The Star Tracker and GPS
AMS-02 identifies high-energy photons and will contribute to the γ-rays sky research field. AMS can determine the photon energy using ECAL (single-particle mode) or Tracker (pair production), the arrival direction with respect to fixed stars using the Star Tracker, and the coordinated universal time (UTC) using the GPS system.
What is γ-ray Astronomy?
The galactic magnetic field (of few µG) traps charged cosmic-rays, as protons or Helia, for nearly 15 million years. During their propagation into the galactic volume, magnetic field bends their paths several times. 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-rays sky. Astronomy, or the study of celestial radiation sources, is possible only with neutral particles such as photons or neutrinos which are not sensitive to magnetic fields (neutrino astronomy is still at the beginning of its history, and is able only to study just one star: the Sun).
The γ-rays are photons of the highest energy, and their sky is being actively studied. Their production is believed to happen in the most energetic ph astrophysical places as Pulsars, AGNi and Gamma-Ray bursts.
Why do we need the Star Tracker?
AMS detects photons in two complementary ways, directly from ECAL and from pair conversion in Tracker. Both these measurements provide the γ-ray energy and the impinging direction with respect to the AMS coordinate system. The Star Tracker determines the orientation of AMS in the sky, more precise than the ISS one. The combination of ECAL/Tracker arrival direction and the Star Tracker orientation estimation will provide the arrival direction of the photon in the sidereal reference frame, i.e., with respect to fixed stars.
Two CCD digital photographic cameras placed on both sides of AMS compose the Star Tracker. We need two because, from time to time, one of the two could point toward the Sun and cannot be used. Star Tracker takes pictures of the sky in a 6 degrees field-of-view. A comparison between the taken picture and stellar maps could reveal the orientation of AMS in the sidereal reference frame. Star Tracker acquires a picture of the sky every 10 seconds, to describe finely the AMS orientation along the 90 minutes ISS orbit.
Why do we need the GPS?
One of the key features of the γ-rays astronomy is the emission variability of sources. The γ-ray emission of objects like Pulsars, Blazars, Gamma-Ray burst evolves rapidly with time. So it is of fundamental importance knowing when a γ-ray has arrived.
The AMS Data Acquisition System (DAQ) associates an UTC time stamp to each event. Two parts compose the AMS time measurement: a GPS UTC time and a fine-time measurement of an AMS DAQ internal clock. The sum of the GPS time and the fine-time of the internal clock provides the arrival time of the particle.
The synchronization between DAQ and GPS consists of a pulse synchronous with an epoch of the GPS constellation time sent every 10 seconds to the AMS DAQ. The pulse restarts the internal fine time clock of the DAQ system. After all checks by the main computer, the UTC time correlated with the last pulse is available in a buffer, to be associated with the internal clock and then to the following events.
Telemetry data provide information on position, velocity and time, the satellite tracking status, mode of the determination of the Figure Of Merit and the GPS clock drifts, elements concurring in the GPS precision in time.
The GPS receiver has been placed on the upper radiator. Its antenna has been fixed on the top of the Transition Radiation detector facing the GPS constellation satellites in the most optimized position.