[time-nuts] GPS receiver testing
SAIDJACK at aol.com
SAIDJACK at aol.com
Wed Oct 31 16:16:45 EDT 2012
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:
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
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-cost Agilent 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 total and 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
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't make 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), via a 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 DUT to 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
Be warned, once you start on above path, you are likely never to stop
searching for the holy grail of references, and measurement equipment..
Hope that helps,
In a message dated 10/31/2012 12:36:55 Pacific Daylight Time,
jlofgren at lsr.com writes:
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
Any help you could provide would be appreciated.
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