The Goldstone Solar System Radar has become a NASA-sponsored facility science instrument for performing scientific observations of nearby asteroids, the surfaces of Venus or Mars, the satellites of Jupiter, and other objects in the solar system. The current instrument is sustained by the resources of the DSN Implementation and Operations Program, but its form is a product of many years of development by the DSN Advanced Systems Program. In the early days of the DSN, the Advanced Systems Program took ownership of the radar capability at the DSN's Goldstone California site and evolved and nurtured it as a vehicle for developing and demonstrating many of the capabilities that would be needed by the Network.
Scientific results abounded as well, but were not its primary product. Timely development of DSN capabilities was the major result. Preparations for a radar observation at the DSN Technology Development Field Site bore many resemblances to those for a spacecraft planetary encounter, since the radar observations could only be successful during the few days when the Earth and the radar target were closest together.
In the conventional formulation of the radar sensitivity equations, that sensitivity depends upon the Aperture, Temperature, Power, and Gain of the system elements. Here, aperture refers to the effective size, or collecting area and efficiency of the receiving antenna, and temperature is a way of referring to the noise in the receiving system, where a lower temperature means a lesser noise; power refers to the raw power level from the transmitter, and gain is the effective gain of the transmitting antenna, which depends in turn upon its size, its surface efficiency, and the frequency of the transmitted signal. Where the same antenna is used to both transmit and receive, the antenna size and efficiency appear twice in the radar equations.
Significant improvements to the DSN's capability for telemetry reception were to come from the move upward in frequency from S-band (2 GHz) to X-band (8 GHz) on the large 64-m antennas. Performance of these antennas at the higher frequencies and the ability to successfully point them were uncertain, however, and these uncertainties would best be removed by radar observations before the first spacecraft with X-band capabilities were launched. The radar had obvious benefit from the large antenna and the higher frequency. The first flight experiment for X-band was scheduled to be on MVM 73. Successful radar observations from the Goldstone 64-m antenna demonstrated that the challenge of operating the large antennas at the higher X-band frequency could be surmounted.
High-power transmitters were needed by the DSN for its emergency forward-link functions, but were plagued by problems such as arcing in the waveguide path when power densities became too high. High-power transmitters were essential for the radar to "see" at increased distances and with increased resolution. Intense development efforts at the DSN Technology Development Field Site could take place without interference or risk to spacecraft support in the Network. Successful resolution of the high-power problems for the radar under the Advanced Systems Program became the successful implementation of the high-power capability needed by the Network.
Low-noise amplifiers were needed by the DSN to increase data return from distant spacecraft. Low-noise amplifiers were essential for the radar to enable it to detect echoes from increasingly distant targets or to provide for increased resolution of already detectable targets. The appetite of the radar system for increasingly lower noise levels provided a motivator for the Advanced Systems Program to develop the extremely low-noise TWM amplifiers that would be transferred for implementation throughout the DSN.
Digital systems technology was rapidly evolving during this period and would play an increasing role in the developing DSN. Equipment developed by the Advanced Systems Program for its radar application included (1) digital encoders to provide for spatial resolution of parts of the radar echo, (2) computer-driven programmable oscillators to accommodate Doppler effects on the signal path from Earth-to-target-to-Earth, and (3) complex high-speed digital signal processing and spectrum analysis equipment. Much of the digital technology learned this way would transfer quickly to other parts of the signal processing work under the Advanced Systems Program and eventually into the operational DSN. Some of the elements would find direct application, such as the programmable oscillators, which became essential for maintaining contact with the Voyager 2 spacecraft, following a partial failure in its receiver soon after launch. And the signal analysis tools would be called on many times over the years to help respond to spacecraft emergencies.
Some of the products of the early radar observations (see adjacent Figure) were both scientific in nature and essential for providing information for the planning and execution of NASA's missions. One notable "first" is the direct measurement of the astronomical unit (AU), which is the mean orbital radius of the Earth, and, it sets the scale size for the distances in the solar system. The measurement was made in support of preparations for Mariner 2 to Venus and provided a correction of 66,000 km from conventional belief at that time. It also allowed corrections that brought the mission into the desired trajectory for its close flyby of the planet. The radar system was also used in qualifying potential Mars landing sites for the Viking landers, and continues to provide information about the position and motion of the planets, which is used to update the predicted orbits for the planets of the solar system.
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