For roughly 100 bursts a year, the LAT will obtain burst locations accurate to better than 10 arcmin. GBM burst locations will not approach that accuracy, but their value to the GLAST mission is nevertheless considerable.

Burst locations in the GBM are calculated by comparing count rates in detectors which are facing in different directions, as was done with BATSE. For the 150 GRBs which are expected to trigger the GBM each year there will be a three-stage location refinement: onboard, ground-automated, and ground-manual.

• The onboard location is the least accurate and is used to repoint the spacecraft to allow the LAT to detect delayed high-energy emission from GRBs which occurred outside its FOV. This crude location will be obtained within seconds of the trigger, and needs to be accurate only to about 20º to insure coverage by the LAT after repointing.
• The ground-based automated response occurs in near real-time and is based on the Global Coordinates Network (GCN) system. This location will be accurate to a few degrees (depending on burst intensity) and will provide coordinates for rapid follow-up by ground-based instruments.
• The ground-based manual location will be provided days after the trigger, with a human operator choosing background and source intervals for maximum sensitivity and the best possible burst location to be entered in the burst catalog. An accurate spectral model will be obtained, and careful generation of error boxes will reduce the systematic error associated with these locations to about 1.5º.

Determining real-time locations for GRBs can significantly increase the number of bursts detected by GLAST in two ways: If the burst is outside the LAT FOV the spacecraft can be reoriented, allowing observation of delayed high-energy emission (as was seen with EGRET in GRB 940217). If the burst occurs within the LAT FOV the location provided by the GBM may be used to enhance the statistical significance in the LAT. The field-of-view of the LAT is contained within that of the GBM so that any burst seen by the LAT will also be viewed by the GBM. There will be many bursts for which the LAT will register only a few high-energy photons, but the counting rates in the GBM detectors will allow a rapid onboard evaluation of an angular region of a few degrees. The burst position on the sky can be searched in the LAT, thus improving the sensitivity of the LAT and increasing the number of rapidly available LAT burst locations.