Isometric drawing of the IMAGE observatory. Click here for a larger figure.

The IMAGE Spacecraft: Design Features

The IMAGE observatory is a spin-stabilized spacecraft that measures 2.25 meters (7.4 feet) in diameter and 1.52 meters (4.99 feet) in height and weighs 494 kg (1087 pounds) (including instruments). Viewed from either end, it has the form of a regular octagon. Arrays of high-efficiency, dual-junction gallium-arsenide solar cells attached to the spacecraft's eight side and two end panels provide power to the scientific instruments and subsystems, which together require an orbit-averaged power of 250 Watts. (When the spacecraft is in eclipse, power is supplied by a Super Nickel-Cadmium battery.) The instruments are located on a payload deck in the middle of the spacecraft; subsystems for electrical power, communications, command & data handling, and attitude determination and control are mounted in four bays below the payload deck. Cutouts in the side panels accommodate the instrument apertures, the deployers for the Radio Plasma Imager's (RPI) radial antennas, and radiators used for thermal control. Extending above and below the spacecraft, parallel to its spin axis, are two 10-meter axial antennas for the RPI; the RPI's four radial antennas, positioned 90 degrees apart, are deployed in the spin plane. These thin (0.321 mm) beryllium-copper antennas each extend 250 meters from the spacecraft.

IMAGE has a nominal spin period of 2 minutes (= a spin rate of 0.5 0.01 rpm); its spin axis is perpendicular to the orbital plane. Spin rate and spin-axis orientation are controlled and maintained by a single magnetic torque rod according to attitude information provided by a sun sensor and star tracker. Attitude knowledge is accurate to within 0.1 degree.

How IMAGE Communicates with the Ground

One of the DSN 34-meter dish antennas at Goldstone, California*

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.)

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. The real-time data are primarily used by the Communications Research Laboratory in Tokyo, Japan, and the National Oceanic and Atmospheric Admistration's Space Environment Center, together with data from other sources, to prepare space weather forecasts. Communications with the IMAGE spacecraft are handled principally by the Deep Space Network ground-stations at Goldstone in California's Mojave Desert and near Madrid, Spain.

The key design features of the IMAGE spacecraft are summarized in Table 1.

The IMAGE spacecraft was built by Lockheed Martin Missiles & Space (LMMS) of Sunnyvale, California.

Launch and Orbit

IMAGE was launched from the Western Range (Vandenberg AFB) on a Boeing Delta II 7326-9.5 launch vehicle. Launch occurred on March 25, 2000, at 12:34:43 PST. (Launch photos.) The Delta II placed IMAGE into an elliptical polar orbit with an apogee altitude of 7.2 Earth radii (45,922 km) and a perigee altitude of 1000 km. The initial apogee is at 40 degrees north latitude. During the first year of the two-year mission, the apogee will precess from 40 to 90 degrees north latitude--that is, to a position directly over the north pole; it will continue to precess until it returns to ~40 degrees by the end of the mission. Because of the Earth's annual motion about the Sun, IMAGE will be able to view each local time from apogee twice during the mission. IMAGE completes one orbit every 14.2 hours.

*Photo courtesy of Goldstone Deep Space Communications Complex

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