= Orienting small satellites with GPS =

GPS Block IIf and GPS III satellites are specified to light up the Earth out to 5 degrees from edge of earth (EoE).
That is 95 degrees from vertical.  The GPS satellites are in a 12 hour orbit, which is GEO radius (42164 km) times (12/24)^2/3^ or 26562 km.  If α is the angle off nadir (the line between the satellite and the center of the Earth) then by the law of sines  sin(α)/26562 = sin(95°)/6371,   so α = 15.4°.

This [[attachment:LockheedGPSAntennaPanel.pdf | Lockheed document ]] specifies the SSV (space service volume) out to 23° for the L1 channel and 26° for the L2 and L5 channels.  More to learn, but by phase-comparing the signals from antennas on the sides of a cubesat sized satellite, it should be possible to estimate angular orientation to the strongest GPS satellites;  crude orientation from the direction to the strongest average signal (for detumbling), and precision orientation to the strongest available satellite with more sophisticated CDMA signal selection for specific satellites.  As a backup, some phototransistors can also help with detumbling.




see papers at http://www.lockheedmartin.com/us/products/gps/gps-publications.html