The ALFA constellation communications and navigation support system

The proposed ALFA space-based imaging interferometer mission consists of 16 identical small satellites with dipole antennas and low frequency radio receivers, distributed in a spherical array 100 km in diameter, and placed in a nearly circular distant retrograde orbit about the Earth, with a typical range of 10⁶ km. Each satellite communicates directly with a small (11 m), low-cost antenna at each DSN site, providing parallel data paths and continuous coverage. The mission communications and navigation support system consists of an X-band frequency division multiplex (FDM) TT&C segment that uses a common 50 bps uplink channel and 17 downlink channels of 500 kbps each. The TT&C segment supports noncoherent precision Doppler ranging. Intersatellite ranging in the navigation support subsystem is accomplished by using the common four tone ranging technique over a UHF channel in the ISM (Industrial Scientific Medical) band