LAT baseline design for the GLAST tracker consists of a four-by-four array of tower modules. Each tower module consists of interleaved planes of silicon-strip detectors and tungsten converter sheets. Silicon-strip detectors are able to more precisely track the electron or positron produced from the initial gamma-ray than previous types of detectors. SSDs will have the ability to determine the location of an object in the sky to within 0.5 to 5 arc minutes. The pair conversion signature is also used to help reject the much larger background of charged cosmic rays. The high intrinsic efficiency and reliability of this technology enables straight forward event reconstruction and excellent resolution with small tails. These ease-of-use properties will maximize the mission science return for guest observers.
In each module, there are 19 pairs of planes of silicon - in each pair, one plane has the strips oriented in the "x-direction", while the other has the strips oriented in the perpendicular "y-direction". When a particle interacts in the silicon, its position on the plane can therefore be determined in two dimensions. The third dimension of the track is determined by analyzing signals from adjacent planes, as the particle travels down through the telescope towards the calorimeter.