I found what was causing the apparent ramps in the message offset time for the
Motorola mode receivers and the Z38xx receivers. Here is the updated and
corrected info. Note that a couple of receivers do have some minor ramp
sawtooths in their message timing. They are less than +/- 10 msecs from
nominal.
-------------------------------------------
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 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
and baud rate) these measurements are of the last byte of the message. This
was chosen to assist people trying to sync a clock to a GPS receiver without
relying on the 1PPS pulse. They are also useful to people that want to knowhow
long they have to process a timing message (like to set a sawtooth compensation
delay line) before the next 1PPS pulse comes out.
A few receivers appear to not fully sync their timing message to some
referenceclock. This causes the timing message arrival time to follow a ramp
curve.The ramp size is fairly small (+/- 5 to 10 msecs) in most receivers.
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 -287.9 3.2
Jupiter Pico Zodiac 14.00 11/28/05 -1235.5 0.4 (ramps -1225
.. -1245 over 130 minutes)
Jupiter Pico Motorola 14.00 11/28/05 -298.0 1.8
Motorola UT+ Motorola R5122U1112 -258.2 3.7
Synergy M12+ Motorola P273T12T11 -194.8 1.4
Z3812A SCPI X98-4-A 949.5 0.8 (ramps 945 ..
955 over 16 seconds)
Z3801A SCPI 19.2K 3543A 973.0 2.0
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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