Radio star flux density expressions for accurate Antenna Gain measurements

This paper reports on measurements and analysis to resolve discrepancies in Earth station G/T measurements when using radio stars as calibration sources. The radio star flux expressions for Omega, Orion, and Virgo have been investigated to supplement the results previously reported by Ekelman and Abler (see IEEE Antennas and Propagation Society International Symposium Digest, vol.1, p.172-5, 1996), which considered Cassiopeia, Taurus, and Cygnus. Recognizing the need for accurate sources to measure Antenna G/T and Gain from locations in the southern hemisphere, the data for Omega, Orion and Virgo were collected (with Cassiopeia-used as the reference) and analyzed to determine accurate flux density expressions to determine Antenna Gain. In addition, an independent method for measuring the Antenna Gain was used to confirm that the Gains found based on matching Cassiopeia results are correct.

Antenna Gain measurements using improved radio star flux density expressions

Radio stars have been used to measure the Gain of large Antennas for many years. The straightforward expressions required, and the simple, direct measurements involved have made this method the most accurate available for this class of Antennas. Radio astronomers provided the bases for the technique by measuring the flux density of various celestial sources having strong radio emission at frequencies from 100 MHz to 100 GHz. Many papers have documented measurements which have lead to refinements and improvements in the flux density expressions, as well as other contributing factors such as angular extent corrections and polarization effects. The existing equations extracted from current literature are presented, along with data to support new and improved flux density expressions that provide more accurate results. A solution is presented to discrepancies found when radio sources are used to measure Antenna Gain. Flux density expressions for radio sources Cassiopeia A (Cas A), Taurus A (Tau A), and Cygnus A (Cyg A) were examined in published data. Using Cas A as a reference, the flux expressions for Tau A and Cyg A were adjusted to eliminate the characteristically lower-than-expected results obtained when these two radio sources are used to measure Antenna Gain. With these new, improved expressions, Cas A, Tau A, and Cyg A give similar Gain results, to within an observable average of /spl plusmn/0.04 dB, for both C-band and Ku-band Antennas.

Single Antenna Attitude Algorithm for Nonuniform Antenna Gain Patterns

The objective of this research is to investigate a new method for improving the accuracy of single Antenna attitude systems based on GPS signal strength measurements. To achieve this objective, a predictive GPS signal strength model is proposed and developed. The model consists of a precisely measured three dimensional receiving Antenna Gain pattern (as fitted to the spacecraft), the distance dependent path loss, the effects of the ionosphere, the GPS satellite transmitting Antenna Gain, and the variation in GPS satellite transmission power. Furthermore, an algorithm is developed to provide an estimate of single-axis attitude solution based on this predictive GPS signal strength model. The performance of this new algorithm is evaluated and compared aGainst two other single Antenna attitude approaches using ground data and flight data. Results show that the new single Antenna attitude algorithm is capable of providing attitude accuracy of 10 deg rms for static terrestrial platforms with a zenith pointing Antenna. Furthermore, 15 deg rms from the FedSat satellite negative velocity pointing Antenna configuration has been achieved.

Single Antenna attitude algorithm for non-uniform Antenna Gain patterns

DOI: 10.2514/1.19428 The objective of this research is to investigate a new method for improving the accuracy of single Antenna attitude systems based on global positioning system signal strength measurements. To achieve this objective, a predictive global positioning system signal strength model is proposed and developed. The model consists of a precisely measured 3-D receiving Antenna Gain pattern (as fitted to the spacecraft), distance dependent path loss, effects of the ionosphere, global positioning system satellite transmitting Antenna Gain, and variation in global positioning system satellite transmission power. Furthermore, an algorithm is developed to provide an estimate of single-axis attitude solution based on this predictive global positioning system signal strength model. The performance of this new algorithmisevaluatedandcomparedwithtwoothersingleAntennaattitudeapproachesusinggrounddataand flight data. Results show that the new single Antenna attitude algorithm is capable of providing an attitude accuracy of 10 deg rms for static terrestrial platforms with a zenith pointing Antenna. Furthermore, 15 deg rms from the FedSat satellite negative velocity pointing Antenna configuration has been achieved.

Single Antenna attitude algorithm for non-uniform Antenna Gain patterns

DOI: 10.2514/1.19428 The objective of this research is to investigate a new method for improving the accuracy of single Antenna attitude systems based on global positioning system signal strength measurements. To achieve this objective, a predictive global positioning system signal strength model is proposed and developed. The model consists of a precisely measured 3-D receiving Antenna Gain pattern (as fitted to the spacecraft), distance dependent path loss, effects of the ionosphere, global positioning system satellite transmitting Antenna Gain, and variation in global positioning system satellite transmission power. Furthermore, an algorithm is developed to provide an estimate of single-axis attitude solution based on this predictive global positioning system signal strength model. The performance of this new algorithmisevaluatedandcomparedwithtwoothersingleAntennaattitudeapproachesusinggrounddataand flight data. Results show that the new single Antenna attitude algorithm is capable of providing an attitude accuracy of 10 deg rms for static terrestrial platforms with a zenith pointing Antenna. Furthermore, 15 deg rms from the FedSat satellite negative velocity pointing Antenna configuration has been achieved.

Single Antenna Attitude Algorithm for Nonuniform Antenna Gain Patterns

The objective of this research is to investigate a new method for improving the accuracy of single Antenna attitude systems based on GPS signal strength measurements. To achieve this objective, a predictive GPS signal strength model is proposed and developed. The model consists of a precisely measured three dimensional receiving Antenna Gain pattern (as fitted to the spacecraft), the distance dependent path loss, the effects of the ionosphere, the GPS satellite transmitting Antenna Gain, and the variation in GPS satellite transmission power. Furthermore, an algorithm is developed to provide an estimate of single-axis attitude solution based on this predictive GPS signal strength model. The performance of this new algorithm is evaluated and compared aGainst two other single Antenna attitude approaches using ground data and flight data. Results show that the new single Antenna attitude algorithm is capable of providing attitude accuracy of 10 deg rms for static terrestrial platforms with a zenith pointing Antenna. Furthermore, 15 deg rms from the FedSat satellite negative velocity pointing Antenna configuration has been achieved.

Single-Antenna attitude determination for Fedsat with improved Antenna Gain patterns

The GPS receiver on FedSat is a single Antenna unit which supplied by NASA Jet Propulsion Laboratory (JPL) and integrated on the satellite by the Queensland University of Technology (QUT) and other participants in the Cooperative Research Centre for Satellite System. The high-level objectives, for the GPS receiver on the FedSat mission, are to provide an orbit determination capability and an attitude determination capability, whilst operating within strict power budget requirements. The current baseline is a total of 30 minutes of operation per 100-minute orbit.

This paper presents the history of the attitude determination capability for FedSat and an outline of the Antenna Gain pattern tests performed for FedSat. These tests show the great impact that the spacecraft has on the measured Antenna Gain pattern. Extensive data collection and simulation of different Antenna Gain patterns was made using a GPS constellation simulator. The accuracy of the attitude determination problem results, are shown to be within 30 degrees for the signal to noise ratio (SNR) weighted approach and were improved to within 5 degrees when mapping of the SNR to elevation angle approach is taken. Recommendations for new improved approaches, in both the methodology and in the choice of Antenna Gain pattern are made.