Hello, Mr. Hamster_nz !
Thanks you for your time. and I really appreciate it for your kind, Open help !
Mr. Hamster_nz, If you don't mind, Would you describe it in more detail ?
"How did you go from the time of transmission.to pseudoranges if you don't know your location (or at least a rough approximation)."
I have recorded at a fixed position.
Again, Thanks you for your time and kind help, Mr. Hamster_nz.
If you haven't been handed the data from somebody else, when you are tracking the Space Vehicle (SVs) you will end up with their orbital elements (ephemeris), and be able to compute the time on the SV.
Using the time, you can calculate the Earth Centered, Earth Fixed (ECEF) position of each of the SVs, at the time of transmission.
You can treat these as a set of linear equations, and work out approximately where you are.
Only then can you calculate your pseudoranges - (and yes, it is the XYZ of the SV less the XYZ at the approximate position). This should be within about 100m of the position. And yes, you can verify them by checking that the distance between the SV and position divided by the speed of light is close to the difference in time from the fix and the space vehicle.
Location is ( 10796441.72686, -11059550.95575, 21468769.87626) @ 466786.68844515
Location is ( 25624920.03297, 5493913.05874, 6376813.14970) @ 466786.68314817
Location is ( 17562608.84385, 17600783.06751, 9306656.31975) @ 466786.68228779
Location is ( 12436678.35329, 8851464.38357, 21813028.15234) @ 466786.68876178
Location is ( 18466438.20510, 6615544.19174, 17512994.82941) @ 466786.69042944
Location is ( 20893698.98803, -8968194.45015, 14090804.00502) @ 466786.68741330
Solved is ( 3851785.003462, -78309.097013, 5066312.772752) @ 0.003504 (alt 6364739.204313)
Lat/Lon/Alt : 52.934773, -1.164697, 176.8
You can then tweak the pseudoranges to make up for ionosphere delays and other known errors, as you now can calculate how high the SVs are in the sky, so the distance that the GPS signal has traveled through the ionosphere and troposphere (that slow the GPS signals down a small amount).
You can then solve again to get the more accurate fix.