ISSN:0737-2884    

DOI:10.1002/sat.4600120502

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1994

数据来源: WILEY

ISSN:0737-2884    

DOI:10.1002/sat.4600120503

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1994

数据来源: WILEY

摘要:

AbstractPreliminary space flight results of attitude determination using GPS are presented from a spacecraft in low Earth orbit. Relative position measurements accurate to the sub‐centimetre level are made among multiple GPS antennas mounted on the space vehicle. A Trimble Navigation TANS Quadrex (a GPS receiver specially adapted for attitude determination by Stanford University) is used as a differential carrier phase sensor for the flight.Four GPS antennas are mounted on the zenith face of RADCAL, a polar orbiting, gravity‐gradient‐stabilized Air Force Space Test Program Satellite, built by Defense Systems, Inc. The four antennas are equally spaced about the perimeter of the 30 inch diameter cylindrical spacecraft bus.The Quadrex receiver measures the phase of theL‐band GPS carrier (1575 MHz) at each of up to four antennas for up to six GPS satellites simultaneously. From these measurements, an initial assessment of attitude determination in space is performed in post‐processing. For RADCAL, the attitude solution is greatly overdetermined. In a preliminary evaluation of system performance, the system accuracy is determined through measurement self‐consistency. Analysis of the attitude motion in the context of a gravity gradient dynamic model yields further insight into the system

ISSN:0737-2884    

DOI:10.1002/sat.4600120504

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1994

数据来源: WILEY

摘要:

AbstractFrequency stability analysis of on‐orbit Navstar clocks is performed by the Naval Research Laboratory using both the broadcast and the precise post‐processed ephemerides. The phase offset between the Navstar clock and the reference clock is computed from pseudorange measurements obtained by dual‐frequency GPS receivers at the five GPS monitor sites and at the U.S. Naval Observatory precise‐time site. The broadcast ephemerides are generated at the GPS master control station by a Kalman filter using data collected from the five GPS monitor stations. The precise post‐processed ephemerides are generated by the Defense Mapping Agency (DMA) using data collected from the GPS monitor sites and from five additional DMA monitor sites. In this paper the frequency stability is estimated for two Navstar caesium clocks–a Block I caesium clock (Navstar 9) and a Block II caesium clock (Navstar 23)–using both the broadcast and the precise ephemerides. A significant improvement in the estimate of the frequency stability of the Block II clocks has been achieved using the prec

ISSN:0737-2884    

DOI:10.1002/sat.4600120505

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1994

数据来源: WILEY

摘要:

AbstractThe presence of selective availability (SA) on GPS has led to a variety of proposals for mitigation of its effects, for example combining GPS with other sensors or by identifying and removing the SA using redundant information in the position solution. A further method for reducing the impact of SA is the combination of GPS and GLONASS measurements. GLONASS does not have any deliberate degradation.This paper describes several algorithms for combining GPS and GLONASS measurements in real time on a single receiver. There are two least squares solutions using the code phase measurements alone: one with the measurements from both systems weighted equally, and one with the measurements weighted by the covariance of the measurements. Kalman filters with no SA model, and first‐ and second‐order models, using both the code and carrier phase to estimate user position and velocity, are also implemented. Two different adaptive schemes are compared which attempt to identify SA model parameters in real time. The algorithms are compared in terms of their positioning accuracy, computational overhead and robustness. A ten‐channel GPS/GLONASS receiver developed at The University of Leeds is used to provide satellite data for the evaluation of the different me

ISSN:0737-2884    

DOI:10.1002/sat.4600120506

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1994

数据来源: WILEY

摘要:

AbstractThe transmission of satellite signals at two widely spaced frequencies allows correction of the ionospheric delay. Both GPS and GLONASS transmit such signals, and to date there have been no published dual frequency GLONASS studies. This paper shows the fundamental accuracy of the Leeds University multichannel GPS/GLONASS receiver. Details are then given of the processing methods that have been used to obtain the ionospheric group delay using GLONASS L1 and L2 P‐code measurements. Examples of the measured delay are provided and compared with the GPS model. Navigation results are shown using the P‐code phase at L1 only and with a combination of the L1 and L2 measurements. An analysis of the accuracy of the P‐code pseudorange and navigation capability is pres

ISSN:0737-2884    

DOI:10.1002/sat.4600120507

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1994

数据来源: WILEY

摘要:

AbstractRadio occultation studies of the terrestrial atmosphere are possible through the use of signals transmitted by satellites of the Global Positioning System (GPS) and received by one or more other satellites in low earth orbit (LEO). The perturbed phase of the occulted signal gives direct information on the refractivity profile in the region of occultation, from which vertical profiles of density, pressure and temperature can be retrieved. The technique requires the use of the dual GPS frequencies in order to isolate and remove most of the ionospheric effect. Analysis of the effect of the ionosphere and methods of removing it will be presented. For the recovery of atmospheric profiles, two major issues are addressed. The first is how accurately can refractivity be retrieved for a region in which there is a large horizontal refractivity gradient; the second considers the separation of temperature and moisture in the lower troposphere. Based on model simulations, the capability of GPS to provide atmospheric profiles is assessed.

ISSN:0737-2884    

DOI:10.1002/sat.4600120508

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1994

数据来源: WILEY

摘要:

AbstractThis paper describes in some detail the infrastructure and concept of two multi‐site reference station differential GPS (DGPS) options being offered to marine users by Racal Survey. The infrastructure of the Racal SkyFix system is briefly outlined, followed by descriptions of Racals' internetting and network capabilities.Comparisons between the two systems are made, including infrastructure, differential data management, user coverage, datalinks and positioning performance in various parts of the world. The paper describes the design of the systems and the tests undertaken to assess their functionality, before going on to compare their performances under a variety of conditions.Emphasis is placed on the technical issues associated with multi‐reference‐station DGPS, including error sources, weighting observations and monitoring of the data. The paper sets out to describe the fundamental approach of the systems to achieve high data integrity which is necessary to develop the systems to their next phase.Having described the two options, the paper uses examples of various combinations of reference stations to illustrate the potential of the system.Real‐time operations with the internetting and network systems are then used to show the increased performance of the two options over ordinary single station DGPS and lead into the conclusions section. The conclusions summarize the paper's findings and make recommendations for the future development of the system.The object of the developments so far has been to improve the quality of real‐time offshore DGPS positioning. The internetting and network options have provided real‐time results to illustrate the benefits and, together with SkyFix, offer significant developments in the ability of DGPS to provide accurate and reliable positioning over vast areas of the world.The ability to compare two multi‐reference‐station DGPS systems over large areas using co‐sited reference stations is very unusual. The opportunity has been taken to compare and report on the performance o

ISSN:0737-2884    

DOI:10.1002/sat.4600120509

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1994

数据来源: WILEY

摘要:

AbstractSuccessful GPS on‐the‐fly (OTF) carrier phase ambiguity resolution for precise positioning at the centimetre level has already been demonstrated. This has usually been in good observation conditions, e.g. over short distances, lots of satellites, P‐code pseudoranges available, and small unmodelled errors. In order for GPS to fulfil the rigorous ICAO Cat. III precise landing navigation requirements centimetre‐level accuracy must also be obtained in more realistic conditions, e.g. A‐S on, high‐unmodelled errors and less than six satellites. Integrating GPS with other sensors, e.g. INS, is likely to aid ambiguity resolution in such conditions, but there are limitations. After discussing critically the OTF methods, approaches are examined which will provide the precise accuracy, reliability and integrity required. Combining GPS with one, or more, pseudolites to provide an extra geometrical constraint to aid ambiguity resolution is described. Flight tests using different configurations of pseudolites are carried out a

ISSN:0737-2884    

DOI:10.1002/sat.4600120510

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1994

数据来源: WILEY

摘要:

AbstractBefore a major new 500 kV transmission line was placed in service between Oregon and the San Francisco, California area, a series of high‐voltage short circuit tests were performed on the transmission system. The short circuit faults produced a brief ‘delta function’ shock to the regional power system. This paper discusses the use of Global Positioning System (GPS) synchronized equipment for the measurement and analysis of key power system quantities.Before the testing, the authors installed two GPS synchronized phasor measurement units (PMUs). The PMUs use GPS to globally synchronize the measurement of the state vector of the power system, the complex voltages of substation busses. Complex voltage means the magnitude and the relative phase angles of substation voltages. GPS provides the global time reference marker for phase angle measurement. One electrical degree of the 60 Hz waveform equals about 46 μs. Across short transmission lines (less than 50 km), measurements may need to be made to 0.1 electrical degree, which translates to clock synchronizations of roughly 5 μs.The PMUs recorded the dynamic response of the power system phase angles when the northern California power grid was excited by the artificial short circuits. Power system planning engineers perform detailed computer generated simulations of the dynamic response of the power system to naturally occurring short circuits, e.g. faults caused by lightning. The computer simulations use models of transmission lines, transformers, circuit breakers, and other high voltage components. This work will compare computer simulations of the same event with field measu

ISSN:0737-2884    

DOI:10.1002/sat.4600120511

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1994

数据来源: WILEY