[time-nuts] Re: Composite sky GPS - with an added digression

[glenlist]

Hi Jim

In this case +/- 30m in H and +/- 15m in V would be quite acceptable. A 
bit of calculator work and I would be able to determine the minimum 
spatial diversity required for those fixes.

yeah the overlap at 0dB ratio  would be  very low or zero. except  for 
regions of the lobe that are way down in sensitivity (far down enough 
that they may not be included in the solution- IIRC L1 = 30dB is 
available fo an isotropic antenna  which provides for substantial off 
axis resolution PROVIDING the spreading gain substantially exceeds that, 
which it does not  - L1 chip rate 1 Mcps, data =50bps, so gain ~13dB  
ALTHOUGH I see L5 has 10x the chipping rate with consumate increases in 
spreading gain, so L5 should be in theory able to provide more fixes as 
a higher wanted/unwanted ratio is possible. substantial. wow L5 is good.).

So, the multi antenna combination likely works OK for this application 
where antennas have true diversity of sky view.

 Â Now, some research and I find that the multi receiver solution is 
supported using the UBLOX RTK toolkit and the raw output receivers , and 
less work to do using the full raw which includes the pseudoranges for 
the 9 series.  That would seem a simple solution for high performance fixes.


On 2/07/2023 2:42 am, Lux, Jim via time-nuts wrote:
> On 6/30/23 6:40 PM, glen english LIST via time-nuts wrote:
>> Hi Jim
>>
>> Seems this question of mine is well covered in  the ether.
>>
>> Yes, might not be as bad as I think. I'll write a program to compute 
>> various scenarios and also will be interested to see how the fixes 
>> get affected  with loss of signal over some AZEL patch of the sky.
>>
>> The other thing- these narrowband antennas, over 24 MHz ish, I would 
>> suspect they are NOT phase  coherent, and that a combined pair of 
>> signals over identical sky would be anythign but flat frequency 
>> response.
>
> Oh, they're probably phase coherent "enough" - and it may not make 
> much difference.
>
> Say your antennas are separated by a meter - that's 5 wavelengths at L 
> band (20cm). If they were ideal, you'd get a series of fans to form 
> grating lobes - the fans would be long in the direction crossing the 
> line between antennas, and narrow in the direction parallel to the 
> baseline between antennas.
>
> Non ideal antennas make the grating lobes wiggle or be wavy along 
> their long axis. Of course what YOU care about are the nulls (and to a 
> lesser extent the phase smoothness as you traverse the pattern).  I've 
> looked a lot of these kind of models and your saving grace is that the 
> nulls are deep only when the signals from the antennas are equal 
> strength, which doesn't really happen much.
>
> If you were trying to do real time kinematic surveying to millimeters, 
> and are depending on smooth phase response - yeah, probably not going 
> to work.  Those folks obsess about apparent phase center displacements 
> of millimeters over a hemisphere.  Good multiband choke ring or 
> artichoke antennas are where it's at.
>
> Or, if you need precise position calculation, then the "multiple 
> receivers and post process" is probably a better approach, because 
> that can explicitly address that the antennas are not co located.
>
>