Global Positioning System Ground Research Articles

Estimating precipitable water vapour using the global positioning system and radio occultation over Ethiopian regions

ABSTRACT The Global Positioning System (GPS) is a space-based radio positioning system, which is capable of providing continuous position, velocity, and time information to users anywhere on, or near, the surface of the Earth. The main objective of this work was to estimate the integrated precipitable water vapour (IPWV) using ground GPS and Low Earth Orbit (LEO) Radio Occultation (RO) to study spatial-temporal variability. For LEO-GPS RO, we used Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) datasets. We estimated the daily and monthly mean of IPWV using six selected ground-based GPS stations for the years 2012 to 2016. The main perspective for selecting the years 2012 to 2016 is that, continuous data were available during these periods at all Ethiopian GPS stations. We studied temporal, seasonal, diurnal, and vertical variations of precipitable water vapour using GPS observables extracted from the precise geodetic GAMIT-GLOBK software package. Finally, we determined the cross-correlation of our GPS-derived IPWV values with those of the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-40 Interim reanalysis and of the second generation National Oceanic and Atmospheric Administration (NOAA) model ensemble Forecast System Reforecast (GEFS/R) for validation and static comparison. There are higher values of the IPWV range from 30 to 37.5 millimetres (mm) in Gambela and Southern Regions of Ethiopia. Some parts of Tigray, Amhara, and Oromia regions had low IPWV ranges from 8.62 to 15.27 mm. The correlation coefficient between GPS-derived IPWV with ECMWF and GEFS/R exceeds 90%. We conclude that there are highly temporal, seasonal, diurnal, and vertical variations of precipitable water vapour in the study area.

Alternative Approach for Tsunami Early Warning Indicated by Gravity Wave Effects on Ionosphere

The rapid displacement of the ocean floor during large ocean earthquakes or volcanic eruptions causes the propagation of tsunami waves on the surface of the ocean, and consequently internal gravity waves (IGWs) in the atmosphere. IGWs pierce through the troposphere and into the ionospheric layer. In addition to transferring energy to the ionosphere, they cause significant variations in ionospheric parameters, so they have considerable effects on the propagation of radio waves through this dispersive medium. In this study, double-frequency measurements of the Global Positioning System (GPS) and ionosonde data were used to determine the ionospheric disturbances and irregularities in response to the tsunami induced by the 2011 Tohoku earthquake. The critical frequency of the F2 layer (foF2) data obtained from the ionosonde data also showed clear disturbances that were consistent with the GPS observations. IGWs and tsunami waves have similar propagation properties, and IGWs were detected about 25 min faster than tsunami waves in GPS ground stations at the United States west coast, located about 7900 km away from the tsunami’s epicenter. As IGWs have a high vertical propagation velocity, and propagate obliquely into the atmosphere, IGWs can also be used for tsunami early warning. To further investigate the spatial variation in ionospheric electron density (IED), ionospheric profiles from FORMOSAT-3/COSMIC (F3/C) satellites were investigated for both reference and observation periods. During the tsunami, the reduction in IED started from 200 km and continued up to 272 km altitude. The minimum observed reduction was 2.68 × 105 el/cm3, which has happened at 222 km altitude. The IED increased up to 767 km altitude continuously, such that the maximum increase was 3.77 × 105 el/cm3 at 355 km altitude.

A Study of the Topside Plasma Blob and Adjacent Bubble Near Sunrise in Low-Latitude Ionosphere During the Main Phase of August 2003 Storm

In this paper, a plasma blob and an adjacent plasma bubble in low-latitude region at 245°E region near sunrise recorded by the Defense Meteorological Satellite Program (DMSP) F13 satellite around 840 km on 18 August 2003 were presented. On the previous local night, the blob and adjacent bubble at 265°E region were also detected by the first satellite of the Republic of China (ROCSAT-1) at about 600 km. Total Electron Content (TEC) derived from ground-global positioning system (GPS) measurements also showed both the evident enhancements and the decreases adjacent to the enhancements, at one low-latitude station near 265°E region. Based on the characteristics of the blob, we conclude that the blob and adjacent bubble near sunrise recorded by DMSP were the remnant of the blob and bubble formed on the previous night, which were detected by ROCSAT-1 satellite and ground-based TEC variations. The long duration of the blob and bubble during storm main phase may be related to the prompt penetration electric field.

Assessing the performance of GPS tomography at retrieving water vapour fields during landfalling atmospheric rivers over southern California

AbstractAccurate monitoring of water vapour evolution is essential to gain a better understanding of the atmospheric river movement, as well as predicting the severity and life cycle of heavy precipitation. The use of hundreds of ground Global Positioning System (GPS) stations in southern California provided a good opportunity to monitor the water vapour changes associated with landfalling atmospheric rivers. Based on a tomographic technique, the 15 min‐resolution water vapour density fields of southern California were reconstructed for February 2019. The assessments by radiosonde and the European Centre for Medium‐Range Weather Forecasts' (ECMWF) Re‐Analysis Interim (ERA‐Interim) database reanalysis show that tomography can retrieve the density profiles with an overall accuracy of about 1.1 g·m–3. In addition, both assessments indicate that the tomographic solutions of altitudes > 6 km are not reliable as relative root mean square error can reach up to 500% in the uppermost layer. Nevertheless, tomographic integrated water vapour is consistent with those derived from radiosonde and ERA‐Interim because their discrepancies are basically < 3 mm. The tomographic density fields were then applied to monitor water vapour evolution associated with two atmospheric river landfalls that occurred on February 2 and February 13, 2019. Results show that both atmospheric rivers made landfall from the northwest and passed over southern California in a southeasterly direction. The series of density profiles reveals that the high‐elevation mountains can block the atmospheric river movement, resulting in the concentration of water vapour at the foot of the mountains.

Airborne Lidar and Electro-Optical Imagery along Surface Ruptures of the 2019 Ridgecrest Earthquake Sequence, Southern California

AbstractSurface rupture from the 2019 Ridgecrest earthquake sequence, initially associated with the Mw 6.4 foreshock, occurred on 4 July on a ∼17 km long, northeast–southwest-oriented, left-lateral zone of faulting. Following the Mw 7.1 mainshock on 5 July (local time), extensive northwest–southeast-oriented, right-lateral faulting was then also mapped along a ∼50 km long zone of faults, including subparallel splays in several areas. The largest slip was observed in the epicentral area and crossing the dry lakebed of China Lake to the southeast. Surface fault rupture mapping by a large team, reported elsewhere, was used to guide the airborne data acquisition reported here. Rapid rupture mapping allowed for accurate and efficient flight line planning for the high-resolution light detection and ranging (lidar) and aerial photography. Flight line planning trade-offs were considered to allocate the medium (25 pulses per square meter [ppsm]) and high-resolution (80 ppsm) lidar data collection polygons. The National Center for Airborne Laser Mapping acquired the airborne imagery with a Titan multispectral lidar system and Digital Modular Aerial Camera (DiMAC) aerial digital camera, and U.S. Geological Survey acquired Global Positioning System ground control data. This effort required extensive coordination with the Navy as much of the airborne data acquisition occurred within their restricted airspace at the China Lake ranges.

A Neural Network‐Based Ionospheric Model Over Africa From Constellation Observing System for Meteorology, Ionosphere, and Climate and Ground Global Positioning System Observations

AbstractThe first regional total electron content (TEC) model over the entire African region (known as AfriTEC model) using empirical observations is developed and presented. Artificial neural networks were used to train TEC observations obtained from Global Positioning System receivers, both on ground and onboard the Constellation Observing System for Meteorology, Ionosphere, and Climate satellites for the African region from years 2000 to 2017. The neural network training was implemented using inputs that enabled the networks to learn diurnal variations, seasonal variations, spatial variations, and variations that are connected with the level of solar activity, for quiet geomagnetic conditions (−20 nT ≤ Dst ≤ 20 nT). The effectiveness of three solar activity indices (sunspot number, solar radio flux at 10.7‐cm wavelength [F10.7], and solar ultraviolet [UV] flux at 1 AU) for the neural network trainings was tested. The F10.7 and UV were more effective, and the F10.7 was used as it gave the least errors on the validation data set used. Equatorial anomaly simulations show a reduced occurrence during the June solstice season. The distance of separation between the anomaly crests is typically in the range from about 11.5 ± 1.0° to 16.0 ± 1.0°. The separation is observed to widen as solar activity levels increase. During the December solstice, the anomaly region shifts southwards of the equinox locations; in year 2012, the trough shifted by about 1.5° and the southern crest shifted by over 2.5°.

Assimilation of Multiple Data Types to a Regional Ionosphere Model With a 3D‐Var Algorithm (IDA4D)

AbstractFor the purpose of building a regional (bound 20–60°N in latitude and 110–160°E in longitude) ionospheric nowcast model, we investigated the performance of IDA4D (Ionospheric Data Assimilation Four‐Dimension) technique considering International Reference Ionosphere model as the background. The data utilized in assimilation were slant total electron content (STEC) from 27 ground GPS (Global Positioning System) receiver stations and NmF2 (ionospheric F2 peak density) from five ionosondes and COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) Data Analysis Archive Center. The period analyzed covered both geomagnetic quiet and disturbed days (15–18 March 2015). Assimilations were run under the following data combinations (cases): (1) GPS‐STEC's only; (2) GPS‐STEC's and NmF2's from five ionosondes; (3) only NmF2's from five ionosondes; and (4) GPS‐STEC's and NmF2's from both five ionosondes and COSMIC. Results showed that under case 1 the root‐mean‐square error (RMSE) in STEC reduced by 44% over the background International Reference Ionosphere values and on averaged over all ionosonde stations in the analysis RMSE values of foF2 (F2layer critical frequency) reduced by 21%. Furthermore, foF2 RMSE values under Case 2 were 36% smaller than those under Case 1. Under Case 4, IDA4D performance improved even further in areas not covered by GPS and ionosonde measurements. Therefore, IDA4D is a potential candidate for regional ionosphere modeling that exhibits improved performance with assimilation of different data types.

Climatology of quasi-two day oscillations from GPS-derived total electron content during 1999–2015

The paper presents the climatology of eastward- and westward-propagating quasi-2-day oscillations (QTDOs) with zonal wave numbers 1–4 using the global ionospheric total electron content (TEC) derived at the International GNSS Service (IGS) from the network of ground Global Positioning System (GPS) receivers. The climatologies of space-time spectra and amplitudes of QTDOs in TEC during 16 years from December to February (DJF) (1998/99–2014/15) and 17 years from June to August (JJA) (1999–2015) were investigated. The space-time spectra (spectral signature) and the mean amplitude of all wave modes (1–4) of QTDOs exhibit strong latitudinal, hemispheric, inter-annual and solar-cycle variations. Latitudinaly, strengths of the spectral signature and amplitude values diminish gradually from maxima at equatorial ionization anomaly (EIA) crest towards high latitude regions. The spectral signature amplify (weaken) in the solar maximum (minimum) periods. Likewise, the amplitudes of both the eastward- and westward-propagating QTDOs attain peak values during the maximum solar activity phases of the solar cycle. Seasonally, the spectral signature and amplitude values reveal summer amplification in both hemispheres as opposed to winter suppression. However, the spectral signatures (amplitudes) in the Southern Hemisphere summer are stronger (larger) than that in the Northern Hemisphere summer. Moreover, the spectral signatures and amplitudes associated with the westward-propagating oscillations are dominant over those propagating eastward. The hemispheric and latitudinal mean differences in the long-term mean daily values of amplitude of E1 (W1) QTDOs exhibit high values in the southern (northern) hemisphere near the December (June) solstices that follow the solar declination. The global spatial mean amplitudes of E1 and W1 QTDOs are significantly higher during DJF than in JJA throughout the period of study with a few exceptions revealing the annual anomaly phenomenon in TEC which is more notable during solar maxima. Moreover winter anomaly is distinctly captured in the northern hemisphere as reflected by higher amplitudes of E1 and W1 QTDOs in summer solstice (JJA) than in winter solstice (DJF) consistent with the antecedent understanding.

Dynamical Coupling Between Hurricane Matthew and the Middle to Upper Atmosphere via Gravity Waves

AbstractDuring 30 September to 9 October 2016, Hurricane Matthew traversed the Caribbean Sea to the east coast of the United States. During its period of greatest intensity, in the central Caribbean, Matthew excited a large number of concentric gravity waves (GWs or CGWs). In this paper, we report on hurricane‐generated CGWs observed in both the stratosphere and mesosphere from spaceborne satellites and in the ionosphere by ground Global Positioning System receivers. We found CGWs with horizontal wavelengths of ~200–300 km in the stratosphere (height of ~30–40 km) and in the airglow layer of the mesopause (height of ~85–90 km), and we found concentric traveling ionospheric disturbances (TIDs or CTIDs) with horizontal wavelengths of ~250–350 km in the ionosphere (height of ~100–400 km). The observed TIDs lasted for more than several hours on 1, 2, and 7 October 2016. We also briefly discuss the vertical and horizontal propagation of the Hurricane Matthew‐induced GWs and TIDs. This study shows that Hurricane Matthew induced significant dynamical coupling between the troposphere and the entire middle and upper atmosphere via GWs. It is the first comprehensive satellite analysis of gravity wave propagation generated by hurricane event from the troposphere through the stratosphere and mesosphere into the ionosphere.

Pre-earthquake ionospheric anomalies before three major earthquakes by GPS-TEC and GIM-TEC data during 2015–2017

The dual-frequency satellite-based measurements from Global Positioning System (GPS) may provide feasible ways of studying and potentially detecting of earthquake (EQ) related anomalies in the ionosphere. In this paper, GPS based Total Electron Content (TEC) data are studied for three major M > 7.0 EQs in Nepal and Iran-Iraq border during 2015–2017 by implementing statistical procedures on temporal and spatial scale. Previous studies presented different time interval of pre-seismic ionospheric anomalies, however, this study showed that EQs ionospheric precursors may occur within 10 days. Furthermore, the ionospheric anomalies on the suspected day occurred during UT = 08:00–12:00 h before the main shock. The Global Ionospheric Map TEC (GIM-TEC) data retrieved over the epicenter of M7.8 (Nepal EQ) showed a significant increase of 6 TECU on April 24, 2015 (one day before the main shock), which is recorded by the ground GPS station data of Islamabad (station lies within the EQ preparation zone). Furthermore, the spatial GIM-TEC result imply significant anomalies over the epicenter during the time interval between UT = 08:00–12:00 h (LT = 13:00–17:00). For M7.3 (Nepal EQ), the TEC anomalies were detected on May 10, 2015 (2 days before the event) in the temporal data. The spatial TEC data imply the huge clouds over the epicenter at about UT = 08:00–12:00 h on May 10, 2015, that may be associated with this EQ in the quiet geomagnetic storm conditions. Similarly, temporal and spatial TEC showed anomaly on November 3, 2017, during UT = 08:00–12:00 (9 days before the Iran-Iraq border EQ) after implementing the statistical method on it. Conversely, there exists a short-term but low magnitude TEC anomaly synchronized with a geomagnetic storm on November 7–8, 2017 (4 to 5 days prior to M7.3 Iran-Iraq border EQ). The diurnal and hourly GIM-TEC and VTEC data also imply the execution of ionospheric anomalies within 10 days prior to all events. All these positive anomalies in TEC may be due to the existence of a huge energy from the epicenter during the EQ preparation period.

GPS precise point positioning for UAV photogrammetry

AbstractThe use of Global Positioning System (GPS) precise point positioning (PPP) on a fixed‐wing unmanned aerial vehicle (UAV) is demonstrated for photogrammetric mapping at accuracies of centimetres in planimetry and about a decimetre in height, from flights of 25 to 30 minutes in duration. The GPS PPP estimated camera station positions are used to constrain estimates of image positions in the photogrammetric bundle block adjustment, as with relative GPS positioning. GPS PPP alleviates all spatial operating constraints associated with the installation and the use of ground control points, a local ground GPS reference station or the need to operate within the bounds of a permanent GPS reference station network. This simplifies operational logistics and enables large‐scale photogrammetric mapping from UAVs in even the most remote and challenging geographic locations.

Assessment of spatial and temporal TEC variations derived from ionospheric models over the polar regions

A comprehensive evaluation of Global Positioning System (GPS) Klobuchar, Galileo broadcast NeQuick2, international reference ionosphere (IRI) and global ionospheric map (GIM) models in estimating ionospheric total electron content (TEC) is performed using GPS-derived, constellation observing system for meteorology, ionosphere, and climate and JASON-2 TECs under various solar conditions in the Arctic and Antarctic. The performances of the four ionospheric models are first analysed for the overall polar regions. In addition, according to the temporal and spatial characteristics of the polar regions, the accuracies of the four models are evaluated for the polar days and nights, the Antarctic ice sheet (AIS) and the Arctic Ocean (AO), the Weddell Sea Anomaly (WSA) as well as for ionospheric storm. The results show that Klobuchar, NeQuick2, IRI2016 and GIM can mitigate the ionospheric delay by 28.69–29.41%, 44.57–56.09%, 43.38–55.99% and 67.17–77.56%, respectively. The performances of the four models during the polar days are obviously worse than those during the polar nights. In the AIS and AO, the Galileo broadcast NeQuick outperforms the GPS broadcast Klobuchar, and the root-mean-square error of IRI2016 performs almost the same as NeQuick2, but IRI2016 has a greater deviation. In addition, the GIM model can only mitigate the ionospheric delay by 46.81–56.72%, which is lower than other regions due to the lack of GPS ground station observations. Under the WSA conditions, the four models underestimate the real TEC to varying degrees, and the night-time deviations of the Klobuchar, NeQuick2 and IRI2016 models are significantly greater than the daytime deviations. The relative accuracy of four ionospheric models is lower during the ionospheric storm period than that in the ionospheric quiet period, especially the NeQuick2 and IRI2016 over the Antarctic.

Quantifying damage from wild pigs with small unmanned aerial systems

ABSTRACTWild pig (Sus scrofa) population expansion and associated damage to crops, wildlife, and the environment is a growing concern in the United States. The destructive rooting behavior of wild pigs indicates where they have foraged and their general presence on the landscape. We used aerial imagery with a small unmanned aerial system to assess damage of corn (Zea mays) fields by wild pigs in the Mississippi Alluvial Valley of Mississippi, USA, during the 2016 growing season. Images were automatically classified using segmentation‐based fractal texture analysis and support vector machines. We assessed the accuracy of automated classification with 5,400 Global Positioning System ground reference points collected in the fields. Classification accuracies for identification of damaged and nondamaged areas were between 65% and 78%. In general, automated classification underestimated the area of damage present within fields. Kappa values ranged from 0.26 to 0.51, on a scale of 0.0–1.0. Small unmanned aerial systems overcome limitations of existing methods because they can survey an entire field rapidly and without significant field labor. © 2018 The Wildlife Society.

Determination of Ocean Tide Loading Displacement by GPS PPP with Priori Information Constraint of NAO99b Global Ocean Tide Model

ABSTRACTThe published global ocean tide models show good agreement in deep oceans and exhibit differences in complex coastal areas, along with subsequent Ocean Tide Loading Displacement (OTLD) modeling differences. Meanwhile, OTLD parameters (amplitudes and phase lags) derived by Global Positioning System (GPS) Precise Point Positioning (PPP) approach need long time to converge to a stable state and show poor precision of S2, K1, and K2 constituents. Based on the fact that no constraint is imposed in the current kinematic solution, a new method is put forward, in which global ocean tide model predictions are taken as the priori information constraints to speed up the convergence rate and improve the accuracy of the GPS-derived OTLD parameters. First, the data of tide gauge from 01 January 2014 to 31 December 2016 are used to generate the harmonic parameters to evaluate the accuracy of six global ocean tide models and a regional ocean tide model (osu.chinesea.2010). Osu.chinesea.2010 model shows good agreement with the tide gauge results, while NAO99b model presents relatively large difference. The predictions from osu.chinesea.2010 and NAO99b model are employed as reference and the prior information, respectively. Second, continuous observations of 12 GPS sites during 2006–2013 in Hong Kong are collected to generate three dimensional OTLD amplitudes and phase lags of eight constituents using PPP with prior information constraints approach and harmonic analysis. Third, comparing the convergence time of eight constituents from PPP without and with priori information constraints approaches, the results show that the new method can significantly improve the convergence rate of OTLD amplitude estimates which obtain a certain level of stability seven years earlier than that derived by the PPP without priori information constraints. Precision of OTLD parameters derived by the new method is about 1 mm. By comparing with the precision of single PPP approach, the accuracy of eight constituents has been improved, especially for S2, K1, and K2 constituents. Finally, through comparing the different correction effects of OTLD estimates on the coordinates and their time series of the ground GPS stations, the results show that OTLD estimates derived by the new approach have similar influence as the osu.chinasea.2010 ocean tide model. The new method provides an effective means to improve the convergence and precision of the GPS-derived OTLD parameters, and achieve a similar correction as the high precision ocean tide model.

Positioning performance of the NTCM model driven by GPS Klobuchar model parameters

Users of the Global Positioning System (GPS) utilize the Ionospheric Correction Algorithm (ICA) also known as Klobuchar model for correcting ionospheric signal delay or range error. Recently, we developed an ionosphere correction algorithm called NTCM-Klobpar model for single frequency GNSS applications. The model is driven by a parameter computed from GPS Klobuchar model and consecutively can be used instead of the GPS Klobuchar model for ionospheric corrections. In the presented work we compare the positioning solutions obtained using NTCM-Klobpar with those using the Klobuchar model. Our investigation using worldwide ground GPS data from a quiet and a perturbed ionospheric and geomagnetic activity period of 17 days each shows that the 24-hour prediction performance of the NTCM-Klobpar is better than the GPS Klobuchar model in global average. The root mean squared deviation of the 3D position errors are found to be about 0.24 and 0.45 m less for the NTCM-Klobpar compared to the GPS Klobuchar model during quiet and perturbed condition, respectively. The presented algorithm has the potential to continuously improve the accuracy of GPS single frequency mass market devices with only little software modification.

Analysis of decade-long time series of GPS-based polar motion estimates at 15-min temporal resolution

We present results from the generation of 10-year-long continuous time series of the Earth’s polar motion at 15-min temporal resolution using Global Positioning System ground data. From our results, we infer an overall noise level in our high-rate polar motion time series of 60 $$\upmu \hbox {as}$$ (RMS). However, a spectral decomposition of our estimates indicates a noise floor of 4 $$\upmu \hbox {as}$$ at periods shorter than 2 days, which enables recovery of diurnal and semidiurnal tidally induced polar motion. We deliberately place no constraints on retrograde diurnal polar motion despite its inherent ambiguity with long-period nutation. With this approach, we are able to resolve damped manifestations of the effects of the diurnal ocean tides on retrograde polar motion. As such, our approach is at least capable of discriminating between a historical background nutation model that excludes the effects of the diurnal ocean tides and modern models that include those effects. To assess the quality of our polar motion solution outside of the retrograde diurnal frequency band, we focus on its capability to recover tidally driven and non-tidal variations manifesting at the ultra-rapid (intra-daily) and rapid (characterized by periods ranging from 2 to 20 days) periods. We find that our best estimates of diurnal and semidiurnal tidally induced polar motion result from an approach that adopts, at the observation level, a reasonable background model of these effects. We also demonstrate that our high-rate polar motion estimates yield similar results to daily-resolved polar motion estimates, and therefore do not compromise the ability to resolve polar motion at periods of 2–20 days.

Dual-Polarimetric C- and X-Band SAR Data for Coastline Extraction

This study proposes a new metric to process dual-polarimetric coherent and incoherent synthetic aperture radar (SAR) data for coastline extraction purposes. The metric, based on the correlation between co- and cross-polarized channels, allows discriminating land from sea in an unsupervised way. Then, simple image processing is adopted to extract continuous coastline from the binary image. Experiments, undertaken on multipolarization C - (RadarSAT-2 and Sentinel-1) and X -band (Cosmo-SkyMed) SAR data collected in South of Italy together with Global Positioning System ground truth, confirm the soundness of the method which is shown to be both effective (a whole SAR scene is processed in seconds) and accurate (the mean error is less than $5$ and $7$ pixels for RadarSAT-2 and CosmoSkyMed, respectively).

Application of Ground Penetrating Radar Surveys and GPS Surveys for Monitoring the Condition of Levees and Dykes

This paper analyses the possibility of using integrated GPS (Global Positioning System) surveys and ground penetrating radar surveys to precisely locate damages to levees, particularly due to the activity of small fossorial mammals. The technology of intercommunication between ground penetrating radar (GPR) and an RTK (Real-Time Kinematic) survey unit, and the method of data combination, are presented. The errors which may appear during the survey work are also characterized. The procedure for processing the data so that the final results have a spatial character and are ready to be implemented in digital maps and geographic information systems (GIS) is also described.

Relative velocity measurement from the spectral phase of a match-filtered linear frequency modulated pulse.

Linear frequency modulated signals are commonly used to perform underwater acoustic measurements since they can achieve high signal-to-noise ratios with relatively low source levels. However, such signals present a drawback if the source or receiver or target is moving. The Doppler effect affects signal amplitude, delay, and resolution. To perform a correct match filtering that includes the Doppler shift requires prior knowledge of the relative velocity. In this paper, the relative velocity is extracted directly from the Doppler cross-power spectrum. More precisely, the quadratic coefficient of the Doppler cross-power-spectrum phase is proportional to the relative velocity. The proposed method achieves velocity estimates that compare favorably with Global Positioning System ground truth and the ambiguity method.

Determining the precipitable water vapor with ground-based GPS and comparing its yearly variation to rainfall over Taiwan

Water vapor plays an important role in weather prediction. Thus, it would be helpful to use precipitable water vapor (PWV) data from Global Positioning System (GPS) signals to understand weather phenomena. Approximately 100 ground GPS stations that cooperate with approximately 500 ground weather stations were used in this study. The relationship between the PWV and rainfall was investigated by analyzing the amplitude and phase that resulted from harmonic analyses. The results indicated that the maximum PWV amplitudes were between 10.98 and 13.10mm and always occurred at the end of July. The magnitudes of the PWV growth rate were between 0.65 and 0.81mm/yr. These rates increased from 9.2% to 13.0% between 2006 and 2011. The largest peak PWV amplitude occurred in the Western region. However, the largest rainfall amplitude occurred in the Southern region. The presented peak rainfall time agreed with the peak PWV time in the Western, Southern, and Central Mountain regions. Although rainfall decreased with time in Taiwan, this decrease was not large. The greatest rainfall consistently occurred during the months in which typhoons occurred, and the greatest PWV values occurred at the end of July. Although the end of July had the greatest monthly average PWV values, the rainfall magnitude during this period was smaller than that during the typhoons, which only occurred for a few days; the PWV also increased during typhoons. Because this effect was short-term, it did not significantly contribute to the PWV monthly average.