Manuel M. Franco

The electrical conductivities of steel and other candidate materials for shrouds in a beam-waveguide antenna system

This article presents the results of electrical conductivity measurements made at 8.420 GHz on samples of the structural steel material used to fabricate shrouds on a Deep Space Network (DSN) 34-m-diameter beam-waveguide (BWG) antenna. Test results show that the structural steel samples at this microwave frequency had effective conductivities that were about 50 times worse than published dc values and also 230 times worse than the measured conductivities of aluminum test samples. Conductivity data are also presented for other candidate materials that could be used to fabricate BWG shrouds. Of interest are the improvements or degradations that were observed after some of the metal test samples were surface treated, plated, or painted.

Efficiency measurement techniques for calibration of a prototype 34-meter-diameter beam-waveguide antenna at 8.45 and 32 GHz

Efficiency measurements at 8.45 and 32 GHz (X- and Ka-bands) have been carried out on a new 34-m-diameter beam-waveguide antenna now in use at the NASA/JPL Goldstone Deep Space Communications Complex. The use of portable test packages enabled measurements at both the Cassegrain and the beam-waveguide focal points. Radio sources (quasars and Venus) were used as calibrators, and updated determinations of flux and source size correction were made during the period of the measurements. Gain and efficiency determinations as a function of elevation angle are presented, and the effects of the beam-waveguide system and antenna structure are clearly seen. At the beam-waveguide focus, an 8.45-GHz peak efficiency of 72.38% was measured; at 32 GHz, 44.89% was measured. >

Portable microwave test packages for beam-waveguide antenna performance evaluations

Portable microwave test packages used to evaluate a new 34-m-diameter beam-waveguide (BWG) antenna are described. The experimental methodology involved transporting test packages to different focal points of the BWG system and making noise temperature, antenna efficiency, and holography measurements. Comparisons of data measured at the different focal points enabled determinations of performance degradations caused by various mirrors in the BWG system. It is shown that, due to remarkable stabilities and accuracies of radiometric data obtained through the use of the microwave test packages, degradations caused by the BWG system were successfully determined. >

Measurements of a deep space station fractional frequency stability to the 10/sup -15/ level

Techniques are presented for measuring the frequency stability of a deep space station that was configured with a new X-band transmitter subsystem to enable high-quality radio science data to be obtained for the gravity wave experiment. The instrument and test procedures for performing frequency stability measurements in the field are described. Test results from measurements made on various transmit and receive subsystems and the overall transmit-receive end-to-end systems below the antenna feed horn are presented. The subsystem test data proved to be useful for assessing the frequency stabilities of various subsystems. This assessment led to design improvements needed to meet stringent frequency stability requirements for forthcoming gravity wave experiments. >

Beam aberration correction technique for a BWG antenna

A technique for a real-time beam aberration correction scheme for deep space uplink and downlink communications is presented. This technique provides a closed-form relationship between the beam aberration angles and the corresponding feed displacement for a reflector antenna with a beam waveguide (BWG) or multiple reflector system. This algorithm will enable one to accurately predict the movement of the feed in real time for a known beam deviation while the antenna is tracking a spacecraft. It is also useful to determine the feed offset position needed to correct for the beam deviation due to an imperfect antenna mechanical structure, and subreflector and mirror misalignments for a complex multiple reflector system.

The fractional frequency stability of a 34-m-diameter beam-waveguide antenna

Advances in fiber-optic technology have made it possible to develop a new technique to isolate and measure the frequency stability of a large beam-waveguide (BWG) antenna. Through the use of the technique described in the paper, at both 46.5/spl deg/ and 37/spl deg/ elevation angles, under good weather conditions, the degradation of the the fractional frequency stability of signals passing through the antenna path was measured to be between 1.3 and 2.2/spl times/10/sup -15/ for sampling intervals of 1024 s. These stability values apply to the portion of the antenna that includes the main reflector, subreflector, tripod legs, and six BWG mirrors. These test results are believed to be the first successful fractional frequency stability measurements made on the microwave optics portion of a large antenna to a level of 1 or 2 parts in 10/sup 15/. >

Preliminary assessement of the atmospheric optical channel at goldstone (CA)

A number of criteria need to be considered for site selection of a deep space optical link receiver station. Some of the factors used to identify suitable locations include whether site conditions are favorable in the atmospheric optical channel, geographical convenience of the location, and (possible) existing facilities (e.g. roads, power, communication networks etc.). Recently, NASA/JPL has been conducting studies to evaluate whether the NASAs Deep Space Network (DSN) Goldstone, California site is a viable candidate location for an optical deep space communications downlink station. Some reasons in considering Goldstone are quite evident: 1) the existing facilities would ease the integration of an optical ground station into the DSN; 2) it is conveniently near a NASA center (Jet Propulsion Laboratory or JPL); and 3) it is located in a desert region with possible high Cloud Free Line of Sight (CFLOS) statistics. Evaluating location suitability requires characterization of the atmospheric optical channel of the site, namely atmospheric loss and clear sky turbulence statistics. A suite of sensors has been installed at Goldstone to collect the necessary data to produce these statistics. The instruments include a sun-photometer, seeing monitor, and a dust particle profiler. This work presents initial results based on the data gathered at Goldstone to provide a preliminary assessment of the atmospheric optical channel and its implication on the data throughput for a hypothetical optical deep space mission.

Precision Pulsar Timing with NASA's Deep Space Network

Millisecond pulsars (MSPs) are a class of radio pulsars with extremely stable rotation. Their excellent timing stability can be used to study a wide variety of astrophysical phenomena. In particular, a large sample of these pulsars can be used to detect low-frequency gravitational waves. We have developed a precision pulsar timing backend for the NASA Deep Space Network (DSN), which will allow the use of short gaps in tracking schedules to time pulses from an ensemble of MSPs. The DSN operates clusters of large dish antennas (up to 70-m in diameter), located roughly equidistant around the Earth, for communication and tracking of deep-space spacecraft. The backend system will be capable of removing entirely the dispersive effects of propagation of radio waves through the interstellar medium in real-time. We will describe our development work, initial results, and prospects for future observations over the next few years.