The Star Tracker and GPS

 
Motivation 

During the propagation of charged cosmic rays through the galaxy the galactic magnetic field (~microgaus) bends their path several times on their 15 million year journey. Therefore, when a cosmic ray reaches Earth, its arrival direction does not point to its source. This means that there are no stars in the cosmic ray sky. In order to study celestial radiation sources one has to analyze neutral particles such as neutrons or photons.

Gamma rays are photons of the highest energy. They are believed to be produced in the most energetic astrophysical places such as pulsars, active galactic nuclei (AGN) and gamma-ray burts. AMS-02 will map these sources by measuring incoming photons.

AMS-02 detects photons in two ways that complement each other. First, directly with the ECAL and secondly, indirectly from pair conversions in the tracker. Both measurements determine the photon's energy and incoming direction. 

The Star Tracker

StarTracker01_300.jpgIn order to create a effectivly create a map of the cosmic ray sky, the orientation of AMS-02 in relation to the fixed stars needs to be known to a high precision. The Star Tracker on AMS-02 allows this measurement to a higher degree than the ISS Star Tracker. Combining the orientation of AMS-02 with the incident angle of the photon provides the arrival direction of the photon in a sidereal reference frame.

The Star Tracker system is composed of two CCD cameras facing in opposite directions. Two cameras are needed as one could be pointing towards the sun and could not be used. Each camera has a field-of-view of six degrees. By comparing the pictures taken with stellar maps, the orientation of AMS-02 can be determined. The Star Tracker takes a picture every 10 seconds in order to finely map the orientation during the 90 minute orbit of the ISS.

The GPS

Receiver_GPS_003_300.jpgIn addition to the exact orientation, AMS-02 also need to know the time of the event to a high accuracy. The reason for this is that the gamma ray emission of objects such as pulsars, gamma-ray bursts and AGNs evolve quickly with time.

Because the event time is so important, the AMS-02 Data Aquisition System (DAQ) assigns a UTC time stamp to each event. The time measurement is composed of two elements: a GPS UTC time and fine-time measurement by the AMS DAQ internal clock. The sum of both provides the arrival time of the incoming particle.

Both clocks are synchronized with help of a time signal sent from the GPS constellation to the DAQ clock every ten seconds. This resets the internal clock to zero. In addition, the UTC time provided by the last GPS pulse is saved in a buffer and attached to all subsequent events.

Furthermore, telemetry data on the position, velocity, time, satellite tracking status and GPS clock drifts is provided.

The GPS receiver has been placed on the upper radiator, with its antenna fixed to the top of the TRD. The antenna has been placed in such a way, that it is facing the GPS constellation satellites in the most optimized position.