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

[Lux, Jim]

On 6/29/23 10:36 PM, glen english LIST via time-nuts wrote:
> talking about GNSS, someone here might know....
>
> I have a problem which I can see either one hemisphere or the other, 
> but not both simultaneously  - from the one antenna.
>
> It takes two antennas. Consequently, my 3D fixes are in the toilet.
>
> Is there a mode where two receivers  separately correlate all the sats 
> they can see independently and then talk to each other to use the sats 
> from the 'other' receiver to get a better all sky 3D fix ? or get  
> enough raw data out of them and post process elsewhere ?
>
> glen.


Not easily - you might be better off just running the two antennas into 
a power combiner.

But, there's a variety of software that will take raw observables and 
combine them to make a single fix.  GipsyX at JPL does this, but it is 
non-trivial to do.

There's also RTGx (Real Time Gipsy) which can probably do it on the fly 
(RTGx is part of GIPSYx). Or, at least, I know it can combine 
observables from the signals from multiple antennas at the same time.  
Granted it's all one receiver, usually, with multiple input channels, 
but I'm pretty sure that RTGx doesn't care - it would help if both 
receivers share a common clock.  (isn't that a particularly Time-Nuts 
kind of statement in general)?


https://gipsyx.jpl.nasa.gov/

In theory, if you've got a research application, you can get a free 
non-commercial license from JPL.


You can get a pdf from here:

https://www.sciencedirect.com/science/article/pii/S0273117720302532

Willy Bertiger, Yoaz Bar-Sever, Angie Dorsey, Bruce Haines, Nate Harvey, 
Dan Hemberger, Michael Heflin, Wenwen Lu, Mark Miller, Angelyn W. Moore, 
Dave Murphy, Paul Ries, Larry Romans, Aurore Sibois, Ant Sibthorpe, Bela 
Szilagyi, Michele Vallisneri, Pascal Willis,

GipsyX/RTGx, a new tool set for space geodetic operations and research,

Advances in Space Research,
Volume 66, Issue 3,
2020,
Pages 469-489,
ISSN 0273-1177,
https://doi.org/10.1016/j.asr.2020.04.015.


Full disclosure, I'm the project manager of NASA's SunRISE mission: 
we're using RTGx on board the six space vehicles AND doing post 
processing of the observables on the ground to get position to ~1 meter 
and time to ~1 ns. The Precision Orbit Determination (POD) also uses 
GIPSYx/RTGx, and gives us the clock bias estimate. SunRISE is a 
constellation of 6 small space vehicles (called the observatory) that 
will record radio signals between 100 kHz and 23 MHz in an orbit about 
300km above GEO.  The six space vehicles fly about 1-15 km apart, and we 
use interferometery to image the source of RF from Type II and Type III 
bursts from the Sun. Type IIs come with CMEs.  We're launching in 2025, 
theoretically.  The space vehicles are all built and tested, and are 
basically in storage "waiting for our ride". Basically in the last few 
weeks.

There will be some papers coming out over the next months (SmallSat in 
August, BigSky in March) describing various aspects of how the design 
came together and was tested. There's been a series of papers at BigSky, 
and a set of 4 posters at AGU last year that describe it fairly well.

Giving plenty of credit to Time-Nuts - the conversations on this list 
have been *invaluable* in feeding into making SunRISE a reality.  After 
all, what we're doing with SunRISE is basically a GPS observed 
oscillator (we don't adjust the oscillator), and there's been a LOT of 
discussion on the list about that.  In fact, those discussions are 
partly why we don't discipline - we don't have to worry about backing 
out the disciplining.

And if that's not cool enough - We've been working on concepts for 
50-100 satellites out at L4 or L5, and there's no handy GNSS out there 
for position nav and timing. So now, we need to know relative position 
to a meter, time to a nanosecond, and orientation relative to the sky, 
completely within the observatory in space.

Solar radio bursts are bright (up to 40 dB over galactic background), 
and the Sun doesn't put out much energy in that band, except from the 
bursts.  But there's a lot of interest in low frequency radio astronomy 
for planetary, or beyond solar system - you can't observe it from the 
Earth's surface, because the ionosphere either blocks it, or is 
sufficiently non-isotropic that imaging doesn't work.