[time-nuts] GPS receiver time message offsets to 1PPS

Mark Sims holrum at hotmail.com
Mon Jul 25 18:51:01 EDT 2016


Here are the results of measuring the difference between the time code in a GPS receiver time message and the arrival time of the last byte of the message.  Negative values mean that the receiver sends the timing message after the 1PPS pulse that it describes. The table also shows the standard deviation of the message arrival times.


The test configuration was a Compaq N610C laptop (2 GHz Pentium, hardware serial port) running Linux Ubuntu Mate 15.10).  The time of the messagearrival was from the system clock synced to NTP.  The receivers were tested in native binary protocol and, if supported, NMEA.  The com port was runningat the default value for the receiver.  If not specified that was 9600:8:N:1Data was collected after the receiver had been tracking satellites for at least 1 hour and averaged over 4 hours.  The timings were also checked andagree with those on a 1.2 GHz Raspberry PI 3 and a quad-core 3 GHz box.


Although measuring the arrival time of the first byte of the timing messagemakes more technical sense (less variation due to timing message length) thesemeasurements are of the last byte of the message.  This was chosen to assistpeople trying to sync a clock to a GPS receiver without relying on the 1PPSpulse.  They are also useful to people that want to know how long they haveto process a timing message (like to set a sawtooth compensation delay line) before the next 1PPS pulse comes out.


Several receivers appear to not sync their timing message to some referenceclock.  This causes the timing message arrival time to follow a ramp curve.


Device                Protocol       Firmware           (msg-arrival)    sdev
                                                                                  (msecs)
=============  =====  ================  ========     =====
Thunderbolt     TSIP         App:3.0 GPS:10.2     -52.9      6.2   (gold box)
Thunderbolt     TSIP         App:2.22 GPS:10.2    -67.2     10.2   (red box)
Resolution-T    TSIP         GPS:1.26            -140.0      5.4
Res-T SMT       TSIP         GPS 2.7             -475.0     63.7
Starloc II      TSIP         App:1.10 GPS:1.2    -267.5      7.2
Trimble NTWx?   TSIP         GPS 10.4             -44.9      1.2 
Nortel NTPx     TSIP         GPS 10.1             -61.8      1.4
Nortel NTGx     TSIP         GPS 10.5             -57.2      2.4


Ublox LEA-6T    binary        6.02 (36023)       -242.7      4.4
Ublox LEA-6T    NMEA          6.02 (36023)       -198.7     11.1
Ublox Neo6M     binary        7.03 (45969)       -211.6      9.3
Ublox Neo6M     NMEA          7.03 (45969)       -149.0      8.4
Ublox 7         binary        1.00 (59842)       -186.9      2.6
Ublox 7         NMEA          1.00 (59842)       -152.9      7.9
Ublox NEO-8M    binary        2.01 (75350)       -192.7      9.1
Ublox NEO-8M    NMEA          2.01 (75250)       -182.4      8.0

V.KEL SIRFIII   binary        GSW3.2.5           -432.8      6.1
V.KEL SIRFIII   NMEA          GSW3.2.5           -427.6      5.0

Navspark Mini   Venus8        1.7.27 15.8.18     -139.3      7.1

Adafruit Ultimate  NMEA                          -428.2     39.8

Skylab SKM53    NMEA                             -492.0     37.8  (has short jumps to usually -180)

Jupiter-T       Zodiac        93.07             -1263.6      3.3
Juptier-T       Motorola      93.07              -774.5      4.7  (ramps -274 .. -1274 over 1000 secs)
Jupiter Pico    Zodiac        14.00 11/28/05    -1236.9      0.4  (ramps -1225 .. -1245 over 130 minutes)
Jupiter Pico    Motorola      14.00 11/28/05     -771.0      4.7  (ramps -1271 .. -271 over 1000 secs)

Motorola UT+    Motorola      R5122U1112          250.5      5.0  (ramps -250 .. +750 over 900 seconds)
Synergy M12+    Motorola      P273T12T11          300.5      1.1  (ramps -200 .. +800 over 1000 seconds)

Z3812A          SCPI          X98-4-A            1450.5      0.6  (ramps 950 .. 1950 over 1000 secs)
Z3801A          SCPI 19.2K    3543A              1341.5      2.0  (ramps 941 .. 1841 over 1000 secs)
Symmetricom     UCCM-P 57.6K  1.0.0.2-01         -115.5      3.6  (ramps -109 .. -119 over 10 seconds)
Trimble         UCCM 57.6K    3.0.0.11-0         -278.5      3.1
 

 		 	   		  


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