Create python script for orbital position prediction based on TLE

[kodsurf]

1) TLE can be obtained by API

https://celestrak.org/NORAD/elements/gp.php?CATNR=25544&FORMAT=JSON

CATNR is ID of object in NORAD database

Object ID can be searched by the name using https://celestrak.org/satcat/search.php

Example :

substitute CARNR with PEGASUS ID :

https://celestrak.org/NORAD/elements/gp.php?CATNR=42784&FORMAT=JSON

To obtain TLE of PEGASUS

JSON

Or text

https://celestrak.org/NORAD/elements/gp.php?CATNR=42784&FORMAT=TLE

Output of simple HTTP GET request is

3 lines of strings.

2) Capture those strings with python (using curl http request)

And use : https://rhodesmill.org/pyephem/

pyephem to calculate object possition (lat/lon/alt)

Example : https://space.stackexchange.com/questions/4211/calculate-satellite-coordinates-from-tle-data

3) Use obtained lat,lon,alt ( and possible other data)

to create NMEA messages and print them to UART in a loop over time.

[kodsurf]

I added python script to simulate NMEA over orbit: https://github.com/RobertK66/obc_1769_core/commit/9c10894b416563b53adac0c0dfe166f4c7dabbe6

currently only for RMC message (will add another messages latter)

Looks ok but some unnessesary bytes received

---

Update : aaaah this is "SysEvent" that checksum is wrong

[kodsurf]

I struggle to understand how OBC NMEA parser and checksum calculation works

Example : I take a valid nmea message from https://orolia.com/manuals/VSP/Content/NC_and_SS/Com/Topics/APPENDIX/NMEA_GGAmess.htm "$GPGGA,123519.00,4807.038,N,01131.000,E,1,08,0.9,545.4,M,-164.0,M,,,,*47"

Then I simply print out this message to COM port in the loop every 1s

What I receive on OBC is SysEvent String

Why so ?

[kodsurf]

looks ok to me

https://github.com/RobertK66/obc_1769_core/commit/afa6182b956470684ed9892adeaead80ffd5bbe1

[kodsurf]

Only one thing left before I mark this python task as completed.

With UTC timestamp that I supply for RMC message - I never reach line 348 because day < 1

I compile RMC message according to https://orolia.com/manuals/VSP/Content/NC_and_SS/Com/Topics/APPENDIX/NMEA_RMCmess.htm

example of MY string of NMEA RMC message

Maybe this is not what OBC RMC parser expects ?

This is how I compile UTC string

input : yy - year (2022) mon - month (11) dd - date (3) hh - hours (6) mm -min (30) ss - sec (5)

I checked OBC code

Looks like It is the same format that I supply. But why is then day is wrong ?

[kodsurf]

Error is in date compilation of RMC message in python script.

"1102022" - WRONG

Correct format is ddmmyy - 03112022 ( or 3112022???)

What if it is first January? 01012023 or 112023

[kodsurf]

http://www.ae.utexas.edu/courses/ase389p7/projects/svatek/commands/RMC.html

Jevgeni please use Google before posting something stupid on issues.

Correct nmea message format with description is given at link above.

3.11.2022 is 031122

[kodsurf]

https://github.com/RobertK66/obc_1769_core/commit/350f1099ea445e6fe7cfdec55f9cdce1ad13262f

I fixed date compilation in RMC message. Now everything works correctly as it seems with RMC parser of OBC.

[RobertK66]

Is the Script available somewhere? You could share it in the tool folder, if you want.....

[kodsurf]

It is on feature/gps-integrate

Utilities/python scripts

[RobertK66]

I will keep only the long lat values in OBC code and integrate this into develop without the debug stuff....