Mixed-mode GPS network processing for deformation monitoring applications in the equatorial region

The Global Positioning System (GPS) can be utilised in a wide range of deformation monitoring applications. During the past few years a methodology has been developed for processing data collected by GPS networks consisting of a mixed set of single-frequency and dual-frequency receivers. The strategy is to deploy a few permanent, 'fiducial' GPS stations with dual-frequency, geodetic-grade receivers surrounding an 'inner' network of low-cost, single-frequency GPS receivers. Such a configuration offers considerable flexibility and cost savings for deformation monitoring applications, which require a dense spatial coverage of GPS stations, and where it is not possible, nor appropriate, to establish permanent GPS networks using dual-frequency instrumentation. The basis of the processing methodology is to separate the dual-frequency, 'fiducial' station data processing from the baseline processing involving the inner (single-frequency) receivers located in the deformation zone. The dual-frequency GPS network is used to generate a file of 'corrections', analogous to Wide Area DGPS correction models for the distance dependent biases. These 'corrections' are then applied to the double-differenced phase observations from the inner receivers to improve the baseline accuracies (primarily through empirical modelling of the residual atmospheric biases that otherwise would be neglected). The performance of this configuration under severe ionospheric conditions in the equatorial region has been investigated by simulating such a two-stage network using data collected in the Hong Kong GPS Active Network. A description of the processing strategy, together with a discussion of the results, is presented

Processing mixed-mode GPS networks for deformation monitoring applications

The Global Positioning System (GPS) can be utilised in a wide range of deformation monitoring applications. During the past few years a methodology has been developed for processing data collected by GPS networks consisting of a mixed set of single-frequency and dual-frequency receivers. The strategy is to deploy a few permanent GPS stations with dual-frequency, geodetic-grade receivers surrounding an 'inner' network of low-cost, single-frequency GPS receivers. The dual-frequency GPS network is used to generate a file of 'corrections', analogous to Wide Area DGPS correction models for the distance dependent biases. These 'corrections' are then applied to the double-differenced phase observations from the inner receivers to improve baseline accuracy (primarily through empirical modelling of the residual atmospheric biases that otherwise would be neglected). The performance of this configuration has been investigated by simulating such a two-stage GPS network using data collected in different geographical regions

Mixed-mode GPS deformation monitoring - A cost-effective and accurate alternative?

During the past few years a methodology has been developed for processing data collected by GPS networks consisting of a mixed set of single-frequency and dual-frequency receivers. The strategy is to deploy a few permanent GPS stations with dual-frequency, geodetic-quality receivers surrounding an 'inner' deformation monitoring network of low-cost, single-frequency GPS receivers. The dual-frequency GPS network is used to generate a file of 'corrections', analogous to Wide Area DGPS correction models for the distance dependent biases. These 'corrections' are then applied to the double-differenced phase observations from the inner receivers to enhance baseline accuracy, primarily through empirical modelling of the residual atmospheric biases that otherwise would be neglected. Moreover, epoch-by-epoch baseline solutions are preferred in order to detect deformational signals in (near) real-time. Data from two continuous GPS networks have been used to investigate the performance of this configuration under severe ionospheric conditions and in different geographical regions. In the mid-latitude region the L1 baseline repeatability has clearly been improved by 40-50%, while an improvement of about 20% has been achieved in the equatorial region. The findings also indicate that the proposed procedure is sensitive to extreme ionospheric conditions, such as those experienced in close proximity to the geomagnetic equator during solar cycle maximum periods

Tropospheric delay corrections to differential InSAR results from GPS observations

Differential Interferometric Synthetic Aperture Radar (DInSAR) techniques have been recognised as well-suited for ground deformation monitoring applications. However, the spatially and temporally variable delay of the radar signal propagating through the atmosphere represents a major limitation to accuracy. The dominant factor to be considered is the tropospheric heterogeneity, which can lead to misinterpretation of differential InSAR results. In this paper a between-site and between-epoch double-differencing algorithm for the generation of tropospheric corrections to DInSAR results based on GPS observations is proposed. In order to correct the radar results on a pixel-by-pixel basis, the GPS-derived corrections have to be interpolated. Using GPS data from the SCIGN network it has been found that the inverse distance weighted and kriging interpolation methods are more suitable than the spline method. Differential corrections as much as several centimetres may have to be applied in order to ensure sub-centimetre accuracy for the DInSAR result and it seems optimal to estimate the tropospheric delay from GPS data at 5-minute intervals. The algorithm and procedures described in this paper could easily be implemented in a CGPS data centre. The interpolated image of between-site, single-differenced tropospheric delay can be provided as a routine product to assist radar interferometry

Tropospheric corrections to SAR interferometry from GPS observations

Interferometric Synthetic Aperture Radar (InSAR) techniques have been recognised as an ideal tool for many ground deformation monitoring applications. However, the spatially and temporally variable delay of the radar signal propagating through the atmosphere is a major limitation to accuracy. The dominant factor to be considered is the tropospheric heterogeneity, which can lead to misinterpretation of InSAR results. In this paper a between-site and between-epoch double-differencing algorithm for the generation of tropospheric corrections to InSAR results based on GPS observations is tested. In order to correct the radar results on a pixel-by-pixel basis, the GPS-derived corrections have to be interpolated. Using experimental data it has been found that the inverse distance weighted and kriging interpolation methods are more suitable than the spline interpolation method. Differential corrections as large as several centimetres may have to be applied in order to ensure sub-centimetre accuracy for the InSAR result. The algorithm and procedures described in this paper could easily be implemented in a continuous GPS network data centre. The interpolated image of between-site, single-differenced tropospheric delays can be derived as a routine product to assist radar interferometry

PPP versus DGNSS

Is Precise Point Positioning (PPP) a viable alternative to Differential GNSS (DGNSS) techniques? In this article we look at the current status of PPP and its potential

Experiences with a mixed-mode GPS-based volcano monitoring system at Mt. Papandayan, Indonesia

During the past few years a methodology has been developed for processing data collected by GPS networks consisting of a mixed set of single-frequency and dual-frequency receivers. The strategy is to deploy a few permanent, 'fiducial' GPS stations with dual-frequency, geodetic-grade receivers surrounding an 'inner' network of low-cost, single-frequency GPS receivers. Such a configuration offers considerable flexibility and cost savings for geodynamic applications such as volcano deformation monitoring, which require a dense spatial coverage of GPS stations, and where it is not possible, nor appropriate, to establish permanent GPS networks using dual-frequency instrumentation. This configuration has recently been tested at the Mt. Papandayan volcano in West Java, Indonesia. The two-stage network design consists of an inner network of four single-frequency Canadian Marconi (CM) GPS receivers surrounded by three dual-frequency Leica CRS1000 GPS receivers. The inner network logged and transmitted GPS data from the 'slave' stations located on the volcano, to a base station. The combined processing of the CM and Leica receiver data was performed offline so as to investigate the performance of such a mixed-mode system. The basis of the processing methodology is to separate the dual-frequency, 'fiducial' station data processing from the baseline processing involving the single-frequency receivers on the volcano. The data processing for the former was carried out using a modified version of the Bernese software, to generate a file of 'corrections' (analogous to Wide Area DGPS correction models for the distance dependent biases -- primarily due to atmospheric refraction). These 'corrections' will then be applied to the double-differenced phase observations from the inner receivers to improve the baseline accuracies (primarily through empirical modelling of the residual atmospheric biases that otherwise would be neglected). A description of the field testing (and its challenges) during February-March 2000, together with a discussion of the results are presented

Sparse network: Wide-area, sub-decimeter positioning for airborne LiDAR surveys

Airborne LiDAR surveys are amongst the most advanced means of producing high-resolution, accurate surface elevation models used for many applications in surveying and civil engineering. Precise geolocation and orientation (or georeferencing) of the LiDAR instrument with a combination of on-board GNSS and inertial sensors at the times when the measurements are made provides the key to high-quality elevation products. The usual practice deploys reference GPS/GNSS land receivers in the area where the aircraft will be flying, to obtain a precise trajectory by short-baseline differential GNSS techniques. This could mean installing and operating receivers at many sites during a flight mission if the area surveyed is a large one. We have tried a different approach: using as reference receivers those of a sparse network of Continuously Operating Reference Stations (CORS) in New South Wales known as CORSnet-NSW, and a wide-area GNSS technique for obtaining the aircraft trajectory with sub-decimeter accuracy even with baseline lengths of several hundred kilometers. We show that this is comparable in precision and accuracy to the short-baseline method, but without the cost and logistical complications. This opens up a new level of operational capability, allowing flexibility for weather conditions and priority response applications. The tests described here were organized and conducted by the NSW government's Land and Property Management Authority, in collaboration with the University of New South Wales, in June 2009

Testing sub-decimeter, kinematic wide-area positioning for airborne LiDAR surveys using the CORSnet-NSW network.

We have studied the possible advantages of a wide-area approach (long-baseline differential positioning with GPS) for the precise kinematic trajectory determination of aircraft in support of airborne scanning lidar altimeter surveys, over the usual and more labor and resource intensive short-baseline approach with locally deployed ground receivers. In this form of remote sensing, the GNSS data are used to find very precisely the aircraft position and, combining it with inertial data, the aircraft orientation, in order to georeference the scanning laser measurements within very strict tolerances. If proved useful, the adoption of the wide-area approach, compared to present practice, could result in a substantial reduction of costs and in more flexibility when confronted with changing weather conditions or dealing with priority response situations. Such situations, at present, may require postponing a survey, or redeploying ground receivers and personnel on short notice. We have conducted three successful tests: two with data collected during the survey of large areas in the northeast of the state of New South Wales, in Australia, and a third one with data from a system calibration flight over a pre-surveyed area around the Bathurst airport, also in that state. These tests were organized and conducted by the NSW Government’s Land and Property Management Authority (LPMA), in collaboration with the University of New South Wales, in June of 2009 and July of 2010. The baselines from the reference stations to the aircraft were as long as 1100 km. The wide-area reference stations used in the tests are part of CORSnet-NSW, a network of continuously operating reference stations run by LPMA in the state of New South Wales. As of September of 2010 this network consisted of 43 stations; and the goal is to reach a total of 70 by 2012. All receivers in the network collected data at the rate of 1 Hz; on the aircraft 2 Hz data were collected. The solutions were calculated in post-processing mode, at 2 Hz. To verify the quality of the aircraft trajectories determined by the wide-area technique, they were compared to the customary short-baseline solutions with local reference stations set up within a few kilometers of the flight path of the airplane. Finally, the digital elevation model (DEM) obtained from the calibration flight data and a precise wide-area GNSS trajectory was compared to the DEM made with the usual short-baseline method. In all cases the agreement was excellent

CORSnet-NSW and airborne LiDAR: A match made in heaven

CORSnet-NSW is a rapidly growing network of Global Navigation Satellite System (GNSS) Continuously Operating Reference Stations (CORS) providing fundamental positioning infrastructure for New South Wales. Currently consisting of over 60 permanent stations tracking multiple satellite constellations, CORSnet-NSW will expand to over 100 stations within the next two years. Airborne LiDAR surveys produce very accurate, high-resolution surface elevation models which are used for many surveying and civil engineering applications as well as for flood prevention and mitigation or monitoring coastal erosion and land subsidence. The key to producing high-quality elevation products is very precise geolocation and orientation of the LiDAR instrument during the survey, obtained with a combination of on-board GNSS and inertial sensors. The usual practice is to deploy temporary GNSS reference stations in the survey area, utilising the short-baseline differential GNSS technique to obtain a precise aircraft trajectory. If the area surveyed is large, this generally means installing and operating receivers at many sites during a flight mission. We show that by combining the positioning infrastructure provided by CORSnet-NSW with a wide-area GNSS technique, the aircraft trajectory can be obtained with subdecimetre accuracy, even with baseline lengths of several hundred kilometres. It is shown that this is comparable in precision and accuracy to the short-baseline method, but without the cost and logistical complications of having to deploy and operate one’s own reference receivers during a mapping mission. This opens up a new level of operational capability for airborne LiDAR, allowing flexibility for weather conditions and priority response applications