Shuttle Laser Altimeter Research Articles

Towards accurate determination of surface height using modern geoinformatic methods: possibilities and limitations

Over recent decades several modern geoinformatic height-finding methods have emerged, including global positioning system (GPS), interferometric radar, and airborne laser scanner (ALS) or lidar. In conjunction with the conventional survey and photogrammetric method, they have found wide applications that demand varying levels of accuracy. In this paper, the principles of each method are briefly summarized. The discussion then concentrates on the accuracy level achievable with each method. The factors that affect the accuracy, wherever possible, are comprehensively evaluated. This review has revealed that the highest accuracy achievable is still with the levelling method, followed by the photogrammetric method. This situation is likely to change in light of real-time kinematic GPS coupled with ALS. In contrast to the imaging methods that are suited to obtain highly accurate, fine-resolution digital elevation models (DEMs) at a local scale, GPS is the most efficient at obtaining heights at spots or along lines accurately. ALS is the only method applicable to acquisition of subsurface heights in vegetated areas. These airborne methods are complementary to their space-borne counterparts, such as Shuttle Radar Topographic Mapping and Shuttle Laser Altimeter, both being ideal in obtaining DEMs at the regional and global scales. The synergistic use of GPS with lidar offers the best hope in obtaining cm-level accuracies essential for monitoring ground subsidence and tectonic uplift. Height measurements accurate to subcentimetres needed for national levelling surveys are possible only after the tropospheric path delay is externally calibrated using Raman lidar data during GPS data analysis.

Validation of surface height from shuttle radar topography mission using shuttle laser altimeter

Spaceborne Interferometric SAR (InSAR) technology used in the Shuttle Radar Topography Mission (SRTM) and spaceborne lidar such as Shuttle Laser Altimeter-02 (SLA-02) are two promising technologies for providing global scale digital elevation models (DEMs). Each type of these systems has limitations that affect the accuracy or extent of coverage. These systems are complementary in developing DEM data. In this study, surface height measured independently by SRTM and SLA-02 was cross-validated. SLA data was first verified by field observations, and examinations of individual lidar waveforms. The geolocation accuracy of the SLA height data sets was examined by checking the correlation between the SLA surface height with SRTM height at 90 m resolution, while shifting the SLA ground track within its specified horizontal errors. It was found that the heights from the two instruments were highly correlated along the SLA ground track, and shifting the positions did not improve the correlation significantly. Absolute surface heights from SRTM and SLA referenced to the same horizontal and vertical datum (World Geodetic System (WGS) 84 Ellipsoid) were compared. The effects of forest cover and surface slope on the height difference were also examined. After removing the forest effect on SRTM height, the mean height difference with SLA-02 was near zero. It can be further inferred from the standard deviation of the height differences that the absolute accuracy of SRTM height at low vegetation area is better than the SRTM mission specifications (16 m). The SRTM height bias caused by forest cover needs to be further examined using future spaceborne lidar (e.g. GLAS) data.

Combined Infrared Stereo and Laser Ranging Cloud Measurements from Shuttle Mission STS-85

Multiangle remote sensing provides a wealth of information for earth and climate monitoring, such as the ability to measure the height of cloud tops through stereoscopic imaging. Further, as technology advances so do the options for developing spacecraft instrumentation versatile enough to meet the demands associated with multiangle measurements. One such instrument is the infrared spectral imaging radiometer, which flew as part of mission STS-85 of the space shuttle Columbia in 1997 and was the first earth-observing radiometer to incorporate an uncooled microbolometer array detector as its image sensor. Specifically, a method for computing cloud-top height with a precision of ′620 m from the multispectral stereo measurements acquired during this flight has been developed, and the results are compared with coincident direct laser ranging measurements from the shuttle laser altimeter. Mission STS-85 was the first space flight to combine laser ranging and thermal IR camera systems for cloud remote sensing.

Enhanced geolocation of spaceborne laser altimeter surface returns: parameter calibration from the simultaneous reduction of altimeter range and navigation tracking data

The accurate geolocation of a laser altimeter’s surface return, the spot from which the laser energy reflects on the Earth’s surface, is a critical issue 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 (1 σ) level. Vertical accuracies, on the order of 1 m (1 σ) 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.

Improvements In Spaceborne Laser Altimeter Data Geolocation

For many science applications of laser altimetry, the preciselocation 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 combinationwith precise knowledge of spacecraft position, instrument tracking points referenced to thespacecraft center of mass, spacecraft attitude, laser orientation, observation and attitude data timetags. An approach that simultaneously estimates the geometric and dynamic parameters of the orbit andlaser range measurement model by a combined reduction of both spacecraft tracking and laseraltimeter surface range residuals is applied to produce improved pointing, orbit and range bias solutionsand therefore improved geolocation. The data acquired by the Shuttle Laser Altimeter (SLA)-01 and 02missions 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 apreliminary SLA-01 data analysis are presented along with a brief description of the methodology and itsapplication to future spaceborne missions.

A comparison of ocean topography derived from the Shuttle Laser Altimeter-01 and TOPEX/POSEIDON

To assess the utility of laser altimetry for studies in dynamical oceanography, the authors present a comparison of the Shuttle Laser Altimeter (SLA)-01 and the TOPEX/POSEIDON (T/P) radar altimeter on global and regional scales. They compare all /spl sim/1.1 million SLA-01 range measurements over the oceans to the CSRMSS95 gridded mean sea surface model and find the overall root mean square (RMS) difference to be 2.33 m. The misfit was improved significantly by removing the mean and trend from individual SLA-01 profiles, often resulting in RMS differences less than 1 m. They also show that in regions where sea surface height varies dramatically over relatively short horizontal scales such as across major subduction zones, SLA-01 as capable of resolving rapid changes in sea surface height. Finally, they examine a number of coastal zones and illustrate SLA-01's ability to track continuously across the land-sea interface, even in regions of dramatic coastal topography. The dominant sources of radial error in the SLA-01 data are time-interval unit (TIU) error (/spl plusmn/0.75 m) and radial orbit error (1 m RMS). Therefore, limitations of the SLA-01 data set are caused primarily by system constraints as opposed to instrumentation error. Their results indicate that future laser altimeters will provide valuable information regarding ocean topography. In particular, laser altimetry data can be used to supplement orbiting microwave ranging systems in coastal areas where such sensors are incapable of maintaining lock across the continent-ocean transition.

Satellite Laser Altimetry. On-Orbit Calibration Techniques for Precise Geolocation.

In the thirty years since the launch of the Skylab radar altimeter, satellite-based altimetry has proven to be a powerful tool to map the Earth and other planets. In order to fully exploit an orbiting altimeter, it is necessary to calibrate certain parameters not only before launch, but also after the altimeter is 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 Shuttle 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 the several missions that have flown laser altimeters to date and for laser altimeter missions which will launch in the near future.

Observations of the Earth's topography from the Shuttle Laser Altimeter (SLA): Laser-pulse Echo-recovery measurements of terrestrial surfaces

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 Shuttleattitude information, a mid-latitude (+ 28.5° to −28.5°), georeferenced database of topographic ground control 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 instrument in 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.

Space Shuttle Precision Orbit Determination in Support of SLA-1 Using TDRSS and GPS Tracking Data

On January 11, 1996, the Space Shuttle Endeavor, mission STS-72, was launched, carrying aboard the first of four Shuttle Laser Altimeter (SLA) experiments. In support of SLA-1, precise orbits have been computed from Tracking and Data Relay Satellite System (TDRSS) Doppler observations. In some cases, these data were combined with Global Positioning System (GPS) pseudorange observations. Traditionally, the Tracking and Data Relay Satellite (TDRS) orbits themselves have been the dominant source of error in Shuttle orbit determination during quiescent attitude periods. However, a new technique utilizing TOPEX/Poseidon’s (T/P) precise orbit knowledge plus the TDRSS-T/P Doppler tracking is used to significantly reduce the TDRS orbit errors. That approach, along with improved modeling and parameterization have allowed us to compute precise Shuttle orbits from TDRSS-Shuttle Doppler tracking data. Orbit overlap comparisons indicate these Doppler-derived orbits have a meter level (1σ) radial precision, and they agree radially with the combined Doppler and GPS derived orbits at the 1.5 m level (1σ).