[hpsdr] GPS Receiver

[Tom Clark, K3IO]

Phil Harman wrote:
> I'm looking for a GPS receiver with a 1pps output suitable as a source 
> for my 10MHz GPS locked reference. I came across this new Garmin 
> receiver - looks very nice physically but not sure if the 1pps spec at 
> ~1uS is satisfactory.
>
> The device is a GPS 18 and the spec is here
>
> <http://www8.garmin.com/manuals/425_TechnicalSpecification.pdf>
>   
Phil -- I can't speak to the newest Garmin units, but some years ago I
tested their GPS-20 and I found it to be pretty bad with 1PPS noise at
the 2+ usec level. One of the serious problems was that there was no
mode where the receiver can be locked in the "zero-d" navigation mode.
For clock use, the best procedure (and the only on which gets you below
100 nsec) is to do a "self survey" to determine your position, and then
lock the receiver to that position so that the data from all the
available solutions goes into the determination of time. If you don't
force "zero-d", typical receivers will show ~300 nsec RMS noise (and we
routinely get < 10 nsec noise for the Motorola engines).

You might find the materials that Rick (W2GPS) and I have posted on
http://gpstime.com to be of interest. In particular, Rick's paper
evaluating the Motorola (now iLotus) M12+ (now the M12M) at the US Naval
Observatory and my presentations at the VLBI Technical Operations
Workshops (especially TOW2007 having the most recent data) of interest.
We have found that the M12M is the best small engine we have tested. The
M12M now has a 40+ MHz effective clock rate so that the pulse-to-pulse
(sawtooth) error due to clock quantization is now at the 25 nsec level
(as compared to the earlier Motorola receivers with a 9.54 MHz clock and
104 nsec pk-to-pk jitter, even in zero-D mode). In my most recent TOW
paper I reveal the basic schematic that Rick uses in his commercial CNS
Clock-II to remove even the 25 nsec sawtooth jitter so that the 1PPS has
~2 nsec noise (and ~20 nsec diurnal ionosphere signature).

Hope that helped -- 73 & Seasons Greetings, Tom

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