[time-nuts] OT: Far-out space navigation from sideways satnav signals

[Magnus Danielson]

Peter,

On 04/18/2013 09:15 PM, Peter Monta wrote:
> Hi Magnus,
>
>> Would not an antenna with a deep zero focus on the earth center help to
> reduce earth-noise (ground temperature noise as well as man-made noise)?
>
> It might, although you'd need a large antenna to generate the angular
> resolution needed to reject Earth noise while listening to a GPS bird near
> the Earth's limb.  If you make the tradeoff of using GPS satellites further
> from the limb (say at their max separation of ~5 degrees looking from lunar
> distance), hoping for less Earth noise, then you're working with a fainter
> sidelobe.  I'm sure the proper tradeoff is known.  The whole problem sounds
> much easier if you're merely at GEO.

GEOs have it simpler. Recall that the side-lobe near the earth is 
actually quite strong, since the GPS birds antenna makes a first degree 
compensation of distance-difference path-loss compensation, so it has 
stronger output to the sides than straight down. So, if you look at 
signals near the earth, you can see quite good signal.

As for GPS antenna with null, you can mount antennas on a linear boom 
and then sum them together with suitable delay, optimize for around 1,4 
GHz as it is in the middle between L1 and L2.

> I wonder if there's any advantage in combining far-away GPS with X-ray
> pulsar navigation (XNAV), which is said to be good to a few kilometers,
> though long integration times are needed.  For example, the rough system
> time from XNAV could enable very long (arbitrarily long?) coherent
> integration for very faint GPS signals obtained with a gain antenna pointed
> at the Earth.

Multiple sources if feasable to include all the detectors.

Cheers,
Magnus