AMS will be able to detect high-energy gamma rays. Unlike all of the charged particles that AMS studies, gamma rays are unaffected by magnetic fields; they do not change directions as they fly through space. Thus, AMS can localize every gamma ray it sees; maybe this photon came from Aldebaran, this one came from Sco X-1, these came from GRS 1915+105, and so on. We first carefully measure the gamma ray's direction through the detector. Then se also measure the detector's position in space.
The Space Station, which is large and fairly flexible, cannot measure its own position very well. So, AMS carries its own small telescope, called a "star tracker". It photographs the stars and compares this image to a sky map. With this information, we can calculate AMS's position very accurately. This is similar to what you do, outdoors, if you orient yourself by the stars. You look for a pattern - like the Big Dipper, and Polaris, the North Star - and compare it to your memory of what the stars look like. With this you can accurately point the way North.
Many astrophysical objects (like pulsars, gamma-ray bursts, and supernovae) change or evolve very rapidly. It is important to know exactly at what time a gamma ray arrives, so AMS can compare its data with other experiments (HETE for example). AMS will carry a small GPS (Global Positioning System) unit to keep its clock in sync.
The star tracker is a pair of small optical telescopes with CCD cameras. They are on opposite sides of AMS, so at least one of them can always see the stars.