Charles C. Counselman

Miniature Interferometer Terminals for Earth Surveying: Ambiguity And Multipath with Global Positioning System

With the recent launching of several satellites of the global positioning system (GPS), a variety of schemes based on radio interferometry have been proposed for the accurate determination of relative positions of receiving terminals on the ground. Provided that the integer-cycle ambiguities of the interferometric phase observations can be correctly resolved, the baseline vector extending from the antenna of one terminal to that of another should be determinable with uncertainty much smaller than the 19-cm wavelength of the GPS transmissions. We propose a method of ambiguity resolution that is suitable for observations made with antennas of low directive gain. Such antennas are compact, but the feasibility of their use has been questioned because observations with them are susceptible to multipath interference. For short-baseline interferometric observations of GPS our method yields correct ambiguity resolution despite severe multipath interference and significant sky blockage, even when instability of the frequency standards governing the separate receiving terminals limits the time span of coherent integration to five minutes.

Very-Long-Baseline Radio Interferometry: The Mark III System for Geodesy, Astrometry, and Aperture Synthesis

The Mark III very-long-baseline interferometry (VLBI) system allows recording and later processing of up to 112 megabits per second from each radio telescope of an interferometer array. For astrometric and geodetic measurements, signals from two radio-frequency bands (2.2 to 2.3 and 8.2 to 8.6 gigahertz) are sampled and recorded simultaneously at all antenna sites. From these dual-band recordings the relative group delays of signals arriving at each pair of sites can be corrected for the contributions due to the ionosphere. For many radio sources for which the signals are sufficiently intense, these group delays can be determined with uncertainties under 50 picoseconds. Relative positions of widely separated antennas and celestial coordinates of radio sources have been determined from such measurements with 1 standard deviation uncertainties of about 5 centimeters and 3 milliseconds of arc, respectively. Sample results are given for the lengths of baselines between three antennas in the United States and three in Europe as well as for the arc lengths between the positions of six extragalactic radio sources. There is no significant evidence of change in any of these quantities. For mapping the brightness distribution of such compact radio sources, signals of a given polarization, or of pairs of orthogonal polarizations, can be recorded in up to 28 contiguous bands each nearly 2 megahertz wide. The ability to record large bandwidths and to link together many large radio telescopes allows detection and study of compact sources with flux densities under 1 millijansky.

Multipath-rejecting GPS antennas

Multipath interference limits the speed and accuracy of determining position by differential Global Positioning System (DGPS) techniques. A geodetic surveyor for example, requires multipath interference rejection of about 36+20log/sub 10/sin/spl epsiv/ dB, where /spl epsiv/ is the elevation angle of the satellite being observed. Signal processing in a GPS receiver cannot satisfy this requirement. A receiving antenna is required that can sufficiently reject signals arriving from below the horizon. Available antennas have inadequate rejection, and brute-force methods of improving them, e.g., by enlarging their ground-planes, are impractical. A compact, ground-planeless, dual-band GPS antenna with improved multipath rejection has been designed and field-tested. This antenna resembles a vertical post rather than a horizontal platter; within its 0.1-m diameter, 0.4-m tall randome is a vertical array of turnstile elements. In field tests, a three-element array antenna rejected multipath better than a 0.5-m diameter ground-plane antenna by an average of 5 dB. A five-element array antenna appears superior to a 0.9-m diameter ground-plane antenna.

Miniature interferometer terminals for earth surveying

SummaryA system of miniature radio interferometer terminals is proposed for the measurement of vector baselines with uncertainties ranging from the millimeter to the centimeter level for baseline lengths ranging, respectively, from a few to a few hundred kilometers. Each terminal would have no moving parts, could be packaged in a volume of less than 0.1 m3, and could operate unattended. These units would receive radio signals from low-power (<10 w) transmitters on Earth-orbiting satellites. The baselines between units could be determined virtually instantaneously and monitored continuously as long as at least four satellites were visible simultaneously. Acquisition of the satellite signals by each terminal would require about one minute, but less than a second of signal integration, and the collection of only a few kilobits of data from two receiving units would suffice to determine a baseline. Different baseline lengths, weather conditions, and desired accuracies would, in general, dictate different integration times.

Astronomical Applications of Differential Interferometry

Intercomparison of radio signals received simultaneously at several sites from several sources with small mutual angular separation provides a powerful astrometric tool. Applications include tracking the Lunar Rover relative to the Lunar Module, determining the moons libration, measuring winds in Venuss lower atmosphere, mapping Mars radiometrically, and locating the planetary system in an inertial frame.

Venus winds are zonal and retrograde below the clouds

Winds in the lower atmosphere of Venus, inferred from three-dimensional radio interferometric tracking of the descents of the Pioneer day and north probes, are predominantly easterly with speeds of about 1 meter per second near the surface, 50 meters per second at the bottom of the clouds, and more than 200 meters per second within the densest, middle cloud layer. Between about 25 and 55 kilometers altitude the average flow was slanted equatorward, with superimposed wavelike motions and alternating layers of high and low shear.

Transcontinental baselines and the rotation of the earth measured by radio interferometry

Nine separate very-long-baseline interferometry (VLBI) experiments, carried out in 1972 and 1973 with radio telescopes 3900 kilometers apart, yielded values for the baseline length with a root-mean-square deviation about the mean of less than 20 centitneters. The corresponding fractional spread is about five parts in 108. Changes in universal time and in polar motion were also detertnined accurately from these data; the root-mean-square scatter of these results with respect to those based on optical methods were 2.9 milliseconds and 1.3 meters, respectively. Solid-earth tides were apparently detected, but no useful estimate of their amplituide was extracted.

Geodesy by Radio Interferometry

By the use of radio-interferometric observations of extragalactic radio sources it is possible to determine all three components of an interferometer “baseline” vector, which extends from one radio antenna to another, with respect to an inertial coordinate system. Uncertainties of only a few millimeters have now been achieved with this technique in determinations of a short (~1 km) baseline vector. Centimeter-level uncertainties are expected to be achieved in intercontinental baseline measurements through the application of recently developed passive microwave remote sensing techniques to the calibration of atmospheric propagation delay variations. Baseline vector determinations may be made with these uncertainties with the use of some portable antennas as small as 3 meters diameter, if ~100-MHz-bandwidth tape recording systems, which are now being built, are used to record the received signals.

Orbit-orbit resonance capture in the solar system.

A realistic model involving mutual gravitation and tidal dissipation for the first time provides a detailed explanation for satellite orbit-orbit resonance capture. Although applying directly only to Saturns satellites Titan and Hyperion, the model reveals general principles of resonance capture, evolution, and stability which seem applicable to other orbit-orbit resonances in the solar system.

SCIENTIFIC USES OF PULSARS.

The recently discovered celestial sources of pulsed radio energy can be used to test general relativity, to study the solar corona, and to determine the earths orbit and ephemeris time. The vector positions and transverse velocities of pulsars can be measured with radio interferometers; in combination with pulse-arrival-time data, the distance determination will yield the average interstellar electron density.