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NASA NTRS1992

Flight test evaluation of precision-code differential GPS for terminal approach and landing

A flight test evaluation conducted at the NASA Ames Research Center of precision-code differential GPS for terminal approach and landing is described. Differential corrections to satellite range and range-rate measurements are computed in a stationary ground reference system, uplinked to the test aircraft, and employed to correct corresponding airborne satellite data. Differentially corrected range and range-rate data, combined with inertial data, are then utilized with satellite position and velocity to compute aircraft position and velocity employing an eight-state extended Kalman filter.

Mcnally, B. D.↗

NASA NTRS1991

Thermal imbalance force modelling for a GPS satellite using the finite element method

Methods of analyzing the perturbation due to thermal radiation and determining its effects on the orbits of GPS satellites are presented, with emphasis on the FEM technique to calculate satellite solar panel temperatures which are used to determine the magnitude and direction of the thermal imbalance force. Although this force may not be responsible for all of the force mismodeling, conditions may work in combination with the thermal imbalance force to produce such accelerations on the order of 1.e-9 m/sq s. If submeter accurate orbits and centimeter-level accuracy for geophysical applications are desired, a time-dependent model of the thermal imbalance force should be used, especially when satellites are eclipsing, where the observed errors are larger than for satellites in noneclipsing orbits.

Vigue, Yvonne↗

NASA NTRS1994

GPS Measurements Show Subdaily Changes In Earth Rotation

Report presents analysis of data from 3-week worldwide Global Positioning System (GPS) tracking experiment conducted from January 22 through February 14, 1991. Focus of analysis upon detection and interpretation of subdaily variations in rate of rotation of earth.

Lichten, Stephen M.↗

NASA NTRS1994

Digital GPS-Signal Processor With P-Code/No-P-Code Option

Size, power, and cost reduced by exploiting commonality. Digital signal processor for Global Positioning System (GPS) receiver set to operate in "code" mode when P code known, or in "codeless" mode when P code not known. In codeless mode, processor performs full-quadrature processing, resulting in signal-to-noise ratio (SNR) 6 dB greater than SNR's of processors not performing at full quadrature.

Thomas, J. Brooks↗

NASA NTRS1994

Daily GPS-Derived Estimates Of Axis Of Rotation Of Earth

Report describes study in which data gathered by worldwide network of 21 Global Positioning System (GPS) receivers during 3-week experiment in January and February 1991 used to estimate location of axis of rotation of Earth.

Lindqwister, Ulf J.↗

NASA NTRS1994

Optimal Preprocessing Of GPS Data

Improved technique for preprocessing data from Global Positioning System receiver reduces processing time and number of data to be stored. Optimal in sense that it maintains strength of data. Also increases ability to resolve ambiguities in numbers of cycles of received GPS carrier signals.

Wu, Sien-Chong↗

NASA NTRS1994

Global Geodesy Using GPS Without Fiducial Sites

Global Positioning System, GPS, used to make global geodetic measurements without use of fiducial site coordinates. Baseline lengths and geocentric radii for each site determined without having to fix any site coordinates. Given n globally distributed sites, n baseline lengths and n geocentric radii form polyhedron with each site at vertex and with geocenter at intersection of all radii. Geodetic information derived from structure of polyhedron and its change with time. Approach applied to any global geodetic technique.

Heflin, Michael B.↗

NASA NTRS1991

GIS, GPS, political history, and geo-demography of the Aramamisi Yanomamo expansion

This year a joint US-Venezuelan research project was initiated to locate, identify, and begin long term research on the ecology of the Siapa Basin and the cultural adaptations of Yanomamo communities to this habit, a poorly mapped region of southern Venezuela. During two separate field trips, representing over 500 hours of flying time, local informants were asked to guide a helicopter to villages and other areas of anthropological interest. The location of each place was established with a GPS instrument. Both the Trimble Navigation TransPak and Magellan Nav 1000 Pro were used on these trips. A brief summary of the results and preliminary comparisons of both instruments will be presented.

Chagnon, Napoleon A.↗

NASA NTRS1992

Precision GPS ephemerides and baselines

The emphasis of this grant was focused on precision ephemerides for the Global Positioning System (GPS) satellites for geodynamics applications. During the period of this grant, major activities were in the areas of thermal force modeling, numerical integration accuracy improvement for eclipsing satellites, analysis of GIG '91 campaign data, and the Southwest Pacific campaign data analysis.

Source record↗

NASA NTRS1994

GPS survey of the western Tien Shan

This report summarizes the background, field work, data collection and analysis, and future plans associated with a collaborative GPS experiment in the Tien Shan of the former Soviet Union. This project involves the amalgamation of two, separately funded projects, which were proposed separately by PIs Hamburger and Reilinger (NSF number EAR-9115159 and NASA number NAG5-1941) and Molnar and Hager (NSF number EAR9117889 and NASA number NAG5-1947). In addition, the work is being conducted under the auspices of the US-USSR Agreement on Cooperation in the Field of Environmental Protection, with support from the United States Geological Survey.

Molnar, Peter H.↗

NASA NTRS1993

First results from the Fennoscandian GPS networks

Temporal correlations were measured of data obtained by the Swedish Permanent GPS Network for Positioning (SWEPOS). The model for correlations, r(Delta t) = exp(-(absolute value of (Delta t))/tau), can be used to predict precision of averaged values for different values of tau. Results indicate that tau is approximately equal to 1 day (r less than 1% after 5 days). The results show that the network can be used in geophysical applications such as the DOSE investigation on postglacial rebound.

Johansson, J. M.↗

NASA NTRS1994

A GPS coverage model

The results of several case studies using the Global Positioning System coverage model developed at Ohio University are summarized. Presented are results pertaining to outage area, outage dynamics, and availability. Input parameters to the model include the satellite orbit data, service area of interest, geometry requirements, and horizon and antenna mask angles. It is shown for precision-landing Category 1 requirements that the planned GPS 21 Primary Satellite Constellation produces significant outage area and unavailability. It is also shown that a decrease in the user equivalent range error dramatically decreases outage area and improves the service availability.

Skidmore, Trent A.↗

NASA NTRS1994

Some applications of GPS timing information

GPS Satellites transmit two independent signals. One is C/A code (course acquisition signal) which could be utilized by all civilian users. This service is called a Standard Positioning Service (SPS). Another is P code (precise signal) that could be only accessed by those authorized users. The service is called a precise positioning service (PPS). SPS C/A code users can navigate with accuracy of approximately 100 m and timing accuracy of about 250-300 ns (1 sigma). Such timing accuracy is not enough for precision time and frequency value transfer. For this reason we must do some research work to improve the SPS C/A code accuracy which SPS users can get. This paper presents our recent progress on timing information analysis, its data processing and its timing and frequency measurement applications.

Zhang, Yinbai↗

NASA NTRS1994

A comparison of several GPS data reduction methods

For several years, discussions have been ongoing about how to analyze GPS time information in order to get the most accurate and precise time comparison between two locations. Many similar studies were done several years ago before the constellation was dominated by Block II satellites. With the Block II satellites and the use of Selective Availability (S/A), time comparisons have lost some precision and accuracy. This paper presents the results of research that was performed to find not only the best of several time comparison methods, but also to determine to what degree precision and accuracy have declined due to S/A.

Chadsey, H.↗

NASA NTRS1994

Comparison of LASSO and GPS time transfers

The LASSO is a technique which should allow the comparison of remote atomic clocks with sub-nanosecond precision and accuracy. The first successful time transfer using LASSO has been carried out between the Observatoire de la Cote d'Azur in France and the McDonald Observatory in Texas, United States. This paper presents a preliminary comparison of LASSO time transfer with GPS common-view time transfer.

Lewandowski, W.↗

NASA NTRS1994

Global reference frame: Intercomparison of results (SLR, VLBI and GPS)

The terrestrial reference frame (TRF) is realized by a set of positions and velocities derived from a combination of the three space geodetic techniques, SLR, VLBI and GPS. The standard International TRF is constructed by the International Earth Rotation Service in such a way that it is stable with time and the addition of new data. An adopted model for overall plate motion, NUVEL-1 NNR, defines the conceptual reference frame in which all the plates are moving. In addition to the measurements made between reference points within the space geodetic instruments, it is essential to have accurate, documented eccentricity measurements from the instrument reference points to ground monuments. Proper local surveys between the set of ground monuments at a site are also critical for the use of the space geodetic results. Eccentricities and local surveys are, in fact, the most common and vexing sources of error in the use of the TRF for such activities as collocation and intercomparison.

Ma, Chopo↗

NASA NTRS1995

GPS survey of the western Tien Shan

There were two major developments in 1994 in our collaborative GPS experiment in the Tien Shan of the Former Soviet Union (FSU). Both were motivated by our expectation that we will ultimately obtain better science at lower cost if we involve our colleagues in the FSU more deeply in (1) the collection and (2) the analysis of data. As an experimental test of the concept of having our local collaborators carry out the field work semi-autonomously, we sent 6 MIT receivers to the Tien Shan for a period of 3 months. To enable our collaborators to have the capability for data analysis, we provided computers for two data analysis centers and organized a two-week training session. This report emphasizes the rationale for deeper involvement of FSU scientists, describes the training sessions, discusses the data collection, and presents the results. We also discuss future plans. More detailed discussion of background, general scientific objectives, discussions with collaborators, and results for the campaigns in 1992 and 1993 have been given in previous reports.

Hager, Bradford H.↗

NASA NTRS1994

Hydro-isostatic deflection and tectonic tilting in the central Andes: Initial results of a GPS survey of Lake Minchin shorelines

Sufficiently large lake loads provide a means of probing rheological stratification of the crust and upper mantle. Lake Minchin was the largest of the late Pleistocene pluvial lakes in the central Andes. Prominent shorelines, which formed during temporary still-stands in the climatically driven lake level history, preserve records of lateral variations in subsequent net vertical motions. At its maximum extent the lake was 140 m deep and spanned 400 km N-S and 200 km E-R. The load of surficial water contained in Lake Minchin was sufficient to depress the crust and underlying mantle by 20-40 m, depending on the subjacent rheology. Any other differential vertical motions will also be recorded as departures from horizontality of the shorelines. We recently conducted a survey of shoreline elevations of Lake Minchin with the express intent of monitoring the hydro-isostatic deflection and tectonic tilting. Using real-time differential Global Positioning System (GPS), we measured topographic profiles across suites of shorelines at 15 widely separated locations throughout the basin. Horizontal and vertical accuracies attained are roughly 30 and 70 cm, respectively. Geomorphic evidence suggests that the highest shoreline was occupied only briefly (probably less than 200 years) and radiocarbon dates on gastropod shells found in association with the shore deposits constrain the age to roughly 17 kyr. The basin-side pattern of elevations of the highest shoreline is composed of two distinct signals: (27 +/- 1) m of hydro-isostatic deflection due to the lake load, and a planar tilt with east and north components of (6.8 +/- 0.4) 10(exp -5) and 9-5.3 +/- 0.3) 10(exp -5). This rate of tilting is too high to be plausibly attributed to steady tectonism, and presumably reflects some unresolved combination of tectonism plus the effects of oceanic and lacustrine loads on a laterally heterogeneous substrate. The history of lake level fluctuations is still inadequately known to allow detailed inferences of crust and mantle rheology. However, it is already clear that the effective elastic plate thickness is closer to 40 km than the 60-70 km crustal thickness in the central Andes and the effective viscosity is less than 5 10(exp 20) Pa s.

Bills, Bruce G.↗