Airborne laser altimeter systems have been recently developed by NASA engineers and scientists which facilitate sub-meter accuracy (and 1-10 cm precision) measurements of Earth surface topography at spatial sampling scales as small as 1 m, and typically in the 2-15 m range (ie. footprint diameters 2 to 15 m in size, with contiguous coverage along track).
NASA airborne laser altimeter systems are now capable of measuring the round-trip travel time of individual laser pulses as well as the backscattered laser "echoes" that are received by the altimeter receiver electronics. Indeed, current NASA efforts are centered around using the acquired laser echoes and various algorithms to quantify within-footprint local slopes, roughness, albedo variations, and perhaps most importantly, the vertical structure of any vegetation (ie. trees) within individual footprints. Efforts to develop state of the art echo recovery methods are led by Dr. David Harding, and engineering aspects of this new capability are spearheaded by J. Bryan Blair, both of NASA's Laboratory for Terrestrial Physics. The SLICER instrument has been designed to accomodate full echo recovery in a narrow multibeam swath of topography, and will be used in 1995 to comprehensively measure the summits of Mt. Rainier and Mt. Saint Helens. Measuring the ground beneath the trees on the flanks of dormant volcanoes has proven to be difficult with most topographic remote sensing approaches, so the NASA Laser Altimeter Facility (SLICER and RASCAL) will explore how well the sub-canopy topography of volcanoes can be extracted from SLICER data.
In order to make airborne laser altimetry (ALA) a viable remote sensing approach for quantifying and monitoring the complex topography of large volcanoes, extremely careful measurement of the platform attitude and trajectory is required. Kinematic GPS (dGPS) methods have recently demonstrated sub-meter positional and vertical precisions associated with laser altimeter transects across Greenland, and sub-meter vertical accuracies are now achievable. NASA aircraft have been able to refly transects to within a few tens of meters on the ground for long distances using a GPS-based aircraft navigational system developed by Wayne Wright of NASA's Wallops FLight Facility, and William Krabill and colleagues at NASA/WFF have developed capabilities for producing 20 cm (vertical) trajectories for aircraft platforms using dGPS methods.
Laser altimetry requires a clear line of sight to the intended target, precision pointing knowledge (i.e., better than 0.1 degree in roll,pitch, and yaw), and accurate knowledge of the platform trajectory relative to a center of mass coordinate reference frame. Multibeam laser altimeter systems such as SLICER (now 3-5 beams, but to be expanded to 20 beams in early 1996) and RASCAL (around 66 beams across track at low altitude) coupled to dGPS tracked aircraft with ring laser gyro systems now make routine topographic monitoring of deforming or otherwise active surfaces possible. This approach is now being adopted as part of NASA projects associated with active or recently active volcanoes such as Rainier and Mt. St. Helens, as well as Pico (Azores) and others.
In order to facilitate objective and systematic comparisons of laser altimeter datasets for complex volcanoes, it is possible to employ mathematical technique to "model" the observed topography of the volcano in question. By acquiring a radial array of profiles or narrow swaths of topography for a volcano such as Rainier, a cylindrical coordinate frame can be established, and the azimuthally and radially varying topography can then be fit using Bessel functions of the second kind. This Cylindrical Harmonic Analysis Technique (CHAT) permits purely objective comparison of derived properties of volcanoes, including total volume, surface area, etc. Garvin [1995] describes the method in the context of a select few volcanoes (Garvin, 1995, in J. Geol. Soc. London).
Other terrain analysis methods can be used to interpret high spatial resolution cross-sections of volcanoes, including analysis of the slope frequency distribution for each feature, computation of local roughness from laser altimeter echoes, and polynomial shape analysis. See Garvin [1995] and other references for more details.
Laser altimeter datasets for various volcanoes under study by NASA scientists include: Mt. St. Helens, Crater Lake (Mt. Mazama), Pico (Azores), Skjaldbreidur (Iceland), Surtsey (Iceland), Capelhinos (Azores), and in the future, plans call for laser altimeter topographic measurements of Hawaiian volcanoes, those of Mexico (Colima, Popocatepetl), and others in the Azores and Icelandic islands.
The panchromatic and multispectral satellite imaging system developed by the French is called SPOT, and presently SPOT 2 and 3 satellites are in order collecting orbital imaging data in the visible and near-IR portions of the spectrum at 20 m (multispectral) and 10 m spatial resolution for 60 x 60 km areas. Because the SPOT satellite has the flexibility to tilt its sensors, stereoscopic imaging is possible, and we are attempting to employ stereogrammetry of a pair of SPOT panchromatic scenes of the FOGO caldera in the Cape Verde Islands off of the western coast of Africa. For more details of the SPOT digital imaging system, please refer to the SPOT web page.
This sensor is to be flown aboard Endeavor on STS-72, presently scheduled for 30 Nov. 1995 lift-off. Although SLA-01 represents an engineering pathfinder for Earth orbital laser altimetry of land, ocean, ice and clouds, it offers the potential of longer wavelength topographic coverage of relatively inaccessible volcanic areas in Africa, Australia, and South America. SLA will acquire topographic profiles made up of 100 m diameter footprints spaced every 750 m along the Shuttle Orbiter's nadir track, and after removal of a radial orbit and correction for pointing, topographic profiles with 10 m (avg.) precision should be achieve, with potentially 2-5 m quality data in areas where relief is lower and orbital/attitude data is of highest quality. Dr. Jack Bufton is the Principal Investigator and Chief Engineer for SLA and J. Garvin is the Lead scientist.