ASIRAS (Airborne SAR/Interferometric Radar System)
ASIRAS is an airborne SAR-altimeter instrument of ESA in preparation for CryoSat validations campaigns. The objectives are to increase the confidence level in the expected instrument performance and to validate the measurement/processing concepts prior to the CryoSat implementation and launch - and to use the instrument after the spaceborne mission launch in underflights during the commissioning phase of the CryoSat mission for calibration validation analysis. 1) 2)
The SIRAL (SAR/Interferometric Radar Altimeter) instrument of CryoSat, built by Alcatel Alenia Space, happens to be a new technology development with rather demanding observation requirements in accuracy; namely to observe ice sheet interiors and the ice sheet margins for sea ice and other topography. SIRAL is a nadir-looking radar instrument operating in Ku-band. In along-track direction, the lateral resolution is enhanced by means of Doppler filtering, while in the across-track direction, interferometric techniques are applied. While this concept had been verified by simulations, no dedicated experimental validation had been performed.
ASIRAS was built by Radar Systemtechnik (RST) of Rorschacherberg, Switzerland with the support of the Alfred Wegener Institute (AWI) and Optimare for the implementation and operation on an aircraft. The instrument design is very similar to the D2P (Delay-Doppler Phase-monopulse Radar) system, an airborne system of JHU/APL, which demonstrated the new technology in varies test flights over Greenland in the spring and summer of 2000. - However, with the D2Ps range resolution of ~40 cm, it became clear that if one could improve on this value, then it would be possible to provide a better opportunity to understand radar pulse penetration within the first few meters of snow pack and therefore provide a better tool for validating the CryoSat retrievals. 3) 4) 5) 6)
ASIRAS is essentially a Ku-band altimeter but with a high pulse repetition frequency such that it is phase sensitive and pulse-coherent. Dual receive antennas oriented adjacent to one another in across-track form a single-pass interferometer. The carrier frequency of the radar is 13.5 GHz and the bandwidth is 1 GHz. The antennas are realized as aperture coupled single linear polarized microstrip patch arrays each consisting of 8 x 32 elements with a careful amplitude taper in both coordinates.
The ASIRAS system consists of the radar altimeter instrument, the antenna module (containing 2 antennas), a control computer, and two data storage PCs. GPS-signals and information on aircraft attitude, ground speed and vertical velocity is required for accurate ASIRAS echo data post processing. The instrument transmits LFM (Linear Frequency Modulated) pulsed signals that are "fully deramped" during reception. The radiated chirped signals are internally followed by a reference signal that is delayed by the radar signal roundtrip delay time. If received signal and reference signals are mixed the output signals would consist - in case of a single radar target - of one frequency line at the frequency difference, Δf. Since the terrain surface represents a distributed but limited scattering area; hence, a limited spectrum is obtained at the output of the deramp process.
Table 1: Overview of ASIRAS instrument parameters
Figure 1: Configuration of the ASIRAS assembly (image credit: RST)
To achieve high sampling resolution of about 4.5 m in the along-track direction (with the aircraft about 1 km above the target surface), the system uses the delay-doppler concept to form beams. The echoes illuminated by these beams are corrected for the effect of slant range as the instrument passes over and samples in any given surface location (the "SAR" component of the radar). Using the difference in phase between the echoes received by the two antennas, the cross-track location of the point of closest approach can be pinpointed (the "interferometric" component of the radar). To reduce fluctuation in echoes due to thermal noise and speckle, an averaging of the echoes is required. This is known as the multi-look concept. Aircraft position and attitude are measured with a DGPS (Differential Global Positioning System) and an INS (Inertial Navigation System). 7)
Figure 2: Block diagram of the ASIRAS instrument (image credit: RST)
Although the launch of the CryoSat spacecraft failed in October 2005 (Eurockot launch failure), the scientific objectives of the CryoVEx validation experiments are still of undiminished importance. In Feb. 2006, ESA received the green light from its Member States to build and launch a CryoSat recovery mission, CryoSat-2, which is due for launch in 2009.
CryoVEx (CryoSat Validation Experiment) campaigns:
ASIRAS has been successfully flown on several measurement campaigns by the CryoSat Validation and Retrieval Team (CVRT). The instrument turned out to be a reliable "workhorse" for radar altimeter data acquisition. It has been flown on two different aircraft types and in low and in high altitude operational modes. Upgrades concerning in particular real-time display capabilities, robustness of computer equipment and reduction of recorded data volume are in progress. Plans for further campaigns up to the year 2012 exist. This may also include measurement flights for the SENTINEL mission.
Table 2: CryoSat preparation campaigns with list of activities, teams and locations
A number of initial test flights with ASIRAS were conducted in the region of Bremerhaven, Germany and the North Sea during 2003. In one test, ESA was able to demonstrate pulse-to-pulse phase coherence from reflections obtained from cranes when bad weather prevented a direct corner reflector over flight.
• CryoVEx (CryoSat Validation Experiment) 2003 campaign): In the spring 2003, an ESA/NASA funded and CVRT coordinated campaign saw DNSC (formerly KMS) operating their laser scanner (with auxiliary instrumentation) combined with the JHU/APL operated D2P Ku-band instrument onboard an Air Greenland Twin Otter. This campaign, known as CryoVEx 2003, covered a significant coverage of land ice in Greenland (EGIG line) and Svalbard, as well as sea ice around the Fram strait, Svalbard and north of Alert, Canada (Alert, at 82º 28' N and 62º 30' W, is the northernmost permanent settlement of the world). The D2P system employed two receive channels to allow cross-channel phase measurements. In this mode the two channels operated as a cross-track interferometer to support geo-location of the reflecting surface over sloping terrain. 8) 9) 10)
• CryoVEx 2004 campaign (spring/autumn): During the last two weeks of March 2004, an important step forward was made in the preparations for the CryoSat Arctic validation campaigns. For the first time, the ESA ASIRAS instrument was flown and tested on an aircraft over the snow and ice expanses of Svalbard, an archipelago of islands lying in the extreme north of Europe just 12º from the North Pole.
This was immediately followed by the spring and autumn Arctic campaigns over Svalbard, Greenland and Devon Island. This logistically demanding campaign incorporated for the first time several surface teams placed at various sites conducting in-situ activities. Also the opportunity was available to test the ASIRAS radar penetration via the use of test corner reflectors (Figure 6) positioned a few meters above the snow surface and installed prior to ASIRAS over flight with accurate positioning communicated to the ASIRAS flight team.
Figure 3: CryoVEx 2004 and Bay of Bothnia 2005 campaign sites and traverses (image credit: ESA)
The specific aim of this campaign was to acquire the first data sets of the instrument over snow and ice and to make sure that the instrument could be operated under the icy temperatures and high winds prevalent in this part of the world. Coincident laser and interferometric radar altimeter measurements were taken, in order to understand the penetration of CryoSat radar signal into polar sea ice and continental ice caps and to quantify uncertainty in the CryoSat measurements.
For redundant calibration purposes, the German aircraft Polar 4 of DLR / AWI was equipped with a laser scanner (ALS), the ASIRAS instrument, a single-beam laser and two DGPS receivers. During CryoVEx 2004, two campaigns took place. Flights were performed in Svalbard across Austfonna, on the Greenland Ice Sheet along the EGIG line (central Greenland), and on the Devon ice cap (Canadian Arctic). 11)
• CryoVEx 2005 campaign: In early 2005 an extra measurement mode was added to ASIRAS allowing the combined use of ASIRAS with laser scanners, which have operating ranges below 600 m, to characterize the penetration of the radar signal. This mode (LAM-SAR) was tested in the field in March 2005 in a dedicated test in the Bay of Bothnia, Finland.
• CryoVEx 2006 campaign: The campaign took place between May 11 and 15, 2006 north of Alert/Ellesmere Island. During the field campaign, a wide range of snow and ice measurements have been performed on two main study sites. Site 1 was a patch of rather level multiyear ice, while Site 2 was very level first year ice. Measurements comprised snow and ice thickness drilling, levelling of surface elevation, and snow pit studies. On each site, two corner reflectors were installed to support airborne radar altimeter measurements with ESAs ASIRAS (Airborne Synthetic-Aperture Interferometric Radar Altimeter System). Coincident flights of ASIRAS (aboard a Twin Otter of Air Greenland) and the HEM bird (helicopter) were performed on May 11 and May 12. For the Twin Otter, the coincident flight was only a small part of a much larger survey. 12) 13)
• CryoVEx 2007 campaign: This mission involves an extensive ground and air campaign in preparation of the CryoSat-2 mission. 14)
The ground campaign consists of the Arctic Arc Expedition, part of the International Polar Year (IPY 2007-2008). The expedition's two Belgian explorers, Alain Hubert and Dixie Dansercoer, 'stepped' onto the sea ice off the coast of Siberia on the 1 March 2007 and have so far covered a staggering 2,500 km each pulling a 130 kg sledge holding supplies and equipment. Along the way these two intrepid explorers are contributing to the preparation of the CryoSat-2 mission by measuring snow depths at regular intervals. These data in turn will be used by scientists to assess how well snow conditions can be predicted using existing climate models as well as inputs to methods for improving the accuracy of CryoSat-2 maps of sea-ice thickness.
In addition, a group of eight scientists were transported by helicopter to the remote Austfonna ice cap on April 12, 2007. As part of the CryoVEx 2007 campaign, they spent one month making measurements of snow and ice properties along long transects that criss-cross the ice sheet surface.
As the ground experiments are carried out, measurements are also being taken (April 2007) from the air by the Alfred Wegner Institute (AWI). ASIRAS is flown on the Do-228 aircraft in Svalbard. By comparing the airborne data with ground measurements, scientists will test and verify novel methods for retrieving ice-thickness change from the CryoSat-2 satellite mission, scheduled for launch in 2009.
Figure 4: ASIRAS antenna assembly mounted underneath the cabin door (image credit: RST)
Figure 5: The ASIRAS instrument on the aircraft (image credit: RST)
Figure 6: Corner reflector being erected on Bay of Bothnia sea ice on March 14th 2005 (image credit: AWI)
1) H. Lentz, W. Borisch, H.-M. Braun, "ASIRAS, An Airborne Radar Altimeter With Very High Spatial Resolution," Proceedings of the Advanced RF Sensors for Earth Observation 2006 (ASRI), Workshop on RF and Microwave Systems, Instruments & Sub-Systems, ESA7ESTEC, Noordwijk, The Netherlands, Dec. 5-6, 2006
3) R. K. Raney, "The Delay/Doppler Radar Altimeter," IEEE Transactions on Geoscience and Remote Sensing, Vol. 36, No 5, Sept. 1998, pp. 1578-1588
4) R. K. Raney, W. H. F. Smith, "The Delay-Doppler Altimeter: More Precision and a Smaller Footprint," 4th Weikko A. Heiskanen Symposium in Geodesy, The Ohio State University, Columbus, OH, USA, Oct. 1-4, 2002
5) C. J. Leuschen, R. K. Raney, "Initial Results of Data Collected by the APL D2P Radar Altimeter Over Land and Sea Ice," JHU/APL Technical Digest, Vol. 26, No 2, 2005, pp. 114-122, URL: http://www.jhuapl.edu/techdigest/td2602/Leuschen.pdf
6) R. Cullen, M. W. J. Davidson, M. R. Drinkwater, C. R. Francis, C. Haas, R. L. Hawley, C. M. Mavrocordatos, E. M. Morris, W. Rack, G. Ratier, P. Viau, D. J. Wingham, "ESA's new range of radar altimeter s for the extraction of geophysical parameters from land, sea ice and ocean surfaces," Symposium: 15 Years of Progress in Radar Altimetry, Venice, Italy, March 13-18, 2006, URL: http://epic.awi.de/Publications/Cul2006a.pdf
7) R. L. Hawley, E. M. Morris, R. Cullen, U. Nixdorf, A. P. Shepherd, D. J. Wingham, "ASIRAS airborne radar resolves internal annual layers in the dry-snow zone of Greenland," Geophysical Research Letters, Vol. 33, L04502, doi:10.1029/2005GL025147, 2006, URL: http://epic.awi.de/Publications/Haw2006a.pdf
8) K. Keller, S. M. Hvidegaard, R. Forsberg, N. S. Dala, H. Skourup, L. Stenseng, "Airborne Lidar and Radar Measurements over Sea Ice and Inland Ice for CryoSat validation: CryoVEx 2003," 2004, Technical Report No.25. National Survey and Cadastre, Denmark, ISBN 87-7866-414-4, ISSN 0908-2867
10) R. K. Raney, C. J. Leuschen, "Simultaneous Laser and Radar Altimeter Measurements over Land and Sea Ice," Proceedings of IGARSS 2004, Sept. 20-24, 2004, Anchorage, AK, USA
12) CryoVEx 2006, Field Report, AWI, Nov. 2006, URL: http://www.awi.de/fileadmin/user_upload/Research/Research_Divisions/Climaex2006.pdf
This description was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" - comments and corrections to this article are welcomed by the author.