[time-nuts] GPS antenna length correction

Michael Perrett mkperrett at gmail.com
Mon Apr 28 20:21:28 EDT 2014

As one responsible for the architecture and design of many military vehicle
GPS based navigation systems I will "un-simplify" my previous email. The
GPS solution (position or Doppler) is always made at the center of the GPS
antenna; that's where the lines of intersection (3D) from the individual
satellites occur (tri-lateration). You can account for the position
difference between the antenna and the receiver, vehicle center of mass or
another sensor (such as an inertial measurement system, velocimeter,
gravitometer etc.) by application of the lever arm vector. This co-locates
the GPS position with that device. This has the *function* of making the
sensor position and the GPS position both act as if they were at the IMU

An example of this is if you have the GPS antenna located on a ships mast,
and the IMU (whose position defines ships position) located on the bridge,
in order for the navigation Kalman Filter to properly compute the ships
location the lever arm correction must be applied. In this case the antenna
can be several tens, or even hundreds, of meters from the IMU. This needs
to be a vector as can been seen when the ship roles to one side or another.

A second, uncorrelated, correction is for the time (delay) that the RF
signal travels along the transmission line (C x Vf  x L). If that's needed
you can compute the delay, hence phase shifts.

Hopefully this helps,

Michael / K7HIL

On Mon, Apr 28, 2014 at 5:00 PM, Chris Albertson
<albertson.chris at gmail.com>wrote:

> On Mon, Apr 28, 2014 at 3:53 PM, Bill Beam <wbeam at gci.net> wrote:
> >
> > 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.
> >
> Yes, of course.  The GPS receiver reports the location of the antenna.  If
> you need to know some other location you have to apply some (x,y,z) offset
> from the antenna.   Actually I doubt anyone would care about the location
> of the receiver.   They might want to know the location of the vehicle's
> center of mass or the location of some display screen.    The cable delay
> accounts only for the offset in time.
> Time and location really are different things, especially because a cable
> can take a non-straightline path or even be coiled up.
> --
> Chris Albertson
> Redondo Beach, California
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