Ulf J. Lindqwister

A global mapping technique for GPS‐derived ionospheric total electron content measurements

A worldwide network of receivers tracking the transmissions of Global Positioning System (GPS) satellites represents a new source of ionospheric data that is globally distributed and continuously available. We describe a technique for retrieving the global distribution of vertical total electron content (TEC) from GPS-based measurements. The approach is based on interpolating TEC within triangular tiles that tessellate the ionosphere modeled as a thin spherical shell. The high spatial resolution of pixel-based methods, where widely separated regions can be retrieved independently of each other, is combined with the efficient retrieval of gradients characteristic of polynomial fitting. TEC predictions from climatological models are incorporated as simulated data to bridge significant gaps between measurements. Time sequences of global TEC maps are formed by incrementally updating the most recent retrieval with the newest data as it becomes available. This Kalman filtering approach smooths the maps in time, and provides time-resolved covariance information, useful for mapping the formal error of each global TEC retrieval. Preliminary comparisons with independent vertical TEC data, available from the TOPEX dual-frequency altimeter, suggest that the maps can accurately reproduce spatial and temporal ionospheric variations over latitudes ranging from equatorial to about ±65°.

Monitoring of global ionospheric irregularities using the Worldwide GPS Network

A prototype system has been developed to monitor the instantaneous global distribution of ionospheric irregularities, using the worldwide network of Globa Positioning System (GPS) receivers. Case studies in this pape indicate that GPS receiver loss of lock of signal tracking may be associated with strong phase fluctuations. It is shown that a network-based GPS monitoring system will enable us to study the generation and evolution of ionospheric irregularities continuously around the globe under various solar and geophysical conditions, which is particularly suitable for studies of ionospheric storms, and for space weather research and applications.

Global ionosphere perturbations monitored by the Worldwide GPS Network

For the first time, measurements from the Global Positioning System (GPS) worldwide network are employed to study the global ionospheric total electron content (TEC) changes during a magnetic storm (November 26, 1994). These measurements are obtained from more than 60 world-wide GPS stations which continuously receive dual-frequency signals. Based on the delays of the signals, we have generated high resolution global ionospheric maps (GIM) of TEC at 15 minute intervals. Using a differential method comparing storm time maps with quiet time maps, we find that significant TEC increases (the positive effect) are the major feature in the winter hemisphere during this storm (the maximum percent change relative to quiet times is about 150%). During this particular storm, there is almost no negative phase. A traveling ionospheric disturbance (TID) event is identified that propagates from the northern subauroral region to lower latitudes (down to about 30°N) at a speed of ∼460 m/s. This TID is coincident with significant increases in the TEC around the noon sector. We also find that another strong TEC enhancement occurs in the pre-dawn sector in the northern hemispheric subauroral latitudes, in the beginning of the storm main phase. This enhancement then spreads into almost the entire nightside. The nighttime TEC increase in the subauroral region is also noted in the southern hemisphere, but is less significant. These preliminary results indicate that the differential mapping method, which is based on GPS network measurements, appears to be a powerful tool for studying the global pattern and evolution process of the entire ionospheric perturbation.

Ionospheric total electron content perturbations monitored by the GPS global network during two northern hemisphere winter storms

The global evolution of two major ionospheric storms, occurring on November 4, 1993, and November 26, 1994, respectively, is studied using measurements of total electron content (TEC) obtained from a worldwide network of ground-based GPS receivers. The time-dependent features of ionospheric storms are identified using TEC difference maps based on the percent change of TEC during storm time relative to quiet time. The onset of each ionospheric storm is indicated by the appearance of auroral/subauroral TEC enhancements which occur within 1 hour of the beginning of the geomagnetic storm main phase. Significant TEC enhancements (> 100%) are observed in the winter northern hemisphere. The rate at which TEC enhancements appear is found to correlate with gradients in the Dst index. The large scale ionospheric structures identified during the storms are (1) nightside auroral/subauroral enhancements which surround the auroral oval, (2) dayside (around noon) high-latitude and middle-latitude enhancements associated with traveling ionospheric disturbances, and (3) conjugate latitudinal enhancements. For the November 1993 storm, a short positive phase (about 15 hours) is followed by a long negative phase (∼60 hours). In the November 1994 storm, we have identified the clear signature of a traveling ionospheric disturbance (TID) which propagated at a speed of ∼460 m/s from ∼60° N to ∼40° N. The motion of this disturbance appears to conserve angular momentum.

A comparative study of ionospheric total electron content measurements using global ionospheric maps of GPS, TOPEX radar, and the Bent model

Global ionospheric mapping (GIM) is a new and emerging technique for determining global ionospheric TEC (total electron content) based on measurements from a worldwide network of Global Positioning System (GPS) receivers. In this study, GIM accuracy in specifying TEC is investigated by comparison with direct ionospheric measurements from the TOPEX altimeter. A climatological model (Bent model) is also used to compare with the TOPEX altimeter data. We find that the GIM technique has much better agreement with TOPEX in TEC measurements, compared with the predictions of the climatological model. The difference between GIM and TOPEX in TEC measurements is very small (less than 1.5 TEC units (TECU)) within a 1500-km range from a reference GPS station. The RMS gradually increases with increasing distance from the station, while the Bent model shows a constant large RMS, unrelated to any station location. Within a 1000-km distance of a GPS site (elevation angle > 25°), GIM has a good correlation (R > 0.93) to TOPEX with respect to TEC measurements. The slope of the linear fitting line to the data set from two TOPEX cycles is 44.5° (near the ideal 45°). In the northern hemispheric regions, ionospheric specification by GIM appears to be accurate to within 3-10 TECU up to 2000+ km away from nearest GPS station (corresponding to ∼1° elevation angle cutoff). Beyond 2000 km, GIM accuracy, on average, is reduced to the Bent model levels. In the equatorial region, the Bent model predictions are systematically lower (∼5.0 TECU) than TOPEX values and often show a saturation at large TEC values. During ionospheric disturbed periods, GIM sometimes shows differences from TOPEX values due to transient variations of the ionosphere. Such problems may be improved by the continuous addition of new GPS stations in data-sparse regions. Thus, over a GPS stations measurement realm (up to 2000 km in radius), GIM can produce generally accurate TEC values. Through a spatial and temporal extrapolation of GPS-derived TEC measurements, the GIM technique provides a powerful tool for monitoring global ionospheric features in near real time.

Global Ionospheric TEC Variations During January 10, 1997 Storm

The ionospheric storm evolution process was monitored during the January 10, 1997 magnetic cloud event, through measurements of the ionospheric total electron content (TEC) from 150 GPS stations. The first significant response of the ionospheric TEC to the geomagnetic storm was at 0300 UT as an auroral/subauroral enhancement around the Alaskan evening sector. This enhancement then extended to both noon and midnight. Around 0900 UT, the enhancement at noon broke from the subauroral band and moved to lower latitudes. This day side northern hemisphere enhancement also corresponded to a conjugate geomagnetic latitude enhancement in the southern hemisphere and lasted about 5 hours. At 1500 UT a large middle latitude enhancement appeared over Mexico and the southern US, and persisted until 2200 UT. The enhancement was probably caused by the equatorward neutral wind which pushed the plasma up. On the basis of this assumption, the kinetic energy of the neutral wind which caused the middle latitude enhancement is estimated as ∼4.1×l09 Joules. This is about 0.03% of solar wind energy impinging on the magnetosphere and about 3% of the energy deposited on polar cap ionosphere. After 2000 UT, a negative phase gradually became stronger (especially in the southern hemisphere), although the northern subauroral enhancement persisted one more day. The entire ionosphere gradually recovered to normal on January 12. Thus, large middle latitude enhancement, equatorward motion of the dayside enhancement (probably related to a TID), the persistence of the subauroral enhancement, and the conjugate features at both hemispheres are the main characteristics of this storm.

Few millimeter precision for baselines in the California permanent GPS Geodetic Array

A microcomputer-based, closed-loop electronic temperature control system and method for an electrically heated cold weather garment having a plurality of independently heated zones. In one preferred embodiment the system comprises a potentiometer for selecting the desired temperature at which a plurality of areas of the garment are to be maintained, a thermistor for determining a reference temperature of the interior areas of the garment, and a plurality of conductive heating wires operable to heat each area of the garment independently. The system further includes a plurality of solid-state semiconductor power devices for controllably allowing current flow through each heating wire and, a plurality of current sensing circuits for sensing the current flow through each heating wire. The microcomputer receives information from the potentiometer, the thermistor and the current sensing circuits, and determines therefrom the level of heating required for each zone. The microcomputer then generates and applies a plurality of pulse width modulated output signals to the solid-state switches to thereby control the amount of heat produced by each heating wire, thus controlling the temperature of each zone.

Ionospheric specification algorithms for precise GPS-based aircraft navigation

The Federal Aviation Administration (FAA) is implementing an aircraft navigation scheme for the United States using the Global Positioning System (GPS) L1 signal (1575.42 MHz). To achieve position accuracies of a few meters, sufficient to allow precision airfield approaches, it will be necessary to broadcast corrections to the direct GPS signal. A significant component of these corrections is the delay in the GPS signal introduced by its propagation through the ionosphere. Ionospheric delay corrections will be derived using a ground network of at least 24 dual-frequency GPS receivers distributed across the continental United States. This network is part of the FAAs wide area augmentation system (WAAS) and will provide real-time total electron content (TEC) measurements. We present a technique for converting these TECs into gridded vertical delay corrections at the GPS L1 frequency, which will be broadcast to users every 5 min via geosynchronous satellite. Users will convert these delays to slant corrections for their own particular lines of sight to GPS satellites. To preserve user safety, estimates of the error in the user delay corrections will also be broadcast. However, the error algorithm must not resort to excessive safety margins as this reduces the expected accuracy, and thus utility, of the navigation system. Here we describe an error algorithm and its dependence on various factors, such as user location with respect to the WAAS ground network and ionospheric conditions.