Christian Thom

Digital Airborne Photogrammetry—A New Tool for Quantitative Remote Sensing?—A State-of-the-Art Review On Radiometric Aspects of Digital Photogrammetric Images

The transition from film imaging to digital imaging in photogrammetric data capture is opening interesting possibilities for photogrammetric processes. A great advantage of digital sensors is their radiometric potential. This article presents a state-of-the-art review on the radiometric aspects of digital photogrammetric images. The analysis is based on a literature research and a questionnaire submitted to various interest groups related to the photogrammetric process. An important contribution to this paper is a characterization of the photogrammetric image acquisition and image product generation systems. The questionnaire revealed many weaknesses in current processes, but the future prospects of radiometrically quantitative photogrammetry are promising.

Effect of small-scale atmospheric inhomogeneity on Positioning accuracy with GPS

Global Positioning System (GPS) measurements through a field of km-size atmospheric boundary layer (ABL) inhomogeneities with a 10-ppm index of refraction excess have been simulated and inverted. Biases of up to 1–2 cm in height, 1–5 mm in horizontal, and ∼5 mm in zenith tropospheric delay (ZTD) are found, in either static or dynamic atmospheres, using 24-h solutions and estimating ZTD parameters. For 1-h sessions the scatter can increase by a factor of up to 5. These biases are attributed to the inadequacy of standard mapping functions. The use of numerical weather prediction (NWP) models and additional sounding techniques is discussed as a means of improving mapping functions. Raman lidars are thought to offer the highest potential for this purpose and for external calibration of both hydrostatic and wet path delay.

Raman lidar for external GPS path delay calibration devoted to high accuracy height determination

Abstract A scanning Raman lidar for the external correction of the wet path delay of GPS data is under development. Simulations of lidar signal returns for nighttime and daytime operations at 355 and 532 nm show that a millimeter precision on wet path delay is achievable. A simple inversion method to retrieve water vapor and nitrogen concentration profiles is presented. An absolute calibration procedure for instrumental factor and atmospheric transmission is discussed.

Multilateration with the wide-angle laser ranging system: ranging performance and first ground-based validation experiment

A wide-angle laser ranging system has been developed for high-precision multilateration. A laser transmits short pulses in a widely diverging beam toward a network of cube-corner retroreflectors (CCRs). Returned echoes are detected, amplified, digitized, and timed. Impact of instrumental error sources, atmospheric refraction, and scintillation are discussed. The precision of the correlation method for the estimation of times of arrival and a least-squares adjustment method for the estimation of relative CCR coordinates is studied. Data from several ground-based experiments are analyzed. The positioning precision achieved is of 1-2.3 mm in relative radial coordinates and 1.8-3.8 cm in relative transverse coordinates. Results are consistent with a covariance analysis, demonstrating that, as follows: 1) systematic instrumental errors are well corrected; 2) outliers arising from coincident echoes are properly detected; 3) models for forward and inverse problems and estimation techniques are correct. A numerical simulation program based on these models and techniques can be used for optimizing instrumental aspects, providing extensive simulations in various conditions and processing data from future airborne experiments to which the system is devoted.

A wide-angle airborne laser ranging system for millimetre accuracy subsidence measurements

A new geodesy technique based on multilateration is presented. A wide-angle laser ranging system has been developed and validated experimentally. A single-shot accuracy of 1 cm is achieved. A terrestrial experiment conducted with this instrument achieved a relative positioning accuracy of 1-2 mm for a linear network of ground-based retroreflectors. Results are compatible with theoretical predictions and covariance analysis. They validate both the instrument and data processing methods. The next step will be an aircraft experiment over a two-dimensional network. Une nouvelle technique geodesique basee sur le principe de multilateration est presentee. Elle est articulee autour dun telemetre laser en champ large qui a ete valide experimentalement. Une precision en distance relative de 1 cm a ete obtenue. Cet instrument a ete mis en oeuvre dans une experience au sol et a permis datteindre une precision de positionnement relatif dun reseau rectiligne de retroreflecteurs de 1-2 mm. Ces resultats sont compatibles avec des predictions theoriques et une analyse de covariance. Ils valident donc a la fois linstrument et la methode de traitement. La prochaine etape consiste en un test en avion au-dessus dun reseau bidimensionnel.

Calibration of Wet Tropospheric Delays in GPS Observation Using Raman Lidar Measurements

Water vapor measurements from a Raman lidar developed conjointly by the IGN and the LATMOS/CNRS are used for documenting the water vapor heterogeneities and correcting GPS signal propagation delays in clear sky conditions. We use data from four 6 h-observing sessions during the VAPIC experiment (15 May–15 June 2004). The retrieval of zenith wet delays (ZWDs) from our Raman lidar is shown to agree well with radiosonde (0.6 ± 2.5 mm) and microwave radiometers (−6.6 ± 1.2 and 6.0 ± 3.8 mm) retrievals.

Scanning Raman lidar for tropospheric water vapor profiling and GPS path delay correction

The design of a ground based and transportable combined Raman elastic-backscatter lidar for the remote sensing of lower tropospheric water vapor and nitrogen concentration is described. This lidar is intended to be used for an external calibration of the wet path delay of GPS signals. A description of the method used to derive water vapor and nitrogen profiles in the lower troposphere is given. The instrument has been tested during the ESCOMPTE campaign in June 2001 and first measurements are presented.

First aircraft experiment results with the wide-angle airborne laser ranging system

The first aircraft experiment with the Wide-Angle Airborne Laser Ranging System has been conducted in May 1998 over an air base in France equipped with a network of 64 cub-corner retroreflectors. The ranging system was operated from the Avion de Recherche Atmospherique et de Teledetection of CNES/IGN/INSU. Data have been collected during two 4-hour flights. The paper describes the data processing methods and presents the first experimental results. The precision is of 2 cm on the difference of vertical coordinates from two sets of 3 X 103 distance measurements, which is consistent with simulations and a posteriori covariance. The precision is mainly limited by the smallness of the number of efficient measurements remaining after a drastic data sorting for outliers. Higher precision is expected for future experiments after some instrumental improvements (achieving higher link budget) and measurement of aircraft attitude during the flight.