

     TRIAD, the first drag-free satellite, was designed and built by the 
Johns Hopkins Applied Physics Laboratory and launched 2 Sept 1972.  The 
satellite was in three sections separated by two booms.  The central section 
housed the DISCOS Disturbance Compensation System, which consisted of a proof 
mass of special non-magnetic alloy housed within a spherical cavity.  The 
proof mass flew a true gravitational orbit, free from drag and radiation 
pressure.  Teflon microthrusters kept the body of the satellite centered 
around the proof mass, thereby flying the entire satellite drag free.

     TRIAD was one of the APL-designed Navy Navigation Satellites.  The 
2nd-generation operational navigation satellites flying today (NOVA) use a 
single-axis version of DISCOS.  TRIAD was also the sixth APL satellite to be 
powered by an RTG (APL flew the first nuclear power supply in space, in 1961).

     Further info on TRIAD, DISCOS, etc. can be found in "Spacecraft Design 
Innovations in the APL Space Department," Johns Hopkins APL Technical Digest, 
Vol. 13, No. 1 (1992).