[time-nuts] OT - GPS and North

[bg at lysator.liu.se]

> Mark Spencer wrote:
>> Would Time Difference of Arrival techniques combined with an array of
>> four closely spaced antennas work with Gps signals as a means of
>> determing the orentation of the antenna array vs the gps satellites ?
>> (I'm thinking traditional TDOA techniques may not work with gps
>> signals.)
>>
>
> There are commercial products available:
> http://www.hemispheregps.com/Default.aspx?tabid=379
> http://www.hemispheregps.com/Default.aspx?tabid=412
>
>
> These devices claim less than 1 degree RMS heading accuracy.
>
> Here the antennas are integrated and a fixed 0.5 meter apart.  They do
> (or did) make a board that could be used with separate antennas, but I
> can not find it right now.
>
> I have no idea of how the math works for computing heading.
>
> Gary
>

Any (almost) pair of GPS-receivers with phase measurement outputs can be
used to make an attitude GPS receiver - sometimes called a GPS compass.
You need decent antennas for good results.

This is a special case of a phase ambiguity problem with moving base
receiver and extremely short baseline. If the antennas are mounted at a
fixed relative position, knowing the baseline gives a simpler problem.

I have read papers about using only one receiver and one antenna. The
trick is then to use the antenna diagram and SNR from the currently
tracked satellites to estimate an orientation. I have seen no commercial
product trying this. Accuracy was not spectacular - a few degrees - if I
remember correctly. Would need a very stable environment to work. A
groundapplication (say car moving in urban environment) will influence the
received SNRs to randomly to make a one antenna approach possible.

Most GPS manufacturers have attitude GPS receivers. If not dedicated its a
special case for their RTK capable versions.

--

    Björn