[time-nuts] GPS antenna length correction
wbeam at gci.net
Mon Apr 28 18:53:59 EDT 2014
On Mon, 28 Apr 2014 13:10:05 -0700, Chris Albertson wrote:
>On Mon, Apr 28, 2014 at 6:52 AM, Brian Lloyd <brian at lloyd.com> wrote:
>> ..., additional antenna cable introduces a fixed
>> delay value and hence a fixed constant that gets added to each path
>> regardless of direction. It seems to me that this would produce a much
>> "fuzzier" solution to position and/or variation in timing. Knowing cable
>> length and propagation velocity, would allow the software to subtract that
>> constant from all ranges and thus provide a more correct position and time
>> solution. Is this not the case?
>I think you have it exactly correct. The antenna location determines the
>geometry and the cable length adds only a fixed offset. Many GPS receivers
>have a command where you can tell them a "cable delay". But also you
>have to think about the other end of this. How long is the cable that is
>used for the output FROM the GPS receiver. This cable introduces a delay
>also. You might even have a distribution amplifier of at least a TTL chip
>acting as a driver or maybe a MAX232 chip doing level conversion.
>There is a total system delay. But really this only matters if you are
>keeping absolute time of day. Delay does not mater for frequency
>measurement or for time interval measurement.
As mentioned earlier the position of the receiver is indeterminate unless
the vector displacement from the antenna to the receiver is accounted for.
Accounting for the cable delay will only correct the absolute time.
Imagine a 100m antenna feed line; the receiver could be anywhere within
100m of the antenna (even above it or at it). The algorithm that computes position
needs to know this. It is possible the programmer assumes that the receiver
is directely below the antenna; but I don't know what programmers assume.
It would be safest to display the position of the antenna. This discussion
suggests that manufacturers should make it clear that the position is of the
antenna or otherwise....
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