I believe for the ICBMS they use stellar navigation and inertial navigation system, or maybe GPS-aided inertial guidance
Here's a quote
Here's a quote
* 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.