Galileo's Perijove Raise Maneuver
The Perijove Raise (PJR) maneuver changes Galileo's orbit so that its closest approach to Jupiter is raised high enough to avoid repeated passage through Jupiter's radiation belts. Without this change, Galileo would rapidly accumulate a fatal radiation dose. In general, perijove must be kept near or beyond the orbit of the moon Europa.
This timeline outlines the events surrounding PJR: the main mission events, the time at which important commands (usually, that an event has a green light to "go") are uplinked, Galileo's downlink data rate, station coverage from the Deep Space Network (DSN), and the name of the spacecraft software sequence currently in command.
Why does the spacecraft data rate drop to zero during such an important event?
Galileo turns significantly off-earth for the PJR burn (by 46.6 degrees). This reduces the signal strength reaching Earth by approximately 75%. The reduced power makes telemetry possible only with the most capable station (Canberra) and then only when that station's capability is at its greatest. All of the most important PJR events (turns to and from the earth, spinup and spindown, and the burn itself) are scheduled over Canberra with 8 bps telemetry planned. Telemetry of marginal quality may be possible for a few hours at the peak of the Goldstone viewperiod, and the station will try to receive it. Telemetry isn't possible over the third station, Madrid. The spacecraft communications is set so that station will receive only the carrier from the spacecraft (meaning zero telemetry data rate). However, the carrier by itself verifies Galileo is alive and allows measurement of spacecraft velocity (doppler).
What are ERT and TRM?
ERT stands for Earth Received Time, and TRM stands for Transmission time. This is different from SCET, or Spacecraft Event Time, which is the time at which the spacecraft actually does something. SCET differs from ERT and TRM by the time it takes light to travel one way to the spacecraft, the one-way light time (OWLT). If a command is sent to the spacecraft, it will be received by the spacecraft at the TRM + OWLT. Similarly, when something happens on board the spacecraft, the ground stations won't know about it until the ERT of the event, or SCET+OWLT.