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Primary Burst Alert




HETE Burst Alert Stations

The HETE-2 satellite continuously transmits a 300 bps packetized data stream at a specially allocated frequency at 137.96 MHz. This constantly repeated data stream contains the times and locations of recent bursts detected by one of the three instruments (FREGATE, WXM and SXC cameras). When a new burst occurs, this stream is purged and a burst alert is transmitted within a few seconds of an instrument trigger. Shortly thereafter, an initial estimate of the coordinates of X-ray positions will be inserted into the data stream.

HETE-2's low orbit allows the broadcasts from a low power transmitter to be picked up by a simple ground antenna and receiver. These burst alert stations must be located within the footprint of the satellite's path, or within 15 degrees latitude of the equator.

Messages are transmitted to the MIT Control Center and arrive less than 5 seconds after a burst alert has been issued. Summaries of burst alerts are forwarded, as appropriate, to the GRB Coordinates Distribution Network as soon as they are received from the spacecraft. thereby allowing early follow-on observations.

An SGS station: click to magnify!

An inexpensive ground station

The Burst Alert Station was designed to be relatively inexpensive, and consists of:

  • a VHF receiver (an AR3030 or a Drake R8 short-wave radio specially equipped to hear VHF signals)
  • omnidirectional antenna (e.g., a quadrifilar antenna by Directive Systems) and preamplifier
  • a PC or Sparc Station with an audio port.
One such system developed at MIT costs less than $3000 for the entire package.

Burst Alert Station Locations

The map below gives the locations and range of sensitivity of a possible set of HETE Burst Alert Stations that will allow nearly 24 hour coverage. The orbit track represents the HETE mission goal of an inclination of 2 degrees.

SGS locations: click on one to find out more about it!

Site Longitude Latitude Pass length*
Kwajalein 167.717 E 8.717 N 13.10 m
Christmas Island 157.1 W 1.9 N 13.93 m
Hiva Oa
(French Polynesia) 		139 W 9.78 S 12.93 m
Galapagos Island
(Ecuador) 		91.1 W 0.72 S 13.98 m
Cayenne
(French Guiana) 		51.9 W 4.9 N 13.71 m
Natal
(Brazil) 		35.13 W 5.47 S 13.64 m
Ascension Island
(UK) 		15W 7 S m
Franceville
(Gabon) 		13 E 1 S m
Malindi
(Kenya) 		40.19 E 3.00 S 13.86 m
Bangalore
(India) 		77.38 E 12.58 N m
Singapore 103.83 E 1.33 N 13.96 m
Koror
(Palau) 		134.5 E 7.33 N 13.36 m
Based on 625 km x 625 km orbit, 2 degree inclination)