[time-nuts] GPS antenna length correction

Tom Van Baak tvb at LeapSecond.com
Tue Apr 29 00:24:56 EDT 2014


> 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.

No.

What the algorithm computes is position -and- time at the antenna.
It doesn't care where the receiver is, or how long the cable is.
The GPS solution gives the place and time of the antenna only.

And then,

1) If you want to translate the antenna solution to some other physical place, then apply a dX,dY,dZ correction.
2) If you want to translate the antenna solution to some other physical time, then apply a dT correction.

For example,

1) If the antenna is on top of a pole but you actually want the true location of the base of the pole, you apply a spatial correction.
2) If the antenna is on top of your roof but you want the true time at a BNC jack in your lab, you apply a temporal correction.

The confusion,

The GPS timing receivers most of us use make it easy to apply a temporal correction, but not a spatial correction.
Consequently, your lab receiver can be configured to pulse the true time at center of the front panel BNC.
But, your lab receiver cannot be configured to show the true location of the center of the front panel LCD.
Instead, the LCD is stuck showing the location of your distant antenna.

/tvb





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