Abstract: A system (10) for providing ephemeris data of a medium to high orbit satellite (12) relative to the Earth (30), where the satellite (12) is in an orbit substantially greater than 20,000 kilometers. The inventive system (10) includes plural transmitters (14, 16, 18, 20, 22, and 24) located within 20,000 kilometers of the Earth for transmitting information relating to the ephemeris of each transmitter relative to the Earth and time. This information is received and processed onboard the satellite (12) to provide ephemeris data. In a specific implementation, the transmitters (14, 16, 18, 20, 22, and 24) are GPS transmitters. One or more of the transmitters may be a ground or air based pseudolite. The transmitters have a geometric dilution of precision of six or less. Each pseuodlite transmitter includes a power supply sufficient to ensure that the signal received by the medium to high orbit satellite is of sufficient signal strength (e.g., -170 dBw to -160 dBw).