[time-nuts] GPS antenna length correction

"Björn Gabrielsson" bg at lysator.liu.se
Mon Apr 28 16:22:17 EDT 2014


Michael,

Lever arms are used to move a measurement to where its needed. Typically
when an INS uses a GPS measurement where the antenna is some meters away
from the INS calculation reference point. Its not applicable to the
antenna cable length problem.

The GPS needs to calculate x,y,z and t for each epoch. However the
receiver can't distinguish between the antenna-cable delay and a
bias/time-error in the receiver clock. And as a result the GPS 1PPS output
will be biased by this delay. x,y,z is not affected by the time-delay
between the antenna phase-center and the receiver.

/Björn

> The ability to correct the position depends on the receiver. Some
> receivers
> have a correction known as the lever arm correction. This is the vector
> difference between the antenna centroid and there is also a variable to
> enter the cable length. Between these two corrections the receiver time
> and
> position is the same as that at the antenna centroid.
>
> These corrections are normally found on kinematic and military receivers
> but may be on any - check the specs / owners manuals.
>
> Michael / K7HIL
>
>
> On Mon, Apr 28, 2014 at 8:20 AM, Hal Murray <hmurray at megapathdsl.net>
> wrote:
>
>>
>> brian at lloyd.com said:
>> > As I think about the geometry of satellite position and path length,
>> it
>> > seems to me that, since the geometry is determined by the antenna
>> position
>> > and not the receiver position, 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? Does it do something simpler but "good
>> > enough"?
>>
>> If you just do the normal calculations, you get the location of the
>> antenna
>> as though you were there a while ago.  If you know the delay in the
>> cable,
>> you can correct the time.  There is no way to correct the position.
>>
>>
>> --
>> These are my opinions.  I hate spam.
>>
>>
>>
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