[time-nuts] GPS Filter

Bob Camp lists at rtty.us
Tue Mar 8 12:59:02 UTC 2011


Hi

Since the idea here is to keep an existing GPSDO running, the DGPS input stream isn't an option. Same goes for per sat corrections. You just don't have access to the "stuff" you need to get at inside the beast.

Bob

On Mar 8, 2011, at 1:30 AM, Magnus Danielson wrote:

> On 03/08/2011 05:22 AM, Hal Murray wrote:
>> 
>>> Since you are after timing off of the sat's, having antennas that move,
>>> either physically or electrically seems like a problem. Any shift in the
>>> effective antenna location as you tracked the satellite would be "exciting"
>>> to compensate for. There was an early paper published based on doing this
>>> (early 80's).
>> 
>> You can correct for the antenna orientation.  (That's what software is for.
>> :)  Radio astronomers have been doing it forever.
>> 
>> I think it's simple, at least in the nice/common cases.  If the antenna
>> geometry has a point that everything swivels around, consider that to the the
>> location of the antenna.  I think that covers the typical alt-az mount:
>>   http://en.wikipedia.org/wiki/Altazimuth_mount
>> The point is where those two axes intersect.  Now just fudge the coax delay
>> to correct for the time/distance from the real antenna location to that
>> point.  That's the before location coax delay (in there with the ionospheric
>> delay) rather than the post GPS antenna-to-box delay.
>> 
>> Of course, it gets a bit more complicated than that if you want to track
>> several satellites in real time.  That probably takes an antenna per
>> satelite.  But again, VLBI  geeks have been doing that sort of math for ages.
> 
> You would need to have a DGPS input stream generated in order to compensate sat for sat. If you don't have a DGPS input to the GPS receiver you are fairly stuck with the shifts...
> 
> Cheers,
> Magnus
> 
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