NASA - National Aeronautics and Space Administration

Detailed topographic maps of very high accuracy are produced by airborne laser altimeter terrain mapping. The unique capabilities of this new technique yield more comprehensive and precise topographic information than traditional methods. Airborne laser altimeter data can be used to accurately measure the topography of the ground, even where overlying vegetation is quite dense. The data can also be used to determine the height and density of the overlying vegetation, and to characterize the location, shape, and height of buildings and other manmade structures.

The method relies on measuring the distance from an airplane, or helicopter, to the Earth’s surface by precisely timing the round-trip travel time of a brief pulse of laser light. The travel-time is measured from the time the laser pulse is fired to the time laser light is reflected back from the surface. The reflected laser light is received using a small telescope that focuses any collected laser light onto a detector. The travel-time is converted to distance from the plane to the surface based on the speed of light. Typically a laser transmitter is used that produces a near-infrared laser pulse that is invisible to humans. The laser light reaching the ground surface is completely safe. It can not cause any eye damage to a person who might be looking up at the plane as it flies overhead.

In July 2005, the Johns Hopkins University, Applied Physics Laboratory began “Pathfinder Airborne Radar Ice Sounder (PARIS)” funded under the NASA Instrument Incubator Program (IIP). The primary objective of this project was the first feasibility demonstration of successful radar sounding of ice sheet layering and bottom topography from a high-altitude platform. Major contributing factors included a high-fidelity 150-MHz radar, supported by along-track partially- coherent processing. “High-fidelity” implies very wide dynamic range, extreme linearity, and very low sidelobes generated by the transmitted pulse. “Partially- coherent processing” implies the delay-Doppler technique, previously proven in airborne radar altimeter and low-altitude radar ice sounding embodiments. The radar was mounted on the NASA P-3, and deployed on a mission over the Greenland ice sheet in the spring of 2007. Data were recorded on board as well as displayed in flight on a quick-look processor. The data subsequently were processed in the laboratory to quantify performance characteristics, including dynamic range, sidelobe level control, and contrast improvement from the delay-Doppler algorithm.

The transmit waveform is a 5-MHz bandwidth chirp at a 150-MHz operating frequency with a trapezoidal envelope. Such severe weighting is essential to reduce the ringing commonly associated with the initial on-off transition of weakly-weighted waveforms. The 180-W (peak) linear-FM pulse has ~6 MHz bandwidth. The amplifier is class AB to help ensure the high linearity needed to suppress the internal clutter (sidelobes) generated by the chirp waveform. Laboratory measurements of the driver and power amplifier show excellent linearity with a two-tone third-order inter-modulation of at least -26 dBc at peak power.

There is no down conversion or IF signal within the receiver, greatly simplifying the design, and eliminating most potential sources of distortion and intermodulation. Upon reception, the radar A/D operates on the RF signal directly out of the LNA. The sample rate is well below Nyquist, but it is chosen so that the resulting spectra shift an alias of the main signal to offset baseband in a clear channel. The receiver includes variable attenuators to adjust the voltage range of the signal input to the analog-to- digital converter as well as sensitivity time control (STC) to increase the effective dynamic range of the response as a function of depth of penetration. The overall noise figure of the receiver is less than 5.5 dB with a gain of over 60 dB and a 45 dBm third-order intercept point.

The digital components consist of a field programmable gate array (FPGA) radar synchronizer, a direct digital synthesizer (DDS), and an under-sampling analog-to-digital converter (ADC). All components of the digital subsection are clocked by a stable 66.6 MHz reference oscillator. The radar data are time-tagged by reference to GPS. The flights included passes over the summit ridge, from which results show internal layering, and the bottom profile at several km depth.

Developed by Dr. John Degnan under the Instrument Incubator Program, the MMLA is designed to detect single photon returns reflected from targets of interest and determine their height. This instrument is comprised of an optical bench, transmit and receive optics, computer-controlled iris, spatial and spectral filters, stray-light baffles, interface optics to a micro-laser transmitter, photo detector, and CCD camera.

Critical to progress in understanding and modeling ice sheets are a better characterization of what ice sheets are doing at present, how fast they are changing, what are the driving processes controlling these changes, and how we can better represent these processes in numerical models to derive more realistic predictions of the evolution of glaciers and ice sheets in the future. Chief among these measurements, are detailed, enhanced and sustained measurements of ice sheet elevation, at high spatial resolution, with high vertical accuracy, over the entire ice sheets. These measurements provide critical information about long-term ice sheet dynamics (mass balance trends) and short-term variability (precipitation, ablation events, surface lowering of an accelerating glacier, etc.).

Ideally suited to making these measurements is GLISTIN, a Ka-band single pass interferometric synthetic aperture radar (InSAR). Proposed also as a spaceborne mission concept [1], the airborne GLISTIN-A serves as a proof-of-concept demonstration and science sensor. Key features include:

1. The Ka-band center frequency maximizes the single-pass interferometric accuracy (which is proportional to the wavelength), reduces snow penetration (when compared with lower frequencies), and remains relatively impervious to atmospheric attenuation.

2. Imaging capabilities that are important for mapping large areas. Imaging allows features to be tracked with time for estimation of ice motion and reduces data noise when measuring topographic changes over rough surfaces of glaciers and coastal regions of ice sheets.

GISMO is a concept for a spaceborne radar system designed to measure the surface and basal topography of terrestrial ice sheets and to determine the physical properties of the glacier bed. Our primary objective is to develop this new technology for obtaining spaceborne estimates of the mass of the polar ice sheets with an ultimate goal of providing essential information to modelers estimating the mass balance of the polar ice sheets and estimating the response of ice sheets to changing climate. Our technology concept employs VHF and P-band interferometric radars using a novel clutter rejection technique for measuring the surface and bottom topographies of polar ice sheets. Our approach will enable us to reduce signal contamination from surface clutter, measure the topography of the glacier bed, and paint a picture of variations in bed characteristics. The technology will also have applications for planetary exploration including studies of the Martian ice caps and the icy moons of the outer solar system. We have recently shown that it is possible to image a small portion of the base of the polar ice sheets using a SAR approach. Through the concept developed here, we believe that, for the first time, we can image the base and map the 3-dimensional basal topography beneath an ice sheet at up to 5 km depth.

GISMO is a NASA Instrument Incubator Project.

The Airborne Topographic Mapper (ATM) is a scanning LIDAR developed and used by NASA for observing the Earth's topography for several scientific applications, foremost of which is the measurement of changing arctic and antarctic icecaps and glaciers. It typically flies on aircraft at an altitude between 400 and 800 meters above ground level, and measures topography to an accuracy of ten to twenty centimeters by incorporating measurements from GPS (global positioning system) receivers and inertial navigation system (INS) attitude sensors.

The ATM instruments are based at NASA's Wallops Flight Facility (WFF) in Virginia. They commonly fly aboard the NASA P3-B based at WFF and have flown aboard other P-3 aircraft, the NASA DC-8, several twin-otters (DHC-6), and a C-130; they can fly on most Twin Otter/King Air-class aircraft. The ATM has flown surveys in Greenland nearly every year since 1993. Other uses have included measurement of sea ice, verification of satellite radar and laser altimeters, and measurement of sea-surface elevation and ocean wave characteristics. The altimeter often flies in conjunction with a variety of other instruments. The ATM has been participating in NASA's Operation IceBridge since 2009.

The Airborne Multi-angle Imaging SpectroRadiometer (AirMISR) is an airborne instrument for obtaining multi-angle imagery similar to that of the satellite-borne Multi-angle Imaging SpectroRadiometer (MISR) instrument, which is designed to contribute to studies of the Earth's ecology and climate. AirMISR flies on the NASA ER-2 aircraft. The Jet Propulsion Laboratory in Pasadena, California built the instrument for NASA.

Unlike the spaceborne MISR instrument, which has nine cameras oriented at various angles, AirMISR utilizes a single camera in a pivoting gimbal mount. A data run by the ER-2 aircraft is divided into nine segments, each with the camera positioned to a MISR look angle. The gimbal rotates between successive segments, such that each segment acquires data over the same area on the ground as the previous segment. This process is repeated until all nine angles of the target area are collected. The swath width, which varies from 11 km in the nadir to 32 km at the most oblique angle, is governed by the camera's instantaneous field-of-view of 7 meters cross-track x 6 meters along-track in the nadir view and 21 meters x 55 meters at the most oblique angle. The along-track image length at each angle is dictated by the timing required to obtain overlap imagery at all angles, and varies from about 9 km in the nadir to 26 km at the most oblique angle. Thus, the nadir image dictates the area of overlap that is obtained from all nine angles. A complete flight run takes approximately 13 minutes.