Monitoring Glaciers with Airborne and Spaceborne Laser Altimetry

by

James B. Garvin, NASA Goddard Space Flight Center

Abstract

As with all dynamic landscapes, glaciers display telltale morphologic features that provide quantitative evidence of their deformation rates and of the basic physics of their motion. High spatial and vertical resolution topographic information is of unquestionable value when the magnitudes and rates of surficial deformation on active glaciers must be inferred, but prior to the advent of advanced optical and microwave topographic remote sensing devices over the past decade, establishing a time series of topographic measurements for any glacier would have involved an unfathomable amount of effort. Furthermore, traditional topographic measurement techniques are frequently inadequate for describing the diverse suite of topographic elements that are found on the upper surface of a glacier. Measurement of longitudinal cross-sections of active temperate glaciers by means of geodetic airborne laser altimetry was first initiated by a joint team of NASA and USGS scientists in the late 1980's. An early version of the high pulse repetition rate Airborne Terrain Mapper (ATM) profiling laser altimeter was operated from a NASA P-3 aircraft in an effort to quantify the geometry of the terminus regions of several outlet glaciers in southern Iceland. Combining high precision aircraft attitude sensing equipment (laser ring gyros) with kinematic GPS tracking methods to facilitate sub-meter level removal of aircraft platform motions facilitated 15-20 cm RMS vertical accuracy observations of Breiðamerkurjökull (S. Iceland) with an along track sampling frequency of approximately 1 m. Direct detection of all major crevasses was achieved in this pathfinding demonstration of the possibilities associated with low-altitude geodetic airborne profiling laser altimetry. Advances in airborne laser altimeter instrumentation at NASA's Goddard Space Flight Center over the past seven years have provided a variety of approaches for monitoring glaciers on a routine basis. By developing cross-track scanning methods, as well as full echo recovery techniques, swaths of local glacier surface topography can be acquired with 1-5 m sampling scales. Swath widths currently exceed 100 m, and can be as large as 200 m. High altitude laser altimeter systems can synthesize footprints of various sizes (i.e., from 5 to 70 m in diameter), while measuring the complete echo resulting from transmitted laser pulses which interact with various features on glacier surfaces. Most recently, NASA deployed an array of three geodetic airborne laser altimeter sensors to Iceland, Jan Mayen, Svalbard, and Greenland. Included in this flight campaign was the SLICER large-footprint echo recovery lidar, the scanning ATM system, and a 2000 pulses per second ATM Profiler; in addition, the University of Kansas Ice Penetrating HF Radar was included. Both low and high altitude laser altimeter swaths were acquired for active outlet and icecap glaciers in Iceland and on Jan Mayen. Sub-meter accuracy topographic transects of the surging Sylgjujökull glacier (W. Vatnajökull, Iceland), of the Grímsvötn sub-glacial caldera, and of the ice cauldrons in the western portion of Vatnajökull were acquired between 27 May and 2 June, 1996. Laser altimeter profiles made up of footprints at the scale of those to be acquired as part of the ICESAT component of NASA's orbiting Earth Observing System (EOS) were acquired; 30 m and 70 m diameter footprints were obtained in order to quantify how orbital laser altimeter echoes might be used to detect crevassed zones in glaciers at the margin of ice sheets or ice caps. Simultaneous acquisition of dense swaths of scanning laser altimetry will provide the ground truth (from the air). This Geoscience Laser Altimeter System (GLAS) emulation dataset will be released to the Earth Science community in a few months for detailed analysis.

NASA's strategy for monitoring important dynamic landscapes such as glaciers revolves around an array of passive imaging sensors (Landsat's ETM+, MODIS, ASTER, etc.), augmented by GLAS, which will orbit the Earth with the primary objective of sampling the polar icesheets and associated glaciers over a five year period in order to measure the mass balance of these important reservoirs. As presently designed, GLAS is a high vertical precision profiling system with a 70 m diameter footprint and with a sub-10 cm vertical precision. The GLAS sensor will be placed into a repeat orbit in ~ 2002, and in combination with the time series dataset of airborne laser altimeter observations of the Greenland ice sheet (i.e., under development since 1991 by R. Thomas and W. Krabill), it should provide the world glacier monitoring community with a potentially rich dataset of extremely precise profiles and spot elevation measurements for many high-latitude glaciers. With the apparent approval of the Shuttle Radar Topography Mission or SRTM, the prospects for 30 m per pixel digital elevation models of all continental glaciers lying between 60 N and 60 S latitudes are rapidly improving. As planned, the SRTM mission is to fly in late 1999 or early 2000, utilizing a novel C-band radar interferometry approach (InSAR) to produce nearly global DEM coverage of the Earth's land areas. While the vertical resolution of the InSAR DEM's will probably be ~ 10-15 m, the seamless 30 m gridding of this dataset could serve as an important foundation for future monitoring studies of temperate glaciers.

Further opportunities for spaceborne topographic remote sensing of glacier systems worldwide are emerging; NASA's fledgling Earth System Science Pathfinder (ESSP) program will solicit proposals for cost-constrained missions every two years, and the first selections were made in February 1997, with two missions to be launched around 2000. Polar orbiting satellite remote sensing systems are either in place or soon will be, some of which are ideally suited for glacier topographic and spatial monitoring. The Canadian RADARSAT system is a prime example. Indeed, Garvin, Williams and others will be utilizing monthly RADARSAT C-band SAR images of Iceland and Jan Mayen to track the spatial fluctuations of particularly dynamic glaciers in these regions over a three year period.

As a pathfinder to the next generation of orbital laser altimeter systems for Earth remote sensing, NASA recently flew the first of four Shuttle Laser Altimeter (SLA) experiments. In January of 1996, the SLA-01 experiment acquired over 80 hours of Earth surface observations during its week-long mission. While SLA-01 was restricted to an equatorial orbit (+28.5 N. and S. latitudes), orbital observations of ice covered surfaces in the Himalayas, at the summit of Mauna Kea volcano, and in other high relief mid-latitude localities suggests that future SLA spaceflight experiments could contribute to sub-arctic glacier topographic monitoring studies. Indeed, the SLA-02 experiment was successfully carried out as part of Space Transportation System-85 (STS-85) in August 1997 in a 57 inclination orbit. SLA-02 was the first spaceflight demonstration of a 100 pulses per second (i.e., sub-100 m along track sampling) laser altimeter system in space, and will pave the way for future orbital scanning laser altimeter sensors. Designs are also underway at NASA for a wide-swath airborne laser altimeter system to be known as the Laser Vegetation Imaging Sensor (LVIS) (c.f. chief engineer J. Bryan Blair), which will be able to achieve 0.5 to 1.0 km wide swaths of sub-meter precision topography from moderate to high altitude aircraft platforms. The LVIS system could be utilized to develop baseline topographic reference maps (DEM's) of climatically important glacier systems in North America, Iceland, Greenland etc. over the next few years.

Finally, NASA is presently supporting a pathfinder study of the changing ice volume at the Mt. Rainier stratovolcano. This investigation, under the leadership of J.B. Garvin, involves the combination of geodetic airborne laser altimetry with airborne InSAR on an annual basis in order to quantify and model the volume changes in ice and snow deposits in the summit region of the edifice. A successful 1995 field season resulted in a laser altimeter model of the volume of the summit of Rainier (Sept. 1995), and airborne InSAR DEM's from August of 1995 are now in hand. Early evidence suggests that volumetric changes from 1994 to 1995 are insignificant. At the conclusion of this 3-year study, a geodetic database of annual laser altimeter measurements of the summit of Rainier will be available, as well as mathematical models of the summit topology and volume.

NASA has a profound interest in developing new remote sensing tools for monitoring dynamic landscapes and especially those that are coupled to short term climate change.

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