Radio Rangefinder System (RDS)

This measured range from an altitude of 40 km (about 400 km from the runway) all the way to touchdown. The on-board component of the RDS, known as 17M900, consisted of four redundant interrogators and four antennas, weighing a total of 85.5 kg. Up to an altitude of 4 km they sent paired pulses to six distance-measuring equipment units (DME) at Baykonur, which then transmitted paired pulses back to the orbiter on a different frequency. The time required for the round trip of this signal was then translated by the orbiter into distance to the transponder. The system indirectly also provided data on elevation and azimuth.

Three of the ground terminals were deployed off one end of the runway and the other three off the opposite end. One terminal in each set of three was located along the runway centerline, about 20 km away, and the other two were deployed on either side within 60 km from the runway. Each of the six terminals had a unique coded

reply, allowing Buran’s Biser-4 computers to select and use distance measurements from the three terminals whose positions provided the best accuracy. Each of the three selected terminals was interrogated 60 times per second, nearly four times the rate for standard en route distance-measuring equipment used in aviation at the time. Each terminal transmitted via two antennas, one horizontally polarized and the other vertically polarized, enabling Buran to receive a strong signal over its circularly polarized antennas despite extreme pitch or roll maneuvers. When the orbiter reached an altitude of 4 km, the RDS interrogators switched to distance-measuring units on either end of the runway that had the same precision as a microwave landing system. The RDS has no equivalent on the US Space Shuttle Orbiter.