Global Positioning System Ground Research Articles

GPS radio occultation with CHAMP: an innovative remote sensing method of the atmosphere

The availability of GPS (Global Positioning System) radio signals has introduced a new promising remote sensing technique for the Earth’s atmosphere. GPS-based radio occultation exploits GPS signals received onboard a Low Earth Orbiting (LEO) satellite for atmospheric limb sounding. Temperature and water vapour profiles with high accuracy and vertical resolution can be derived from these measurements. The GPS radio occultation technique requires no calibration, is not affected by clouds, aerosols or precipitation, and the occultations are almost uniformly distributed over the globe. The radio occultation experiment aboard the German CHAMP (CHAllenging Minisatellite Payload) satellite provided its first measurements in February 2001. Since then up to 250 daily occultation measurements were performed. About 70% of these were successfully processed to yield atmospheric temperature and water vapour profiles. The CHAMP radio occultation experiment demonstrated the immense potential of this technique for the global monitoring of stratospheric temperature, the tropopause region and the global humidity distribution in the troposphere. All of these are important variables of the climate system. In this paper the GPS radio occultation experiment onboard CHAMP will be described. At GeoForschungsZentrum (GFZ) Potsdam an operational system was established to process CHAMP radio occultation data, orbit data, and GPS ground station observations in an automatic and operational way to derive profiles of atmospheric parameters. The CHAMP temperature bias compared to ECMWF analyses is less than 1.5 K between the tropopause layer and the 30 hPa level, and less than 0.5 K between 12 and 20 km at mid and high latitudes. Further comparisons between CHAMP atmospheric profiles and radiosonde data as well as applications of CHAMP radio occultation data for atmospheric research and climate monitoring are given.

Combining ionosonde with ground GPS data for electron density estimation

Dual frequency Global Positioning System (GPS) receivers provide integrated total electron content (TEC) along the ray path (slant TEC, affected by a bias). By inverting this observable, it is possible to obtain the vertical total electron content with some assumptions about the horizontal structure of the ionosphere. The large number of permanent receivers distributed around the world provide enough information to obtain such TEC observables with high spatial and temporal resolutions. Nevertheless, the geometry (mainly vertical) of the ground GPS observations does not allow to solve the vertical structure of electron density of the ionosphere. Mixing different kinds of complementary data in a tomographic context helps to overcome this problem. Several works have obtained successful results achieved by combining occultation and ground GPS data to estimate the local three-dimensional structure of ionospheric electron density. This paper proposes the use of just ground data to obtain similar or better results. To do this, the ground GPS data are mixed with vertical profiles of electron density derived from ionosonde data instead of GPS occultation observations. In this paper, the complementarity between vertical profiles of electron density (estimated using the NeQuick model) and ground GPS data (from GPS IGS permanent network) are shown as well as the performance of the resulting combination.

CHAMP rapid orbit determination for GPS atmospheric limb sounding

In preparation of the CHAMP small satellite LEO (Low Earth Orbit) mission to be launched on July 15, 2000, the GeoForschungsZentrum Potsdam (GFZ) enhanced its high precision orbit determination capabilities in order to rapidly produce precise LEO and GPS (Global Positioning System) orbital products. These orbits shall be available at 3 hour intervals with a latency of 2 hours after the actual observation for the application of GPS atmospheric limb sounding. Rapid orbit determination will exploit spaceborne GPS high-low satellite-to-satellite tracking data, fast delivery GPS ground tracking data from a global network and on-board accelerometer data for surface force modelling. Rapidly available ground based laser ranges support the orbit determination task. Because of the timeliness requirements of the orbital products, the availability of input data in space and time for POD may be sub-optimal. It is planned to apply dynamic orbit integration and an either separate or combined adjustment for CHAMP and GPS satellites' routine rapid orbit determination. For quality control purposes and during the initial mission phase, when the available force models, in particular the gravity field model, are not accurate enough, the CHAMP orbit can also be computed in a purely kinematic/geometric approach not using any dynamical model.

GPS radio occultation with CHAMP: Atmospheric profiling utilizing the space‐based single difference technique

The GPS (Global Positioning System) radio occultation experiment onboard the German CHAMP (CHAllenging Minisatellite Payload) satellite was successfully started in February 2001. By the end of July 2001 more than 11,000 globally distributed vertical profiles of atmospheric temperature were derived. Operational data processing is based on the double difference method to correct for satellite clock errors. This technique requires a continuous data stream from a global network of GPS ground stations. Termination of the GPS Selective Availability mode on May 2, 2000 reduced the GPS clock errors by orders of magnitude, thereby rendering space‐based single differencing feasible. First results using this single difference technique without direct use of GPS ground station data are presented. The comparison of two data sets consisting of 436 occultations, one processed with space‐based single differencing, the other with classical double differencing, yields no discernible temperature bias below 30 km altitude, a standard deviation of <0.6 K below 20 km and <1.2 K below 30 km. The comparison of both data sets with corresponding meteorological analyses shows nearly identical results with differences in mean and standard deviation of <0.15 K up to 30 km.

Low-cost GPS-based volcano deformation monitoring at Mt. Papandayan, Indonesia

The global positioning system (GPS) can be utilised to detect ground deformations of the surface of a volcano. Ground deformation monitoring is considered one of the most effective tools for investigating the behaviour of active volcanoes. The decreasing cost of GPS hardware, together with the increased reliability of the technology, facilitates such demanding applications. GPS ground deformation measurements can be continuous, automatic, conducted in all weather conditions, and provide three-dimensional positioning results. Higher computing power also means that the complex mathematics required to process GPS baselines can be easily handled in near real time. During the past few years a methodology has been developed for processing data collected by GPS networks consisting of a mixed set of single-frequency and dual-frequency receivers. The strategy is to deploy a few permanent, ‘fiducial’ GPS stations with dual-frequency, geodetic-grade receivers surrounding an ‘inner’ network of low-cost, single-frequency GPS receivers. Such a configuration offers considerable flexibility and cost savings for geodynamic applications such as volcano deformation monitoring, which require a dense spatial coverage of GPS stations, and where it is not possible to establish permanent GPS networks using only expensive dual-frequency instrumentation. This configuration has recently been tested at the Mt. Papandayan volcano in West Java, Indonesia. The two-stage network design consists of an inner network of four single-frequency Canadian Marconi (CM) GPS receivers surrounded by three dual-frequency Leica CRS1000 GPS receivers. The inner network logged and transmitted GPS data from the ‘slave’ stations located on the volcano, to a base station. The combined processing of the CM and Leica receiver data was performed off-line so as to investigate the performance of such a mixed-mode system. The basis of the processing methodology is to separate the dual-frequency, ‘fiducial’ station data processing from the baseline processing involving the single-frequency receivers on the volcano. The data processing for the former was carried out using a modified version of the Bernese software, to generate a file of ‘corrections’ (analogous to Wide Area DGPS correction models for the distance dependent biases – primarily due to atmospheric refraction). These ‘corrections’ were then applied to the double-differenced phase observations from the inner receivers to improve the baseline accuracies. A description of the field testing (and its challenges) during February–March 2000, together with a discussion of some of the results are presented.

Monitoring of Integrated Water Vapour from ground-based GPS observations and their assimilation in a limited-area NWP model

Detailed short-range numerical weather prediction (NWP) requires observational information of good quality and with high spatial/temporal resolution. Due to various limitations, important quantities such as tropospheric water vapour are often inadequately covered by conventional observations from radiosondes or by meteorological satellite systems. The Global Positioning System (GPS) is developing into a powerful source of humidity information for fine-scale regional models. Since May 2000, the GeoForschungsZentrum in Potsdam has been processing the data from a dense network of German GPS ground stations in near-real time. The final product, Integrated Water Vapour, has been monitored at the Deutscher Wetterdienst, and preliminary assimilation experiments have been carried out to test the impact of the new observations in the operational non-hydrostatic limited-area NWP model.

Analysis and validation of GPS/MET radio occultation data in the ionosphere

Global Positioning System (GPS) radio occultation signals received by a low Earth orbit (LEO) satellite provide information about the global distribution of electron density in the ionosphere. We examine two radio occultation inversion algorithms. The first algorithm utilizes the Abel integral transform, which assumes spherical symmetry of the electron density field. We test this algorithm with two approaches: through the computation of bending angles and through the computation of total electron content (TEC) assuming straight line propagation. We demonstrate that for GPS frequencies and for observations in LEO, the assumption of straight‐line propagation (neglecting bending) introduces small errors when monitoring the F2 layer. The second algorithm, which also assumes straight‐line propagation, is a three‐dimensional (3‐D) inversion constrained with the horizontal structure of a priori electron density fields. As a priori fields we use tomographic solutions and the parameterized real‐time ionospheric specification model (PRISM) when adjusted with ionosonde data or ground‐based GPS vertical TEC maps. For both algorithms we calibrate the occultation data by utilizing observations from the part of the LEO that is closer to the GPS satellite. For inversions we use dual‐frequency observational data (the difference of Ll and L2 phase observables) which cancel orbit errors (without applying precise orbit determination) and clock errors (without requiring synchronous ground data) and thus may allow inversions to be computed close to real time in the future. The Abel and 3‐D constrained algorithms are validated by statistically comparing 4 days of inversions with critical frequency (ƒ0F2) data from a network of 45 ionosonde stations and with vertical TEC data from the global network of GPS ground receivers. Globally, the Abel inversion approach agrees with the ƒ0F2 correlative data at the 13% rms level, with a negligible mean difference. All tested 3‐D constrained inversion approaches possess a statistically significant mean difference when compared with the ionosonde data. The vertical TEC correlative comparisons for both the Abel and 3‐D constrained inversions are significantly biased (∼30%) by the electrons above the 735‐km LEO altitude.

Correction of single frequency altimeter measurements for ionosphere delay

This study is a preliminary analysis of the accuracy of various ionosphere models to correct single frequency altimeter height measurements for ionospheric path delay. In particular, research focused on adjusting empirical and parameterized ionosphere models in the parameterized real-time ionospheric specification model (PRISM) 1.2 using total electron content (TEC) data from the Global Positioning System (GPS). The types of GPS data used to adjust PRISM included GPS line-of-sight (LOS) TEC data mapped to the vertical, and a grid of GPS derived TEC data in a Sun-fixed longitude frame. The adjusted PRISM TEC values, as well as predictions by IRI-90, a climatological model, were compared to TOPEX/Poseidon (T/P) TEC measurements from the dual-frequency altimeter for a number of TIP tracks. When adjusted with GPS LOS data, the PRISM empirical model predicted TEC over 24 1 h data sets for a given local time to within a global error of 8.60 TECU rms during a midnight centered ionosphere and 9.74 TECU rms during a noon centered ionosphere. Using GPS derived sun-fixed TEC data, the PRISM parameterized model predicted TEC within an error of 8.47 TECU rms centered at midnight and 12.83 TECU rms centered at noon. From these best results, it is clear that the proposed requirement of 3-4 TECU global rms for TOPEX/Poseidon Follow-On will be very difficult to meet, even with a substantial increase in the number of GPS ground stations, with any realizable combination of the aforementioned models or data assimilation schemes.

Real-time APL prototype of a GPS system

The Global Positioning System (GPS) consists of a constellation of 24 high-altitude satellites with very accurate atomic clocks, along with a global network of satellite tracking stations and sophisticated ground processing stations, that together provide precise navigation coordinates to any user who possesses a small, readily available GPS receiver.The precision that is achieved depends on[1] the number of GPS satellites in view of the user and the geometries involved;[2] the design of the user's receiver (e.g., military or civilian);[3] the presence of nearby objects that may interfere with or degrade the reception of signals from one or more satellites;[4] and other factors.In most cases civilian users (whose receivers can read only unencrypted, intentionally corrupted GPS signals) can expect navigation accuracy on the order of 50-100 meters. The advertised accuracy for users with military receivers, which read the encrypted GPS signals, is 16 meters. Coalition forces in Desert Shield / Desert Storm relied heavily on GPS, and U.S. military authorities praised GPS effusively for its effectiveness as a force multiplier in that conflict.Loral Federal Systems (formerly IBM Federal Systems Division) in Gaithersburg, Maryland has been intimately involved with GPS from its concept validation phase through operations and maintenance. People in this group analyzed the physical dynamics, designed the mathematical algorithms and associated software, and performed the system integration work that culminated in the GPS ground control segment.APL was used over the years in most major aspects of GPS design and analysis. Early in the full-scale development phase, APL was used to model the initial GPS system design, to perform design tradeoffs, to model and test candidate algorithms, and to perform error analyses to estimate future system performance. In later phases APL was used to monitor and analyze actual GPS system performance, to compare it with theoretically achievable performance, and to analyze and isolate anomalies [Allan].

Errata: Minimizing Selective Availability Error on Satellite and Ground Global Positioning System Measurements

Errata: Minimizing Selective Availability Error on Satellite and Ground Global Positioning System Measurements

Minimizing selective availability error on satellite and ground global positioning system measurements

Consider the NASA Deep Space Network DSS-13 antenna rate loop model (see Fig. 2; for details of the model see Ref. 2). It consists of the antenna structural model and models of elevation and azimuth drives. The 35-state rate loop model has three poles at zero and is reduced while using balanced antigrammians; their singular values are shown in Fig. 3. By deleting the states with the largest singular values, the model has shrunk from 90 states to 27 states. The reduced model preserves the full-model properties, as illustrated by the frequency-response plots in Fig. 4.

Precise tracking of remote sensing satellites with the Global Positioning System

The Global Positioning System (GPS) can be applied in a number of ways to track remote sensing satellites at altitudes below 3000 km with accuracies of better than 10 cm. All techniques use a precise global network of GPS ground receivers operating in concert with a receiver aboard the user satellite, and all estimate the user orbit, GPS orbits, and selected ground locations simultaneously. The GPS orbit solutions are always dynamic, relying on the laws of motion, while the user orbit solution can range from purely dynamic to purely kinematic (geometric). Two variations show considerable promise. The first one features an optimal synthesis of dynamics and kinematics in the user solution, while the second introduces a novel gravity model adjustment technique to exploit data from repeat ground tracks. These techniques, to be demonstrated on the TOPEX/Poseidon mission in 1992, will offer subdecimeter tracking accuracy for dynamically unpredictable satellites down to the lowest orbital altitudes. >