TABLE OF CONTENTS Telecommunications in space differs from the earthbound version in two major respects: 1) its long range, which may be anything from a few hundred to several billion kilometers, and 2) the potentially large relative velocity between transmitter and receiver, so that Doppler shift becomes significant ( 50 kHz in the S-band for low Earth orbit), requiring complex frequency-tracking loops in the receiver. Also, spacecraft in low orbit see very limited communications coverage from any single surface station. A station that can track to within 5 of the horizon will view a spacecraft in a 300-km orbit for only 6.5 min, even for a zenith pass. At the opposite extreme, distant spacecraft move very slowly against the background of the fixed stars, thus, the pass time is essentially governed by the rotation of the Earth. Signals from distant spacecraft, because they are very weak, require tracking by large, specialized equipment, such as NASAs Deep Space Network (DSN).
These factors complicate spacecraft design because of the mismatch between the rates of data acquisition and return. In low Earth orbit (LEO) a spacecraft may collect data throughout the orbit period of perhaps 95 min. Given only one downlink station, the spacecraft can dump data only a few times per day. Clearly, the downlink data rate must be many times that of the acquisition rate even with onboard processing and compression of the data. Power limitations and range restrict the rate at which data can be returned from a spacecraft at another...
