[time-nuts] GPS receiver testing

[John Lofgren]

Thank you very much for the thorough reply.

Best Regards,
John

From: SAIDJACK at aol.com [mailto:SAIDJACK at aol.com]
Sent: Wednesday, October 31, 2012 3:17 PM
To: John Lofgren; time-nuts at febo.com
Subject: Re: GPS receiver testing

John,

thanks for your email, I am replying to Time Nuts as well as there is a lot of knowledgeable folks here that can help.

In terms of a GPSDO tutorial, take a look at the HP papers linked on the JLT website under the "Links Of Interest" and "Related Whitepaper" sections:

  http://www.jackson-labs.com/index.php/support

In terms of how to measure the 1PPS accuracy and how to set up the equipment, see the paper labeled "Critical evaluation of the Motorola M12+ receiver" on that page. Explains how to set up the Agilent counter I mention below.

There is a lot of threads in the time nuts archives discussing the pro's and con's of different equipment, but let me give you a quick guide to what worked well for me:

For 1PPS measurements and frequency stability down to the ~2E-010 level per second you can get a low-costAgilent 53132A counter.

In order to check GPS position accuracy, you may want to get a timing GPS receiver that can do position-averaging using Auto Survey features. Such as the Trimble Thunderbolts, the JLT Mini-JLT GPSDO, or the HP58503A units. The JLT unit is the only unit using WAAS augmentation, so it probably has a much quicker and more accurate position indication than the other units mentioned. Let it average the position of your antenna for a day or two, and you will likely have an accuracy at the centimeter level (horizontal) and at the foot level vertical. Beware of different GPS datums, e.g. MSL versus GPS height indications etc.

You can use those GPSDO as a reference for your counter as well.

The above equipment can be had with a couple of days shipping time from Ebay, at around $1500 to $2000 totaland will serve you well for a very long time, and the resolution of the counter (150ps) is likely much higher than the GPS sawtooth error from the GPS you mentioned.

If you need much more accuracy and resolution, get a Wavecrest DTS time interval analyzer from Ebay for around $800, those have picosecond averaged noise floors, femtosecond hardware resolution, and 10ps single shot resolution.

In order to measure the 1PPS stability and accuracy, you would input the GPSDO reference 1PPS and your GPS 1PPS into the counter, and set it to T1 to T2 time interval measurement can capture that data. You may or may not use an external 10MHz reference for the counter doing this measurement, it shouldn'tmake a difference to your results.Then download Ulrich Bangerts excellent freeware "Plotter" program to do the time-stability analysis (search Google for "Bangert Plotter" and you will find his website).

Please note that you may or may not want to use the GPS receiver sawtooth correction data on your dataset, either manually (using Excel to subtract the offset error), viaa delay line, or other mechanism in your system. It will make a significant difference in your stability.

To measure frequency, feed the GPSDO reference into the ref-in port of the counter, your DUTto the A input, then use the "offset" feature to remove the carrier frequency, then capture the frequency offset of your source to a file, and again use Ulrich's plotter to give you the time-stability info etc.

Be warned, once you start on above path, you are likely never to stop searching for the holy grailof references, and measurement equipment..

Hope that helps,
Said



In a message dated 10/31/2012 12:36:55 Pacific Daylight Time, jlofgren at lsr.com writes:
Hello Said,

I'm familiar with your postings on the Time Nuts list, so I thought I'd ask your advice.  I searched the Time Nuts archive, but didn't come up with what I was looking for (reference to a good GPS tutotial).

We have a GPS project in-house that requires us to characterize receiver module performance.  We have a Litepoint IQ-Nav box with several stored scenarios, but no other signal generators with GPS personalities in them.  We need to measure position accuracy and time accuracy.  We may also need to get some characterization of the 1 PPS output.

I know that you do these types of measurements frequently.  Could you point me to a good reference on correctly performing these tests and, maybe, describe the equipment you are using?  We're checking through Agilent and R&S application papers, but you seem to have a lot of the required knowledge readily available.

Any help you could provide would be appreciated.


Best regards,

John