Inflight Operations The inflight operations of a spacecraft involve guidance, navigation, and control. In space usage, these
terms have specific meanings. Guidance refers to the determination of which way a probe should go to achieve a desired end
position, such as a planetary intercept or a rendezvous with another vehicle (see guidance and control systems). Navigation refers to the process of determining exactly where a probe is at any given time and where it will later be along that same course. Control refers to means of altering the flight path of a probe, usually by means of small rockets. Guidance is accomplished by computing a space vehicle's end position compared to a desired condition and then using the differences to determine what changes in current motion would result in smaller final differences. The future position is computed by propagating the vehicle's "state vector" (current position and velocity) forward in time, taking all gravitational influences into account as well as smaller perturbations due to atmospheric drag, solar wind, spacecraft venting, and similar disturbances. Significant course deviations can also be introduced by imprecise launch-vehicle performance. A certain amount of state-vector dispersion must be expected due to imperfect executions of maneuvers. Measuring the actual position of a space vehicle is a complex task. Powerful radar stations on Earth can track
satellites out to several thousand kilometers by bouncing radar beams off the satellites. Some satellites have transponders that automatically echo such a transmitted radar pulse, which greatly facilitates tracking. Satellites in GEO, however, usually can be detected from Earth only by their own radio transmissions, by optically tracking the satellites through powerful telescope cameras, or by using very powerful radar pulses. For vehicles in deep space, tracking is accomplished through the analysis of returned radio signals, in terms both of line-of-sight to the probe and of Doppler shifts (see Doppler effect) of the signal from the probe as it passes through changing gravitational fields. These measurements are compared to computer models of where the probe would be traveling if a certain initial position were assumed. From this comparison a best estimate of position can then be determined. A space vehicle's own attitude, or pointing direction, is determined onboard. Precise angles can be measured relative to an outside inertial frame of reference by means of periodic star sightings, and gyroscopes are used to measure any variations that occur subsequently. A precise knowledge of attitude is required to perform proper course changes. Once required velocity changes have been computed, the vehicle must rotate itself in space in order to point its propulsion system in the proper direction and then perform the firing at the precise moment for which it was computed. The velocity change actually executed can be observed by accelerometers on the probe. Following such correction maneuvers, additional periods of tracking may occur so that more navigation can refine the knowledge of the probe's trajectory and end point, and additional course corrections can then be performed as needed. Radio communications are required to command a probe and to receive information about its status and about the findings of its instruments. Information received over a radio link is called telemetry. Control is exercised by sending coded instructions that are received by the spacecraft, interpreted by a circuit called the command decoder, and then executed as necessary by the probe's computer autopilot. For deep-space probes the round-trip time of radio signals can become excessive. Round-trip time to the Moon, for example, is only a matter of seconds, but it can reach tens of minutes for Mars probes and many hours for probes in the outer solar system. Direct real-time commanding therefore cannot always be accomplished, and a great deal of flexibility and apatory programs are involved in preparing for such distant probes. The use of ground tracking stations is very different for LEO satellites and for deep-space vehicles. The latter move through the Earth's sky very slowly and can remain in sight of a single tracking site for up to 12 hours. Because of the low altitude and relatively great speed of LEO satellites, however, they quickly cross the sky of any ground site, moving from horizon to horizon in five or six minutes. This explains why satellites near Earth spend most of their time out of radio contact even though 10 to 20 tracking sites are available for use, whereas vehicles millions of kilometers out in space can be continuously monitored by only a handful of sites strategically spread around the globe. NASA's deep-space network has three main sites, at Goldstone Tracking Station in California, Madrid in Spain, and near Canberra in Australia. In order to overcome this geographical restriction and to reduce the expense of maintaining worldwide radio stations, both NASA and the Russian space program have developed geosynchronous relay satellites. Known in the United States as the Tracking and Data Relay Satellite System, the network of three satellites and three spares now routinely relays data from manned and some unmanned satellite missions. The equivalent Russian system is called Luch ("ray").