[time-nuts] GPS receiver testing

SAIDJACK at aol.com SAIDJACK at aol.com
Wed Oct 31 16:16:45 EDT 2012


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_ 
(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-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 
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'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 
etc. 
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,
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






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