CHAPTER 7 SYSTEMS: MOTION CONTROL SYSTEM—NAVIGATOR OF THE HEAVENS 126 antennas mounted on the S0 truss (Figures 9 and 10). These receivers are SIGI units, manufactured by Honeywell (Morristown, NJ). SIGI receivers are common in satellites today, although the ISS was the first to use it operationally. GPS Antennas Figure 10. GPS antenna array on S0 truss. The rectangle traced out by the four antennas is 3 x 1.5 m (~10 x 5 ft). For the most part, the SIGI receivers determine position in the same way a GPS receiver determines position in a vehicle on Earth. The receiver can triangulate a position of the ISS, as well as compute a time error to the microsecond level between the computer clock running on the ISS and that on board the synchronized atomic clocks on the GPSs, by receiving coded signals from at least four of the 24 operational GPSs that orbit above the ISS. The position is provided to the navigation software within the USOS GNC MDM to correct the navigation filters, if necessary, whereas the time error is occasionally adjusted by MCC-H to slowly adjust the on-board clock of the ISS (see Chapter 5). Similar satellite navigation equipment is also installed in the Russian Segment, which determines its own state vector and shares it with the USOS GNC flight software. The SIGI can also determine acceleration however, this is normally only used during reboost maneuvers due to the relatively infrequent maneuvers of the ISS, and, even then, only occasionally. Additionally, tracking by ground radar and Tracking and Data Relay Satellite ranging can be used to determine the orbit of the ISS, although typically these data are used only by the ground. In some rare cases, both MCC-H and MCC-M may command new state vectors to the USOS and/or Russian Segment software when the ground solution is determined to be better or when the satellite navigation equipment in either segment is offline.