Abstract: A three-axis accelerometer (4) mounted on board a spacecraft (10) proximate a fuel tank (3) is used to derive an estimate of the amount of fuel (2) remaining within the tank (3). The output of the accelerometer (4) is converted (at 7) to a signal representing the rate of change of the polarity of the acceleration; amplitude information is ignored. This signal is then filtered by bandpass filter (8) or Fourier transform means and applied against an empirically derived function (stored within storage medium 9) relating frequency of oscillation to fuel (2) remaining in the tank (3). The function is stored for three axes; information from one axis is accessed, corresponding to the vector of a perturbing force such as caused by a thruster (11) firing. Some or all of the above described items can be located on board the spacecraft (10) itself. The accelerometer (4) can also be used directly to estimate the force imparted by the thruster, from the expression F=MA.

Abstract: An r.f. phase modulator operable at up to extremely high frequencies. A circulator (1) having input, output, and isolated ports (2, 3, 4, respectively), channels an r.f. carrier (15) in a prescribed direction. Fitting into an open end of the output port (3) is a moving reflective surface (6) coupled via a moving plunger (7) fixedly mounted at one end thereof to a housing (5) fixedly mounted with respect to the circulator (1). A modulating signal (9) is applied to the plunger (7), causing the reflective surface (6) to move linearly within the output port (3). This change in path length of the carrier (15) produces linear phase modulation thereon. The plunger (7) can comprise a magnetostrictive material (7A), a piezoelectric material (7B), a thermally expansive material (7C), or a combination thereof. The invention can be used for temperature compensation and phase stabilization as well as phase modulation.

Abstract: The method of placing a spacecraft having on-board propulsion devices with self-start capabilities into a series of successively higher transfer orbits to attain from a low earth orbit a final earth orbit or earth escape trajectory. Incremental thrusting impulses limited to a thrusting level at which the acceleration of the spacecraft is more than 0.01 g at beginning of thrusting, but is no more than 0.3 g at termination of thrusting are applied to the spacecraft. The propulsion propellant is ignited upon the near completion of each orbit at the perigee altitude, the latter being substantially common to all of the transfer orbits. The duration of each propellant burn is selectively varied during the transit from one transfer orbit to the next higher orbit and results in the spacecraft being propelled into successively higher energy orbits or trajectories until the desired orbit or escape trajectory is attained.