[time-nuts] Re: GPS Elevation Mask Values.

[Keelan Lightfoot]

I still feel like an important point is being overlooked here: The receiver
can not directly tell where in the sky the satellite is. It's impossible to
determine that from a single point of observation. All the receiver can
discern is the distance to the satellite. The sky plot is a prediction, not
an observation.The sky plot is calculated from the almanac data received
from the GPS constellation as a digital payload.  No combination of
antenna, cable, or receiving conditions will affect the data contained
within the almanac message. There are no satellites over the north pole,
the receiver is just drawing a picture that shows it over the north pole
based on the almanac data and a bunch of math used to calculate the
predicted azimuth and elevation for the satellite. All of the satellites
could be clustered over the north pole and just artificially delaying their
signals (oh dear I've just made a flat earth argument), but as long as the
almanac data showed them on their appropriate orbits, the sky plot would
look perfectly normal because it's just a parallel construction.

That means that either:

1. The GPS constellation is transmitting bad data in the almanac. Very
unlikely.
2. Certain receiver firmwares have a bug in the formulas used to calculate
the azimuth and elevations of satellites. Very likely. The fact that the
plot noodles towards the north pole tells me that some equation is being
pushed past a limit, and it's producing bogus data.

If you turn on the $GPGSV NMEA-0183 message, you can see the azimuths and
elevations the receiver is calculating. I'll belabour the point that these
are just calculations based on the almanac, and your current position and
the current time. The only actual observed value contained in the $GPGSV
message is the SNR value. So if the receiver reports that a satellite is
flying over the north pole, or that it is somehow observable when it's in
the southern hemisphere, it's because of bad math, not bad observations.

- Keelan

On Tue, Nov 16, 2021 at 9:04 PM Dan Kemppainen <dan at irtelemetrics.com>
wrote:

> Hi,
>
> I've recorded a log of the NMEA data from the three GPS Modules. I ran
> the Ublox through Ucenter, and the skyview plot doesn't show any tracks
> above the north pole (except the ones completely over and on the other
> side). The others are still running, but I expect the results to be
> similar.
>
> I spent some time this evening reading the documents on LH, but couldn't
> quite figure out how to get it to read the that data. I'd like to run
> this data through LH to see if it displays that same finger of data from
> before.
>
> Can anyone advise on how this data can be replayed in LH, if it's possible?
>
> Currently,  I was able to start a log of the Ublox module within LH6.
> That appears to save the same data as the terminal program that was used
> for logging yesterday while also displaying the az/el chart. (Maybe this
> will answer the question).
>
> Thanks!
> Dan
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