# Initial condition for SGP4 in satellites without GPS, before the uplink of TLE data

I believe that the SGP4 model for orbit estimation for LEO satellites uses TLE data as initial condition. Some CubeSats chose to use up-link TLE data than installing GPS.

My question is for the case when just after launch and detumbling, there is no communication has been made between the CubeSat and Ground station and the satellite has no TLE data, then how the ADCS would work without position of satellite. As there is no GPS and SGP4 would not work until someone feed it TLE.

If somehow the communication is possible then by up-link the TLE the SGP4 would run, I understand but for having that communication the satellite has to be in proper orientation and for that ADCS (sun model and IGRF model) need position in orbit. Sort of chicken-egg problem.

• Not all ADCS needs position knowledge - see b-dot and variants. And there are such things as omnidirectional antennas. Any cubesat project would be wise to include one. – pericynthion Jan 8 '18 at 7:10
• @pericynthion Sure, the b-dot control law does not require position knowledge but for pointing the antenna, the position knowledge is required. Also I am taking about cubesats with monopole antenna, where the satellite antenna need to be parallel with ground station antenna for communication.My question is how to ensure first TLE up-link. – Sumit Agrawal Jan 8 '18 at 8:03
• @SumitAgrawal perhaps pointing to that big blue marble below (nadir) could be done with a few optical sensors or a fisheye lenses on some low-resolution cameras, along with some creative programming. There will be times during each orbit when the Earth is fairly fully illuminated. It might be a slow process, but it would not require any absolutes. – uhoh Jan 8 '18 at 8:07
• @pericynthion there are certainly antennas called omnidirectional, but one might need at least two to cover all of $4 \pi$. As far as I know, real "omni" antennas have low directionality within a given plane, but not a sphere. No single passive antenna can receive a given polarization type from all directions at the same time. If I'm wrong, I'd certainly like to know about it! Maybe there is some 3D fractal design that can do it? – uhoh Jan 8 '18 at 10:50