The InterPlanetary Network (IPN) is a group of spacecraft equipped with gamma ray burst (GRB) detectors.
The precision for determining the direction of a GRB in the sky is improved by increasing the spacing of the detectors, and also by more accurate timing of the reception.
Typical spacecraft baselines of about one AU (astronomical unit) and time resolutions of tens of milliseconds can determine a burst location within several arcminutes, allowing follow-up observations with other telescopes.
In addition, gamma ray bursts are brief flashes (often as little as 0.2 seconds) that occur randomly across the sky.
Some forms of gamma ray telescope can generate an image, but they require longer integration times, and cover only a fraction of the sky.
A sky position is derived, and distributed to the astronomical community for follow-up observations with optical, radio, or spaceborne telescopes.
[citation needed] The Vela group of satellites was originally designed to detect covert nuclear tests, possibly at the Moon's altitude.
Other spacecraft (such as the OGO, OSO, and IMP series) had detectors for Earth, Solar, or all-sky gamma radiation, and also confirmed the GRB phenomenon.
The Helios-2 spacecraft carried a detector with precision time resolution to a Solar orbit that took it over one AU from Earth.
[citation needed] Compton and Ulysses were joined briefly by Mars Observer in late 1992, before that spacecraft failed.
Although Wind was in Earth orbit, like Compton, its altitude was very high, thus forming a short but usable baseline.
Interpolation between the two units usually gave a general sky direction for bursts, which in many cases could augment the IPN algorithm.
Once a GRB was detected, operators could spin the spacecraft within hours to point the X-ray telescopes at the coarse location.
Because distant probes require sensitive ground antennas for communication, they introduce a time lag into GRB studies.
Large ground antennas must split time between spacecraft, rather than listen continuously for GRB notifications.
A new generation of spacecraft are designed to produce GRB locations on board, then relay them to the ground within minutes or even seconds.
Should a GRB trigger the gamma detectors, X-ray masks report sky coordinates to ground stations.
When a GRB triggers the gamma detectors, generating a crude position, the spacecraft spins relatively rapidly to use its focusing X-ray and optical telescopes.
But should a burst happen to occur in its telescope field of view, its position and characteristics can be recorded with high precision.
In 2007 AGILE was launched and in 2008 the Fermi Gamma-ray Space Telescope and although these are Earth orbiters, their instruments provide directional discrimination.