Jack L. Bufton

Laser altimetry measurements from aircraft and spacecraft

The techniques involved in the design and application of laser altimeter instruments are reviewed, including a description of the instrument subsystems required for the range and waveform measurements. Laser pulse transmitters based on the relatively novel technology of diode-pumped solid-state lasers are considered. Various factors affecting laser altimeter instrument performance are discussed. These include the receiver signal-to-noise ratio, atmospheric propagation, and altimeter platform effects. Some examples of laser altimeter data are presented to illustrate the variety of possible instrument applications. >

Airborne measurements of laser backscatter from the ocean surface

Laser backscatter data for the ocean surface near nadir have been acquired from an airborne lidar platform. The unique capability of this lidar instrument to scan both transmitted laser beam and receiver field of view up to 15° off nadir have made these data sets possible. Backscatter data were collected on eight separate missions using laser wavelengths at 337 and 532 nm and 9.5 μm. Statistics of the mean, standard deviation, and probability density function of backscatter were computed and analyzed in terms of prior analytical work that relates backscatter to wind speed and mean-square wave-slope statistics. We found the full width at half-maximum of the Gaussian-shaped mean backscatter pattern to range from 11 to 24° and the normalized standard deviation at a nadir-viewing angle to range from 0.1 to 0.6. We calibrated mean backscatter at nadir for the ocean surface in terms of an effective Lambertian reflectance by comparison of beach sand and ocean backscatter. Results were 16 and 24% reflectance on two missions where calibration was possible. Our data are compared with prior laser backscatter measurements and the general literature on optical scattering from the ocean surface.

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 Earths 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 Earths 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 (DEMs) with stated vertical accuracies on the order of 10–16 m, SLAs 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.

Comparison of Vertical Profile Turbulence Structure with Stellar Observations

Vertical profiles of microthermal turbulence structure have been obtained from balloon flights to altitudes near 25 km above mean sea level. Comparison of the observed turbulence structure with meteorological data and simultaneously acquired stellar scintillation data has been successful. For this comparisonintegrals over the observed turbulence structure were computed as required by theory. Turbulence and wind velocity data were also employed to predict successfully stellar irradiance spectra.

Measurements of Turbulence Profiles in the Troposphere

Temperature structure coefficients were measured with balloon-borne temperature sensors. Data converted to refractive-index-structure coefficients are reported. These extend knowledge of this coefficient to the upper troposphere. The results are discussed with reference to possible meteorological origins for turbulence.

Airborne lidar for profiling of surface topography

A lidar system is described that measures laser pulse time-of-flight and the distortion of the pulse waveform for reflection from earth surface terrain features. This instrument system is mounted on a high-altitude aircraft platform and operated in a repetitively pulsed mode for measurements of surface elevation profiles. The laser transmitter makes use of recently developed short-pulse diode-pumped solid-state laser technology. Aircraft position in three dimensions is measured to submeter accuracy by use of differential Global Positioning System receivers. Instrument construction and performance are detailed.

The Geoscience Laser Altimetry/Ranging System

The Geoscience Laser Altimetry/Ranging System (GLARS) is a planned highly precise laser distance-measuring system to be used for geoscience measurements requiring extremely accurate geodetic observations from a space platform. The system combines the attributes of a pointable laser ranging system making observations to retroreflectors placed on the ground with those of a nadir-looking laser altimeter making height observations to ground, ice sheet, and oceanic surfaces. In the ranging mode, centimeter-level precise baseline and station coordinate determinations will be made on grids consisting of 100 to 200 targets separated by distances from a few tens of kilometers to about 1000 km. These measurements will be used for studies of seismic zone crustal deformations and tectonic plate motions. Ranging measurements will also be made to a coarser, but globally distributed, array of retroreflectors for both precise geodetic and orbit determination applications. In the altimetric mode, relative height determinations will be obtained with approximately decimeter vertical precision and 70-100-m horizontal resolution. Altimetric profiles consisting of nearly contiguous spots will be available when the system is operated at 40 pulses per second. The height data will be used to study surface topography and roughness, ice sheet and lava flow thickness, and ocean dynamics. Waveform digitization will provide a measure of the vertical extent of topography within each footprint.

Aerosol and cloud backscatter at 1.06, 1.54, and 0.53 µm by airborne hard-target-calibrated Nd:YAG/methane Raman lidar

A lidar instrument was developed to make simultaneous measurements at three distinct wavelengths in the visible and near infrared at 0.532, 1.064, and 1.54 mum with high cross-sectional calibration accuracy. Aerosol and cloud backscatter cross sections were acquired during November and December 1989 and May and June 1990 by the NASA DC-8 aircraft as part of the Global Backscatter Experiment. The instrument, methodology, and measurement results are described. A Nd:YAG laser produced 1.064- and 0.532-mum energy. The 1.54-mum transmitted pulse was generated by Raman-shifted downconversion of the 1.064-mum pulse through a Raman cell pressured with methane gas. The lidar could be pointed in the nadir or zenith direction from the aircraft. A hard-target-based calibration procedure was used to obtain the ratio of the system calibration between the three wavelengths, and the absolute calibration was referenced to the 0.532-mum lidar molecular backscatter cross section for the clearest scattering regions. From the relative wavelength calibration, the aerosol backscatter cross sections at the longer wavelengths are resolved for values as small as 1% of the molecular cross section. Backscatter measurement accuracies are better than 10(-9) (m sr)(-1) at 1.064 and 1.54 mum. Results from the Pacific Ocean region of the multiwavelength backscatter dependence are presented. Results show extensive structure and variation for the aerosol cross sections. The range of observed aerosol cross section is over 4 orders of magnitude, from less than 10(-9) (m sr)(-1) to greater than 10(-5) (m sr)(-1).

Satellite laser altimetry of terrestrial topography: vertical accuracy as a function of surface slope, roughness, and cloud cover

Analysis of the sensitivity of laser ranging errors to surface conditions indicates that predicted single-shot range errors are primarily dependent on surface slope. Range errors are less sensitive to variations in surface roughness or reflectivity. Values of total surface slope and roughness for nine terrestrial landforms, derived from digital elevation data at a 186 m length scale, vary from 2/spl deg/ to 40/spl deg/ and 0.8 to 15 m, respectively, at a 90% frequency of occurrence. This range of surface morphologies yields a variation in single shot laser ranging error from 0.4 to 8 m, assuming system parameters for the proposed Topographic Mapping Laser Altimeter (TMLA) and a nominal 30% surface reflectivity. The total elevation accuracy of data obtained via satellite laser altimetry, although dominated by the range error, is also a function of additional error sources, including orbit ephemeris, atmospheric, and calibration errors. Averaging of multiple laser measurements improves the vertical accuracy of the elevation data by statistical reduction of random errors. During a three-year mission, two to three laser measurements will be acquired, on average, for each 200-m footprint at low to moderate latitudes, accounting for the latitudinal variation of ground track spacing and cloud cover. For high-latitude regions, the narrow spacing of satellite ground tracks in a polar orbit will provide frequent repeat observations yielding, on average, 4 to 25 measurements of each footprint over the Antarctic and Greenland ice sheets. Averaging of these multiple repeat observations at high latitude will yield an improvement in vertical accuracy by a factor of two to five. >

Scintillation statistics measured in an earth–space–earth retroreflector link

An experiment is described for the measurement of scintillation in a vertical path from an earth-based laser transmitter to the GEOS-III satellite and back to an earth-based receiver telescope. Measurements of the normalized variance, probability density function, and power spectral density of scintillation are presented. These results are discussed in terms of recent analytical results.