Geodesy Papers | Ice Papers | Vegetation Papers

Geodesy-Lidar Related Publications

Abstract. Meter-precision topographic measurements of a diverse suite of terrestrial surfaces have been accomplished from Earth orbit using the Shuttle Laser Altimeter (SLA) instrument flown aboard the Space Shuttle Endeavour in January of 1996. Over three million laser pulses were directed at the Earth by the SLA system during its ~80 hours of nadir-pointing operation at an orbital altitude of 305 km (+/- 10 km). Approximately 90% of these pulses resulted in valid range measurements to ocean, land, and cloud features. Of those which were fired at land targets, 57% resulted in valid surface ranges, the remainder being cloud tops, false alarms, or missed shots. The SLA incorporated an electronic echo-recovery system into a pulsed, time-of -flight laser altimeter instrument in order to capture and characterize the vertical structure within each 100 m diameter surface footprint. The echoes recorded by SLA demonstrate aspects of the vertical structure of the nearly ubiquitous vegetation cover on the planet, as well as sensitivity to local slopes, surface reflectivity, and vertical ruggedness. With a vertical resolution of 0.75 m and horizontal sampling at 0.7 km length scales, SLA provides a new form of high vertical accuracy topographic data for studying problems related to the dynamics of the Earth's surface. Assessment of the error budget associated with the SLA experiment suggests that ~2.8 m (RMS) precision was achieved for ranging measurements to oceanic surfaces, for which there are over 700,000 examples. With the availability of a precision radial orbit and post-flight Shuttle attitude information, a mid-latitude (+ 28.50 to -28.5°), georeferenced database of topographic groundcontrol point elevations has been achieved using SLA data, consisting of ~344,000 land measurements. Each of these measurements is geolocated to within 1-2 SLA footprints(100-200 m) on the Earth's surface, with vertical errors that approach the limits of resolution (0.75 m) of the instrumentin topographically benign regions. When compared to available Digital Elevation Models (DEM's) with stated vertical accuracies on the order of 10-16 m, SLA's measurements differ by no more than 11 m to 46 m RMS in rugged terrain. We have computed a total vertical roughness parameter for all multi-peaked SLA echoes using a multi-Gaussian decomposition technique and have observed a very high degree of correlation of this parameter with global landcover classes. In some cases (~6%), SLA echoes clearly resolve both the ground surface and vegetation canopy within a single footprint, suggesting that the modal height of equatorial vegetation is ~18 m. The global distribution of total vertical roughness varies from ~5 m to 60 m, with a mean value of 27 m and a standard deviation of 12 m. SLA successfully served as a pathfinder for high vertical resolution orbital topographic remote sensing instrumentation, and demonstrated the first high resolution echo-recovery laser altimeter observations over land surfaces.

Abstract. Altimetry from the Mars Observer Laser Altimeter (MOLA), an instrument on board the Mars Global Surveyor (MGS) spacecraft, has been analyzed for the period of the MGS Science Phasing Orbit-1 (SPO-1) mission phase. Altimeter ranges have been used to improve significantly the orbit and attitude knowledge of the spacecraft by the use of crossover constraint equations derived from short passes of the MOLA data. These constraint equations differ from traditional crossover constraints and exploit the small footprint associated with laser altimetry. The rationale for using this technique with laser altimetry over sloping terrain is laid out and evidence of the resulting benefit is presented.

Abstract. In the thirty years since the launch of the Skylab radar altimeter, satellite-based altimetry has proven to be a pwoerful tool to map the Earth and other planets. In order to fully expoit an orbiting altimeter, it is necessary to calibrate certain parameters not only before launch, but also after the altimeter is already in orbit. Over the years, techniques have been worked out for on-orbit calibration of radar altimeters. Our use of Earth-orbiting satellite laser altimetry began in 1996 with the Satellite Laser Altimeter. Although laser altimetry presents unique opportunities, it also requires new on-orbit calibration techniques. These techniques are still evolving and include the integration of multiple tracking data types with planned pointing maneuvers over oceans and waveform analysis. This paper describes on-orbit calibration techniques for several missions that have flown laser altimeters to date and for laser altimeter missions which will launch in the near future.

Abstract. For many science applications of laser altimetry, the precise location of the point on the Earth’s surface from which the laser energy reflects is required. The laser surface return geolocation is computed from the laser altimeter’s range observation in combination with precise knowledge of spacecraft position, instrument tracking points referenced to the spacecraft center of mass, spacecraft attitude, laser orientation, observation and attitude data time tags. An approach that simultaneously estimates the geometric and dynamic parameters of the orbit and laser range measurement model by a combined reduction of both spacecraft tracking and laser altimeter surface range residuals is applied to produce improved pointing, orbit and range bias solutions and therefore improved geolocation. The data acquired by the Shuttle Laser Altimeter (SLA)-01 and 02 missions constitute a valuable pathfinder data set to test algorithms in preparation for the upcoming VCL (Vegetation Canopy Lidar) and ICESat (Ice, Cloud and Elevation Satellite) missions. Results from a preliminary SLA-01 data analysis are presented along with a brief description of the methodology and its application to future spaceborne missions.

Abstract. The accurate geolocation of a laser altimeter’s surface return, the spot frm which the laser energy reflects on the Earth’s surface, is a critical isse in the scientific application of these data. Pointing, ranging, timing and orbit errors must be compensated to accurately geolocate these data. Detailed laser altimeter measurement models have been developed and implemented within precision orbit determination software providing the capability to simultaneously estimate the orbit and geolocation parameters from a combined reduction of altimeter range and spacecraft tracking data. In preparation for NASA'’s future dedicated Earth observing spaceborne laser altimeter missions, the Vegetation Canopy Lidar (VCL) and the Ice, Cloud and land Elevation Satellite (ICESat), data from two Shuttle Laser Altimeter (SLA) missions have been reprocessed to test and refine these algorithms and to develop the analysis methodologies for the production and verification of enhanced geolocation products. Both direct altimetry and dynamic crossover data have been reduced in combination with navigation tracking data to obtain significant improvement in SLA geolocation accuracy. Residual and overlap precision tests indicate a factor of two improvement over the previously released SLA Standard Data Products, showing 40-m RMS horizontal and 26-cm RMS elevation geolocation precision for the long SLA-01 arcs. Accuracy estimates by comparing SLA profiles to Digital Elevation Models show horizontal positioning accuracy at the 60-m (1σ) level. Vertical accuracies, on the order of 1 m (1σ) for low slope surfaces are now dominated by the ± 75-cm one-way range resolution of the instrument. Comparable relative improvements are also observed in the analysis of the SLA-02 data. The analyses show that complex temporal variations in parameters (i.e., pointing) can be recovered and not just simple biases. The methodology and results obtained from the detailed analysis are discussed in this paper, along with their applicability to VCL and ICESat.

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