Proton gyroharmonic echoes observed by the Radio Plasma Imager The IMAGE satellite contains a sweep-frequency radio sounder, the Radio Plasma Imager (RPI), used to probe the surrounding plasma environment. We studied signals detected by RPI during the inbound part of the orbit, at altitudes ranging from 1500 km to 20,000 km. IMAGE satellite main page The IMAGE spacecraft has three antennas for S-band communication with the ground: a medium-gain helix antenna and two low-gain omni-directional antennas. One of the omni antennas is mounted on the aft end panel of the spacecraft; the other is mounted together with the helix antenna on the forward panel. The helix antenna is used to transmit data from the spacecraft to the ground; the co-mounted omni antenna is used to receive uplinked commands and data. (The second omni antenna was switched off after the spacecraft attained its final orbital orientation.) IMAGE data delivery system Uplink normally occurs once a week at a data rate of 2 kbps. Downlink of stored science, engineering, and housekeeping data occurs once every 14.2-hour orbit at a rate of 2.28 Mbps. In addition to the playback of stored data, the IMAGE spacecraft also continuously transmits real-time data at a nominal rate of 44 kbps. Radio plasma imager The RPI instrument is a low-power radar which operates in the radio frequency bands which contain the plasma resonance frequencies characteristic of the Earth's magnetophere (3 kHz to 3 MHz). RPI can locate regions of various plasma densities by observing radar echos from the plasma that are reflected where the radio frequency is equal to the plasma frequency. Personnel in VLF group Don Carpenter, dlc@nova.stanford.edu Maria Spasojevic, maria@nova.stanford.edu Umran Inan, inan@nova.stanford.edu Tim Bell, bell@nova.stanford.edu