GPS Height Research Articles

How to improve the accuracy of height data from bird tracking devices? An assessment of high-frequency GPS tracking and barometric altimetry in field conditions

BackgroundIn the context of rapid development of wind energy infrastructure, information on the flight height of birds is vital to assess their collision risk with wind turbines. GPS tags potentially represent a powerful tool to collect flight height data, yet GPS positions are associated with substantial vertical error. Here, we assessed to what extent high-frequency GPS tracking with fix intervals of 2–3 s (GPS remaining turned on between fixes), or barometric altimetry using air pressure loggers integrated in GPS tags, improved the accuracy of height data compared to standard low-frequency GPS tracking (fix interval ≥ 5 min; GPS turned off between fixes).ResultsUsing data from 10 GPS tag models from three manufacturers in a field setting (194 tags deployed on free-living raptors), we estimated vertical accuracy based on periods when the birds were stationary on the ground (true height above ground was approximately zero), and the difference between GPS and barometric height in flight. In GPS height data, vertical accuracy was mainly driven by noise (little bias), while in barometric data, it was mostly affected by bias (little noise). In high-frequency GPS data, vertical accuracy was improved compared to low-frequency data in each tag model (mean absolute error (AE) reduced by 72% on average; range of mean AE 2–7 vs. 7–30 m). In barometric data, vertical accuracy did not differ between high- and low-frequency modes, with a bias of − 15 to − 5 m and mean AE of 7–15 m in stationary positions. However, the median difference between GPS and barometric data was smaller in flight positions than in stationary positions, suggesting that the bias in barometric height data was smaller in flight. Finally, simulations showed that the remaining vertical error in barometric and high-frequency GPS data had little effect on flight height distributions and the proportion of positions within the collision risk height range, as opposed to the extensive noise found in low-frequency GPS data in some tag models.ConclusionsBarometric altimetry may provide more accurate height data than standard low-frequency GPS tracking, but it involves the risk of a systematic error. Currently, high-frequency GPS tracking provides highest vertical accuracy and may thus substantially advance the study of wind turbine collision risk in birds.

Variance–covariance analysis of two high-resolution regional least-squares quasi-geoid models

This paper investigates the full variance–covariance (VC) matrix of two high-resolution regional quasi-geoid models, utilizing a spherical radial basis function parameterization. Model parameters were estimated using weighted least-squares techniques and variance component estimation (VCE) for data weighting. The first model, known as the “RCR model,” is computed through the remove–compute–restore method, incorporating various local gravity and radar altimeter datasets. The second model, the “combined model,” includes the GOCO05s satellite-only global geopotential model as an additional dataset with a full-noise VC matrix. Validation of the noise VC matrix scaling for each quasi-geoid model is achieved by comparing observed and formal noise standard deviations of differences between geometric and gravimetric height anomalies at GPS height markers in the Netherlands. Analysis of the noise VC matrix of height anomalies at grid nodes reveals significantly smaller formal noise standard deviations for the RCR model compared to the combined model. This difference is attributed to VCE assigning larger weights to the GOCO05s dataset, which exhibits greater noise standard deviations for the specific spatial scales used. Additionally, the formal noise standard deviations of height anomaly differences, relevant for GNSS-heighting, favor the RCR model. However, the disparity between the two models is smaller than implied by the height anomaly noise standard deviations. This is due to the combined model’s noise autocorrelation function displaying a longer correlation length (67 km) in contrast to the RCR model’s (17 km). Consequently, the combined model exhibits a greater reduction in noise variance for height anomaly differences relative to white noise compared to the RCR model.

Real-time estimation of altitude and airspeed under large airflow angles in flight test

In the flight test, many test subjects need to enter the large airflow angle. Due to the large airflow angle, the position error of air data sensor is big, and the altitude and airspeed of the aircraft are not accurate, which not only affects the effectiveness of the test results, but also affects the safety of the test flight. This paper proposes two real-time altitude and airspeed estimation methods, including the GPS method and constant wind field method. The GPS method is commonly used in the airspeed calibration, which mainly uses the GPS height change to correct the static pressure, but it depends on the aircraft total pressure sensor to get the airspeed. The constant wind field method directly calculates the true airspeed not depending on the pressure measurement, but it is greatly affected by the wind stability. During the flight test, using this method, the real-time monitoring can provide pilots with accurate altitude and airspeed reference.

Real Time Kinematic (RTK) heave as a replacement of Motion Reference Unit (MRU) heave in hydrographic surveying works

Heave is one of the major contributors to errors in water depth measurements. Motion Reference Unit (MRU) measures the heave signal with high-level accuracy as well as other ship motions. Unfortunately, MRU has been reported to have some of drawbacks such as; heave drift error, influence of ship motion dynamic, In addition to its very expensive price Real-Time Kinematic (RTK) GPS uses dual frequency receiver and carrier phase differential technique under kinematic solution and provides very accurate position in all three components in real time. In addition, RTK GPS also calculates the low frequency changes in water level such as tidal oscillation. This research is an attempt to investigate the possibility of using the RTK GPS data to deduce the heave signal from the GPS height (tides - heave) instead of heave from MRU to correct the water depth. Moreover, to examine to what extent, RTK heave can be used as a backup to satisfy the International Hydrographic Organization (IHO) survey order standards. A comparison between the extracted RTK heave and MRU heave signals revealed a good agreement with a strong direct correlation of 0.96. As shown in figure 1 RTK heave as a replacement for MRU heave in hydrographic surveying was statistically validated with the use of a lot of methods of analysis such as Test of the Normality, Paired Samples T Test, Wilcoxon Signed Ranks Test, Heave Signals Frequency Adjustment , Descriptive Statistics for the Two Heave Signals , Descriptive Statistics for Each Signal Individually , Correlation and Trend, Analysis Between the Two Signals, Scatter Diagram and Trend, Standard Deviation and Uncertainty for Soundings, Characteristics of the Difference between Two Signals , Comparing the Surfaces by TIN Model. The results of this analysis provided the possibility of using RTK heave as a replacement for MRU heave in hydrographic surveying. Therefore, RTK GPS is not only used to provide precise position or tidal oscillations but also based on this study, can be used to measure heave accurately to correct the depth satisfying IHO survey standards. Received: 28 January 2023 Accepted: 26 April 2023 Published: 22 May 2023

Determination of the height reference surface for the Republic of Albania by using global geopotential models

Abstract For the development of various geodetic tasks within a state, determining the Height Reference Surface by the geoid model is extremely important. Considering this, one of the main task of geodesy is to determine the geoid, which is defined as an equipotential surface of the Earth’s gravity field, as a result, it corresponds on average to the sea level. The aim of this study is to analyze the best-fitting geoid model for the territory of the Republic of Albania. In this study, DFHRS (Digital Finite Element Height Reference Surface) method was used (www.dfhbf.de), developed by Reiner Jäger [Jäger R. State of the art and present developments of a general approach for GPS-based height determination. East Africa: University of Applied Sciences, Faculty Geoinformationswesen, Department of Surveying and Geomatics. Paper Presented at the First Workshop on GPS and Mathematical Geodesy in Tanzania (Kilimanjaro Expedition 1999); 1999] to determine the most suitable geoid model for the territory of Albania. This approach allows the conversion of ellipsoidal heights determined by GNSS into the standard heights, which refer to the height reference surface (HRS) of an orthometric. The DFHRS is defined as continuous HRS in arbitrarily large areas by bivariate polynomials over an irregular grid [Jäger R, Schneid S. Online and postprocessed GPS heighting based on the concept of a digital height reference surface (DFHRS), in vertical reference systems. In: IAG Symposium. Cartagena, Colombia, Heidelberg: Springer; 2001, vol 124:203–8 pp]. The DFHRS approach uses a wide range of input data (Geometric and Physical) and in our case, there were 151 GPS/levelling height data as well as physical derivatives from different global geopotential models. The main focus of this study is placed on the calculation of the most suitable geoid model for the territory of Albania using global geopotential models (EGM96, EGM2008, EIGEN04, EIGEN6C4 and European Gravimetric Geoid Model 1997 (EGG97)). After analyzing the results and comparing the models among themselves, the Albanian DFHRS-EIGEN6C4 model was selected as the most suitable model for the territory of Albania.

Constraints on Secular Geocenter Velocity From Absolute Gravity Observations in Central North America: Implications for Global Melting Rates

AbstractLong series of absolute gravity observations at multiple sites provide a direct method of measuring the velocity of the geocenter caused by surface mass transport related to climate change. Annual absolute gravity observations and available GPS vertical data at eight sites in central North America from 1995 to 2010 are used to solve for geocenter velocity while correcting for glacial isostatic adjustment. The precision of the secular gravity trends and GPS heights at our sites, used to estimate geocenter motion, was improved by correcting for local and global water storage attraction and loading. Corrections for the loading and attraction effects of regional ice‐mass loss, the atmosphere and non‐tidal ocean were found to be minimal. Our results provide a direct measurement of the change in geocenter velocity over the time‐interval 2002–2010 compared to the time‐interval 1995–2003 of 0.68 ± 0.28 mm/yr away from North America. This suggests similar velocities in the negative Z (0°N) and positive Y (90°E) directions to those found by other studies.

Detection of GPS-levelling datum variation using heterogeneous data: A case study in Sri Lanka

Geodetic datum is a fundamental reference surface in positioning, and plays an important role in relation to the survey activities of a country. Sri Lankan horizontal datum was established in 1999 (SLD99) using GPS technology, and the vertical datum was fixed averaging 6 years of tidal observations from 1884 to 1890. This research investigates potential inconsistencies of Sri Lankan GPS-levelling datum. For the analysis, heterogeneous data such as long-term tidal, GPS, levelling, global geoid heights and Mean Dynamic Topography (MDT) around the coastal region were used. Tidal analysis reveals that the tidal MSL around the country is linearly varied with an increasing rate of 3 mm per year. By analysing GPS, levelling, MDT and global geoid heights, it is found that there is around 1.9 m vertical deviation in Sri Lankan GPS datum with respect to the latest realization of ITRF, and the MSL based levelling datum is capable of representing global geoid features.

Application of an improved clustering approach on GPS height time series at CMONOC stations in Southwestern China

The hydrological, geological and meteorological conditions in southwestern China are relatively complex, so that the land surface deformation presents various features. Using 58 Crustal Movement Observation Network of China (CMONOC) stations across four provinces in Southwestern China, we adopt an improved clustering algorithm to classify 49 stations into 12 clusters with different similarity levels. Our results show that the average annual signals of GPS stations within each cluster have strong consistency, while obvious differences exist among the 12 clusters, indicating that clustering algorithm helps to describe surface deformation features more accurately in regions with complex conditions. We then combine other earth observation techniques, such as the Gravity Recovery and Climate Experiment (GRACE) satellite datasets and surface loading models (SLM), and observe that GPS, GRACE and SLM have strong correlation in their monthly displacement series at GPS stations. After excluding non-clustered stations according to our previous clustering results, the correlation coefficients of GPS/GRACE and GPS/SLM are enhanced. Also, the RMS reduction rates of GPS coordinate time series have been improved after deducting displacements obtained from GRACE and SLM, thus the clustering algorithm proves to be effective in improving the consistency of three techniques in joint detection of surface deformation. Moreover, the vertical displacements of four riverside GPS stations in the Three Gorges Reservoir (TGR) area show significant negative correlation with water level of TGR, hence we conclude that the Three Gorges Dam (TGD) may directly affect the consistency of GPS annual signals of its upstream and downstream GPS stations.Graphical

Terrestrial Signature of a Recently-Tidewater Glacier and Adjacent Periglaciation, Windy Glacier (South Shetland Islands, Antarctic)

Contemporary retreat of glaciers is well visible in the West Antarctic region. The aim of this study is to identify, map and quantify terrestrial glacial and periglacial landforms developed in front of Windy Glacier (Warszawa Icefield, King George Island, South Shetlands), which recently turned from being tidewater to land-terminating, and on near-by Red Hill. The study is based on an orthophoto map and a DEM elaborated with a use of images obtained during a UAV BVLOS photogrammetric survey in 2016, Google Earth Pro images from 2006 and an orthophoto map from 1978/1979. The geomorphological map obtained includes 31 types of landforms and water bodies, grouped into: (1) glacial depositional landforms, (2) fluvial and fluvioglacial landforms, (3) littoral and lacustrine landforms, (4) solifluction landforms, (5) other mass movement landforms, (6) patterned ground, (7) debris flows, landslides and mudflows, (8) water bodies, (9) other (bedrock, boulders, glacial ice, snow patches, and not recognized surface). Most area is occupied by glacial lagoon, fluvial and fluvioglacial landforms, not recognized surfaces and littoral landforms. Between 2006 and 2016 the glacier deposited a well-developed patch of fluted moraine with small drumlins. We recognize the glacial-periglacial transition zone between 41 and 47 m GPS height above which solifluction landforms and sorted patterned ground dominate. Advantages of UAV and BVLOS missions are highlighted and problems with vectorization of landforms are discussed. Distinction between flutes and small drumlins is shown on length-to-elongation and length-to-width diagrams and critical reference to previous geomorphological mappings on King George Island is presented.

Interannual Variability of GPS Heights and Environmental Parameters over Europe and the Mediterranean Area

Vertical deformations of the Earth’s surface result from a host of geophysical and geological processes. Identification and assessment of the induced signals is key to addressing outstanding scientific questions, such as those related to the role played by the changing climate on height variations. This study, focused on the European and Mediterranean area, analyzed the GPS height time series of 114 well-distributed stations with the aim of identifying spatially coherent signals likely related to variations of environmental parameters, such as atmospheric surface pressure (SP) and terrestrial water storage (TWS). Linear trends and seasonality were removed from all the time series before applying the principal component analysis (PCA) to identify the main patterns of the space/time interannual variability. Coherent height variations on timescales of about 5 and 10 years were identified by the first and second mode, respectively. They were explained by invoking loading of the crust. Single-value decomposition (SVD) was used to study the coupled interannual space/time variability between the variable pairs GPS height–SP and GPS height–TWS. A decadal timescale was identified that related height and TWS variations. Features common to the height series and to those of a few climate indices—namely, the Arctic Oscillation (AO), the North Atlantic Oscillation (NAO), the East Atlantic (EA), and the multivariate El Niño Southern Oscillation (ENSO) index (MEI)—were also investigated. We found significant correlations only with the MEI. The first height PCA mode of variability, showing a nearly 5-year fluctuation, was anticorrelated (−0.23) with MEI. The second mode, characterized by a decadal fluctuation, was well correlated (+0.58) with MEI; the spatial distribution of the correlation revealed, for Europe and the Mediterranean area, height decrease till 2015, followed by increase, while Scandinavian and Baltic countries showed the opposite behavior.

Comprehensive Analysis of Mass Loading Effects on GPS Station Coordinate Time Series Using Different Hydrological Loading Models

This paper comprehensively analyzes the effect of surface mass loading on GPS height time series in terms of weighted root mean square (WRMS), annual timescale, velocity and noise properties. Considering the complexity of the hydrological process, we adopt the same atmospheric loading and nontidal ocean loading estimated from the TUGO-m barotropic model (ATMMO) and different hydrological loadings estimated from the Global Land Data Assimilation System (GLDAS) and the Global Land Surface Discharge Model (LSDM). The results indicate that the combined ATMMO & GLDAS outperforms the combined ATMMO & LSDM in reducing the WRMS of GPS height times series due to the annual phase asynchrony between GPS height and ATMMO & LSDM. Furthermore, cross-wavelet transform-based (XWT-based) semblance analysis is adopted to quantitatively express the annual phase relationship between GPS height time series and different loading model combinations. The results show that (1) mass loading is the main driving force for the annual fluctuation in GPS height, and (2) the annual term of ATMMO & GLDAS better matches the GPS height time series than ATMMO & LSDM. Consequently, it is concluded that the LSDM may not work well in Eurasia with its current accuracy. Finally, we investigate the impacts of the mass loading correction on site velocity and noise properties. For most sites, the velocity change can reach 0.1 mm/year after the loading correction, indicating that the influence of mass loading should not be ignored in the construction of millimeter-level earth reference frames. However, for some sites, the velocity uncertainty is increased due to undesirable changes in noise properties, suggesting that using the mass loading model to correct GPS height time series is potentially feasible but not sufficient.

Evaluation of Global Gravitational Models Based on DGPS/leveling Data over Baghdad University (IRAQ)

GPS becomes one of the most technical favorite geodesy methods. A major dilemma in GPS survey lies in oval-based elevations, while orthometric heights are commonly used in engineering practice. Converting GPS heights into orthometric heights by applying a precise geode model is therefore necessary. This paper aims to model the local geoid in the using study region (h-H). Compared with many global gravitational models (GGMs) and three of them were selected, namely EGM96, EGM84, and EGM2008. The evaluations of the accuracy of these models and then choose the most accurate and closer to the local geode In the field of work, which is mounted on the campus of the University of Baghdad in the capital of Iraq. Techniques require the use of GGM to represent global differences or long wavelengths of The gravity field of the Earth; the gravitational field of the Earth takes into account much of the direct information about the distributions of mass density in the Earth. It then provides the basis for exploring oil and gas in this respect, and the detection of geodesics forms the basis for exploring minerals. The EGM08 is the closer GGMs to the observed local geodetic dataset, in terms of the difference of geoid undulation that have a maximum of) 0.4011m), a minimum of) 0.3996m), an average of [-0.111256m], and a standard deviation of[ ±0.152859m].

Mapping mean lake surface from satellite altimetry and GPS kinematic surveys

Lake water height is a key variable in water cycle and climate change studies, which is achievable using satellite altimetry constellation. A method based on data processing of altimetry from several satellites has been developed to interpolate mean lake surface (MLS) over a set of 22 big lakes distributed on the Earth. It has been applied on nadir radar altimeters in Low Resolution Mode (LRM: Jason-3, Saral/AltiKa, CryoSat-2) in Synthetic Aperture Radar (SAR) mode (Sentinel-3A), and in SAR interferometric (SARin) mode (CryoSat-2), and on laser altimetry (ICESat). Validation of the method has been performed using a set of kinematic GPS height profiles from 18 field campaigns over the lake Issykkul, by comparison of altimetry’s height at crossover points for the other lakes and using the laser altimetry on ICESat-2 mission. The precision reached ranges from 3 to 7 cm RMS (Root Mean Square) depending on the lakes. Currently, lake water level inferred from satellite altimetry is provided with respect to an ellipsoid. Ellipsoidal heights are converted into orthométric heights using geoid models interpolated along the satellite tracks. These global geoid models were inferred from geodetic satellite missions coupled with absolute and regional anomaly gravity data sets spread over the Earth. However, the spatial resolution of the current geoid models does not allow capturing short wavelength undulations that may reach decimeters in mountaineering regions or for rift lakes (Baikal, Issykkul, Malawi, Tanganika). We interpolate in this work the geoid height anomalies with three recent geoid models, the EGM2008, XGM2016 and EIGEN-6C4d, and compare them with the Mean Surface of 22 lakes calculated using satellite altimetry. Assuming that MLS mimics the local undulations of the geoid, our study shows that over a large set of lakes (in East Africa, Andean mountain and Central Asia), short wavelength undulations of the geoid in poorly sampled areas can be derived using satellite altimetry. The models used in this study present very similar geographical patterns when compared to MLS. The precision of the models largely depends on the location of the lakes and is about 18 cm, in average over the Earth. MLS can serve as a validation dataset for any future geoid model. It will also be useful for validation of the future mission SWOT (Surface Water and Ocean Topography) which will measure and map water heights over the lakes with a high horizontal resolution of 250 by 250 m.

Accuracy detection of Satellite and InSAR Technology in the Deformation Monitoring in Civil Engineering

This paper focuses on the application of Satellite Navigation Positioning System (such as GPS and BeiDou Navigation Satellite System) and InSAR technology in healthy monitoring in civil engineering. Considering the high cost of Satellite Positioning measurement and blind spot of InSAR measurement, it is reasonable to combine both technologies for accomplishing regionally area and focused points monitoring. During the study, the control network points were positioned with GPS measurement technique and height differences were supported with precise levelling measurements. As the result of measurement campaigns, the height differences were determined from the GPS measurements and InSAR measurements respectively. Later on, deformation analysis using the height differences according to provided data from the GPS and the data from the InSAR system were carried out separately. The comparison between GPS and InSAR results will be shown in the paper.

Interpolation and Statistical Analysis for Evaluation of Global Earth Gravity Models Based on GPS and Orthometric Heights in the Middle of Iraq

The regions around the world need to perform their results based on the local geoid. However, each region has different ground topography based on the amount of gravity in this region. Nowadays, the recent global Earth's gravity model of 2008 is successfully used for different purposes in geosciences research. This research presents an overview of the preliminary evaluation results of the new Earth Gravitation Model (EGM08) in the middle of Iraq. For completeness, the evaluation tests were also performed for EGM96 by examining 31 stations distributed over four Iraqi provinces. The national orthometric heights were compared with the GPS /leveling data obtained from these stations. This study illustrated that the GPS /leveling based on EGM08 data was better than that based on EGM96 data in terms of reducing the root mean square error (RMSE) of the differences between the orthometric heights and GPS/leveling data. The standard deviation (SD) values for the national orthometric heights and GPS heights were about 4 and 26cm, respectively. The results also show that there is a small difference in hight ranged (0.0013 - 0.1333 m) in Karbala, (0.0023 – 0.0062 m), in Najaf and (0.0173 – 0.0703 m), in Babylon. Due to the flat area, better results were obtained in Karbala and Najaf than Babylon. The EGM08 geoid method has shown to yield very close results to reality for various projects, thus its accuracy is acceptable.

Comparative Analysis of Change between Ellipsoidal Height Differences and Equivalent Orthometric Height Difference

Height is an important component in the determination of the position of a point. The study aimed at performing a comparative analysis of change between ellipsoidal height differences and the equivalent orthometric height difference of points. A hi-target Differential Global Positioning System (DGPS) was used to acquire GPS data with an occupation period of thirty (30) minutes on each point, which were processed using Hi-target Geomatics Office (HGO) software to obtain the ellipsoidal heights. An automatic level instrument was used to acquire leveling data, which were processed using the height of collimation method to obtain the orthometric heights. A total of fifty (50) points were occupied as common points for both the GPS and levelling observations at 20-meter intervals. The accuracy of the height difference was determined using standard deviation with the ellipsoidal height difference as 53.59cm and the orthometric height as 53.07cm respectively. A Root Mean Square Error value of 0.0621m was obtained as the accuracy of the change between the two height differences. Statistical analysis using the independent-sample Z test was used to analyze the data at a 5% significant level. The result shows no significant difference in the performance of the two height systems. It is worthy to note that GPS and spirit levelling height differences can be used interchangeably for any heighting in short distances for surveying and engineering applications.

RESEARCH ON GPS HEIGHT FITTING BASED ON ABC OPTIMIZED LSSVM

Abstract. Aiming at the problem that the fitting parameters of the least squares support vector machine fitting method are difficult to select, a method of introducing the artificial bee colony algorithm into the least squares support vector machine to establish a high-precision region fitting model is proposed. The artificial bee colony algorithm can perform global tracking search on the parameters in the least squares support vector machine, imitate the honey collecting process of the bees, and use the primary value of the parameters as the honey source, and the average square error predicted by the least squares support vector machine as the target. The function value is determined by iterative update within a certain range to determine the optimal parameters, and finally a GPS height fitting model with higher precision is established. Experimental analysis, compared with the conventional least squares support vector machine fitting method, the accuracy of the fitting model constructed by the ABC-LSSVM combination method is improved by 45.4%. At the same time, the combined method is better than the particle swarm optimization fitting method and BP neural network. The legal convergence effect is higher and the stability is better. The effective feasibility of the ABC-LSSVM combination method in the construction of GPS height fitting model is proved, which provides a certain reference value for the establishment of GPS height fitting model.

Modelling Orthometric Heights from a Combination of Ellipsoidal Heights and Gravimetric Geoid Model in Rivers State, Nigeria

Many applications in geodesy, hydrography and engineering require geoid-related heights. Spirit leveling which is the traditional means of obtaining geoid- or mean sea level-related heights is slow, time-consuming and costly. Global Navigation Satellite Systems (GNSS) offer faster and relatively cheaper way of obtaining geoid-related heights when geoidal undulation is applied to ellipsoidal heights. However, difficulties involved in determining acceptable geoid height have seriously hampered the application of GNSS for leveling in Rivers State, thus necessitating the need to develop an acceptable geoid model which will serve as a means of conversion of GNSS-delivered ellipsoidal heights to their orthometric heights equivalent. In pursuance of this objective, a detailed gravimetric geoid has been evaluated for Rivers State, Nigeria. The computation of the geoid was carried out by the traditional remove-restore procedure. The Earth Geopotential Model 2008 (EGM08) was applied as the reference field for both the remove and restore parts of the procedures; spherical Fast Fourier Transform (FFT) was employed for the evaluation of the Molodenskii’s integral formula for the height anomaly, (ζ) to yield the quasi-geoid; while the Residual Terrain Modelling (RTM) was done by prism integration. The classical gravimetric geoid over Rivers State was obtained from the rigorously evaluated quasi-geoid by adding the quasi-geoid to geoid (N - ζ) correction it. The minimum and maximum geoid height values are 18.599 m and 20.114 m respectively with standard deviation of 0.345 m across the study area. Comparison of the gravimetric geoidal heights with the GPS/Leveling-derived geoidal heights of 13 stations across Rivers State, Nigeria showed that the absolute agreement with respect to the GPS/leveling datum is generally better than 7 cm root mean squares (r.m.s) error. Results also showed that combining both GPS heights and the computed Rivers State geoid model can give orthometric heights accurate to 3 cm post-fit using a 4-parameter empirical model. The geoid model can thus serve as a good alternative to traditional leveling when used with GPS leveling, particularly for third order leveling in the study area.

Research on GPS precise point positioning algorithm with a Sea Surface Height Constraint

Marine surveying does not receive full benefits from RTK because there are lack of near reference stations along the coastal lines. Compared with RTK, PPP can realize positioning with just a single station, which can allow a wider operation range in marine applications, but its elevation positioning accuracy is significantly lower than the horizontal direction. For some ships with relatively static offshore operations, the vertical movement of ships is mainly affected by the tide, and the fluctuation of tide can be obtained by global tide models, etc. Therefore, we propose a GPS PPP method with a sea surface geodetic height constraint to make full use of sea tide motion information to improve PPP effect. In this paper, we establish the Sea Surface Height constraint PPP (SHC-PPP) model. It is noted that the GPS height is relative to WGS-84 reference ellipsoid, thus we convert the tide fluctuation information to the sea surface height relative to WGS-84 reference ellipsoid by adding a fixed value, which is the geodetic height of GPS receiver at the first epoch obtained in advance by using the post-positioning PPP minus the corresponding epoch's water level data predicted by FES2014. Validated by kinematic experiments, our SHC-PPP method can improve the vertical positioning accuracy over conventional PPP (PPPC) by 79%.

Online spraying quality assessment system of plant protection unmanned aerial vehicle based on Android client

We develop an online spraying quality assessment system of plant protection unmanned aerial vehicle (UAV) based on Android Client. The built-in location and motion system of mobile phone are used to acquire the GPS data, speed and height of the UAV placed with Android mobile phone. A pump operation endpoints extraction model based on double-threshold method is developed to determine the pump operation status (on/off) during UAV flight. Adaptive Gaussian Filtering is employed to improve the positioning accuracy. Some assessment indexes including overlap spray and blank spray rates indicating plant protection UAV spraying quality are proposed in this paper. The test results show that: ① the max deviation of the beginning/end point extracted by double-threshold model is 2 s, and the max deviation of total operation time is 4 s; ② adaptive Gaussian Filtering can effectively improve the accuracy of static and dynamic positioning. We take filter parameter σ = 1, to decrease the measured dynamic positioning deviation and avoid clear reduction of measured total distance (when σ > 1) by smartphone; ③ when σ = 1, the difference values of effective spray rate, the overlap spray rate and the blank spray rate between smartphone measuring and UAV recording are only 1.08％, 0.68％and 1.07％, respectively.