Airborne Radar Altimeter Research Articles

Effect of the Local Backscattering Pattern of the Sea Surface to the Reflected Signal of a Precision Airborne Radar Altimeter at Low Altitude

When designing a high-precision airborne radar altimeter, the approach of an analysis of the characteristics of radio signals reflected from a statistically rough sea surface based on its steady-state model becomes invalid. In the measurement of the sea surface profile, a characteristic of the reflection properties is the local backscattering pattern of the radar cross section, which depends on the time and the relative position of the low-altitude carrying vehicle from the surface. The effective width of the local backscattering pattern is determined by the statistical characteristics both of the small-scale roughness and the large-scale roughness of the surface. The location of the maximum value of the local backscattering pattern depends on the derivative of the large-scale roughness of the sea surface, i.e., the shifts in proportion to the slopes of the large-scale surface roughness. These effects must be considered when analyzing the measurement errors of a low-altitude radar altimeter.

A First Assessment of IceBridge Snow and Ice Thickness Data Over Arctic Sea Ice

We present a first assessment of airborne laser and radar altimeter data over snow-covered sea ice, gathered during the National Aeronautics and Space Administration Operation IceBridge Mission. We describe a new technique designed to process radar echograms from the University of Kansas snow radar to estimate snow depth. We combine IceBridge laser altimetry with radar-derived snow depths to determine sea ice thickness. Results are validated through comparison with direct measurements of snow and ice thickness collected in situ at the Danish GreenArc 2009 sea ice camp located on fast ice north of Greenland. The IceBridge instrument suite provides accurate measurements of snow and ice thickness, particularly over level ice. Mean IceBridge snow and ice thickness agree with in situ measurements to within ~ 0.01 and ~ 0.05 m, respectively, while modal snow and ice thickness estimates agree to within 0.02 and 0.10 m, respectively. IceBridge snow depths were correlated with in situ measurements (R = 0.7, for an averaging length of 55 m). The uncertainty associated with the derived IceBridge sea ice thickness estimates is 0.40 m. The results demonstrate the retrieval of both first-year and multiyear ice thickness from IceBridge data. The airborne data were however compromised in heavily ridged ice where snow depth, and hence ice thickness, could not be measured. Techniques developed as part of this study will be used for routine processing of IceBridge retrievals over Arctic sea ice. The limitations of the GreenArc study are discussed, and recommendations for future validation of airborne measurements via field activities are provided.

Assessment of specular radar backscatter from a planar surface using a physical optics approach

Satellite and airborne radar altimeter return signals often contain the specular component primarily in polar regions due to the backscatter from a relatively smooth and planar water or ice surface. It is demonstrated here that the specular backscatter from a planar surface should be estimated taking into account the spherical nature of the incident electromagnetic wave. This article introduces the radar equation derived using the physical optics approach and describes the backscatter as a function of the spatial distribution of the complex Fresnel reflection coefficient across a planar surface. The closed form of the radar equation was obtained for an infinite surface, an annulus and a disk with sizes greater than the radius of the first Fresnel zone.

Spatially extensive estimates of annual accumulation in the dry snow zone of the Greenland Ice Sheet determined from radar altimetry

Abstract. We present estimates of accumulation rate along a 200 km transect ranging in elevation from 2750 to 3150 m in the dry snow zone on the western slope of the Greenland Ice Sheet. An airborne radar altimeter is used to estimate the thickness of annual internal layers and, in conjunction with ground based snow/firn density profiles, annual accumulation rates between 1998 and 2003 are derived. A clear gradient in the thickness of each layer observed by the radar altimeter and in the associated estimates of annual accumulation is seen along the transect, with a 33.6% ± 16% mean decrease in accumulation from west to east. The observed inter-annual variability is high, with the annual mean accumulation rate estimated at 0.359 m.w.e. yr−1 (s.d. ± 0.049 m.w.e. yr−1). Mean accumulation rates modelled using meteorological models overestimate our results by 16% on average, but by 32% and 42% in the years 2001 and 2002. The methodology presented here demonstrates the potential to obtain accurate and spatially extensive accumulation rates from radar altimeters in regions of ice sheets where field observations are sparse, and accumulation rates greater than several tens of cm.

Measurement of the Wind Vector Over Sea by an Airborne Radar Altimeter Using an Antenna Characterized by Different Beamwidth in the Vertical and Horizontal Planes

In this letter, a possibility of recovering the wind vector over sea using an airborne radar altimeter in a short-pulse scatterometer mode is discussed. The nadir-looking wide-beam antenna, with the modified beam shape forming an ellipse footprint, is assumed for the purpose of analysis. Simultaneous range Doppler discrimination techniques are also used and a measuring algorithm of the sea-surface wind speed and direction is proposed.

Combined airborne laser and radar altimeter measurements over the Fram Strait in May 2002

Knowledge of sea ice thickness is critical for the prediction of future climate, and for assessing the significance of changes in thickness. Sea ice thickness can be calculated from radar or laser satellite altimetry measurements of freeboard. However, a lack of knowledge of snow depth introduces significant uncertainties into these calculations. This paper compares the first coincident airborne laser and radar altimetry data over sea ice, collected during the Laser Radar Altimetry (LaRA) field campaign. LaRA was a flight of opportunity that provided valuable data to explore techniques to validate satellite measurements of ice freeboard, and the possibility of combining laser and radar measurements over snow covered sea ice to calculate the snow depth. Two new methods were created to analyse these data sets: a new radar retracker and a radar power simulator, which models radar returns from the laser data. We present the first quantitative analysis of data from the LaRA laser and radar altimeters, and demonstrate the potential of combining laser and radar altimetry to estimate snow depth. LaRA elevation estimates compare well with elevations from the radar altimeter onboard ERS-2 at the sub-meter level and the study provides lessons for future validation of satellite altimetry data over sea ice. Laser elevations are consistently higher than the radar elevations over snow covered sea ice. As LaRA was a flight of opportunity, no coincident in-situ measurements were available. Nevertheless, the difference between the reflecting surface of the laser and radar is consistent with snow depth from climatology and the analysis techniques developed in this paper will be useful for future radar and laser altimetry comparisons.

Winter accumulation in the percolation zone of Greenland measured by airborne radar altimeter

We here determine the surface elevation and the winter snow accumulation rate along a profile in the percolation zone of the Greenland Ice Sheet from data collected with ESA's Airborne SAR/Interferometric Radar Altimeter System (ASIRAS) in spring 2004. The altimeter data show that in addition to a backscatter peak at the air‐snow interface a dominant second peak occurs. This second peak appears due to the strong scattering properties of the last summer surface layer. A robust re‐tracking algorithm was developed that enables the tracking of both interfaces. Utilizing this algorithm, the winter snow thickness is estimated to 1.50 ± 0.13 m. This compares favorably with field measurements (1.43 ± 0.04 m). The snow depth estimates in combination with snow‐density measurements of 420 kg m−3 give a mean winter mass accumulation rate of 63 cm water equivalent (w.e.) and a spatial variation of ±6 cm w.e. Furthermore a strong correlation is found between surface gradient and accumulation rate, with higher accumulation rate in flatter areas. The approach adopted here has significant potential for remote measurements of winter snow accumulation rate across ice sheets at larger spatial scales.

On airborne measurement of the sea surface wind vector by a scatterometer (altimeter) with a nadir-looking wide-beam antenna

A pilot needs operational information about wind over sea as well as wave height to provide safety for a hydroplane landing on water. Near-surface wind speed and direction can be obtained with an airborne microwave scatterometer, a radar designed for measuring the scatter characteristics of a surface. Mostly narrow-beam antennas are applied for such wind measurement. Unfortunately, a microwave narrow-beam antenna has considerable size that hampers its placement on flying apparatus. In this connection, a possibility to apply a conventional airborne radar altimeter as a scatterometer with a nadir-looking wide-beam antenna in conjunction with Doppler filtering for recovering the wind vector over sea is discussed, and measuring algorithms of sea surface wind speed and direction are proposed. The obtained result can be interesting for many studies in oceanography, meteorology, air-sea interaction, and climate change and for creation of an airborne radar system for amphibious airplane safe landing on the sea surface, in particular for search and rescue operations in coastal areas.

Comparison of retracking algorithms using airborne radar and laser altimeter measurements of the Greenland ice sheet

Compares four continental ice sheet radar altimeter retracking algorithms using airborne radar and laser altimeter data taken over the Greenland ice sheet in 1991. The refurbished Advanced Application Flight Experiment (AAFE) airborne radar altimeter has a large range window and stores the entire return waveform during flight. Once the return waveforms are retracked, or post-processed to obtain the most accurate altitude measurement possible, they are compared with the high-precision Airborne Oceanographic Lidar (AOL) altimeter measurements. The AAFE waveforms show evidence of varying degrees of both surface and volume scattering from different regions of the Greenland ice sheet. The AOL laser altimeter, however, obtains a return only from the surface of the ice sheet. Retracking altimeter waveforms with a surface scattering model results in a good correlation with the laser measurements in the wet and dry-snow zones, but in the percolation region of the ice sheet, the deviation between the two data sets is large due to the effects of subsurface and volume scattering. The Martin et al. model results in a lower bias than the surface scattering model, but still shows an increase in the noise level in the percolation zone. Using an offset center of gravity algorithm to retrack altimeter waveforms results in measurements that are only slightly affected by subsurface and volume scattering and, despite a higher bias, this algorithm works well in all regions of the ice sheet. A cubic spline provides retracked altitudes that agree with AOL measurements over all regions of Greenland. This method is not sensitive to changes in the scattering mechanisms of the ice sheet and it has the lowest noise level and bias of all the retracking methods presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Topographic mapping using a multibeam radar altimeter

An airborne radar altimeter operating at 36 GHz is uniquely capable of measuring the topography of water, land, and ice surfaces. The Multimode Airborne Radar Altimeter (MARA) was designed to combine a narrow transmitted pulsewidth, a high transmitted power level, and a narrow antenna beam to produce a high-precision ranging capability at the nadir of the aircraft platform and at four fixed off-nadir angles out to 12/spl deg/ in the multibeam mode described in this paper, or a single beam scanning between /spl plusmn/22/spl deg/ in the scanning radar altimeter mode. Data collected over water and land surfaces are presented to demonstrate the potential of MARA for topographic mapping with accuracies and precisions of value in dynamic oceanography, geodesy, geology, hydrology, biogeochemistry, biogeography, and glaciology. >

Sea surface mean square slope from Ku‐band backscatter data

Near‐nadir, quasi‐specular backscatter data obtained with a 14‐GHz airborne radar altimeter are analyzed in terms of the surface mean square slope (mss) parameter. The raw mss data, derived from a least squares fitting of a ray optical scattering model to the return waveform, show an approximately linear wind speed dependence over the wind speed range of 7–15 m s−1, with a slope of about one half that of the optically determined mss. Further analysis based on a simple two‐scale scattering model indicates that, at the higher wind speeds, ∼20% of this apparent slope signal can be attributed to diffraction from waves shorter than the estimated diffraction limit of ∼0.10 m. The present slope data, as well as slope and other data from a variety of sources, are used to draw inferences on the structure of the high wavenumber portion of the wave spectrum. The data support a directionally integrated model height spectrum consisting of wind speed dependent k−5/2 and classical Phillips' k−3 power law subranges in the range of gravity waves, with a transition between the two subranges occurring around 10 times the peak wavenumber, and a Durden and Vesecky wind speed dependent spectrum in the gravity‐capillary wave range. With a nominal value of the spectral constant Au = 0.002 in the first k−5/2 subrange, this equilibrium spectrum model predicts a mss wind speed dependence that accords with much of the available data at both microwave and optical frequencies.

Laboratory measurements of the 36 GHz backscattering cross section from water waves

The backscattering cross section per unit area sigma /sub 0/ of a roughened water surface at 36 GHz is needed in the development of new remote sensing instrumentation that will operate at this frequency. One instrument, the multimode airborne radar altimeter (MARA), will illuminate the surface at incidence angles ranging from nadir to 12 degrees off-nadir. Laboratory tests were performed at the Wallops Wind-Wave Tank Facility to determine the variability of sigma /sub 0/ at these angles and as a function of windspeed. The measurement procedures used in a 6-in.-diam aluminum sphere as the calibration source. The results are compared with earlier measurements at this frequency. Little variability with windspeed was found in the cross-section values for 12 degrees off-nadir, while the cross section decreased with windspeed at nadir. The nadir cross section exceeded the off-nadir result by about 7 dB at a windspeed of 10 m/s (at a 10-m effective height above sea level).< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Off-nadir radar altimetry

The characteristics of nadir versus off-nadir altimetry are reviewed and contrasted and a potentially serious problem has been pointed out that has been overlooked by earlier investigators, who focused on the nongeophysical error sources in off-nadir altimetry. Spatial gradients of radar cross section on the sea surface, caused by wind or current gradients or the variation of radar cross section with incidence angle, could introduce significant range errors in off-nadir altimeter. This potentially crippling effect can be overcome by leaving the traditional 13-GHz frequency and implementing the multibeam altimeter at 36 GHz. A multibeam altimeter proposed for the Eos (Earth Observing System) is described as well as a multimode airborne radar altimeter being developed to study problems inherent in off-nadir altimetry. >

A Multi-Sensor Approach to the Interpretation of Radar Altimeter Wave Forms from Two Arctic Ice Caps

Data collected over Svalbard on 28 June 1984 by a 13.81 GHz airborne radar altimeter enabled analysis of signals returned from two relatively large ice masses. Wave forms received over the ice caps of Austfonna and Vestfonna are analysed with the aid of existing aerial photography, radio echo-sounding data, and Landsat MSS images acquired close to the date of the altimeter flight. Results indicate that altimeter wave forms are controlled mainly by surface roughness and scattering characteristics. Wet snow surfaces have narrow 3 dB back-scatter half-angles and cause high-amplitude signals, in contrast to relatively dry snow surfaces with lower-amplitude diffuse signals. Metre-scale surface roughness primarily affects wave-form amplitude and leading-edge slope, this becoming apparent over ice streams on Vestfonna.

A Multi-Sensor Approach to the Interpretation of Radar Altimeter Wave Forms from Two Arctic Ice Caps

Data collected over Svalbard on 28 June 1984 by a 13.81 GHz airborne radar altimeter enabled analysis of signals returned from two relatively large ice masses. Wave forms received over the ice caps of Austfonna and Vestfonna are analysed with the aid of existing aerial photography, radio echo-sounding data, and Landsat MSS images acquired close to the date of the altimeter flight. Results indicate that altimeter wave forms are controlled mainly by surface roughness and scattering characteristics. Wet snow surfaces have narrow 3 dB back-scatter half-angles and cause high-amplitude signals, in contrast to relatively dry snow surfaces with lower-amplitude diffuse signals. Metre-scale surface roughness primarily affects wave-form amplitude and leading-edge slope, this becoming apparent over ice streams on Vestfonna.

A Multi-Sensor Approach to the Interpretation of Radar Altimeter Wave Forms from Two Arctic Ice Caps

Data collected over Svalbard on 28 June 1984 by a 13.81 GHz airborne radar altimeter enabled analysis of signals returned from two relatively large ice masses. Wave forms received over the ice caps of Austfonna and Vestfonna are analysed with the aid of existing aerial photography, radio echo-sounding data, and Landsat MSS images acquired close to the date of the altimeter flight. Results indicate that altimeter wave forms are controlled mainly by surface roughness and scattering characteristics. Wet snow surfaces have narrow 3 dB back-scatter half-angles and cause high-amplitude signals, in contrast to relatively dry snow surfaces with lower-amplitude diffuse signals. Metre-scale surface roughness primarily affects wave-form amplitude and leading-edge slope, this becoming apparent over ice streams on Vestfonna.

Electromagnetic bias of 10‐GHZ radar altimeter measurements of MSL

Electromagnetic bias, the small difference that exists between the radar measured mean sea level and the geometric mean sea level is an important issue in high precision satellite altimetry. Present day satellite altimetry has achieved, with SEASAT-1, a precision of 5 cm rms in the range measurement. Future altimeter designs are expected to improve the range measurement precision to cm rms. In order to exploit the capability of these precise radar altimeters are marine geodesy and oceanography, it is necessary to understand and account for all of the known biases in the range measurement. The electromagnetic bias or the EM bias, which has been attributed to the observed fact that ocean wave troughs tend to be better reflectors of nadir viewing microwave radar energy than ocean wave crests, can be observed with high resolution airborne radar. This report presents the results of the EM bias measurements made by NRL using an airborne radar altimeter operating at 10 GHz with a 1 ns range resolution. Data were taken for various sea states and wind conditions. The experimental results are compared with current theories.

Airborne Radar Altimeter Measurements of Geostrophic and Ageostrophic Winds Over Irregular Topography

Abstract Radar altimeter and inertial wind velocity measurements from the NCAR Sabreliner research aircraft are combined with Defense Mapping Agency high-resolution (∼1 km) digital terrain height maps to obtain height analyses and geostrophic and ageostrophic winds for upper-level jet stream systems situated over land. Results show that given an accurate geographical location of the aircraft over an accurately mapped topography, it is possible to diagnose geostrophically and convectively forced ageostrophic motions over irregular topography.

Radar altimetry operations of the United States Geological Survey in 1950

This paper describes the methods employed in obtaining ground elevations for topographic mapping by the use of an airborne radar altimeter and related equipment. The 1950 program of the U. S. Geological Survey in Alaska is described and some of the results obtained are presented and discussed.