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The High Energy Transient Experiment (HETE) was a series of international missions led by The Massachusetts Institute of Technology (MIT). Its prime objective of HETE-1 was to carry out the first multiwavelength study of gamma ray bursts (GRBs) with UV, X-ray, and gamma ray instruments. A unique feature of the mission was its capability to localise bursts with several arcsecond accuracy, in near real-time aboard the spacecraft. These positions were transmitted to the ground, and picked up by a global network of primary and secondary ground stations (SGS), enabling sensitive follow-up studies.
The HETE spacecraft measured a meter high by half a meter in diameter. HETE-2 retained most of the original HETE-1 design, with significant modifications made primarily to the power system.
The spacecraft consisted of a spacecraft bus, in which the satellite control hardware and the spacecraft computers reside. The four solar panels (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 always pointed away from the sun, and the deployed solar panels were directed toward the sun.
HETE-2 was equipped with instruments sensitive to gamma- and X-radiation. These instruments shared a common field of view of ~1.5 steradians, spanning a detectable energy range of about 3 decades, and were capable of immediate inter-instrument communication.
The gamma-ray instrument, FREGATE, was sensitive to photons in the 6-400 keV energy range. It provided accurate burst triggers and moderate resolution spectra and was built by the CESR in Toulouse, France.
The wide-field X-ray Monitor (WXM) was sensitive in the 2-25 keV energy range, with good energy resolution and localisation accuracy to ~10 arc-minutes or better. The WXM detectors were built by RIKEN.
The soft X-ray camera (SXC) was sensitive in the 0.5-10 keV energy range, with very good energy resolution and localisation accuracy to 30 arc-seconds or better. The SXC was built by the Center for Space Research at MIT
Each HETE-2 instrument operated independently of the others; however, if a burst was detected by one of the instruments, the others were notified and began collecting data at higher temporal and energy resolution. All instruments utilised on board sensors to determine safe and appropriate operational conditions.
The control of the attitude of the spacecraft is the responsibility of the ACS (Attitude Control System) algorithms in the spacecraft computer. The inputs to the ACS were from: 1) two magnetometers, one on the spacecraft body, and one on one of the solar panels; 2) a set of twelve coarse, medium, and fine sun sensors, which tell where the sun in in their fields-of-view; and 3) the optical camera system, which delivered spacecraft drift rate information during orbit night.
Control of spacecraft attitude was done with: 1) three orthogonal torque coils, which used electrical currents in the coils to generate IxB forces around the Earth's magnetic field; 2) a momentum wheel, which nominally spun at roughly 1800 RPM and which could be torqued to change the angular momentum of the spacecraft.
The ACS software consisted 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.
The communications system hardware complement consisted of: 1) an S-band radio, transmitting and receiving at frequencies near 2 GHz, which was used for uplink and downlink of spacecraft data; 2) 5 dual patch transmit-receive S-band antennae, four of which were oriented on the tips of the solar panels and one on the "bottom" (sun-facing side) of the spacecraft, 3) a VHF transmitter, which was used to transmit (only) important scientific and spacecraft-related housekeeping to the ground; 4) a "whip" antenna for the VHF, mounted to one of the solar panels.
The HETE-2 power system hardware consisted of: 1) four solar panels, made of honeycomb aluminum with silicon substrate, each supplying 42W; 2) power box with power point tracker; 3) ~90% efficient 6 battery packs, each made up of a string of 24 1.5V NiCd cells, and each with 1.2 A-hrs capacity
The power box had a sophisticated design which allowed it to charge the batteries ("net positive charging") regardless of orientation or panel deployment state, when the spacecraft was in "safe-hold mode": the safe-hold power was 2W, while the minimum charging power was 7W.
The spacecraft computer system consisted of four identical processor boards: each board contained one T805 transputer, two Motorola 56001 DSPs, and 20 Mbytes of RAM. The processors were 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 allowed for quick and efficient communications between processors. The DSPs serve as the interface to the instruments.
The Radiation Belt Monitor (RBM) detected 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 was monitored for high particle flux, at which time the instruments are turned off.
The Global Positioning System (GPS) receiver, provided by CNES, was used to measure the precise position of HETE-2 on orbit and to provide a good mapping of spacecraft time to UTC.
Mass: 124 kg. 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
Spacecraft: HETE is a sun pointing, Momentum biased spacecraft with attitude controlled to +/- 2 degrees by magnetic torque coils. Actual spacecraft attitude can be determined to within +/- 30 arcminutes with onboard sensors. HETE's multi-processor architecture runs at 80 VAX MIPS with 96 MBytes of EDAC mass memory available for data buffering. Telemetry can be sent/received from three primary ground stations located in Haystack, Mass. , Miyazaki, Japan, and Toulouse, France. S-band uplink is designed for 31.25 kbit/s throughput at 2.092 GHz. Downlink is designed for 250 kbit/s at 2.272 GHz. A network of receive-only secondary ground stations are designed to immediately alert researchers to transient outbursts, thus increasing the chance of observing the event with ground-based telescopes. Transmission occurs in the VHF at 137.9622 MHz. The VHF link can also transmit spacecraft telemetry in the event of an onboard emergency. Payload: HETE provides multiwavelength coverage of high energy transients with three different instruments. Each is capable of operating independently of the other, but during orbit night, when all three instruments operate, the detection of a transient by one instrument will result in collection of data in all three energy bands. The French Gamma Telescope (FREGATE) handles the detection and spectroscopy of gamma-ray bursts and monitors variable X-ray sources. The Wide-Field X-Ray Monitor (WXM), built by a team of Japanese and U. S. Scientists from Riken and Los Alamos National Labs is the prime instrument for detecting X-ray sources. Four Ultraviolet CCD cameras were built by MIT's Centre for Space Research. Three are of identical design, one has an optical filter. In addition to providing the most accurate directional information about transient events, the UV cameras assist with spacecraft attitude determination. The 1024x1024 pixel CCD's were fabricated by MIT's Lincoln Labs. HETE hardware and software have been designed to allow the satellite to operate with virtually no commands from the ground. All instruments can be configured to allow for completely autonomous observations, using on board sensors to determine safe and appropriate operational conditions. Status as of 3/31/97: The HETE spacecraft reached a nominal orbit on Nov.4, along with its piggybacked partner, SAC-B. However, the two spacecraft were not ejected from the Orbital Sciences Corporation Pegasus XL rocket as planned. Telemetry indicated a power failure in the transient power bus of the Pegasus third stage, causing the failure of 3 crucial pyrotechnics to ignite. Thus, HETE remained enclosed in OSC's Dual Payload Attachment Fitting (DPAF). On the morning of Nov.5, operators of a 23m VHF antenna at Wallops Island did receive a transmission while monitoring a frequency used by HETE's emergency beacon. The modulation and duty cycle were correct. Subsequent passes at Los Alamos National Lab and Wallops Island also picked up a similar though much weaker signal. Unable to deploy solar panels to charge it's batteries, the HETE spacecraft expired several days after launch. The relevance of the HETE mission has not been diminished since the loss of the satellite. The resolution of the mystery of Gamma ray bursts remains a high priority in X-ray astronomy, and the resurrection of HETE in the immediate future is a course being actively pursued by the HETE team.
Design Life: 6 months, nothing to preclude 18 months. Total Length: 0.9 m. Maximum Diameter: 0.7 m. Total Mass: 124 kg.
The High Energy Transient Experiment (HETE) is an international mission led by The Massachusetts Institute of Technology (MIT). Its prime objective is to carry out the first multiwavelength study of gamma ray bursts (GRBs) with UV, X-ray, and gamma ray instruments. A unique feature of the mission is its capability to localise bursts with several arcsecond accuracy, in near real-time aboard the spacecraft. These positions are transmitted to the ground, and picked up by a global network of primary and secondary ground stations (SGS), enabling sensitive follow-up studies.
Second High Energy Transient Explorer; built by MIT using leftover parts from the first HETE. This failed to operate because of a Pegasus adapter failure during launch in November 1996. MIT operates the satellite, while the program is managed by NASA GSFC as an Explorer mission of opportunity. HETE's main instrument is FREGATE, the French Gamma Telescope, a hard X-ray spectrometer operating in the 6 to 400 keV energy range. This gamma ray burst detector, together with a Wide Field X-ray Monitor hard X-ray coded mask telescope, is used to trigger searches with the two Soft X-ray Imagers which have 33-arcsecond spatial resolution. This will let astronomers get precise locations for gamma-ray bursts, allowing detailed follow-up with optical instruments. The satellite is in a 595 x 636 km x 2.0 deg equatorial orbit, and sends data to a network of small ground terminals spaced around the equator.