UNDER NORMAL OPERATING CONDITIONS, MARINE DIFFERENTIAL GPS (DGPS) HORIZONTAL POSITIONING accuracies on the order of several meters are achieved in North America. Such accuracies are well within the tolerance of 10 meters (95 percent confidence level) specified by the Canadian and United States Coast Guards, but under high levels of ionospheric activity, significant degradations in DGPS positioning accuracies can occur. Marine DGPS operations in North America are particularly susceptible to such effects, where a feature known as storm enhanced density is observed during ionospheric storm events.
It was previously thought that the mid-latitude North American ionosphere was reasonably benign, with minimal storm effects of relevance for marine DGPS users. However, during ionospheric storms in May and October, 2003, marine DGPS horizontal position accuracies were degraded by factors of 10-30. These degraded accuracies persisted for hours and were well beyond system tolerances specified for marine DGPS users. Such ionospheric activity is not unusual during the years following solar maximum, and is expected to persist for several years.
In this month's column, we examine the impact of ionospheric storms on marine DGPS users and look at a proposed wide-area approach for mitigating large storm-induced positioning errors.--R.B.L.
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In differential GPS (DGPS), range errors are calculated at a reference site with known coordinates and transmitted to remote users. The errors remaining after DGPS processing are the residual atmospheric effects (both tropospheric and ionospheric errors), multipath, and, to a lesser extent, GPS satellite orbit errors. Proper antenna selection and placement can mitigate multipath errors. Atmospheric errors, on the other hand, can be rather large depending on the weather conditions (in the case of the troposphere) and ionospheric activity.
The ionospheric range error depends on both the frequency of the signal and the electron density along the signal path:
I = [+ or -]40.3[[TEC]/[f.sup.2]] (1)
where TEC denotes the total electron content, which is the electron density integrated along the signal path (in electrons per meter squared), f is the signal frequency (in hertz), and the rounded value 40.3 is a function of the electron mass and charge values and the permittivity of free space. The sign, positive or negative, depends on the measurement. The ionospheric group delay is positive, contributing a positive range error to pseudorange measurements whereas the phase of a carrier is advanced by the ionosphere, contributing a negative range error to carrierphase measurements. The ionospheric range error can dominate the DGPS error budget under high levels of ionospheric activity. Additional effects of ionospheric scintillation can cause degradation of GPS receiver tracking performance and, in extreme cases, loss of navigation capabilities entirely.
Mariners worldwide rely on DGPS systems for safety of navigation, hydrographic surveying applications, and exploration/exploitation of marine resources. Marine horizontal position accuracy requirements are 2-5 meters (at a 95 percent confidence level) and 8-20 meters (95 percent) for safety of navigation in inland waterways and harbor entrances/approaches, respectively; horizontal position accuracies of 1-100 meters (95 percent) are required for benefits of resource exploration in coastal regions. Marine DGPS currently assists a diverse range of government, industrial and military applications--these include hydrographic surveying, assistance to vessel traffic management services, search and rescue operations, environmental assessment and clean-up, and underwater mine detection and disposal.
The Canadian Coast Guard (CCG) currently offers marine radiobeacon DGPS services along the Pacific and Atlantic coasts of Canada, in addition to the Great Lakes and St. Lawrence River regions. While DGPS horizontal error bounds are specified as 10 meters (95 percent), marine users typically experience much better accuracies--on the order of several meters (95 percent). However, DGPS operations in Canada and other regions of the world are susceptible to enhanced ionospheric effects associated with geomagnetic storms, which can cause degraded positioning accuracies. To mitigate the impact of such effects, one could use a number of reference stations to spatially model correlated GPS ranging errors. In this article, we describe an investigation of such a wide-area approach for applications to marine users in Canada.
In wide-area differential DGPS (WADGPS), GPS observations from a sparse network of reference stations help model correlated error sources over an extended region. WADGPS services allow consistent levels of positioning accuracy to be achieved at all locations within the coverage area. A growing demand for accurate and reliable DGPS positioning worldwide has spurred the development of several WADGPS services in recent years. Current operational WADGPS systems include the Wide Area Augmentation System (WAAS), a system designed by the U.S. Federal Aviation Administration (FAA) for aircraft precision approach and en-route navigation. Commercial WADGPS systems include the OmniSTAR service, which charges annual user fees.
In this article, we present a description of ionospheric effects on GPS, the theory of WADGPS, and an emerging wide-area DGPS technique to mitigate the impact of large ionospheric gradients on positioning accuracies. Our analyses focus on differential positioning accuracies in the mid- to high-latitude region, where some of the largest TEC gradients in the Earth's ionosphere occur.
Storm Enhanced Density
The focus of our investigation was to evaluate the potential of employing a wide-area positioning algorithm for marine users--and ultimately to mitigate extreme ionospheric effects in the Canadian sector. Prior to presenting this technique and discussing our test results, we will review relevant phenomena in the Earth's ionosphere.
Enhanced ionospheric electric fields are present near the mid- to high-latitudes during geomagnetically disturbed periods, which can lead to depletions and enhancements of electron density in this region. The resulting gradients in TEC can cause large differential ionospheric range errors. Of particular interest is an effect known as storm enhanced density (SED). The Millstone Hill incoherent scatter radar in Massachusetts originally recognized SED in the early 1990s, and the phenomenon has been studied in detail ever since--most recently with satellite data from the Defense Meteorological Satellite Program (DMSP) and Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellites, and with TEC data collected from multiple GPS receivers located across the United States and Canada.
Analysis of the GPS TEC data shows that during geomagnetic disturbances, ionospheric electrons are transported from lower latitudes to higher latitudes, redistributing TEC across latitude and local time (see Figure 1). Gradients as large as 70 parts per million have been observed at geographic latitudes of 45[degrees]-50[degrees] in North America. SED effects can persist for several hours in this region, presenting a significant issue for the CCG DGPS service.
Wide-Area DGPS
For single-reference DGPS, range errors are calculated for each satellite observed at a single reference station and are transmitted as corrections to remote users. Corrections for spatially correlated errors (atmospheric and orbital effects) become less effective with increased baseline length, and DGPS positioning accuracies can be significantly degraded over longer baselines. Limitations therefore exist in both availability and positioning accuracy when using the single-reference DGPS approach.
In the wide-area approach, GPS observations from a sparse network of reference stations are used to model sources of correlated errors over an extended region. Such WADGPS services allow consistent levels of positioning accuracy to be achieved at all locations within the coverage area. Several different approaches and algorithms (including both state-space domain and measurement domain approaches) may be used to derive WADGPS corrections.
In the state-space domain algorithms, different error models are computed for each individual source of error. Typically the satellite clocks, orbits, and ionosphere are modeled separately, and values of parameters describing these corrections are transmitted to users within the area of coverage. Vector corrections are then derived for each satellite observed at the user's location. This approach is mathematically complex and requires an adequate number of reference sites to resolve the various error components (for example, WAAS uses more than 20 reference stations throughout the United States).
