Acquisition of S-Band Telemetry Data during Spacecraft Launch Phases

Whenever one sets out to write a paper dealing with space exploration, the temptation is strong to try to cover everything, from the philosophy of why we are in the space business, to the detailed component description of our own favorite spacecraft . It is therefore, worth while to define the limits of the topic of this paper. The object is to compare the problems encountered in the acquisition of S-band telemetry data from spacecraft during the launch phases, with those encountered in the deep space phases. The paper is also limited to a discussion of the Pioneer, Mariner and Surveyor missions.In the discussion of telemetry mission support, the point of primary concern is data quality, which is directly related to the system signalto- noise ratio (S/N). It is primarily from this point of view that this paper discusses support problems.There were some valid reasons for transferring some telemetry link frequencies from the 215 to 260 MHz band to the 2200 to 2300 MHz band . Of primary importance was the lower noise environment at S-band. Also of importance was the improvement in overall antenna system gain for a given size directional antenna on the ground and on the spacecraft . If you consider system antenna gain as the sum of the transmitting and receiving antenna gains, fixed parabolic sizes will give an increase in system gain proportional to the square of the cormnunication frequency. During launch, an omni-directional spacecraft antenna nrust be used since the spacecraft may not be stabilized and the aspect angles from the multiple ground stations vary widely. Any improvement in system antenna gain with increase in frequency due to the ground antenna alone , is cancelled by the increase in free space attenuation. Even though the transition to S-band improved system performance in the deep space mode, during the launch phases, it did nothing but complicate the acquisition problem with narrower beam widths . For example, the beamwidth of the 85' antennas on the Range decrease from 4 degrees at 230 MHz to 0 . 4 degrees at 2300 .MHz.