Navigation systems

The orbiter’s primary navigational aids were three so-called “gyro-stabilized platforms” (GSPs). Comparable with the Shuttle’s three Inertial Measurement Units (IMUs), they provided inertial attitude and velocity data to the guidance, navigation, and control software. Just like the IMUs, the GSPs were isolated from rotations by four gimbals and used a set of gyros to maintain the platform’s inertial orientation. Attitude data was provided by so-called resolvers and velocity data by a set of accelerometers. Whereas the Shuttle’s IMUs are mounted on a navigation base forward of the flight deck control and display panels, Buran’s GSPs were installed in a small module in the payload bay, attached to the outer wall of the aft flight deck just under the aft-looking window.

Since Energiya’s own GSPs were much more accurately aligned prior to launch than those of Buran, the orbiter primarily relied on data from Energiya’s navigation sensors for accurate azimuth alignment during launch. Buran’s on-board computers continuously compared navigation data originating from Energiya with that obtained by the orbiter’s own sensors and then made the necessary corrections.

In-orbit alignment of the GSPs was conducted with star trackers and a radio altimeter, attached to the right and left sides of the GSP module. While the Shuttle Orbiter also uses star trackers for IMU alignment, the radio altimeter was unique to Buran. The star trackers, concentrated in a so-called Stellar-Solar Instrument (ZSP), measured the line-of-sight vector to at least two stars. Using this information, the on-board computers calculated the orientation between these stars and the orbiter to determine the vehicle’s attitude. Comparison of this attitude with the attitude measured by the GSP provided the correction factor necessary to null the GSP error. The ZSP had a door which was opened after opening of the payload bay doors.

The radio altimeter (Vertical Radio Altimeter or RVV), providing local vertical measurements, acted as a back-up to the star trackers for GSP alignment and thereby increased the reliability of the navigation system. GSP re-alignment required at least two measurements of the local vertical, ideally with an interval of a quarter orbit. However, the RVV only provided reliable information over bodies of water, making it necessary to accurately time the measurements. Shortly after a GSP alignment session, the RVV could also be used for autonomous navigation, updating the vehicle’s state vector. State vector updates were also performed with a Sunrise/Sunset Detection Instrument (PRZS), an optical device that compared the expected and actual moments of sunset and sunrise as seen from the orbiter. This was also unique to Buran.

When orientation was lost completely (e. g., due to a computer failure) and could not be restored with the star trackers or radio altimeter, Buran could rely on an infrared horizon sensor system (Local Vertical Sensor or PMV) reacting to the Earth’s radiation to re-establish orientation to a point where the radio altimeter could take over. Not available on the Space Shuttle, this system could be used during unmanned missions.

Navigational aids to be used for proximity and docking operations in orbit were a radar system known as the Mutual Measurement System (SBI), the Cosmonaut Visual Rangefinder (VDK), and the Visual Navigation Measurement System (NIVS). The latter was permanently mounted on a special porthole in the aft flight deck and had to be manually aligned with the GSP module by the crew.

Just like the Shuttle Orbiter, Buran had three-axis rate gyro assemblies and body – mounted accelerometers to measure angular rates and accelerations for use in flight control algorithms [23].

For entry the accuracy of the GSP-derived state vector was insufficient to guide the spacecraft to a pinpoint landing. Therefore, data from other navigation sensors were blended into the state vector at different phases of entry to provide the necessary accuracy. Because of the requirement to perform unmanned landings, Buran had a more elaborate system of landing navigation aids than the Space Shuttle Orbiter: