Level 1b SAR/SARIn Processor Descriptions for CryoSat and ASIRASRobert Cullen and Duncan WinghamESTEC & University College London

Abstract The algorithms used in the L1b CryoSat (SAR/SARIn) and ASIRAS processors are described. Both simulated and real data acquired over Greenlandfrom ASIRAS are compared and confirm the successful use of SIRAL science measurement simulations and ASIRAS acquired data as pre-launch tools forproving the new processing technique.

5.1 ConclusionWe continue to test the CryoSat SAR/SARin processors CryoSat prior to launch, now with the primary goal of improving our understandingof the data we are likely to encounter (the use of test data has also been used extensively in overall ground segment validation). However,the analysis of data obtained from the successful pre CryoSat launch use of ASIRAS in the CryoVex 2004 campaigns have been invaluablein our understanding in this respect. Although the geometry is different, we see features in the ASIRAS data which have confirmed ourunderstanding in simulating the CryoSat science data.

Fig 2.1. The CroSat/ASIRASprocessing scheme. Userdistributable data are the un-calibrated full bit rate level 1 dataand the processed level 1b.Optional (non distributable) outputdata files can also be generated atseveral stages of the processor(yellow) for diagnostic uses andare shown here to show two stagesfor which understanding of theprocessor data is key.

Input FBR, processor control andsupporting data are ingested andunpacked. The FBR complex de-ramped time domain echoes arethen corrected for instrumentimperfections and other effects [6].A large internal store can holdseveral hundred bursts of unpackedand calibrated burst data at any onetime mainly to determine locationsto steer beams in advance (Fig 2.2).With a set of suitable level 1bsurface locations, beam steeringand azimuth FFT are processed andeach beam is Doppler rangecorrected with respect to its anglewith the velocity vector. Beamslant range corrections aredetermined and are applied oneither side of the rangecompression FFT to avoidunwanted wrapping in range of theechoes. When a full stack of dataper level 1b location has beenprocessed the multi look functionintegrates the echo power, phasedifference and coherencewaveforms (power only for SARmode) and outputs the level 1brecord with supporting parameters.

Fig. 4.2: (a) 4 superimposed 2D FFT power plots as the aircraft approaches the corner reflector (1), is above the reflector (2) and as itrecedes away from the reflector (3 and 4). One can see in the locations of the 4 responses in range follow a parabola, we remove thisrange migration exactly in the processing. Note, for ASIRAS the antennae are mounted such that their bore sights are pitched 2.5° in theforward direction and also what appears to be instrument contamination appears at the back of the range window. Also, apart from theremoval of I and Q waveform biases (that would result in zero frequency spikes) no compensation has been made for instrumentimperfections. (b) Power echo stack data for the snow surface some tens of meters before the corner reflector was reached and (c) powerecho stack for the snow surface with the corner reflector response being 2 meters above the snow surface. Again, apparent instrumentinduced features can also be seen towards the back of the range window. (d) Stacked level 1b power echoes showing the corner reflectorresponse at bin 64 and the surface response at bin ~100. Here the processor was operated 3 times in different configurations providingslightly differing results but improved when the level 1b surface location is shifted as shown in Fig. 4.1. The lower returned power forthe case when no beam steering is used can be seen.

Fig. 2.2 The reason for steering beams: Beams are formed to improve along track sampling of any surface.However, as with all altimeters, single echoes are heavily contaminated with thermal and speckle noise which have to bereduced by the averaging of echoes. This can only be achieved if the beam echo energy have been scattered fromapproximately the same illumination strips on the ground.

Thus, (a) The azimuth FFT forms beams spaced equally in angle. As mentioned, this angle is a function of aperture length(a function instrument PRI and the variable satellite or aircraft speed), carrier wavelength and the number of beams beingformed from a burst. (b) Without beam steering beams will illuminate strips on the ground which do not superimpose.Stacking this data will result in unwanted blurring and thus range measurement error. (c) Level 1b ground locations on theground are determined some few kilometers in advance of the current satellite/aircraft location for the burst beingprocessing. (d) This results for CryoSat in some ~61 (SARIn) and ~244 (SAR) beams illuminating approximately thesame strip on the ground within an acceptable error margin. Following Doppler and slant range corrections and rangecompression, these echo stacks can be integrated to reduce the speckle/thermal noise.

(a) (b)

(c) (d)

2. Processor DescriptionFor SAR/SARIn operation, SIRAL (and ASIRAS) generates pulse limited data similar to that produced on-board by theEnviSat and ERS altimeters prior to instrument range compression, power echo accumulation (for use in on-board surfacetracking) and being transmitted for ground processing. With one main exception the data is almost identical with the burstmode operation of RA-2 which are complex I and Q time domain echoes. The main exception for SIRAL concerns theinstrument pulse repetition frequency which is designed to be high enough such that echoes over some fixed time period(the burst length) are phase coherent. This coherency can be utilised with the purpose of improving the along-trackresolution with respect to the conventional altimeter by forming what are know as along track ‘Doppler’ beams [2] byfiltering filtering rates of change of phase over the burst. Further, with the introduction of a second receive antennapostioned in the across-track direction it is possible to interfere the returned echoes and extract a interferometric phasedifference. The interferometric phase difference can then be utilised in the level 2 processor to convert the phase differenceto an infered angle such that the echo location of the power echo leading edge can be resolved. The level 1b processingscheme is described in both figures 2.1 and 2.2.

The level 1b data files essentially consists of records of 20 sub-blocks containing geolocated and time stamped power echowaveforms with accompanying attitude, position, instrument state/corrections and power echo stack shape characterisation.For SARIn mode, interferometric phase difference and coherence waveforms are also output. Additionally, for all modes,we compute pulse-width limited 1Hz power echoes to assist in range bias determination between all 3 measurement modes.Also contained at 1Hz are geo-corrections which are added at a post processing stage.SIRAL (differences with respect to ASIRAS are described in section 4) is a normal incidence Ku band altimeter with acarrier frequency of 13.6 GHz.With a measurement and tracking bandwidth of 320 Mhz (40 MHz tracking for SARIn) this provides an echo rangeresolution of ~0.46 meters (3.68 metres for SARIn tracking echoes). There are 128 and 512 echo samples available for SARand SARIn respectively leading to range window sizes of ~ 60 metres and ~ 235 metres (the SARIn 40 MHz trackingwindow of 128 samples is ~ 470 meters).The instrument transmits a burst of 64 pulses with a pulse repetition interval (PRI) of 56.6µS , the return echoes of which,as explained above, are phase coherent and allow 64 beams to be produced via the use of an azimuth FFT operation (phasefiltering). Beams are spaced equally in angle, the angle being a function of satellite speed, PRI and number of echoes perburst (these combine to give the aperture length) and the carrier wavelength. This gives an angular spacing of, typically, 0.4milli radians which is variable around the orbit as a function of platform speed.Further information regarding technical SIRAL and associated instrumentation data, processing science data from level 0through to level 2 is described further in [3] (in press) and [4].

1. IntroductionThe CryoSat Synthetic INterferometric Radar Altimeter (SIRAL) has been designed to extend the coverage of conventionalpulse-limited altimeters to improve the measurement of sea ice thickness and freeboard using SAR mode and to improveelevation mapping of the marginal regions of ice sheets with the SARIn mode [1]. There is also a pulse-width limited lowrate mode for use over interior ice sheets.The data acquired by the low rate mode is similar to that of the EnviSat RA-2, with the exceptions that a single operationalKu band and 320MHz bandwidth tracking is suitable for ice sheet interior or ocean tracking. Since users will be familiarwith this type of level 1b data we do not refer further to this data type in this paper but focus on the new Level 1bprocessing schemes for the SAR and SARIn modes for which the data acquired and processed is of a more complex naturethan the traditional radar altimeter processing counterpart.

Since this is the case, we explain the CryoSat Level 1b SAR/SARIn processing by processing simulated level 0 data for 2simple cases, a point target and an ocean like surface and extend this by examining data acquired by the aircraft bourneexperimental ASIRAS radar altimeter which was operated in September 2004 over a number of corner reflectors mountedsome 2 meters above snow surfaces as a part of the CryoVex 2004 campaigns.

ASIRAS is an experimental radar altimeter with similar functionality to SIRAL. Post CryoSat launch, ASIRAS will beoperated on an aircraft to assist in the validation of the CryoSat data. Apart from a number of test flights, ASIRAS, has beenoperated during spring and autumn campaigns during 2004 over a number of regions of interest in the Arctic. The ASIRASLevel 1b processor is an adapted version of the CryoSat Level 1b processor and essentially contains the same processingscheme.

These CryoVex campaigns follow on from the LaRa (2002) and CryoVex 2003 campaigns that utilised the D2Pexperimental radar [2] that proved the concept for the CryoSat mission.

Fig. 4.1: CryoVex 2004 September 14 campaign along the EGIG line. Here the ASIRAS instrument was operated over a corner reflectorerected, amongst some others on the EGIG line, at a location, T21, nominally at 70.544º N, -43.025ºE. (a) The 1Hz DGPS locations (bluedots) of the flight path showing the corner reflector in the top right, (b) localised region of the corner reflector showing two DGPSmeasurements (blue dots), the level 1b ground locations (black dots) at regular spacing of just less than 5m and the known location of thecorner reflector (Star). (c) In order to improve the processed stack data the nearest level 1b location was shifted such that it is at the nearestlocation on the ground track to the reflector. This feature will be used during the CryoSat mission in order to improve results overtransponders since an along-track positioning error has an impact on slant range across the stack and can destroy the return response.

(a) (b) (c)

Level 1b locations

Corner reflector

Shifted L1b location for corner reflector analysis

4. Processed ASIRAS DataThere are a number of differences between the operation and the instrument characteristics of SIRAL and ASIRAS. These should be takeninto account to avoid confusion with the CryoSat simulation plots in section 3 of the poster. These main differences are•Bandwidth: The bandwidth is 1GHz which results in a range bin spacing of ~15cm. Due to processor reasons (range FFT requires power of2 size sampling window) this is further reduced to ~8.7cm.•Elevation: The aircraft has an elevation of about 1 km as opposed to 720 km for CryoSat.•Instrument speed: The average speed of the aircraft for which ASIRAS was the primary payload was ~66m/s as opposed to ~7500 m/s forCryoSat for which SIRAL is the main payload.•Beam footprint spacing: The along-track level 1b sample spacing for CryoSat is of the order of 300 meters. The configuration used toprocess the ASIRAS data results in sampling of about 5 meters (the configuration can be changed within Doppler bandwidth limits).The CryoVex 2004 A and B campaigns took place during May and September respectively over various locations within the Arctic region(Svalbard, Greenland and Devon ice cap, for example). A number of sections of data have undergone an initial analysis. We present one ofthem in this poster. These are described in figures 4.1 and 4.2.

4. References[1] D. J. Wingham, “CryoSat science and mission requirements,” http://cryosat.esa-ao.org/description/data/MRD.pdf, Sept. 1999.[2] R. K. Raney, “The delay Doppler radar altimeter”, IEEE Transactions on Geoscience and Remote Sensing, vol. 36, pp. 1578-1588, 1998.[3] C. R. Francis, “Mission and data description,” CS-RP-ESA-SY-0059, http://cryosat.esa-ao.org/description/data/MDD.pdf, Nov 2001.[4] D. J. Wingham, et al, “CryoSat: A mission to determine the fluctuations in Earth’s land and marine ice fields”, submitted to Advances in Space research,2004.[5] D.J. Wingham, et al,“The mean echo and echo cross product from a beam forming interferometric altimeter and their application to elevationmeasurement”, IEEE Transactions on Geoscience and Remote Sensing, vol. 42 No. 10, pp. 2305-2323, 2004.[6] R. Cullen and L. Rey, “CryoSat Internal Calibration: Processing Methods and data examples”, CryoSat workshop poster, ESA, 2005

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Surface responses

Corner reflector responses

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Instrument effects?

Corner reflector response

Fig 3.1: Point target responses: (a) For SARIn a stack of beam echoes are displayed side by side as a surface plot. The response atrange bin 256 is that from an on-track scatterer placed at a level 1b location. The response at range bin 280 is that from a scatterer placed tothe right of the ground track by 4 km. The effect of antenna pattern modulation can be seen both as a function of beam from forwardlooking beams (beam=0) through nadir looking beams and backward looking beams (beam~55) and as a function of range. (b) Integratingthe power in all the beam echoes per range bin provides this plot. The position in range of the 2 scatterer peak responses are calculated as atest. (c) Here the interferometric phase difference is extracted at the locations in range of the peak power of the 2 scatterers.

Ocean like surface response: (d) A power echo stack showing both the speckle in each beam echo and the antenna pattern modulation asa function of beam. (e) the stacked echo. An analytical expression for this echo shape is described in [5]. Level 2 ‘Re-tracking’ of theseechoes provides the location in range to extract the phase difference and thus resolve the across track echoing point. (f) However, the phasedifference echo is noisy about the power echo leading edge and the phase must be fitted in the level 2 processing using coherence as aweighting function.

In Level 2 processing a phasefitting function is used toextractthe phase difference at theleading edge of the power echo

(a) (b) (c)

(d) (e) (f)

Beam

Beam

Range

Range

3. Simulated CryoSat Science DataIn order to test the CryoSat ground segment a number of data sets were simulated from simple point target reflectors and surfaces throughto more complex surfaces such as those expected over ice sheet margins and sea ice. In some test cases we also applied worse thanexpected simulated instrument errors in order to check retrieval following internal calibration correction. In this paper we consider twosimple cases and consider the more complex case of real data from the CryoVex 2004 ASIRAS data described in the section 4.