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HETE-2 Spacecraft

Spacecraft Overview
Mechanical Structure
Attitude Control
Power Systems
Science Instruments
Picture Gallery
This is a clickable image: click on parts of the spacecraft to learn more about them!

Spacecraft Overview

The HETE-2 spacecraft is a small satellite, measuring roughly a meter high by half a meter in diameter. HETE-2 retains most of the original HETE-1 design, with significant modifications made primarily to the power system. It was developed and constructed by essentially the same MIT team that completed HETE-1, relying in part on key spacecraft design and development consultants.

The spacecraft consists of a spacecraft bus, in which the satellite control hardware and the spacecraft computers reside, and the science payload, which point out one end of the spacecraft. In the drawing above, the four solar panels (which are stowed, parallel to the spacecraft, for launch) are connected at the bottom of the spacecraft bus, and the science instruments point out the top. On orbit, the science instruments will always be pointing away from the sun, and the deployed solar panels will be directed toward the sun.

More information about the various parts of the spacecraft can be found below.

HETE-2 undergoing assembly:

The FREGATE and WXM instruments comprise the top half of the spacecraft,
while the radios, the computer, batteries and power box form the lower structure.
The solar panels will eventually be attached at the lower plate above the Marmon ring.
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Mechanical Structure

The spacecraft structure of HETE-2 is comprised of two parts: the bottom half (closest to the solar panels) is mostly spacecraft hardware (power, communications, attitude control), and the upper half (furthest from the solar panels) is where the science instruments sit.

The mechanical structure of HETE is formed by the spacecraft hardware itself. As seen from the bottom of HETE, the mechanical structure consists of

  1. the Marmon ring, which connects to
  2. the baseplate, on top of which sit
  3. two battery boxes, on top of which sits
  4. the electronics box (spacecraft computer), to which is mounted
  5. the WXM electronics and mask support structure.

All other pieces of spacecraft or science hardware are mounted to this structure.

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Attitude Control System (ACS)

The control of the attitude of the spacecraft is the responsibility of the ACS algorithms in the spacecraft computer. The inputs to the ACS are from

Control of spacecraft attitude is done with

The ACS software consists of a sophisticated set of algorithms which take the current spacecraft orientation and spin and adjusts the torque coil currents and wheel momentum to create a stable orientation.

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The communications system hardware complement consists of

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Power Systems

The HETE-2 power system hardware consists of

The power box has a sophisticated design which allows it to charge the batteries ("net positive charging") regardless of orientation or panel deployment state, when the spacecraft is in "safe-hold mode": the safe-hold power is 2W, while the minimum charging power is 7W.

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Spacecraft Computers

The spacecraft computer system consist of four identical processor boards: each board contains one T805 transputer, two Motorola 56001 DSPs, and 20 Mbytes of RAM. The processors are assigned to the spacecraft and science needs in the following way:
  1. spacecraft processing
  2. SXC
  3. optical cameras
  4. WXM and Fregate

The "links" feature of the transputer allows for quick and efficient communications between processors. The DSPs serve as the interface to the instruments.

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The Radiation Belt Monitor (RBM) detects energetic protons and electrons (with energies > 1 MeV) in low-Earth orbit. Because such particle radiation can damage the spacecraft hardware (especially the WXM), the RBM is monitored for high particle flux, at which time the instruments are turned off.

The Global Positioning System (GPS) receiver, provided by CNES, is used to measure the precise position of HETE-2 on orbit and to provide a good mapping of spacecraft time to UTC.

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Spacecraft Specifications

Mass: 273 lbs.
Envelope:  Fits within cylinder 89cm x 66 cm dia.
Desired orbit:  625 km circular, 0-2 degree inclination
Operating life:  18 months, nothing to preclude 2+ years
Attitude:  Sun pointing. Momentum bias. Attitude controlled to +/- 2 degrees
Data processing:  4 T805 transputers, 8 DSP56001, ~ 100 Mips
Data Buffering:  96 MBytes of EDAC mass memory
Downlink:  250 kbits/sec data rate with overall bit error rate < 2x10-8 from data storage to ground archive.
Uplink:  31.25 kbits/sec data rate, overall bit error rate < 10-8
Radio Frequencies:  S-band uplink (2.092 GHz) and downlink (2.272 GHz) for primary ground stations, VHF downlink only (137.9622 MHz) for burst alert stations.
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