An important subject in AMS02 experiment would be to perform measurements of the arrival time of the photons on a scale of few µsec for the Transient Gamma Sources, mainly Pulsars, Blazars and Gamma Ray Bursts. This may be possible if each trigger correlated with a detected cosmic particle, acquires a precise time stamp provided by a spatial GPS receiver which will also be used to synchronize the internal clocks of the AMS-02 DAQ system.
The TOPSTAR 3000 D GPS receiver characteristics are described. The built in procedures for the integration of the GPS module in the DAQ and Trigger systems are presented. The dependence on the flight orbit, ISS in case of AMS-02 mission is studied. In final, the monitoring requirements and the first results of tests are discussed.
1. AMS-02 DETECTOR
The AMS-02 detector is a typical particle physics detector, to be placed in space on the International Space Station in 2008.
It is composed of the triggering part for the charged and neutral Cosmic Rays (Time-of-Flight and electromagnetic calorimeter), of the tracking detectors such as Silicon Tracker embedded in the 0.7 T superconducting magnet, and the electromagnetic calorimeter for the photon and electron shower reconstruction.
The particle identification is done in the Transition Radiation, Ring Imaging Cherenkov and ToF detectors.
The GPS receiver will be placed on the upper radiator and its antenna will be fixed on the top of the Transition Radiation detector, facing the GPS constellation in the most optimized position.
2. A GLOBAL POSITIONING SYSTEM RECEIVER IN AMS-02
The GPS receiver will provide, with a precision of few microseconds for the temporal data (universal time coordinate - UTC), a time stamp for each physical event, and will synchronize the internal clock system of the AMS-02.
Various physics subjects and scientific goals of AMS : Antimatter, Cold Dark Matter, age of cosmic rays, gamma ray astronomy, primordial black holes, earth’s particle environment and other topics will be addressed and the temporal information will be essential for the transient objects.
3. GPS NETWORK AND GPS RECEIVER : PRINCIPLES AND APPLICATION
The GPS satellite constellation contains 24 satellites arranged in 6 MEO orbital planes with 4 satellites per plane. The GPS network consists of 3 segments : space, control, and passive user (receivers).
3.1. Position Velocity and Time determination
The ranging codes and navigation data are broadcasted on L1 (1,575.42 MHz) and L2 (1,227.6 MHz signals).
Each satellite transmits on its ranging code which permits to be identified and to determine the transit
time (Time Of Arrival). This TOA permits to calculate the satellite-to-user range. Each satellite transmits navigation data allowing the user to determine all the GPS constellation location.
Three dimensional locations require a TOA ranging measurements from at least 4 satellites to define Position, Velocity and Time (PVT determination).
3.2. Coordinate systems and Universal Time Coordinate for GPS network
The Earth-centered Inertial Coordinate System (ECI) with axes pointing in fixed directions with respect to the stars, is used by the GPS satellites to control their orbital flight.
The Earth-centered Earth-fixed coordinate System (ECEF) with x, y, z axes in rotation with the earth, is convenient for the GPS receiver to compute height, latitude and longitude positions.
The World Geodetic System 1984 (WGS84) contains in details a model of the earth’s gravitational irregularities.
The satellite transmissions are referenced to highly accurate atomic frequency standards of onboard satellites, synchronized with a GPS System Time managed from the control segment.
The Universal Time Coordinate (UTC) broadcasted by the GPS constellation is derived from atomic clocks (TAI – International Atomic Time) and Earth’s rotation rate ; GPS time and UTC time were coincident at the January 6, 1980 at 0 hour.
4. INTEGRATION OF TOPSTAR 3000D IN AMS-02, EXPECTED PERFORMANCES
AMS-02 requirements are : a position accuracy in LEO (Low Earth Orbit) of few meters, a time precision accuracy of few ms, the Universal Time Coordinate (UTC) and a monitoring of the degradation C/A (Coarse Acquisition) of the signal at L1 frequency.
The performances of the TOPSTAR 3000 D (Alcatel Alenia™) are : position accuracy in LEO (Low Earth Orbit) less than 10 meters, a velocity accuracy in LEO less than 1 cm/s, a time accuracy with TCXO (Temperature Controlled Crystal Oscillator) less than 1 ms (model GPS-AMS02). The TOPSTAR 3000 D is already installed on scientific satellites like Demeter, HETE-2 and various applications outside science.
5. GPS RECEIVER INTEGRATION IN AMS-02 DATA ACQUISITION SYSTEM
5.1. Functional architecture
The TOPSTAR functional architecture includes : a RF (radio frequency) module containing the TCXO and managing signal reception and analogue to digital transformation, a SP (signal processing) module to manage visibility and satellites tracking, a LOC (localisation) module (Diogene navigator on TOPSTAR to calculate an accurate localisation, a RS422 serial port to send a PPS (Pulse Per Second) and data in output and read data in input from the onboard system.
The communication scheme, in Figure 1, between GPS receiver and Data Acquisition System of AMS uses : the GPS module with a RS422 interface, the GPSE board to transmit the commands to the GPS receiver and Tele-commands coming from the JMDC via the USCM (DAQ computers).
The GPSE board collects and distributes PPS to the Trigger System and sends Telemetry (or Tele-data) to the JMDC by the way of the USCM.
From the GPSE board, the gates for the PPS and STIME Tele-data coming from the GPS could be opened or close on demand. The Simplex-Str4500 TM is a hardware and software system for orbital flight simulation. The objective is to control and command the receiver under the same conditions as on orbital flight with an ISS flight scenario loaded from a SPIRENT system.
The PC Windows XP Pro runs the graphical SIMPLEX software and loads the ISS scenario into the STR4500. Some configurations of the GPS constellation (number of visible satellites, orientation of their orbital flight, position compared to the horizon…) are more favourable than others. A valid UTC time acquisition initialized with Cold Start needs from 9 to 22 minutes to be operational depending on the orbit.
5.2. GPS receiver software integration in AMS DAQ system
To control and monitor the GPS receiver, two modes are defined : a standard mode and a monitoring mode.
The standard mode is considered as the normal way to activate the GPS ; it includes the following steps : initialization by a WMODE Tele-command (TC), exploitation of the PPS (Pulse Per Second) for internal synchronization, exploitation of the STIME measurement (TM) to extract the UTC time stamp for each event. In monitoring mode, the operation of the GPS will be followed in time, with a frequency that will be defined and changed if necessary.
The AMS-02 project will need particular information concerning the GPS, the status of its acquisition channels, and the quality of the signal received in the L1 band : extraction of the tracking status and raw measurement of channels of the acquisition, analysis of the PVT (position, velocity and time) computed by the navigation software, extraction of a general status of the TOPSTAR 3000 D receiver, and the L1 signal quality.
5.3. Standard mode : PPS and UTC Time correlation
A PPS synchronous with a 1-second epoch of the GPS constellation time is sent every second to the AMS DAQ. The PPS, via the GPSE card, restarts the internal clock of the trigger system. This PPS (still via the GPSE card) opens a communication gate to a buffer which gets the TM STIME.
After all checks by the main computer, the UTC time correlated with the last PPS is available in buffer and to be associated with the internal clock, in view to provide a stamp of a physical event.
6. GPS RECEIVER MONITORING
The evaluation of the quality of the operational mode of the GPS receiver implies a regular on-line monitoring. This monitoring is related to three topics : hardware status (verification on hardware configuration), software running (initialization during software upload and N2G navigator status) and the time measurement precision.
6.1 Monitoring of the time precision
Monitoring of the precision in time is based on Tele-data providing information on position, velocity and time, the satellite tracking status, mode of the determination of the Figure Of Merit and the GPS clock drifts.
The calculation of time is closely related to the calculation of the position ; the GPS receiver compares at any time the results of the “snapshot” and the “orbital navigator” algorithms. The measurement with lowest estimated error is selected.
Typically, the result of the algorithm “snapshot” is selected as long as the “navigator” did not converge. A TFOM (Time Figure Of Merit) is calculated to estimate the precision of the time provided by the GPS receiver and is sent in output to the onboard system.
This TFOM depends for example on the current phase of the acquisition, as the status of the receiver (in phase of carrier alignment, code alignment, or acquisition), the signal/noise ratio, the estimated value of DOP (Dilution Of Precision), the estimation of error rate depending on the ionosphere, the information provided by the onboard system (Almanach of the GPS constellation, the local onboard time).
The analysis in time presented in Figure 2 shows that the fraction of a second drifts in regular way, illustrating the drift between UTC time and the GPS receiver internal time.
With each PPS, the telemetry of the STIME provides the UTC time, the internal time of the receiver, and a TFOM. This TFOM varies depending on the number of satellites in view and their capture.
In conclusion, in context of simulations performed during implementation tests, the obtained time precision and the stability of the TOPSTAR 3000 D receiver are in conformity with the AMS-02 requirements.
1- TOPSTAR3000-N2G data Communication Interface – Alcatel (2003) DES.T3000.CI.1416
2- TOPSTAR3000 User’s Manual with N2G Navigator – Alcatel (2003) DES.T3000.MU.1441