HETE Burst Alert Network
One of the key features of the HETE-2 satellite is its ability to
calculate precise localizations of GRBs on board within seconds of
burst onset, and then to transmit the burst localizations to
the ground as soon as they have been calculated.
The HETE-2 satellite utilizes a low-rate VHF transmitter to
continuously broadcast the burst information;
on the ground, an array of listen-only
secondary ground stations (SGS)
receive the data and transmit them to the MIT Control Center.
Once received at MIT, burst information are immediately
relayed to the
GRB Coordinate Distribution Network (GCN)
at the Goddard Space Flight Center for distribution to interested
ground observers.
Burst information downlinked by HETE
Information about a GRB will come to the ground in two ways:
- The results of real-time analyses performed on the spacecraft
are transmitted via the VHF to the secondary ground stations.
These results will be of moderate quality, as the spacecraft
processors are computationally limited.
- The raw data taken by all science instruments are transmitted
via the S-band data link. With these data, sophisticated
ground analyses can refine the burst position by factors
of 2-5x.
Burst analysis procedures
The analysis of HETE burst data occurs in several different stages
on several different fronts:
- Real-time analyses are performed on the spacecraft,
resulting in a quick but unrefined calculation of the
burst localization and spectrum. The results of these
analyses are distributed over the VHF link within seconds
of the burst detection.
- After the downlink of the raw data, automated analysis
engines run through the GRB data and come up with a
refined position. These results will be available 15 minutes
to two hours after the GRB, depending on where in the
spacecraft's orbit the burst occurred.
- Once a human analyst has been notified and reaches a
computer, a manual analysis of the data will be performed.
We expect that the results of such an analysis will
yield a significant improvement over the previous two
methods. The results of this analysis will be available
several hours after the burst.
The GCN messages
The GCN messages devoted to HETE data are the following:
- The HETE_ALERT message indicates that a burst has been
detected. Preliminary brightness and trigger time information
are included in this message, but no localization information
will be available.
This message is meant as a warning of an impending
localization message.
- HETE_FLIGHT messages contain updates of burst intensities
and durations and, most importantly, burst localizations.
There can be multiple versions of this message sent during
a burst: as new or better localization information become
available, they will be relayed to the GCN.
These messages are likely to be of most use to those
GCN sites with automated instrumentation.
- The HETE_FINAL message contains a summary of the best
information available from analyses done on the satellite.
This message should be of general interest to all.
- HETE_GND_ANALYSIS messages will be sent out after
ground analyses of the raw data have been completed.
More than one such message could be distributed.
These messages will give useful a posteriori
updates on the burst position and intensity.
