127 SYSTEMS: MOTION CONTROL SYSTEM—NAVIGATOR OF THE HEAVENS CHAPTER 7 While the position in a particular orbit changes rapidly (8 km/sec [5 miles/sec]), the orbit itself changes little over the course of a day, mostly in the form of a small altitude decrease on the order of 25 to 50 m per day (82 to 164 ft per day) due to atmospheric drag. Because of this, the MCS can easily go for 24 hours or more without a position measurement (also sometimes referred to as a “fix”) to correct its orbit knowledge, although it is rare to go more than 1 hour. The USOS GNC system actually propagates three different orbit positions in memory—one based on measurements from GPS (SIGI) receiver 1, one based on measurements from GPS receiver 2, and one that is calculated by the Russian Segment Terminal Computer and transmitted to the GNC MDM. The software performs a comparison of the three estimates and will vote out the one that does not agree with the other two in order to isolate errors in the system. Normally, the three estimates will agree within a few tens of meters, and the system will automatically select GPS 1 if all three agree. See Figure 11. GPS 1 SELECTED STATE SELECTION GPS 2 RS State Figure 11. Selection of GPS estimates in the USOS GNC software. Attitude Determination Attitude (rotational position) determination is a more complex problem than orbit determination. Sensors are needed to determine the attitude at specific intervals as well as the changes between those times. The US Segment also determines the attitude of the ISS using GPSs, but in a fundamentally different way than that in which orbital position is determined. The GPS antenna array is relatively large, and the distance between the antennas is fixed and known. The position of the antenna array in three-dimensional space can be roughly determined by the attitude processor within SIGI by using the phase difference (i.e., time delay) between GPS signals received by at least three of the four different antennas in the array. These fixes can be infrequent (i.e., more than an hour apart). Tracking angles from the USOS Ku-band communications antenna can also be used as a source of attitude information, although this is considered a backup to the GPS (see Chapter 13). The US Segment has a pair of Rate Gyro Assemblies (RGAs) (Figure 12) mounted in the S0 truss to propagate attitude in between relatively infrequent position fixes. Each RGA consists of three ring laser gyros mounted at 90-degree angles to each other to sense rates about all three axes. Internally, the RGAs measure attitude changes 200 times per second five times per second, that information is provided to the GNC flight software. The GNC flight software updates its attitude knowledge at the same rate using the attitude information to calculate attitude error, which is used by the attitude control function that is described later. 20 cm (8 in.) Figure 12. ISS RGA. A complex attitude-determination filter in the GNC flight software combines the attitude fixes from GPS or the Ku-band antenna in concert with sensed rate changes and generates a highly accurate filtered attitude as well as estimates of RGA misalignment and gyro drift of the RGAs. With this information, the GNC flight software typically knows its attitude to 0.1 or 0.2 degrees accuracy.