A Little Known Satellite Attitude Control Concept

Many of the early satellites were spin-stabilized with their spin axes normal to the orbit plane. The Defense Support Program (DSP) launch detection satellites, which began flying in 1970 and still fly today, are an exception. These satellites are deployed in geosynchronous equatorial orbits with the spin axis pointed along nadir so that an infrared sensor, with its line-of-sight offset from the spin axis, can scan a cone along the earth horizon. They have a hard-mounted, counter-rotating momentum wheel that cancels the spin momentum so even though the satellites spin they are not spin-stabilized. Pointing control is by reaction jets.

In 1991 I gave a briefing at Hughes about a proposed satellite that (like the DSP) would be spinning in a geosynchronous equatorial orbit with its spin axis pointed toward Earth. The zero momentum attitude control approach of the DSP was one option. Harold Rosen (“father” of the synchronous communication satellite) was in attendance. He proposed a different approach: one where the spinning satellite is torqued against a two-axis gimbaled momentum wheel. (His patent is # 5,441,222 “Attitude Control of a Spinning Spacecraft” August 15, 1995.)

By choosing the wheel momentum to be nominally equal and opposite the satellite momentum, angular displacements between the wheel and satellite are small even when the satellite is slewed through large angles. For low slew rates the gimbals can be replaced by a flexure suspension, and torques between the wheel and satellite can be generated using non-contacting linear voice coil actuators. Consequently the satellite is largely isolated from wheel disturbances, which can be critical for sensitive optical payloads. Unlike the DSP, the satellite is spin-stabilized, so reaction jets are not required and there is no fuel consumption or residue to contaminate optics. Possible applications include: launch detection from GEO, space surveillance from LEO or MEO, and meteorology from LEO. Modeling of Rosen’s attitude control system is a most fascinating dynamics/control problem and one that I documented.