KEEPING IN TOUCH
In order to communicate with the Earth, Surveyor will use a 1.5-meter (59-inch) diameter high-gain antenna dish that sits at the end of a 2.0 meter (6.6 foot) long boom attached to the propulsion module. Two rotating joints, called gimbals, hold the antenna to the boom. The gimbals will allow the antenna to automatically track and point at the Earth while the science instruments observe Mars.
One of the high-gain antenna's two main functions will involve receiving command programs sent by the flight operations team on Earth. These programs, called command sequences, will contain instructions to tell Surveyor what functions to perform for time periods up to seven weeks in duration. During commanding periods, data will flow to Surveyor at a rate up to 750 commands per minute (500 bits per second).
The antenna's other main function involves sending data back to Earth. All transmissions broadcast by Surveyor will utilize X- band radio signals near 8.4 gigaHertz. That frequency would be equivalent to 8,400 on an FM receiver if ordinary radio dials reached that high. To complicate matters, Surveyor's radio transmitter will broadcast with a power of only 25 Watts. By the time the signal crosses millions of kilometers of space to reach the Earth, the signal strength will diminish to less than one millionth of one billionth of a Watt. How small is that number? If one could gather energy at that rate to charge a battery, it would take 30 million years to store enough charge to run a wrist watch for one second.
In order to receive the faint signal transmissions sent from Mars by Surveyor, NASA will utilize the gigantic tracking antennas of the Deep Space Network. These 34-meter (112-foot) diameter antennas are located in the Mojave desert, Spain, and Australia. The large size of these Earth-based tracking antennas will allow Surveyor to transmit scientific data at rates up to 85,333 bits per second. In contrast, the average home computer modem functions at 28,800 bits per second.