Gyroscope Drift Rate Experiment

Description

The primary objective was to measure two relativistic effects on nearly perfect gyroscopes. Einstein's General Theory of Relativity precisely predicts both of these effects; the spacecraft measurements are an experimental test of the General Theory of Relativity.

The two measured relativistic effects are the geodetic effect, which is due to the gravitational interaction of the spinning gyroscope and its orbital angular momentum about the earth, and the motional or frame-dragging effect which is due to the gravitational interaction between the spinning gyroscope and the angular momentum of the earth. The geodetic effect is predicted to cause a drift rate of 6.6 arc seconds per year in the plane of the orbit for a gyroscope in a ~640 km circular orbit, and the motional effect is predicted to cause a drift of the 41 milli-arc seconds per year in a direction perpendicular to the plane of the orbit.

At the core of the gyro-telescope is a block of fused quartz 21 inches long bonded to a quartz telescope and containing four gyroscopes plus the drag-free proof mass. This gyro-telescope structure is kept at high vacuum within a 9-foot-long cigar-shaped chamber ("the probe"), which is inserted into a large dewar vessel filled initially with 608 gallons of superfluid helium. The dewar maintains the instrument at a temperature of 1.8K and stays cold for nearly two years. It is also the main structural element of the spacecraft. Within the dewar, surrounding the probe, is a shield formed from superconducting lead foil which almost completely excludes the Earth's magnetic field. Thus, the gyroscopes operate: (1) at low temperature; (2) at low pressure; (3) in low magnetic field; and, (4) in the low gravity of space.

A proof-mass, a quartz sphere identical to a gyro rotor, "floats" within an evacuated cavity near the spacecraft's center of mass. The mass, being shielded from external accelerations, tends to follow an ideal gravitational orbit; by sensing its position and applying thrust forces to make the spacecraft chase after it, the satellite can be made drag-free.

The telescope was focused on the guide star IM Pegasi in order to provide a reference point for measuring the tiny deflections in the gyroscopes' spin axes. The guide star shifts its apparent position as both it and the Sun independently orbit the center of the Milky Way. As seen from the GP-B spacecraft, the apparent position of the guide star is also affected by the spacecraft's orbit around the Earth and the Earth's orbit around the Sun. To account for these motions, IM Pegasi was monitored by a world wide system of radio telescopes.

The telemetry from the satellite contained the essential information to determine the gyroscope drift rate as well as the health and safety of the satellite and its subsystems.

Data stored in the on-board Solid State Recorder (SSR) along with real time data was packaged together, telemetered to the ground and received by NASA GN ground stations. Data may also have been sent at a lesser data rate of 1 Kbps or 2 Kbps to the TDRS satellite constellation (via an on-board antenna) and later telemetered to the ground by TDRSS to Space Network ground stations. This TDRSS data can be sent at the same time as the SSR was receiving data in a different telemetry format. Data received was sent through a Front-End Processor (FEP) where Reed-Solomon checking was performed, the data was de-convolved and IPDU (Internet Protocol Data Unit) headers were assigned to each data packet.