The determination of the position and velocity of a space probe with respect to a target body or a known reference body such as the Earth (navigation) and, based upon this determination, the application of propulsive maneuvers to alter the subsequent path of the probe (guidance).
Space navigation can be viewed as determining the current position and velocity of the probe and then using that determination as the basis of predicting future motion. This determination is made by taking a series of measurements relating to the probe's motion and combining these measurements in such a manner as to make the most accurate estimate of the probe's current position and velocity, taking into account possible small errors or inaccuracies in the measurements themselves. One of the most powerful (and accurate) measurements which can be made is the relative velocity between an Earth tracking station and the space probe itself. This is accomplished by broadcasting an electromagnetic signal which consists of a single tone having a stable frequency to the probe. The probe will receive this signal shifted in frequency in exact proportion to the relative station-probe velocity. This frequency shift is known as the Doppler effect. It is also possible to measure the station-probe distance (or range) using electromagnetic signals.
For missions to the outer planets (Jupiter and beyond) or for a mission to a body such as a comet or asteroid, the position of the target may be sufficiently uncertain as to make a strictly Earth-relative navigation scheme inadequate. Here it is necessary to make measurements that directly involve the target. Typical of these is to obtain optical measurements of the location of the target relative to a star as seen from the probe itself.
In its simplest form, guidance consists of comparing the predicted future motion of the probe against that which is desired, and if these are sufficiently different, executing a propulsive maneuver to modify that future position. Typically, the probe will contain a small rocket motor which can be fired in any desired direction by first rotating the spacecraft away from its cruise orientation and then holding the new attitude fixed while the rocket motor is firing.
Multiplanet missions use an accurate delivery to the first target not only to scientifically explore that planet, but also to use the planet's gravitational attraction to change the course of velocity of the probe advantageously for the next leg of the mission toward the second and subsequent targets. The encounter with a planet becomes in itself a type of guidance correction as the planet changes the path of the probe. See also Guidance systems; Space probe.
