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At least 235 records · Page 13
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NASA NTRS1987

Precise near-earth navigation with GPS: A survey of techniques

The tracking accuracy of the low earth orbiters (below about 3000 km altitude) can be brought below 10 cm with a variety of differential techniques that exploit the Global Positioning System (GPS). All of these techniques require a precisely known global network of GPS ground receivers and a receiver aboard the user satellite, and all simultaneously estimate the user and GPS satellite orbits. Three basic approaches are the geometric, dynamic, and nondynamic strategies. The last combines dynamic GPS solutions with a geometric user solution. Two powerful extensions of the nondynamic strategy show considerable promise. The first uses an optimized synthesis of dynamics and geometry in the user solution, while the second uses a novel gravity-adjustment method to exploit data from repeat ground tracks. These techniques will offer sub-decimeter accuracy for dynamically unpredictable satellites down to the lowesst possible altitudes.

Yunck, T. P.↗
NASA NTRS1988

The limits of direct satellite tracking with the Global Positioning System (GPS)

Recent advances in high precision differential Global Positioning System-based satellite tracking can be applied to the more conventional direct tracking of low earth satellites. To properly evaluate the limiting accuracy of direct GPS-based tracking, it is necessary to account for the correlations between the a-priori errors in GPS states, Y-bias, and solar pressure parameters. These can be obtained by careful analysis of the GPS orbit determination process. The analysis indicates that sub-meter accuracy can be readily achieved for a user above 1000 km altitude, even when the user solution is obtained with data taken 12 hours after the data used in the GPS orbit solutions.

Bertiger, W. I.↗
NASA NTRS1988

Impact of tracking network variation on GPS orbit determination

The accuracy of GPS orbit determination using a continental U.S. tracking network is limited by the localized viewing geometry. Substantial improvement can be gained when supplementary receiving sites are added outside the continental U.S. Covariance analysis shows that, when GPS pseudo-range data are used, adding a site at either Yellowknife in western Canada, or Fairbanks, Alaska, improves the orbits by about 25 percent. A supplementary network of two stations in the Australia/New Zealand region can improve GPS orbit accuracy by a factor of two. Adding Hawaii to a combined U.S. and Australia/New Zealand network improves the accuracy further, to a factor of three over the nominal U.S. network. With GPS carrier phase data, the improvement is not as great; adding Hawaii and Australia/New Zealand to the nominal U.S. network improves orbit accuracy by a factor of two.

Wu, S. C.↗
NASA NTRS1988

Sole means navigation through hybrid Loran-C and GPS

A minimum of four GPS range measurements or two Loran-C time differences are normally required for a position solution for en route navigation, area navigation, and nonprecision approaches. This paper describes a new technique that hybridizes GPS and Loran-C used in the pseudorange mode to process efficiently all available navigation information. Emphasis is placed on combined GPS and Loran-C timing, both for the ground/space facilities and the user. The hybrid system has the potential to solve the GPS and Loran-C integrity problems; more range measurements are available than are required for the navigation solution.

Van Graas, Frank↗
NASA NTRS1988

Deriving a unique reference frame for GPS measurements

Two strategies for deriving a unique reference frame for GPS (Global Positioning System) measurements are discussed. The first strategy utilizes the precise relative positions which have been predetermined by VLBI (very long baseline interferometry) to fix the frame orientation and the absolute scaling, while the offset from the geocenter is determined from GPS measurements. Three different cases are presented under this strategy. The second strategy establishes a reference frame by holding only the longitudinal of one of the tracking sites fixed. The absolute scaling is determined by the adopted gravitational constant (GM) of the earth; and the latitude is inferred from the time signature of the earth's rotation in the GPS measurements. The coordinate system thus defined will be a geocentric earth fixed coordinate system. A covariance analysis shows that geometric positioning to an accuracy of a few centimeters can be achieved with just one day of precise GPS pseudorange and carrier phase data.

Malla, Rajendra P.↗
NASA NTRS1989

GPS-based system for satellite tracking and geodesy

High-performance receivers and data processing systems developed for GPS are reviewed. The GPS Inferred Positioning System (GIPSY) and the Orbiter Analysis and Simulation Software (OASIS) are described. The OASIS software is used to assess GPS system performance using GIPSY for data processing. Consideration is given to parameter estimation for multiday arcs, orbit repeatability, orbit prediction, daily baseline repeatability, agreement with VLBI, and ambiguity resolution. Also, the dual-frequency Rogue receiver, which can track up to eight GPS satellites simultaneously, is discussed.

Bertiger, Willy I.↗
NASA NTRS1989

GPS-aided gravimetry at 30 km altitude from a balloon-borne platform

A balloon-borne experiment, flown at 30 km altitude over New Mexico, was used to test dynamic differential Global Positioning System (GPS) tracking in support of gravimetry at high-altitudes. The experiment package contained a gravimeter (Vibrating String Accelerometer), a full complement of inertial instruments, a TI-4100 GPS receiver and a radar transponder. The flight was supported by two GPS receivers on the ground near the flight path. From the 8 hour flight, about a forty minute period was selected for analysis. Differential GPS phase measurements were used to estimate changes in position over the sample time interval, or average velocity. In addition to average velocity, differential positions and numerical averages of acceleration were obtained in three components. Gravitational acceleration was estimated by correcting for accelerations due to translational motion, ignoring all rotational effects.

Lazarewicz, Andrew R.↗
NASA NTRS1989

Long range geoid control through the European GPS traverse: Final results

The European north-south Global Positioning System (GPS)-traverse proposed by IAG SSG 3.88, should control and improve the European geoid. This traverse follows first order leveling lines, included in the United European Leveling Network. From May to August 1986 and in July 1987, the central and northern part of this traverse (approx. 3000 km) was observed using up to four TI 4100 receivers, covering Austria, Federal Republic of Germany, Denmark, Sweden and Norway. Both traverse parts contain 71 stations with distances of about 50 km. In addition, 8 stations have been occupied for overlapping connections, and traverse links were established for connecting the fundamental stations Wettzell (VLBI and SLR) and Onsala (VLBI). Final results show a GPS observation precision of a few cm for loops of some 100 km circumference. After transformation of the GPS results to geoid heights using the leveled heights, comparisons with different existing gravimetric geoid determinations including geopotential models were performed. In addition, new geopotential models complete to degree and order 360 tailored to gravity data in Europe, and gravimetric geoid solutions using 6 x 10' mean gravity anomalies were investigated. The comparison with GPS and leveling yields rms discrepancies of + or - 0.1...0.2 m over 1000 km traverse sections for the best solutions, but a strong slope is existing in Sweden and southern Norway in almost all solutions, which is probably caused by systematic errors in the available gravity data for Scandinavia. This is confirmed by a new geoid computation at the Danish Geodetic Institute where the slope has disappeared. If this new solution is taken for the northern traverse section and the best solution for the central part, the rms discrepancy reduces to approximately + or - 0.2 m over 3000 km. Thus, a + or - 10 (exp 7) relative height accuracy seems to be achievable over long distances with the GPS/leveling and the gravimetric geoid calculation techniques, applied in this experiment.

Torge, W.↗
NASA NTRS1990

Sole means navigation and integrity through hybrid Loran-C and NAVSTAR GPS

A sole means navigation system does not only call for integrity, but also for coverage, reliability, availability and accuracy. Even though ground monitored GPS will provide integrity, availability is still not sufficient. One satellite outage can affect a large service area for several hours per day. The same holds for differential GPS; a total satellite outage cannot be corrected for. To obtain sufficient coverage, extra measurements are needed, either in the form of extra GPS satellites (expensive) or through redundant measurements from other systems. LORAN-C is available and will, hybridized with GPS, result in a system that has the potential to satisfy the requirements for a sole means navigation system for use in the continental United States. Assumptions are made about the qualification sole means, mainly based on current sole means systems such as VOR/DME. In order to allow for system design that will satisfy sole means requirements, it is recommended that a definition of a sole means navigation system be established. This definition must include requirements for availability, reliability, and integrity currently not specified. In addition to the definition of a sole means navigation system, certification requirements must be established for hybrid navigation systems. This will allow for design and production of a new generation of airborne navigation systems that will reduce overall system costs and simplify training procedures.

Vangraas, Frank↗
NASA NTRS1990

The limits of direct satellite tracking with GPS

The expected accuracy of the direct user orbit solution and the magnitude of the principal error sources are evaluated using the techniques of covariance analysis. Utilization of advanced GPS receivers, measurement calibration methods, and geophysical models developed for high-precision GPS-based geodesy and differential satellite tracking are assumed in order to explore the limiting accuracy of the above technique. In addition, the dependence of user orbit accuracy on such factors as data arc length, the time interval between the end of the ground data arc used for GPS orbit solutions and the beginning of the user data arc, and data types used are studied. For comparison, results from the analysis of a full differential orbit solution are also presented. It is shown that submeter real-time accuracy can be readily achieved for a user above 700 km altitude, even when the user solution is based on a GPS ephemeris that is more than 12 h old.

Bertiger, Willy I.↗
NASA NTRS1990

Navigation accuracies for GPS demonstration on Topex/Poseidon

When Topex/Poseidon, is launched in June 1992, the joint United States/French oceanographic satellite mission will carry a developmental Global Positioning System (GPS) receiver on board to support the planned GPS demonstration experiment. The navigation operations aspect of the demonstration will develop software and operational procedures for the GPS Data Processing Facility (GDPF) at NASA's Jet Propulsion Laboratory. This paper presents simulations and covariance analysis of operational scenarios in support of the GDPF. Multi-orbit filter techniques using GPS carrier phase and P-code pseudo-range are studied both to tune the filter parameters and to determine feasible arc lengths for use in operations. In addition, a baseline operational scenario is presented with performance results given in terms of end-to-end processing times and computational loading based on a prototype version of the operational software.

Williams, B. G.↗
NASA NTRS1990

Minimizing selective availability error on Topex GPS measurements

GPS measurements made at Topex/Poseidon and the accompanying ground tracking sites will be affected by the selective availability. Although in principle the effects may be removed by differencing between receivers observing the same GPS satellites, this requires accurate synchronization of all receiver clocks. In the case of Topex/Poseidon application, there are two sources of imperfect clock synchronization. The first and larger is due to the constantly drifting clock onboard Topex, which may cause a residual effect as large as 10 cm on Topex carrier phase and 1 m on Topex pseudorange. The second is due to light-time differences between receivers observing the same GPS satellites, which may amount to a few mm error. In this paper a data reduction scheme which incorporates a low-order polynomial interpolation and carrier phase smoothing on pseudorange acquired at Topex and ground receivers is described; a simulation analysis is given demonstrating the effectiveness of the scheme for reducing the GPS S/A effects; and comparison with other schemes is discussed.

Wu, S. C.↗
NASA NTRS1990

Codeless GPS Applications to Multi-Path: CGAMP

Cordless Global Positioning System (GPS) Applications to Multi-Path (CGAMP) is meeting the challenge of exploiting the L-band signals from the Global Positioning System (GPS) satellites for the measurement of the impulse response of radio transmission channels over space-Earth paths. This approach was originally suggested by E. K. Smith and has been pursued by J. Lemmon, without an affordable implementation being identifiable. In addition to the high cost of a suitable P code correlating GPS receiver, there is also the major impediment of the often announced Department of Defense policy of selective availability/anti-spoof (SA/AS) that clouds reliable access to the wideband (20 MHz) P channel of the GPS signals without cryptographic access. A technique proposed by MacDoran utilizes codeless methods for exploiting the P channel signals implemented by the use of a pair of antennas and cross correlation signal detection.

Macdoran, P. F.↗
NASA NTRS1989

GPS measurements of strain accumulation across the Imperial Valley, California: 1986-1989

The Global Positioning System (GPS) data collected in southern California from 1986 to 1989 indicate considerable strain accumulation across the Imperial Valley. Displacements are computed at 29 stations in and near the valley from 1986 to 1988, and at 11 sites from 1988 to 1989. The earlier measurements indicate 5.9 +/- 1.0 cm/yr right-lateral differential velocity across the valley, although the data are heavily influenced by the 1987 Superstition Hills earthquake sequence. Some measurements, especially the east-trending displacements, are suspects for large errors. The 1988 to 1989 GPS displacements are best modeled by 5.2 +/- 0.9 cm/yr of valley crossing deformation, but rates calculated from conventional geodetic measurements (3.4 to 4.3 cm/yr) fit the data nearly as well. There is evidence from GPS and Very Long Base Interferometry (VLBI) observations that the present slip rate along the southern San Andreas fault is smaller than the long-term geologic estimate, suggesting a lower earthquake potential than is currently assumed. Correspondingly, a higher earthquake potential is indicated for the San Jacinto fault. The Imperial Valley GPS sites form part of a 183 station network in southern California and northern Baja California, which spans a cross-section of the North American-Pacific plate boundary.

Larsen, Shawn↗
NASA NTRS1990

Rubidium atomic frequency standards for GPS Block IIR

The Rubidium Atomic Frequency Standards (RAFS) were provided for the GPS Block IIR NAVSTAR satellites. These satellites will replenish and upgrade the space segment of the Global Positioning System in the mid 1990s. The GPS RAFS Rb clocks are the latest generation of the high-performance rubidium frequency standards. They offer an aging rate in the low pp 10(exp 14)/day range and a drift-corrected 1-day stability in the low pp 10(exp 14) range. The Block IIR version of these devices will have improved performance, higher reliability, smaller size, and greater radiation hardness. The GPS Block IIR atomic clocks have a natural frequency configuration whereby they output a frequency of about 13.4 MHz that is a submultiple of the atomic resonance of Rb (or Cs). The RAFS operates at a low, fixed C-field for increased stability. The GPS Block IIR RAFS design, including the changes and improvements made, and the test results obtained are described.

Riley, William J.↗
NASA NTRS1990

The use of precise ephemerides, ionospheric data, and corrected antenna coordinates in a long-distance GPS time transfer

Over intercontinental distances, the accuracy of The Global Positioning System (GPS) time transfers ranges from 10 to 20 ns. The principal error sources are the broadcast ionospheric model, the broadcast ephemerides and the local antenna coordinates. For the first time, the three major error sources for GPS time transfer can be reduced simultaneously for a particular time link. Ionospheric measurement systems of the National Institute of Standards and Technology (NIST) type are now operating on a regular basis at the National Institute of Standards and Technology in Boulder and at the Paris Observatory in Paris. Broadcast ephemerides are currently recorded for time-transfer tracks between these sites, this being necessary for using precise ephemerides. At last, corrected local GPS antenna coordinates are now introduced in GPS receivers at both sites. Shown here is the improvement in precision for this long-distance time comparison resulting from the reduction of these three error sources.

Lewandowski, Wlodzimierz W.↗
NASA NTRS1990

GPS/Loran-C interoperability for time and frequency applications: A survey of the times of arrival of Loran-C transmissions via GPS common mode/common view satellite observations

The results from this survey clearly indicate that the Global Positioning System (GPS) time transfer capability is superior to that of the Loran-C system for absolute timing accuracy, and that even with the most careful calibration of the Loran-C receiver delay and propagation path, inexplicable time of arrival (TOA) biases remain which are larger than the variations across all of the transmitters. Much more data covering years would be needed to show that these biases were stable enough to be removed with a one time site calibration. The synchronization of the transmissions is excellent, all showing low parts in 10(exp 13) offsets versus the United States Naval Observatory (USNO) master clock. With the exception of the Searchlight transmitter, all of the transmissions exhibit timing stabilities over the entire period of less than 300 ns RMS which is at the observed levels of GPS under selective availability (SA). The Loran-C phase instabilities take place over a much greater time interval than those being forced onto the GPS signals under SA, providing for better medium to short term frequency stability. Data show that all but the most distant transmitters offer better than three parts in 10(exp 11) stability at this averaging time. It is in the frequency control area where GPS/Loran-C interoperation will offer some synergistic advantages over GPS alone under SA.

Penrod, Bruce↗
NASA NTRS1991

The MARCOR GPS mobile data system

Market research revealed several key demands for an Automatic Vehicle Location (AVL) Global Positioning System (GPS) radio. The demands were for minimization of urban building blockage, easy programmability to minimize mobile data transmission costs, high accuracy for street map level coordination, interface capability with non-digital Specialized Mobile Radios (SMR), and a selling price close to that of alternatives such as Signposts and Loran-C. A team of experts was assembled to surmount these challenges and deliver a GPS radio for $500 to $1000, which operates at high accuracy in an urban environment and is plug-compatible with nearly all vehicle radios. Among the engineering and production breakthroughs described here are a unique Simultrac (Trademark) approach to satellite tracking, enabling up to eight GPS satellites to be used for position determination with a 2-channel receiver, and a receiver-in-a-microphone design. A powerful Application Specific Integrated Circuit (ASIC) allowed GPS to be brought within easy reach of millions of AVL users such as bus, taxi, and delivery vehicle fleets.

Rothblatt, Martin↗