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 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.