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Distorted antenna surfaces forced to produce a uniform wave front. SUBFORMING employs geometric optics in determining subreflector coordinates to match main reflector surface with known distortions. Antenna with distorted paraboloidal reflecting surface forced to produce uniform wave front by using a Cassegrainian geometry with path-length-compensating subreflector. Program written in FORTRAN V for batch execution.
Tilt-measuring apparatus gives precise indication of elevation angle of scientific instrument or other object that must be aimed precisely. In original intended application, apparatus mounted directly on primary reflector of large radio antenna. Sensors and actuator in elevation-angle encoder include motor, shaft-angle encoder, and pendulous accelerometer mounted on shaft. Designed to give zero output when designated reference axis perpendicular to gravity.
Improved method devised for calculating gain of antenna system comprising main reflector and array of feed elements. Method involves much less computation; important advantage when necessary to compute gains repeatedly, with slight variations in design at each iteration, in effort to find optimum design.
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The lunar radio observations and interpretations of Piddington and Minnett (1949) and Gibson (1958) show that the lunar brightness variation with phase at millimeter wave lengths can be used to determine the physical properties of the lunar surface. They found that the millimeter-wave brightness lagged the optical phase, and their interpretation was that the millimeter radiation originates below a surface layer that is a very good thermal insulator. The thickness of this layer could not be determined from observations at one frequency. Observations at different frequencies give different results because of the wave-length dependence of the radio absorption by the surface material. The attenuation in the material increases with decreasing wave length, and therefore it is possible, in principle, to determine surface layer thickness from radio observations at several wave lengths. For this reason, observations of lunar radiation were started at the Naval Research Laboratory at a wave length of 4.3 mm. This is half the wave length used by Gibson (1958) in his earlier studies. The radio telescope used for these observations has been described in detail in a previous publication (Coates 1958). The antenna is a parabolic reflector 10 feet in diameter, and it has a beam width of 6.7 minutes of arc at the wave length of 4.3 mm. This is about one-fifth the diameter of the moon. The receiver was a Dicke-type radiometer.
Examples of large space systems currently being considered by NASA include a large communications antenna system (the Land Mobile Satellite System), a precision antenna system (the Large Deployable Reflector System), and a preliminary concept for NASA's space station. Each system has low natural vibration frequencies, stringent pointing requirements, and, for the antennas, demanding surface accuracy requirements. A review is provided of the progress being made in structural ground tests with respect to surface accuracy, deployment, and erection of large structures. Attention is given to the dynamic loads on large space structures, on-orbit testing, space vibration control devices, and the characteristics of distributed control.
Subreflector for Cassegrain antenna which permits prime-focus feeding of paraboloidal reflector
On ITALSAT Flight 2, the Italian telecommunications satellite, the two L-Ka antennas (Tx and Rx) use two large deployable reflectors (2000-mm diameter), whose deployment and fine pointing functions are accomplished by means of an innovative mechanism concept. The Antenna Deployment & Pointing Mechanism and Supporting Structure (ADPMSS) is based on a new configuration solution, where the reflector and mechanisms are conceived as an integrated, self-contained assembly. This approach is different from the traditional configuration solution. Typically, a rigid arm is used to deploy and then support the reflector in the operating position, and an Antenna Pointing Mechanism (APM) is normally interposed between the reflector and the arm for steering operation. The main characteristics of the ADPMSS are: combined implementation of deployment, pointing, and reflector support; optimum integration of active components and interface matching with the satellite platform; structural link distribution to avoid hyperstatic connections; very light weight and; high performance in terms of deployment torque margin and pointing range/accuracy. After having successfully been subjected to all component-level qualification and system-level acceptance tests, two flight ADPMSS mechanisms (one for each antenna) are now integrated on ITALSAT F2 and are ready for launch. This paper deals with the design concept, development, and testing program performed to qualify the ADPMSS mechanism.
Antenna systems for a Ku-band synchronous orbit communications satellite were considered. A multiple beam offset feed cluster used in conjunction with an offset parabolic reflector was chosen for the antenna realization. For radiating elements in the feed array, polyrod or dielectric rod radiators fed by waveguide are investigated. Polyrod antennas shrink the cross section of the waveguide required, are fairly directive, and can be tapered to reduce sidelobes. As a consequence, it is thought that mutual coupling effects between adjacent elements will be minimal and the resultant physically small elements will allow greater packing densities for the drive elements in the feed array than would otherwise be possible.
Configurations (two) of the reflector-only assemblies, using different width backup cones, are analyzed for RF boresight direction changes and wind distortions. The wider backup cone is best for minimum weight; however, there is an optimum weight which minimizes the RF boresight errors for a wind load that produces the maximum pitching moment of both configurations.
The NASA Langley Research Center’s Experimental Test Range is an indoor anechoic compact range far field test facility used to conduct antenna and electromagnetic radiation measurements. The Experimental Test Range was designed to simulate far field illumination in the facility test volume over a broad band of frequencies by collimating the RF energy from the 26 ft by 26 ft parabolic reflector. The quality of the antenna and radiation measurements are dependent on the uniformity of the far field plane wave generated by the compact range reflector. While this facility is going through several upgrades, this report describes an assessment of the far field plane wave conducted after resurfacing the primary reflector to improve performance and extend the range of frequencies for which this facility can operate. This assessment addresses far field uniformity probe data measured in the test volume across the facility operational frequency bands.
The technologies for optimized, i.e., state of the art, operation of satellite-based communications systems are surveyed. Features of spaceborne active repeater systems, low-noise signal amplifiers, power amplifiers, and high frequency switches are described. Design features and capabilities of various satellite antenna systems are discussed, including multiple beam, shaped reflector shaped beam, offset reflector multiple beam, and mm-wave and laser antenna systems. Attitude control systems used with the antenna systems are explored, along with multiplexers, filters, and power generation, conditioning and amplification systems. The operational significance and techniques for exploiting channel bandwidth, baseband and modulation technologies are described. Finally, interconnectivity among communications satellites by means of RF and laser links is examined, as are the roles to be played by the Space Station and future large space antenna systems.
Initial optical communications experiments with a Vertex polished aluminum panel have been described. The polished panel was mounted on the main reflector of the DSN's research antenna at DSS-13. The PSF was recorded via remotely controlled digital camera mounted on the subreflector structure. Initial PSF generated by Jupiter showed significant tilt error and some mechanical deformation. After upgrades, the PSF improved significantly, leading to much better concentration of light. Communications performance of the initial and upgraded panel structure were compared. After the upgrades, simulated PPM symbol error probability decreased by six orders of magnitude. Work is continuing to demonstrate closed-loop tracking of sources from zenith to horizon, and better characterize communications performance in realistic daytime background environments.
Measurements made on the frequency stability of the DSS-13 34-m diameter Beam Waveguide (BWG) antenna showed that at 46.5 and 37 deg elevation angles, the BWG antenna stability at 12.2 GHz was between 1.3 and 2.2 x 10(exp -15) for tau = 1024 sec and good weather conditions. These frequency stability values apply to the portion of the antenna that includes the main reflector, subreflector, tripod legs, and the six BWG mirrors. The test results reported are believed to be the first known successful measurements of the stability of the microwave optics portion of a large antenna to a level of 1 or 2 parts in 10(exp 15).
The deployable array antenna using an inflatable or thin-membrane structure has been identified as one of the enabling technologies to achieve low-mass, high packaging efficiency, low cost, and reliable deployment for future NASNJPL spacebome high-gain and large aperture antennas. Array antennas, when compared to parabolic reflectors, although suffering from limited bandwidth performance, offer wide-angle beam scanning capability and a more reliable flat “natural” aperture. To demonstrate the feasibility and capability of this low-mass array technology, three antenna concepts using inflatable and thin-membrane structures were initiated in 1997 at JPL and several breadboard units have been successfully developed. These three concepts are (1) the inflatable phased array, (2) the inflatable reflectarray, and (3) frame-supported thin membrane array antenna. All three concepts utilize the printed microstrip antenna technology. Although all three concepts will be briefly presented, this paper will discuss in more detail of the recent development of the second concept. The first concept of inflatable phased arrays that have been constructed are 3 to 5-meter size L-band dual-polarized synthetic aperture radar (SAR) arrays for Earth remote sensing application. They all consist of a rectangular configuration with inflatable cylindrical tubes that support and tension a multi-layer thin-membrane radiating aperture with microstrip patches and microstrip power divider lines. For the second concept, an inflatable reflectarray was developed for future deep-space telecom applications. It is a 3m Ka-band inflatable reflectarray (1.8 kg/m2). This reflectarray uses inflated torus tubes to support and tension a flat-membrane reflectarray surface. The reflectarray surface emulates a curved parabolic reflecting surface. However, because of its flat surface being a “natural surface”, it is much easier to achieve and more reliable to maintain the required surface tolerance than that of a curved parabola during long space flight. For the third concept, an L-band dual polarized SAR array with a 5m x 3m aperture and 2.5 kg/m2 of mass has recently been demonstrated. It consists of seven foldable panels each having a rectangular frame that supports a two-layer thinmembrane microstrip subarray aperture. Each frame is made of light-weight graphite composite material. The chief advantage of this deployable “frame” concept is that each frame is able to rigidly support an appropriate number of T/R modules and phase shifters to achieve the desired power distribution and beam scanning. Several technology challenges, such as the development of rigidizable inflatable tubes, a controlled deployment mechanism, thin-membrane thermal effects, a low-mass inflation system, membrane mounted T/R modules, means to counter surface tolerance issues, etc. are being investigated and will be discussed in the presentation. With foreseeable success in the development of these challenging areas, the inflatable/thin-membrane array antennas could be mature enough in a few years for actual space flight.
The Advanced Communications Technology Satellite (ACTS) to be launched in 1993 is briefly introduced. Its multibeam antenna, consisting of electrically similar 30 GHz receive and 20 GHz transmit offset Cassegrain systems, both utilizing orthogonal polarizations, is described. Dual polarization is achieved by using one feed assembly for each polarization in conjunction with nested front and back subreflectors, the gridded front subreflector acting as a window for one polarization and a reflector for the other. The antennas produce spot beams with approximately 0.3 degree beamwidth and gains of approximately 50 dbi. High surface accuracy and high edge taper produce low sidelobe levels and high cross-polarization isolation. A brief description is given of several Ka-band components fabricated for ACTS. These include multiflare antenna feedhorns, beam-forming networks utilizing latching ferrite waveguide switches, a 30 GHz HEMT low-noise amplifier and a 20 GHz TWT power amplifier.
A printed reflectarray is an antenna similar to a parabolic reflector, but with its reflecting surface capable of being designed either flat or slightly curved for conformal mounting onto an existing structure without adding significant amount of mass and volume to the structure.