Timing Errors of the Garmin GPS 35 HVS
by Oliver Kloes

PLEASE NOTE:
These errors are caused by the GPS 35 only. The STVASTRO is a valuable tool for occultation observers and works fine.


INTRODUCTION:

I have been using the Garmin 35 together with the STVASTRO video time inserter for measuring occultations of stars by the moon and asteroids. I then received a warning to be careful using a GPS receiver for timing. There could be "jumping seconds" and the GPS could give the wrong time. So I contacted other observers, who were using the "35" with the STVASTRO. None of them had ever observed a "jumping second". One of the observers, Steve Preston from Seattle, U.S.A., suggested looking for these errors with the help of the "KIWI" timing software.

So Geoff Hitchcox from Christchurch, New Zealand, joined the hunt for timing errors. During the next five months, with more than 300 e-mails and hundreds of hours testing, we really found "jumping seconds" and solutions for this problem. The following information will be helpful for users of the "35" and the STVASTRO. Even "35" users with other timing purposes will find the information valuable.

To provide an easy overview, I have created the following Q & A:

1. How was testing done?
2. What kind of timing error can happen?
3. What causes these errors?
4. How often will these errors appear?
5. Will there be a fix from Garmin?
6. Can I predict timing errors?
7. What can I do about these timing errors?
8. LINKS:
9. Thanks to:

HOW WAS TESTING DONE?

Steve Preston used a GPS 35 HVS with software version 2.5 with good sky coverage.
Oliver Kloes used a GPS 35 HVS with software version 2.21 with limited sky coverage (30% open sky)
Geoff Hitchcox, using the data from S.P. and O.K., developed KIWI according to the test results. New versions were then tested by O.K. and S.P. During most of the tests, the display of STVASTRO was recorded on a video recorder. Test runs had durations of hours up to days.

WHAT KIND OF TIMING ERROR CAN HAPPEN?

We found different errors:

1. STVASTRO display stuck for one second. Example: Display of seconds reading:
11 12 13 13 15 16
Second "13" is double, second "14" missing.

2. STVASTRO display reading (seconds): 39, 40, 41, 42, 42, 43, 44, 46, 47
Second "42" is double, the seconds "43" and "44" are late for one second, second "45 is missing.

WHAT CAUSES THESE ERRORS?

We know two sources of error:

1.SLOW CALCULATION OF THE GPS 35
According to our tests, it is the "slow" CPU of the GPS 35 that causes these errors. KIWI can now show the time of transmission of the NMEA data (At option#8 KIWI 1.07). Sampling the NMEA data for a period of time, can show the transmission of the data is sometimes very late.

Example:
KIWI screen:
13-03-2003 23:30:47, 479, 485, 627, 629, 772

Meaning:
The NMEA data of second 47 was transmitted the following way:

479 ms from 1PPS, "$" start of the RMC
485 ms from 1PPS, "R" 4th character of RMC
627 ms from 1PPS, LF END of the RMC
629 ms from 1PPS, "$" start of the GGA
772 ms from 1PPS, LF END of the GGA

(To operate correctly with the STVASTRO, the GPS 35 must be programmed to transmit only the RMC and GGA sentences.)

But sometimes you can see this:
13-03-2003 23:29:20, 685, 691, 833, 835
The GGA sentence is completed in the next second AFTER the following 1PPS.

In some cases, the NMEA data overflowed to the following second. The data of second A is shifted to following second B and the GPS transmits the wrong time to the STVASTRO causing jumping seconds.

2. MISSING 1PPS

In rare cases the GPS 35 "forgets" the 1PPS signal, so there are 2 or more seconds between pulses. Each 1PPS signal is timed by the PC, if it is outside a window of plus or minus 50 microseconds (that's a 50 millionths of a second), KIWI gives an alert and logs the details.

If a 1PPS signal is missing for one whole second, jumping seconds will occur.

HOW OFTEN WILL THESE ERRORS APPEAR?

All these errors are very rare!!!

You will get about five in a 12 hour run. So less than 0.02 % of all timings of the GPS are wrong. But you have to be aware that they can happen!

WILL THERE BE A FIX FROM GARMIN?

Unfortunately we don't think so. Garmin was informed about our results, but it seems the limits of the hardware can't be corrected by a new software update. The "35" belongs to an older generation of Garmin receiver, and they may not spend any more research on this unit.

CAN I PREDICT TIMING ERRORS?

We found only one error that shows a pattern.

At a time of XX:00:06, we sometimes found the display (seconds): 04 05 06 06 08 09

We called this specific error "T06" (Triple zero six), because it appears at a time when the display reads three zeros and one six.

This "T06" appears more at even hours than at uneven hours, but you can't count on it. In a typical 12 hour test run, you get two or three of these errors - sometimes none!

Something happens at the sixth second after a full hour. It seems that the "T06" is a timed housekeeping task in the "35" that sometimes cannot be completed in time, like us trying to do 65 minutes work in an hour. It is because someone has programmed too much work for a "small brain" (CPU) used in the Garmin 35. That is why it cannot be fixed by a software upgrade.

Other errors are random.

WHAT CAN I DO ABOUT THESE TIMING ERRORS?

With good sky coverage for the GPS receiver, these errors appear less often than with poor sky coverage. But you can't eliminate these errors completely with an open sky.

You have four options:

1. Be aware timing errors can occur

The easiest and cheapest way. You know now that there can be errors in the timing display. So check your recordings around an occultation for "jumping seconds" carefully, and correct the time in your observation report if an error appears. It's an easy task and should be no problem for any observer. As the errors appear not very often, it is possible you will never get one during an occultation ever. The longest timing error we ever found was about 3 seconds. According to our knowledge today, there is no danger that the STVASTRO will show the wrong time for longer.

2. Use KIWI 1.07 for monitoring GPS errors

You can run KIWI in parallel with the STVASTRO, where it can monitor the output of the GPS 35, and write timing errors to a log file. KIWI's filtering is now very good at hunting down these errors. (This was the hardest task of our testing and Geoff's software writing.)

You have to do some hardware modifications for this setup. Split the data and 1PPS stream from the GPS 35, and send to the STVASTRO and KIWI simultaneously. The 1PPS signal has to be inverted. For more information on how to do this, read wiring.txt. You'll find this text file at the zipped KIWI package at the KIWI website.

3. Use KIWI 1.07 as a "Virtual GPS" (VGPS)

KIWI can now generate its own NMEA data from analysis of the GPS information. The GPS 35 still provides the 1PPS signal, but the NMEA data for the STVASTRO comes from KIWI which finds errors in the "35" NMEA, and corrects if necessary. The GPS still beats the drum of time, KIWI tells which time. Additional to the hardware modifications above, you need to connect the NMEA data from KIWI to the STVASTRO.

4. Buy a Garmin GPS 16

This 1PPS unit can be used with the STVASTRO too. Steve Preston made a continuous test run of 6 (!) days!!! Using KIWI, he detected no timing errors at all! The manufacturer of STVASTRO now recommends the Garmin 16. for the STVASTRO. So if you are considering buying the STVASTRO, and have no GPS, get the "GPS-16".

LINKS:

KIWI timing software.

STVASTRO video time inserter.

Garmin 35 GPS receiver.

Garmin 16 GPS receiver.

If you want to have a closer look at our tests, read this Fault Summary.

THANKS :-)

I want to thank...

Geoff Hitchcox (Christchurch, New Zealand)
for uncounted versions of testing software he provided just hours after I have written a new request. Without him the "timing errors" would still be a mystery. He has advanced KIWI to a very useful timing tool even for users without a STVASTRO video time inserter.

Steve Preston (Seattle, USA)
for running hundreds of hours of tests with his "35 and with the "16" and for contacting Garmin. Without his ideas and his contact to Geoff, this project would never had started.

Stefan Messer (Hofheim am Taunus, Germany)
for helping me constructing the cables and wiring between STVASTRO, Garmin 35 and KIWI. If I got lost in the wiring diagram, he showed me the way back. He was the valuable "third" hand, while I was soldering the connections.

Don Oliver (Houston Texas, USA)
who joined us with a version of the STVASTRO for the TRIMBLE GPS unit. Without Don, "Virtual GPS" would still be a dream.

Oliver Kloes
Eppstein-Bremthal
Germany
2003 August 28