James R Wertz Microcosm

We use navigation* and orbit determination interchangeably to mean determining the satellite's position and velocity or, equivalently, its orbital elements as a function of time. Similarly, we use both guidance and orbit control to mean adjusting the orbit to meet some predetermined conditions. For satellites, orbit control has two important subsets. Orbit maintenance refers to maintaining the orbital elements but not the timing of when the satellite is at a particular location in the orbit Stationkeeping refers to maintaining the satellite within a predefined box, which includes maintaining both the in-track position and the other orbital elements. Altitude maintenance is an example of orbit maintenance in which occasional thruster firings are used to overcome drag and keep the orbit from spiraling downward. Geosynchronous stationkeeping maintains the satellite in a box over one place on the Earth. Stationkeeping in low-Earth orbit includes constellation maintenance, in which each satellite is maintained in a moving box defined relative to the rest of the satellites in the constellation.

The satellite ephemeris is a tabular listing of the position and possibly the velocity as a function of time, usually in electronic form. It is important to distinguish the satellite ephemeris from the solar ephemeris, which lists the relative positions of the

* The origin of the terminology causes some confusion, particularly when reading older sources or references not associated with satellites. Navigation traditionally referred to determining how to get a craft where we wanted it to go. The term guidance was introduced with rockets and missiles to mean computing the steering commands needed to make the rocket go where we wanted it to (thus, a guided missile); control meant carrying out these steering commands to adjust the vehicle's direction of flight Thus, an intercept missile would have a guidance and control (G&C) system, and a space plane or interplanetary spacecraft would have a guidance, navigation, and control (GN&C) system. However, for spacecraft we use navigation to mean orbit determination, guidance to mean orbit control, and control system as a shortened form of attitude control system.

Garth and Sun as the Garth travels in its orbit, and lunar and planetary ephemerides, which provide similar data for other bodies in the solar system.

There are two types of orbit determination, differentiated by timing. Real-time orbit determination provides the best estimate of where a satellite is at the present time and may be important for spacecraft and payload operations, such as accurate pointing at some target Definitive orbit determination is the best estimate of the satellite position and orbital elements at some earlier time. It is done after gathering and processing all relevant observations. Orbit propagation refers to integrating the equations of motion to determine where a satellite will be at some other time. Usually orbit propagation refers to looking ahead in time from when the data was taken and is used either for planning or operations. Occasionally orbits will be propagated backward in time, either to determine where a satellite was in the past or to look at historical astronomical observations in the case of comets or planets.

Traditionally, ground stations from around the world provide tracking data to a mission-operations center. When all data is available, definitive orbit determination provides the best estimate of the orbit This is used to process the payload data for science or observation missions. The best estimate of the oibit is then propagated forward for real-time operations (such as star catalog selection or maneuver timing) and further forward for mission planning.

In 1983 NASA launched the first Tracking and Data Relay Satellite, TDRS, to begin replacing the worldwide ground tracking network.* TDRS provides the same functions as the traditional ground-station network. As the name implies, it tracks low-Earth orbiting satellites and relays data between the satellite and the TDRS ground station in White Sands, NM. As described in Sec. 11.7.2, GPS, GLONASS, and other more autonomous systems are also becoming operational, so orbit determination for future systems will differ significantly from what it has been in the past.

We can think of orbit determination and control as analogous to attitude determination and control. The ADCS subsystem (Sec. 11.1) measures and maintains the spacecraft's orientation about its center of mass. Similarly, the guidance and navigation function, perhaps better thought of as the Orbit Determination and Control Subsystem, or ODCS, measures and maintains the position of the spacecraft's center of mass. Both systems deal with spacecraft dynamics and both have the multiple functions of acquisition, determination, maintenance, and maneuver control.

11.7.1 System Definition Process

Major changes are occurring in the guidance and navigation arena. Traditionally, this has been exclusively a ground-operations activity. However, with the introduction of GPS and advanced onboard computers, several options now exist for autonomous navigation—determining the orbit on board the satellite in real time. We also have the capability to perform autonomous orbit maintenance and control, so the orbit determination and control function will change significantly. Even if we ultimately choose a completely traditional approach, we should evaluate new techniques which may reduce cost and risk for a particular space mission.

Table 11-64 summarizes the process of defining the orbit determination and control function. Each of the steps is described below. Section 11.7.2 then discusses the

* The second TDRS was lost in the Challenger accident in 1986, so the two-satellite operational constellation was not complete until 1988.

principal alternatives for navigation systems, and Sec. 11.7.3 describes the alternatives for maintaining and controlling the orbit The implementation of these in hardware and software is discussed in Sec. 11.7.4

TABLE 11-64. Process for Defining the Guidance and Navigation Subsystem. See text for discussion of each step.


Principal Issues

Where Discussed

1. Define navigation and orbit-related top-level functions and requirements

Mapping and pointing Scheduling

Constellation or orbit maintenance Rendezvous or destination requirements

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