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

[Jim Lux]

On 4/18/13 11:02 AM, Magnus Danielson wrote:
> On 04/18/2013 04:01 PM, Jim Lux wrote:
>> On 4/18/13 4:01 AM, David J Taylor wrote:
>>> An interesting novel use of GPS "stray" signals....
>>>
>>> "ESA’s retired GIOVE-A navigation mission has become the first civilian
>>> satellite to perform GPS position fixes from high orbit. Its results
>>> demonstrate that current satnav signals could guide missions much
>>> further away in space, up to geostationary orbit or even as far as the
>>> Moon. " See:
>>>
>>> http://www.esa.int/Our_Activities/Space_Engineering/Far-out_space_navigation_from_sideways_satnav_signals
>>>
>>>
>>>
>> Interesting..
>>
>> I know there's a bunch of analysis about how to use GPS at the moon. you
>> need to look at the GPS satellites on the far side of the earth, grazing
>> the limb.
>
> 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)?
>

But then you'd need to point it.  A bigger issue might be the sun. 
Either way, the question is whether it makes much difference.   The 
typical GPS receiver has a hemisphere field of view (granted, mostly at 
3K) and the satellite is at 20-40,000 km.
At the moon, your antenna would have a 4-5 degree field of view (if 
pointed at the earth), so 30dB more gain, but looking at a 300K load, as 
well.
At 300,000 km, that's about 10 times the distance, or 40 dB less signal..

So a net of -10dB..
Typical GPS receiver Tsys is 100K or something (in space).

So, 3 times the noise.

Maybe you need more gain (earth is 2 degrees wide viewed from the sun, 
so you could go to a 3 degree beamwidth..  that's like 36 dB gain