Differential GPS Corrections Research Articles

Mapping Dilution of Precision (MDOP) and map-matched GPS

A novel method of map matching using the Global Positioning System (GPS) has been developed which uses digital mapping and height data to augment point position computation. This method reduces the error in position, which is a sum from several sources, including signal delay due to the ionosphere and atmosphere and until recently from ‘selective availability’ (S/A). S/A was imposed by the US military to degrade purposefully the accuracy of GPS, but was switched off on 2 May 2000, and is to be replaced with ‘regional denial capabilities in lieu of global degradation’ (Interagency GPS Executive Board, 2000). Taylor et al. (2001) describe the Road Reduction Filter (RRF) in detail. RRF is a method of detecting the correct road on which a vehicle is travelling. In the work described here, the position error vector is estimated in a formal least squares procedure, as the vehicle is moving. This estimate is a map-matched correction, that provides an autonomous alternative to DGPS for in-car navigation and fleet management. In this paper, a formula is derived for ‘Mapped Dilution of Precision’ (MDOP), defined as the theoretical ratio of position precision using map-matched corrections to that using perfect Differential GPS (DGPS) correction. This is shown to be purely a function of route geometry, and is computed for examples of basic road shapes. MDOP is favourable unless the route has less than a few degrees curvature for several kilometres. MDOP can thus provide an objective estimate of positioning precision to a vehicle driver. Precision estimates using MDOP are shown to agree well with ‘true’ positioning errors determined using high precision (cm) GPS carrier phase techniques. The exact location of a vehicle on a road is essential for accurate surveying applications. These include close range photogrammetry using digital video or still cameras and the verification of digital mapping by measured (GPS and other sensors) trajectories.

Evaluation of the Potential for Power Line Noise to Degrade Real Time Differential GPS Messages Broadcast at 283.5-325 kHz

The new nationwide differential global positioning system network uses the 283.5-325 kHz hand to broadcast differential GPS (DOPS) correction messages. Concem has been expressed that power line corona and gap discharge noise could degrade the performance of DGPS receivers using this band. Previous work on power lines and the AM broadcast band identified corona and gap discharges as broadband noise sources in the LFIMF bands. The potential to locally degrade performance of DGPS receivers relatively close to some power facilities appears possible for certain situations. The extent of any DGPS interference problem will depend on receiver/antenna design and placement, signal strength, power line design, weather conditions, and characteristics of the noise source. Also affecting DGPS receiver performance can be the presence of any nearby non-power line RF noise sources such as electronic devices or equipment internal to the user's vehicle.

Road Reduction Filtering for GPS‐GIS Navigation

A novel method of map matching using the Global Positioning System (GPS) has been developed for civilian use, which uses digital mapping data to infer the systematic position errors of less than 100m which result largely from ‘selective availability’ (S/A) imposed by the US military. Selective availability was switched off on the 2nd of May 2000, and is to be replaced with ‘regional denial capabilities in lieu of global degradation’. The system tracks a vehicle on all possible roads (road centre‐lines) in a computed error region, then uses a method of rapidly detecting inappropriate road centre‐lines from the set of all those possible. This is called the Road Reduction Filter (RRF) algorithm. Point positioning is computed using C/A code pseudorange measurements direct from a GPS receiver. The least squares estimation is performed in the software developed for the experiment described in this paper. Virtual differential GPS (VDGPS) corrections are computed and used from a vehicle's previous positions, thus providing an autonomous alternative to DGPS for in‐car navigation and fleet management. Height aiding is used to augment the solution and reduce the number of satellites required for a position solution. Ordnance Survey (OS) digital map data was used for the experiment, i.e. OSCAR 1 m resolution road centre‐line geometry and Land Form PANORAMA 1:50,000, 50 m‐grid digital terrain model (DTM). Testing of the algorithm is reported and results are analysed. Vehicle positions provided by RRF are compared with the ‘true’ position determined using high precision (cm) GPS carrier phase techniques. It is shown that height aiding using a DTM and the RRF significantly improve the accuracy of position provided by inexpensive single frequency GPS receivers.

A new method of generating differential GPS corrections

In this paper, a simple method is proposed for calculating positions on the ground with a differential global positioning system that does not use conventional correction data. The method employs two low-cost GPS receivers, one of them being at a fixed location. In order to coordinate their operation, both receivers only use three of the highest satellites in sight. The precision obtained with this technique is similar to that calculated by more sophisticated methods.

Generation of differential GPS corrections along with performance of quality control

The DGPS technique can provide considerably better relative positioning accuracy than the stand-alone GPS positioning, but the improvement depends on the distance between the user and the reference station (spatial correlation), the latency of differential corrections (temporal correlation), and the quality of differential corrections. Therefore, how to correctly generate differential corrections as well as their pricision is very important to the DGPS positioning technique. This paper presents a new algorithm for generating differential GPS corrections. This algorithm directly uses code and carrier observations in the measurement model of a Kalman filter, so that it is possible to use a simple stochastic model and to use the standard algorithm of the Kalman filter. The algorithm accounts for biases like multipath errors and instrumental delays in code observations and it shows how differential corrections are differently affected by code biases when dual or single frequency data is used. In addition, the algorithm can be integrated with a real time quality control procedure. As a result, the quality of differential corrections can be guaranteed with a certain probability.

Hughes aircraft's architectural design of the federal aviation administration wide-area augmentation system: An international system

Hughes Aircraft is currently developing under contract with the Federal Aviation Administration (FAA) a GPS-based navigation system that is intended to become the primary navigational aid for commercial aviation during all phases of flight—from enroute through Category I precision approach. This innovative system, named the Wide-Area Augmentation System (WAAS), will make use of a network of reference stations distributed throughout the U.S. National Airspace System. These reference stations will collect GPS measurements and send them to master stations. The master stations will process the data to provide correctional information for each GPS satellite. This information will include as separate components the GPS ephemeris errors, satellite clock bias and ionospheric estimation data. The corrections will be sent to the users by means of a Geosynchronous Earth Orbit (GEO) satellite using a specific signal and data format. WAAS is first expected to provide supplemental radio navigation, and eventually to become the primary system of navigation. The system will add the following features to the current GPS system: a GEO ranging function that will improve availability and reliability; differential GPS corrections that will improve accuracy; and integrity monitoring that will provide and enhance safety. To meet the requirements associated with a primary navigation system, WAAS should be able to provide fault-free position fix with a time availability of 0.999 for Category I approaches, and 0.99999 for domestic enroute, terminal and nonprecision approach phases of flight. This paper presents the improvements that this augmentation system offers to stand-alone GPS in order to become the primary navigational system in the U.S. The paper also covers the features that make this wide-area system a naturally expandable system worldwide that will allow the integration of future local-area differential systems across the world.

Latency Compensation for Differential GPS

Differential GPS (DGPS) has the potential to reduce spherical position error standard deviation to approximately 1 m. However, two practical issues affect the accuracy that can be achieved in real-time applications: long sampling periods and data latency. Low-bandwidth communications channels can result in differential corrections with sampling periods greater than 5 s. A time delay (latency) is inherent between the generation of pseudorange corrections and their application by the user. For real-time applications (e.g., autonomous vehicle navigation), low DGPS correction update rates and data latency are a fundamental performance limitation. This paper analyzes the modeling of differential corrections, uses the resulting optimal models for differential correction latency compensation, and analyzes the resulting accuracy by comparison with the first-order polynomial predictors recommended in the RTCM-104 standard. The comparison shows that, as long as the base station produces appropriate coefficients for the polynomial predictor, there is no significant accuracy improvement achieved by an optimal (i.e., Kalman filter-based) predictor over the RTCM-recommended polynomial predictor.

A new algorithm for generating carrier adjusted differential GPS corrections

A new algorithm for generating carrier adjusted differential GPS corrections

A new algorithm for generating carrier adjusted differential GPS corrections

This paper presents a new algorithm for generating differential GPS corrections. This algorithm directly uses code and carrier observations in the measurement model of a Kalman filter, so that it is possible to use a simple stochastic observation model and to use the standard algorithm of the Kalman filter. The algorithm accounts for biases like multipath errors and instrumental delays in code observations and it shows how differential corrections are differently affected by code biases when dual or single frequency data are used.

Aircraft traffic management on the airport surface using VHF data link for CNS

Aircraft traffic management on the airport surface will require the effective utilization of communications, navigation and surveillance (CNS). This paper describes an experiment which provides a glimpse of future automation. The experiment combines precision surface radar, high-accuracy differential GPS (DGPS) position determination, automatic dependent surveillance (ADS), and ground and airborne automation systems able to intercommunicate via a VHF data link. In particular, this paper describes the air/ground communications provided by the VHF Data Link. Two classes of messages are processed: continual and ad hoc. The former include DGPS corrections, ADS messages and target data. The latter include graphical taxi clearances and hold bar set-reset information. The work described in this paper is the result of a cooperative venture among NASA, Westinghouse-Norden Systems and ARINC. >

Testing the precision lightweight GPS receiver

Innovative field testing techniques are employed at Holloman Air Force Base to help the Global Positioning System (GPS) NAVSTAR Joint Program Office (JPO) test the Precision Lightweight GPS Receiver (PLGR). Characterizing the PLGR's accuracy in dynamic environments is of prime importance but testing also prescribes the evaluation of its ability to receive differential GPS corrections, real time, and its Electronic Counter Counter Measures. To meet these goals, the 46th Test Group provides the C-12 cargo aircraft for flight testing, an instrumented test van for mobile testing, the High Speed Test Track for high velocity testing, a UH-1 helicopter for rotor blade modulation testing, and special PC laptops for ground troop testing. All of these test capabilities utilize Holloman's well instrumented test environments with thousands of surveyed sites validated by the Defense Mapping Agency. This paper emphasizes the testing techniques that are helping to define Test & Evaluation methodologies for the changing world where Global Positioning with NAVSTAR is becoming a reality.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Wide Area Differential GPS Field Study

Inmarsat commissioned the field study to be carried out by Signal Computing Ltd. in association with the Royal Greenwich Observatory. The study aimed to identify the commercial suitability of Wide Area Differential GPS (WADGPS) corrections. The corrections provided to users will be relayed via Inmarsat-3 geostationary satellites and are required to be valid over the footprint of an entire Inmarsat ocean region (approximately one third of the Earth's surface).The study has been conducted within the Inmarsat Atlantic Ocean Region East. Trials took place over a five-month period to achieve a representative set of data. Six widely-distributed collection sites were used to ensure that the maximum duration of satellite data were received and that a representative model for atmospheric effects was obtained. Analysis is restricted to the use of GPS carrier and phase data obtained using the C/A code transmitted by Block II satellites only.This paper presents the analysis of data collected during a four-day trial period. The ability to determine the GPS satellite ephemeris (independent of GPS broadcast ephemeris), Klobuchar model parameters tailored to the oceanic region, and satellite clock corrections using GPS data obtained from this limited deployment of collection sites is addressed. The primary degradation effects on stand-alone GPS positioning of Selective Availability (SA) are corrected using WADGPS, resulting in a significant improvement in position accuracy.

海上航走時におけるDGPSの効果について

An evaluation experiment on the differential GPS (DGPS) was conducted when the manoeuvrability of a small fishing boat was tested at Tateyama Bay in 1993. The pseudo range correction values for the DGPS were measured at a reference station where is in Yokohama during the sea experiment. The DGPS correction and the positioning accuracy were analyzed through a method of post processing. The correction effects of ship position by using DGPS in moving condition are discussed in this paper. It was concluded from the results of experiment that the definite correction effects of DGPS were recognized and high absolute/relative accuracy was verificated.

Coverage of DGPS/Radiobeacons

Differential GPS (DGPS)/radiobeacons are being deployed worldwide because they are a cost-effective way to broadcast DGPS corrections over relatively large areas to mobile marine users. They simply require the addition of a GPS reference station and a minimum shift keying (MSK) modulator to any existing marine radiobeacon. This paper describes the worldwide growth in the number of DGPS/radiobeacons, and it details an algorithm that estimates the coverage of an individual DGPS/radiobeacon. It also includes an appendix that addresses forward error correction and data compression for DGPS/radiobeacons.

The Effect of Selective Availability on Differential GPS Corrections

The newer Block II GPS satellites have the capability to control the accuracy of the Standard Positioning Service signals by a combination of dithering the satellite clock and manipulating the ephemeris data. This capability is called Selective Availability (SA). Differential GPS operation greatly reduces the effects of SA, in addition to the effects of ionospheric, tropospheric, and other slowly-varying errors, for users in a local area around a reference station. A major technical issue in the design of reference stations has been the correction update rate. It has been assumed until recently that new differential GPS corrections are required about once every 10 s to maintain 5–10 m service accuracy. Until the recent launch of the new Block II satellites, there has been no firm evidence that this update rate would be adequate. This paper presents statistical analysis of recently obtained data sets from Block II satellites, specifically position and velocity variations and correlation times. Pseudorange corrections generated by differential reference station equipment using these signals are applied to the pseudoranges from a navigation receiver to obtain estimates of the effect of correction update rate on differential GPS performance.

Extended Differential GPS

The Radio Technical Commission for Maritime Services (RTCM) Special Committee (SC)-104 on Differential GPS has established a standard data format for providing differential GPS services. The satellite pseudorange corrections that are broadcast in the RTCM SC-104 differential GPS navigation message lump together the errors from a number of different sources. This results in a degradation of the differential navigation accuracy if the user is operating at a distance from the differential reference station. Generally, the user must be within 100 nmi of the reference station to benefit fully from the differential GPS corrections. This paper presents an Extended Differential GPS concept that extends the range of operation of the differential GPS message. By using a network of differential GPS stations, one can compute a differential GPS message that provides corrections for the different components of the pseudorange error. This allows the differential GPS navigation accuracy to be extended to a range of 1000 nmi from the master differential station.

Correction Algorithms for Differential GPS Reference Stations

The generation of differential GPS corrections for general navigation purposes must consider the effects of selective availability, ionosphere, data link latency, and processing time. The U.S. Coast Guard Differential GPS Harbor Entrance program has developed a differential GPS reference station system design that successfully addresses these concerns. This paper presents a high-level description of this design and some of the test data that led to the critical design decisions.