I found this on FAS.org. Covers the subject, including the use of GPS.
http://www.fas.org/nuke/intro/missile/basics.htm
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Guidance System
The guidance system in a missile can be compared to the human pilot of an airplane. Every missile guidance system consists of an attitude control system and a flight path control system. The attitude control system functions to maintain the missile in the desired attitude on the ordered flight path by controlling the missile in pitch, roll, and yaw. The attitude control system operates as an auto-pilot, damping out fluctuations that tend to deflect the missile from its ordered flight path. The function of the flight path control system is to determine the flight path necessary for target interception and to generate the orders to the attitude control system to maintain that path.
The operation of a guidance and control system is based on the principle of feedback. The control units make corrective adjustments of the missile control surfaces when a guidance error is present. The control units will also adjust the control to stabilize the missile in roll, pitch, and yaw. Guidance and stabilization corrections are combined, and the result is applied as an error signal to the control system.
The heart of the inertial navigation system for missiles is an arrangement of accelerometers that will detect any change in vehicular motion. An accelerometer, as its name implies, is a device for mea-suring acceleration. In their basic form such devices are sim-ple. For example, a pendulum, free to swing on a transverse axis, could be used to measure acceleration along the fore-and-aft axis of the missile. When the missile is given a forward acceleration, the pendulum will tend to lag aft; the actual displacement of the pendulum form its original position will be a function of the magnitude of the accelerating force. The movement of the mass (weight) is in accordance with Newton's second law of motion, which states that the acceleration of a body is directly proportional to the force applied and inversely proportional to the mass of the body.
Usually there are three double-integrating accelerometers continuously measuring the distance traveled by the missile in three directions--range, altitude, and azimuth. Double-integrating accelerometers are devices that are sensitive to acceleration, and by a double-step process measure distance. These measured distances are then compared with the desired dis-tances, which are preset into the missile; if the missile is off course, correction signals are sent to the control system. If the missile speed were constant, the distance covered could be calculated simply by multiplying the speed by time of flight. But because the acceleration varies, the speed also varies. For that reason, the second integration is necessary.
When targets are located at great distances from the launching site, some form of navigational guidance must be used. Accuracy at long distances is achieved only after exacting and comprehensive calculations of the flight path have been made. Navigational systems that may be used for long-range missile guidance include inertial and celestial.
* Inertial guidance. The simplest principle for guidance is the law of inertia. In aiming a basketball at a basket, an attempt is made to give the ball a trajectory that will terminate in the basket. However, once the ball is released, the shooter has no further control over it. If he has aimed incorrectly, or if the ball is touched by another person, it will miss the bas-ket. However, it is possible for the ball to be incorrectly aimed and then have another person touch it to change its course so it will hit the basket. In this case, the second player has provided a form of guidance. The inertial guidance system sup-plies the intermediate push to get the missile back on the proper trajectory. The inertial guidance method is used for the same purpose as the preset method and is actually a refinement of that method. The inertially guided missile also receives programmed informa-tion prior to launch. Although there is no electromagnetic contact between the launching site and the missile after launch, the missile is able to make corrections to its flight path with amazing precision, controlling the flight path with accelerometers that are mounted on a gyro-stabilized platform. All in-flight accelerations are continuously measured by this arrangement, and the missile attitude control generates corresponding correction signals to maintain the proper trajectory. The use of inertial guidance takes much of the guesswork out of long-range missile delivery. The unpredictable outside forces working on the missile are continuously sensed by the accelerometers. The genera-ted solution enables the missile to continuously correct its flight path. The inertial method has proved far more reliable than any other long-range guidance method developed to date.
* Celestial Reference. A celestial navigation guidance system is a system designed for a predetermined path in which the missile course is adjusted continuously by reference to fixed stars. The system is based on the known apparent positions of stars or other celestial bodies with respect to a point on the surface of the earth at a given time. Navigation by fixed stars and the sun is highly desirable for long-range missiles since its accuracy is not dependent on range. The missile must be provided with a horizontal or a vertical reference to the earth, automatic star-tracking telescopes to determine star elevation angles with respect to the reference, a time base, and navigational star tables mechanically or electrically recorded. A computer in the missile continuously compares star observations with the time base and the navigational tables to determine the missile's present position. From this, the proper signals are computed to steer the missile correctly toward the target. The missile must carry all this complicated equipment and must fly above the clouds to assure star visibility. Celestial guidance (also called stellar guidance) was used for the Mariner (unmanned spacecraft) interplanetary mission to the vicinity of Mars and Venus.
ICBM and SLBM systems at present use celestial guidance.
* Command Guidance Multi-source radio signals that allow a triangulation of position offer an alterna-tive to acceleration measurements. Advanced missile powers dropped radio guidance in the 1960’s and switched to autonomous inertial measuring units, which are carried onboard the missile. The United States considered radio guidance again in the late 1980’s for mobile missiles but dropped the idea in favor of a Global Positioning System (GPS). A radio guidance system could transmit signals from the launch site, or an accurate transmitter array near the launch site to create the signals. Radio command and control schemes, because of the immediate presence of a radio signal when the system is turned on, alert defenses that a missile launch is about to occur. And performance for these systems degrades because of the rocket plume and radio noise. Also, these systems are very much subject to the effects of jamming or false signals.
Global Positioning System (GPS) and the Global Navigation Satellite System (GLONASS) are unlikely ever to be used in the control function of a ballistic missile. The best military grade GPS receivers produce positions with an uncertainty of tens of centimeters. If a missile has two of these receivers in its airframe spaced 10 meters apart, the best angular resolution is roughly in the centiradian range. Theater ballistic missiles [TBMs] require milliradian range angular accuracy to maintain control. However, GPS has significant application for an TBM outfitted with a post-boost vehicle (bus) or attitude control module that navigates a reentry vehicle to a more accurate trajectory."
Btw, the last sentence adresses the issue of accuracy of single staged TBM's, discussed in one of numerous Taiwan Strait Scenario threads.
