us.
-
10
3.1.3 NovaTel (OEM GPS)
The GPS-card is a OEM product manufactured by NovAtel. It is a
12-channel receiver available towork as free standing or
differential GPS. It has great flexibility in areas such as
configuration selection,remote control and specification of output
data and control signals. For the communication it has
twoindependent serial ports, working at baud rates up 115.2k.
3.1.4 TimeSource 100, Symmetricom
TimeSource 100 is a high accuracy, compact 12-channel GPS timing
receiver module that providesflexibility in integrating GPS into
OEM clock systems. Output timing and clock generation functions
areintegrated within the module to allow precisely synchronised
frequency and timing signals to be generated. Theclock generation
is fully programmable enabling up to 3 different digital clock
signals to be produced. It hasthree serial ports, two RS232 and one
RS485. The size of the circuit board is only 72x38x10mm, and
itprovides the user with a very good accuracy to UTC (
-
11
3.1.7 TDS420A Digital oscilloscope
The Tektronix TDS420A is a 4 channel 200MHzdigitizing
oscilloscope that is capable of doing fourindividual measurements
simultaneously. It has a GPIBinterface for communication with other
instruments andalso a VGA-output to which a VGA-monitor can
beconnected. The built-in floppy drive can be used to
storewaveforms, screen data and setup. In the tests theoscilloscope
was basically used to measure the delay from
one channel (the reference) to the three other. A test (see
text) shows that the measured values have an accuracywithin
±2ns.
4 The IRIG Standard.
In the early 1950’s it became apparent that efficient
interchange of test data between the various testranges and
laboratories would require time code standardization. This task of
standardization was assigned tothe Tele Communication Working Group
(TCWG) of the Inter Range Instrumentation Group (IRIG)* inOctober
1956. The original IRIG standards were accepted by the steering
committee in 1960. IRIG document104-60 defined the original IRIG
formats, and this was in 1970 revised and reprinted as IRIG
Document 104-70 and later in 1970 the status of the document was
upgraded to that of a standard. As of this writing, thelatest
publication is IRIG Standard 200-98.[Datum Inc., Timing & Time
Code Reference]*[IRIG STANDARD 200-98]
-
12
4.1 Description of the IRIG B
IRIG is a standard for transmitting time from one system to
another, e.g. for synchronisation of differentsystems located at
different places. The master system sends out an IRIG signal and
the slaves then estimatesthe time upon the received signal. All
IRIG signals base their time transmitting upon frames and words and
theIRIG B uses two words, which are each exactly one second long.
The first word of the time code frame is thetime-of-year in binary
coded decimal (BCD) with notation in days, hours, minutes and
seconds, note that theIRIG standard does NOT include the year
information. The second word is called Straight Binary Seconds(SBS)
and is a straight binary counter that tells the
seconds-time-of-day, resets at midnight. It counts, in otherwords,
from 0 to 86399 and then starts over again. Nowadays manufacturers
of time code generatingequipment normally do not include the
seconds-of-day code, which is also the case with the TrueTime
units.
Figure 6: 1PPS, IRIG-B DC-level and amplitude modulated
As explained above the time of year is coded in BCD, 7 bits for
seconds, 7 bits for minutes, 6 bits forhour and 10 bits for days.
The value of the bits are different from normal BCD-code where they
have the valueof 0,1,2,4,8,16,32… instead 0,1,2,4,8,10,20,40… are
used. A position identifier occurs between decimaldigits in each
group to provide separation for visual resolution. The time
information that can be extractedfrom a frame is the time of the
first pulse in the frame. A frame is 1 second long and consists of
10 sections,every section consists of 10bits. A bit is 10ms long
and can have three values: 1, 0 or position identifier. Aposition
identifier is set for 8ms, a 1 is set for 5ms and a 0 is set for
2ms as shown below.
Figure 7: A bit in IRIG-B is 10ms long
The signal can either be amplitude-modulated or sent as a
DC-level signal. The amplitude-modulatedsignal is modulated with a
1kHz sine wave. The existing GPS receivers at CERN use the 1kHz
amplitudemodulated IRIG-B signal and therefore these were used
during the measurements.
-
13
5 The Tests
To ensure that the GPS-system can provide as good an accuracy as
it is claimed, a number of tests havebeen performed. The output
from the equipment was compared to each other, and also to UTC.
There was alsoa need to check whether different locations of the
antennas changed the accuracy, and if different antennasplaced at
the same position gave different test results. There were also a
number of tests that were done tocheck the performance of the
IRIG-B equipment, such as distribution of IRIG-B over long
distances usingcoaxial cables. All the tests were first done with a
GPS-VME as reference since no better reference unit wasavailable,
but later on the System 2000 was delivered and could then work as
reference in the measurements.As the result of a decision taken by
Bill Clinton, the President of the United States, the SA
(SelectiveAvailability) was turned off on 1st. May 2000 to improve
the accuracy of GPS for civilian use. This did ofcourse also change
the accuracy of the tested equipment and therefore all measurements
were repeated.
-
14
5.1 TrueTime GPS-VME period time
The measurement was done with a Philips PM6681 counter. The
counter was set up to measure theperiod time and then a program was
created in LabView to read and store the data in a text file. Due
to thetriggering and the fact that the information had to be sent
to the computer, the counter could not measure twosubsequent
periods. Ten thousand period times were measured during a 14-hour
period. The result shows thatthe mean value is located around
400ns, which is equivalent to a period of 0.999999600ns
(1-0.999999600=400ns), this value should be near 0ns
(T=1.000000000s). It is likely that the counter was notcorrectly
calibrated or, an offset error was added as a result of trigger
level error. As can be seen from thediagram and the charts, there
is a clear difference between GPS-VME and System 2000. It is
notable that thestandard deviation for System 2000 did not decrease
when SA was turned off.
Table 1
1 – Period time TrueTime GPS-VME System 2000
Selective availability SA on SA off SA on SA off
Mean 423ns 443ns 413ns 507ns
Median 423ns 439ns 404ns 497ns
Standard deviation 54ns 48ns 33ns 37ns
Range 487ns 426ns 151ns 179ns
Minimum 115 134ns 341ns 433ns
Maximum 602 660ns 504ns 612ns
Measurements 10000 10000 10000 10000
Figure 8: GPS-VME connected to a counter Figure 9: System 2000
connected to a counter
-
15
T r u e T ime (GPS21) 1PPS per iod t ime, SA on
0
50
100
150
200-1
80
-168
-156
-144
-132
-120
-108 -9
6
-84
-72
-60
-48
-36
-24
-12 0 12 24 36 48 60 72 84 96
108
120
132
144
156
168
180
δδ t/[ns]
F/[
n]
T r u e T ime (GPS21) 1PPS period t ime, SA off
0
50
100
150
200
-180
-168
-156
-144
-132
-120
-108 -9
6
-84
-72
-60
-48
-36
-24
-12 0 12 24 36 48 60 72 84 96
108
120
132
144
156
168
180
δδ t/[ns]
F/[
n]
System2000 1PPS, period time, SA off
0
50
100
150
200
-180
-168
-156
-144
-132
-120
-108 -9
6
-84
-72
-60
-48
-36
-24
-12 0 12 24 36 48 60 72 84 96 108
120
132
144
156
168
180
δδt/[s]
F/[
n]
System2000 1PPS, period time, SA on
0
50
100
150
200
-180
-168
-156
-144
-132
-120
-108 -9
6
-84
-72
-60
-48
-36
-24
-12 0 12 24 36 48 60 72 84 96 108
120
132
144
156
168
180
δδt/[s]
F/[
n]
-
16
5.2 Two GPS-VME
The goal of this test was to gather information on the relative
stablity of two GPS-VME modules. Laterin this report tests are
conducted to check stablity relative to UTC, using System 2000 from
Datum Inc. asreference.
The first measurement in this test was carried out with a
Philips PM6681 counter. It measured the timefrom the positive edge
of input A to the positive edge of input B. Statistically, B starts
before A every secondpulse, and if this occurs the counter returns
a value near one second instead of a value in nanoseconds. As
aresult of this, the measurement was repeated with a TDS420A
(digital oscilloscope from Tektronix). Theoscilloscope was set up
to measure the delay from Ch1 to Ch2. The oscilloscope returned the
needed values,i.e. it returned a negative time if Ch2 rose before
Ch1. After the SA was turned off by the USA, the test wasrepeated.
The histograms are based on 50 000 measurements.
GPS-VME card specifications from TrueTime indicate that the card
is accurate to within one microsecond to UTC. The measured time in
this test is the relative time between the two units, and not to
UTC.From the diagram one can see that it is within ±800ns and from
the histogram one can see that basically allvalues are within
±500ns.The ID-nr of the devices was GPS-04 and GPS-21.
Table 2
SA on SA off
Mean -8ns -10ns
Median -9ns -80ns
Standard deviation 171ns 169ns
Range 1552ns 1480ns
Minimum -783ns -750ns
Maximum 769ns 730ns
Measurements 50000 50000
Figure 10: Two GPS-VME, jittermeasured relative to each
other.
-
17
GPS21 -> GPS04, 50k meas, 10ns bins, SA on, Oscilloscope
0
200
400
600
800
1000
1200
1400
1600
-800
-750
-700
-650
-600
-550
-500
-450
-400
-350
-300
-250
-200
-150
-100 -50 0 50 100
150
200
250
300
350
400
450
500
550
600
650
700
750
800
δδt/[ns]
F/[
n]
GPS04 -> GPS21, 50k meas, 10ns bins, SA off, Oscilloscope
0
200
400
600
800
1000
1200
1400
1600
-800
-750
-700
-650
-600
-550
-500
-450
-400
-350
-300
-250
-200
-150
-100 -5
0 0 50 100
150
200
250
300
350
400
450
500
550
600
650
700
750
800
δδt/[ns]
F/[
n]
-
18
5.3 3 x GPS-VME jitter test with System 2000 as reference
For this test, there was only one possibility to measure the
jitter of the three signals, to use theTDS420A oscilloscope. The
System 2000 worked as reference and its 1PPS was connected to
channel 1 of theoscilloscope. The 1PPS signal from the GPS-VME
cards was fed into the three other channels of theoscilloscope and
the delay between the reference and the signals was measured. By
studying the diagram, it canbe see that the mean value is located
around 350ns. This offset is caused by the System 2000 that worked
asreference and has its mean value located about 350ns earlier than
the TrueTime-units. The 350ns offset issubtracted from the values
used in charts. When the SA (Selective Availability) was switched
off, the standarddeviation decreased by about 30ns. Both the charts
and statistics show that the output of the GPS-VME cardsimproved
when the SA was turned off. This test shows that the cards will
provide the correct time within±500ns to UTC if they are connected
to the same antenna.
The ID-nr of the devices was GPS-04, GPS-12 and GPS-21
Table 3
Statistics for three TrueTime GPS-VME cards, relative to System
2000
System 2000 à
GPS04System 2000 à
GPS12System 2000 à
GPS21
SA on SA off SA on SA off SA on SA off
Mean 332ns 347ns 317ns 329ns 334ns 345ns
Median 329ns 344ns 303ns 310ns 333ns 340ns
Standard deviation 154ns 120ns 158ns 127ns 155ns 121ns
Range 1438ns 840ns 1377ns 886ns 1373ns 862ns
Minimum -401ns -66ns -331ns -115ns -343ns -86ns
Maximum 1037ns 774ns 1046ns 771ns 1030ns 776ns
Measurements 50000 50000 50000 50000 50000 50000
Figure 11: Three GPS-VME, jitter measuredrelatively to System
2000.
-
19
Sys2000->GPS04, 50k meas, 10 ns bins, SA on.
Oscilloscope.
0
500
1000
1500
2000
-650
-600
-550
-500
-450
-400
-350
-300
-250
-200
-150
-100 -5
0 0 50 100
150
200
250
300
350
400
450
500
550
600
650
700
δδt/[ns]
F/[
n]
Sys2000->GPS12, 50k meas, 10 ns bins, SA on.
Oscilloscope.
0
500
1000
1500
2000
-650
-600
-550
-500
-450
-400
-350
-300
-250
-200
-150
-100 -5
0 0 50 100
150
200
250
300
350
400
450
500
550
600
650
700
δδt/[ns]
F/[
n]
Sys2000->GPS21, 50k meas, 10 ns bins, SA on.
Oscilloscope.
0
500
1000
1500
2000
-650
-600
-550
-500
-450
-400
-350
-300
-250
-200
-150
-100 -5
0 0 50 100
150
200
250
300
350
400
450
500
550
600
650
700
δδt/[ns]
F/[
n]
-
20
Sys2000 -> GPS12, 50k meas, 10ns bins, SA off.
Oscilloscope
0
500
1000
1500
2000
2500
-650
-600
-550
-500
-450
-400
-350
-300
-250
-200
-150
-100 -5
0 0 50 100
150
200
250
300
350
400
450
500
550
600
650
700
δδt/[ns]
F/[
n]
Sys2000 -> GPS04, 50k meas, 10ns bins, SA off.
Oscilloscope
0
500
1000
1500
2000
2500
-650
-600
-550
-500
-450
-400
-350
-300
-250
-200
-150
-100 -5
0 0 50 100
150
200
250
300
350
400
450
500
550
600
650
700
δδt/[ns]
F/[
n]
Sys2000 -> GPS21, 50k meas, 10ns bins, SA off.
Oscilloscope
0
500
1000
1500
2000
2500
-650
-600
-550
-500
-450
-400
-350
-300
-250
-200
-150
-100 -5
0 0 50 100
150
200
250
300
350
400
450
500
550
600
650
700
δδt/[ns]
F/[
n]
-
21
5.4 3xGPS jitter test, 3 different antennas, buildings 864 and
866
The aim of this measurement was to determine whether the
position of the antenna changes the accuracyof the system and if
systems at different locations decode the same time. The two
GPS-systems involved arelocated in buildings 864 and 866, 200m
apart as the crow flies but 400 metres via cable ducts. On top of
thesetwo buildings the antennas are placed. To conduct the
measurements, four coaxial cables installed betweenthese two
buildings were used. Because of the length of the cables there must
be a delay, and to measure this, atest was developed. In building
866 the cables were connected to create two loops. In building 864
a loop wasalso made, and thereby, a 1600m long cable was created. A
1PPS-signal was then sent into the cable and withthe oscilloscope,
the returned signal was compared. The delay was 6.25us/4ways =
1.563us/way (where oneway is 400m in length). When all cables are
included in the formula the delay is td=(1.563 * n)+0.030 [us]where
n is ways. When one cable is used the delay, therefore, is 1.593us.
In the statistics for GPS03 the1.593us delay is subtracted. The
charts and the statistics based on 50 000 data shows that despite
the differentlocations and antennas there are no greater errors
compared with systems with the same antenna. The 80nsdifference
between the mean values from antenna 1 and antenna 2 can be related
to longer antenna cables (seecircuit description). The standard
deviation of the result is, like in the earlier tests, near
120ns.
Table 4
Statistics when three different antennas were used
Sys2000 – GPS04Antenna1
Sys2000- GPS04Antenna2
Sys2000 – GPS03Antenna3
Mean 440ns 359ns 357ns
Median 436ns 357ns 358ns
Standard deviation 117ns 121ns 123ns
Range 851ns 864ns 849ns
Minimum 10ns -78ns -67ns
Maximum 861ns 786ns 782ns
Measurements 50000 50000 50000
Figure 12: Three GPS-VME,connected to three different
antennas.System 2000 worked as reference.
-
22
Sys2000->GPS04,Antenna1, 50k meas, SA off
0
500
1000
1500
2000
2500
-800
-750
-700
-650
-600
-550
-500
-450
-400
-350
-300
-250
-200
-150
-100 -50 0 50 100
150
200
250
300
350
400
450
500
550
600
650
700
750
800
δδt/[ns]
F/[
n]
Sys2000->GPS21,Antenna2, 50k meas, SA off
0
500
1000
1500
2000
2500
-800
-750
-700
-650
-600
-550
-500
-450
-400
-350
-300
-250
-200
-150
-100 -50 0 50 100
150
200
250
300
350
400
450
500
550
600
650
700
750
800
δδt/[ns]
F/[
n]
Sys2000->GPS03,Antenna3, 50k meas, SA off
0
500
1000
1500
2000
2500
-800
-750
-700
-650
-600
-550
-500
-450
-400
-350
-300
-250
-200
-150
-100 -50 0 50 100
150
200
250
300
350
400
450
500
550
600
650
700
750
800
δδt/[ns]
F/[
n]
-
23
5.5 IRIG-B jitter test
This test was done with the PHILIPS PM6681. The IRIG-B code was
generated by a GPS-receiver(GPS-VME) and distributed to a
synchronised generator (VME-SG). The IRIG-B signal was first fed
into afan-out module and, thereafter, into the VME-SG card. The
jitter between the 1pps from the GPS-21 and the1pps from System
2000 was measured. The first chart shows the jitter, and it can be
observed from thediagram that the mean value is located at 3.8us.
For compensation, there is a phase compensation register athex
offset 00D0. The word written to hex offset D0 must be a 16 bit
signed binary number representingmicroseconds of compensation. If
this register is set to minus 4 microseconds it will result in the
secondcolumn and chart shown below (the chart is calculated, not
measured). TrueTime claims that the synchronisedgenerator keeps its
time within two microseconds and typically 1us to UTC. If one does
not consider the offset,the card keeps its time within one micro
second. The ID-nr of the devices was GPS-21 and SGD-01
Table 5
Statistics for VME-SG -> System 2000
Mean 3780ns -220ns
Median 3767ns -233ns
Standard deviation 275ns 275ns
Range 2295ns 2295ns
Minimum 3070ns -930ns
Maximum 5365ns 1365ns
Measurements 50000 50000
Figure 13: Synchronised generatorsynchronised to a GPS-VME
viaIRIG-B.
-
24
IRIG-B transmitted over 1m cable, System 2000 as referece, SA
off
0
100
200
300
400
500
600
700
800
90027
80
2880
2980
3080
3180
3280
3380
3480
3580
3680
3780
3880
3980
4080
4180
4280
4380
4480
4580
4680
4780
4880
4980
5080
5180
5280
5380
δδt/[ns]
F/[
n]
IRIG-B 1m cable, compensated with -3us, System 2000 as referece,
SA off
0
100
200
300
400
500
600
700
800
900
-120
0
-110
0
-100
0
-900
-800
-700
-600
-500
-400
-300
-200
-100 0
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
δδt/[ns]
F/[
n]
-
25
5.6 IRIG-B jitter test with 1600m cable
In the applications where IRIG-B is used, the length of the
cables can exceed 1000m. If the IRIG-Bsignal is sent over a cable,
there will be some distortion of the signal, but one must notice
that the IRIG-B isamplitude modulated with a 1kHz sine wave and it
is easily transmitted over longer distances without anynotable
distorsion. The measurements were done with PHILIPS PM6681 counter.
The IRIG-B was generatedby a GPS-VME card and fed into a fan-out
and from that into a 1600m long cable (four 400m coaxial
cablesconnected to create a loop) and, thereafter, connected to a
synchronised generator, VME-SG2. The jitterbetween the 1PPS from
the System 2000 and the 1PPS from the VME-SG2 was measured. The
first chart,based on 50 000 measurements, shows the jitter when
IRIG-B is distributed through a 1m long coaxial cable,the second
chart shows the jitter when the signal has been transmitted through
the 1600m cable. There is only27ns greater standard deviation when
1600m cable is used compared with 1m, and, this shows that the
cabledoes not distort the signal to a great extent.
One has to pay attention to the fact that the time difference
created by 1600m long cable is 3797-3034=760ns, whilst it should be
near 6.2us. The measured value decreased because the 1PPS of the
VME-SGrose 3.7us before System 2000 and when this signal is delayed
the mean value decreases. The IRIG-B signalwas checked with an
oscilloscope to ensure that it entered the synchronised generator
6.2us later (caused by the1600m long coaxial cable). The received
amplitude modulated IRIG-B signal has its ’on time mark’ at thezero
crossing between two position identifiers. If there is a small
DC-level error added to the signal it wouldchange the zero crossing
either forward or backward depending on the polarity of the
DC-level. A long cableattenuates this DC-level and thereby it
changes the zero crossing. Another possibility would be that
theattenuation of the long cable gives the signal a more flattened
slope and if the receiver circuit does not have thezero crossing at
exactly 0V it will also change the zero crossing and thereby time
offset.The ID-nr of the devices was GPS-12 and SGD-01
Table 6
Statistics for System 2000 à VME-SG
1m coaxial cable 1600m coaxial cable
Mean 3797ns 3034ns
Median 3784ns 3024ns
Standard deviation 277ns 304ns
Range 2296ns 2515ns
Minimum 3068ns 2162ns
Maximum 5364ns 4677ns
Measurements 50000 50000
-
26
Figure 14: Synchronised generator VME-SG synchronised to a
GPS-VME with IRIG-B, 1600m cable.System 2000 as reference
IRIG-B transmitted over 1m cable, System 2000 as referece, SA
off
0
100
200
300
400
500
600
700
800
900
-100
0
-900
-800
-700
-600
-500
-400
-300
-200
-100 0
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
1500
1600
δδt/[ns]
F/[
n]
IRIG-B transmitted over 1600m cable, System 2000 as referece, SA
off
0
100
200
300
400
500
600
700
800
900
-100
0
-900
-800
-700
-600
-500
-400
-300
-200
-100 0
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
1500
1600
δδt/[ns]
F/[
n]
-
27
5.7 Oscilloscope test, Tektronix TDS 420A
To be sure that no error occured when the jitter was measured
with the oscilloscope, this test wasperformed. The oscilloscope was
set to 500ns/div as in all tests. One 1PPS was fed into ch1 and
another intoch2 and ch3. The cables used were identical to ensure
that the pulses entered the channels at the same time.Two delays
was measured with the oscilloscope, dly ch1->ch2 and dly
ch1->ch3. The difference between thesetwo channels should be
zero. The histogram shows that the error generated by the
oscilloscope is ±2ns andtypically (95%) ±1ns. In other words it
does not cause any notable error to the results. In addition to
this onecan also point out that when the same measurements were
done twice, once with the oscilloscope and later withthe PM6681
counter, the histograms created were identical. The histogram below
describes the error related tooscilloscope and is based on 50000
measurements.
Figure 16: Oscilloscope test
Error related to Tektronix TDS 420A (500ns/div), 50k
6970
36622
4447
73941268770733760448286
-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7
δδt/[ns]
F/[
n]
-
28
5.8 TrueTime vs NovAtel (OEM GPS)
As soon as the System2000 precision GPS-reference engine from
Datum Inc. was available andinstalled, it was introduced to the
measurements. Since the only GPS-engines available before this were
theTrueTime units and a unit manufactured by NovAtel there was a
problem to determine which of them providedthe best accuracy and
thereby work as the reference. When System2000 was available, it
could be used asreference and the results show that NovAtel keeps
its time better to UTC than the TrueTime card. When thepresident of
the United States decided to turn off the SA on the first of May
2000, the measurements wererepeated. As expected, better
performance was observed.
Table 8
NovAtel (PM6681) TrueTime (TDS420A)
Selective Availability SA on SA off SA on SA off
Mean 1141ns 1146ns 334ns 345ns
Median 1141ns 1146ns 333ns 340ns
Standard deviation 84ns 29ns 155ns 121ns
Range 836ns 288ns 1373ns 862ns
Minimum 576ns 1011ns -343ns -86ns
Maximum 1412ns 1299ns 1030ns 776ns
Measurements 50000 50000 50000 50000
Figure 17: GPS-VME test Figure 18: NovAtel test
-
29
Sys2000 -> GPS21, 50k meas, 10ns bins, SA off.
Oscilloscope
0
500
1000
1500
2000
2500
-650
-600
-550
-500
-450
-400
-350
-300
-250
-200
-150
-100 -5
0 0 50
100
150
200
250
300
350
400
450
500
550
600
650
700
δδt/[ns]
F/[
n]
Sys2000->GPS21, 50k meas, 10 ns bins, SA on.
Oscilloscope.
0
500
1000
1500
2000
-650
-600
-550
-500
-450
-400
-350
-300
-250
-200
-150
-100 -50 0 50 100
150
200
250
300
350
400
450
500
550
600
650
700
δδt/[ns]
F/[
n]
Sys 2000 -> NovAtel, 50k meas, 5ns bins, SA on. Counter
0
200
400
600
800
1000
1200
1400
1600
-300
-275
-250
-225
-200
-175
-150
-125
-100 -7
5
-50
-25 0 25 50 75
100
125
150
175
200
225
250
275
300
δδt/[ns]
F/[
n]
Sys 2000 -> NovAtel, 50k meas, 2ns bins, SA off. Counter
0
200
400
600
800
1000
1200
1400
1600
-300
-280
-260
-240
-220
-200
-180
-160
-140
-120
-100 -8
0
-60
-40
-20 0 20 40 60 80
100
120
140
160
180
200
220
240
260
280
300
δδt/[ns]
F/[
n]
-
30
5.9 Different GPS-equipments
The table shows that there has been a great development of
GPS-receivers over the past years and thatbetter performance can be
observed. The TrueTime unit was released in 1990 (6000FS), the
NovAtel OEMGPS-card was manufactured 1995 (3000FS). The latest
technology available for tests was a Time Source 100from
Symmetricom, manufactured in 1999 (450FS). The Symmetricom GPS has
by far the best performancewith a standard deviation of 4ns (System
2000 as reference). Then comes the NovAtel, manufactured fouryears
earlier with a standard deviation seven times greater. The oldest
GPS-unit tested, the GPS-VME fromTrueTime, had the worst statistics
with a standard deviation of 121ns. This is thirty times greater
than the newSymmetricom unit.
System 2000 as reference, SA off
TrueTime GPS NovAtel Symmetricom
Mean 345ns 1146ns 244ns
Median 340ns 1146ns 241ns
Standard deviation 121ns 29ns 13ns
Range 862ns 288ns 59ns
Minimum -86ns 1011ns 220ns
Maximum 776ns 1299ns 279ns
Measurements 50000 50000 50000
Price (FS) 6000 3000 450
-
31
Sys2000 -> GPS21, 50k meas, 10ns bins, SA off. Counter
0
500
1000
1500
2000
-450
-400
-350
-300
-250
-200
-150
-100 -50 0 50 100
150
200
250
300
350
400
450
δδt/[ns]
F/[
n]
Sys 2000 -> NovAtel, 50k meas, 10 ns bins, SA off.
Counter
0
200
400
600
800
1000
1200
1400
1600
0
δδt/[ns]
System 2000 -> TimeSource 100, 50 k meas, 1 ns bins, SA off,
counter
0
500
1000
1500
2000
2500
3000
-20
-18
-16
-14
-12
-10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
36 38 40
δδt/[ns]
F/[
n]
-
32
6 Conclusions
The initial goal of this exercise was to verify if the GPS
equipment manufactured by TrueTime couldprovide time of day
information to within 1us, referenced to UTC, as per the
manufacturers specifications. Thiswas proven to be correct.
However, as more equipment was obtained the scope of the
investigation wasexpanded. The key item was the GPS regulated
rubidium oscillator source supplied by Datum Inc. This wasused as
the common reference for all the tests, including the IRIG-B
studies, which again fully confirmed themanufacturer’s claims.
GPS is a fast evolving technology, both in respect to its
increasing performance and decreasing costs.This phenomenon is
clearly presented in section 5.10. The tests have demonstrated that
GPS and IRIG-B canfulfill the requirements necessary for accurate
postmortem time stamping at CERN. For LHC and CNGS, thephilosophy
is proven but it is still too early to decide on specific hardware
implementations, let the marketdecide.
However, it is important that the existing comprehensive test
system should be maintained in a goodworking condition in order to
make meaningful comparison tests with future GPS/IRIG-B
equipment.
-
33
