Multiband Polarimetric SAR in Arctic Scenarios Technology demonstration for potential future capabilities

Ernst Krogager Joint Research Centre Danish Defence Acquisition and Logistics Organization (DALO) Ballerup, Denmark [email protected]

Abstract—In relation to a working group on future capabilities for applications in the Arctic region, the Danish Defence Acquisition and Logistics Organization (DALO) has conducted test campaigns with the multi-band, fully polarimetric F-SAR system owned by the German Aerospace Center (DLR) in order to explore the possibilities that advanced synthetic aperture radar (SAR) systems provide for surveillance, change detection, moving target identification and high resolution imaging. Examples of results and some preliminary conclusions are presented in this paper.

Keywords—SAR; polarimetry; Arctic; surveillance; detection; recognition

I. INTRODUCTION In relation to future capabilities in the Arctic region, the Danish Defence Acquisition and Logistics Organization (DALO) has carried out several test campaigns with the F-SAR system of the German Aerospace Center (DLR), which offered the possibility of exploring the performance of a high- resolution, fully polarimetric SAR system with five frequency bands in the range of 400 MHz to 10 GHz [1][5]. After an initial test in Denmark in October 2014, the main test campaign was held in Greenland from late April to late May 2015 with Fig. 1. Test sites for F-SAR campaign in Greenland April/May 2015. several different test sites and test scenarios designed for investigating the use of advanced, multi-channel SAR systems TABLE I. F-SAR PRINCIPAL PARAMETERS for applications such as surveillance, change detection and target recognition. Fully polarimetric radar systems are still not X C S L P commonly used for military applications, mainly due to added f [GHz] 9.6 5.3 3.25 1.325 0.435 complexity and cost, but nevertheless, the potential benefits of PolSAR Quad Quad Quad Quad Quad fully polarimetric systems must be taken into due consideration InSAR + - + - - for future capabilities [9]. Hence, a main objective of the test BW [MHz] 760 380 300 150 50 power [W] 2500 1000 1250 750 750 campaigns reported here has been to demonstrate and illustrate rg res. [m] 0.25 0.5 0.6 1.0 4.0 how the utilization of information carried by the polarization of az res. [m] 0.1 0.25 0.3 0.5 2.0 the electromagnetic waves could improve the performance of swath [km] 2 to 5, depending on altitude imaging radar systems significantly. The test scenarios included experiments with objects hidden under snow as well as moving targets and boats near icebergs. Detailed ground II. EXPERIMENTS AND TEST SCENARIOS truth was collected in the form of precision GPS measurements with associated photos, and aerial photos were taken during A. F-SAR system description helicopter flights. Examples of results and findings are DLR employed the airborne F-SAR system in X-C-S-L- presented in this paper with a focus on techniques for band and P-band configurations for the mission with technical visualization of multi-band polarimetric SAR imagery. parameters as given in Table 1. Data acquisitions were made with up to three simultaneous polarizations: X-C-L or X-S-L, Fig. 2. L-band SAR image of Kangerlussuaq area around the airport. Parked aircraft can be seen in the taxiway area near the hangars. Colour coding is based on the sphere, diplane, helix decomposition as explained in section III.B. while P-band data acquisition was carried out on separate whereby RGB (red, green, blue) images are generated using the flights after reconfiguration. respective components of the decompositions [9][9]. 1) Pauli decomposition B. Description of Test Sites ° ° []=[SSkk12]] + [ S + k 3 [ S ] (1) Flights with F-SAR were carried out over four test sites sphere diplane(045 ) diplane( ) located at Kangerlussuaq, K-Transect, Kulusuk (Lost Squadron cos 2θθ sin 2 site nearby), Nuuk and Qeqertarsuaq. In this paper, selected [S = (2) ] diplane(θ )  examples from Kangerlussuaq and Qeqertarsuaq are shown. sin 2θθ− cos 2 1) Kangerlussuaq 10 =  Calibration reflectors and man-made objects were [S] sphere  (3) emplaced, and detailed ground truth was collected in the form 01 of photos, videos and precision GPS recordings. Detection of 1 ==+−[][]T changes was demonstrated by repeated flights over a given test kkkk123 SHH SSS VV HH VV2 S HV (4) area, where objects were moved between flights. 2 2) Qeqertarsuaq, Disko Island This test area provided opportunities for collecting SAR image data of the village area, ships near ice, dogsleds, snow scooters, as well as test setups on the ice and snow cover of the Disko Island with tent, corner reflectors and buried test objects, including humans hidden in snow caves.

III. METHODOLOGY

A. Geocoding Regions of interest were selected and cropped from the original datasets. For each region of interest (ROI), polarimetric decompositions were applied, and the resulting images were geocoded based on a digital elevation model (DEM) created from interferometric X-band data collected during initial F-SAR flights at the various test sites.

B. Polarimetric decompositions For the presentation of results, two coherent three- component decompositions are employed in this paper: conventional Pauli decomposition (HH-VV, HV, HH+VV) and Fig. 3. Test area at the Lyngmark Glacier on the Disko Island with Krogager sphere, diplane, helix decomposition (SDH), experimentation area around blue cottage indicated. 2) Sphere, diplane, helix decomposition

ϕ ϕ jjs []=S{Seeksd []]sphere + k [ S diplane(θθ ) + k h [ S} ] helix( ) (5)

10 =   [S] sphere   (6) 01 cos 2θθ sin 2 =  [S] diplane(θ )  (7) sin 2θθ− cos 2 ± 1 θ 1 j =  j 2 [eS] helix(θ )  (8) 2 ±−j 1 = kSs RL (9) = kSSdRRLLmin ( , ) (10) =− kSShRRLL (11)

ϕϕϕ=+−1 π 2 ()RR LL (12)

θ =−+1 ϕϕπ 4 ()RR LL (13)

ϕϕ==1 ϕ + ϕ s RL2 () RR LL (14) As can be seen from (9)-(14), the SDH decomposition is closely related to the components measured directly by circular polarization (or obtained by transforming from the horizontal- vertical linear polarization basis to the right-left circular polarization basis). The use of these decompositions rather than a straightforward use of the measured HH, HV, VV scattering matrix elements allows for interpretations in terms of physical scattering properties. The two considered decompositions have some similarity in terms of physical scattering mechanisms. Thus, the SDH sphere component is identical to the HH+VV term of the Pauli decomposition, and the HH-VV term is representing an even-bounce scatterer, e.g., a dihedral. For a dihedral aligned horizontally or vertically, HH-VV is identical Fig. 4. SDH and Pauli decomposition images of parked aircraft at to the diplane component of the SDH decomposition, while a Kangerlussuaq airport. From top: X (590 MHz BW), S (300 MHz BW), L dihedral with an orientation angle different from 0 or 90 (150 MHz BW). Left column RGB: sphere, diplane, helix. Right column degrees will generate contributions to the HH-VV component RGB: HH-VV, HV, HH+VV. as well as to the HV component. A more complex target with phase information of the decompositions, which is not used for two or more even-bounce contributions will contribute to both the RGB image generation. However, the separation of three the diplane and the helix component of the SDH decomposition components with significant physical merit facilitates the and to HH-VV and HV of the Pauli decomposition [11]. visualization and interpretation of the radar scattering properties. IV. RESULTS B. Qeqertarsuaq A. Kangerlussuaq The test sites at Qeqertarsuaq included an area around the Images of an aircraft parked at Kangerlussuaq airport are harbor village of Qeqertarsuaq as well as an area on the Disko shown in Fig. 4 for three different frequency bands and two Island covered by ice and snow. A cottage in this area was used different polarimetric decompositions. A notable difference as a base for experiments with stationary and moving test between the two representations can be seen in some areas, objects, see Fig. 5. At this test site, we carried out change which are all green in the SDH representation, but partly green detection experiments by digging caves into the snow to the and partly red in the Pauli decomposition. This indicates that northeast of the cottage, where test objects (including persons the associated scattering contributions are due to even-bounce and metallic reflectors) were present during some flights and reflections from structures with some slope relative to the radar absent during others. Likewise, two snow scooters and two geometry. In fact, such information is also included in the dogsleds were used as both stationary and moving test objects.

Fig. 6. SDH images of area around the blue cottage (bright area in the lower left part of the images) at five frequency bands. Also shown is a P-band Fig. 5. Overview of experimentation area around blue cottage. Two snow difference map based on two consecutive passes. Changes due to snow scooters and two dogsleds were used as stationary and moving test objects scooters, dogsledges, persons and metal objects in and out of snow caves are during flights. Snow caves were dug into for experiments with hidden objects. clearly seen. Two persons, a ladder made of aluminum, and two aluboxes forming a dihedral corner structure with mostly double-bounce scattering. The lower right image in Fig. 6 shows a difference map For visualization of changes, difference maps were generated from two consecutive passes of the P-band system. generated based on comparing pairs of images from different The clear blue and red areas correspond very well with flights. Further examples for the upper part of the scene in Fig. positions, where changes were made at the test site shown in 6 are shown in Fig. 8 - Fig. 13, which include difference maps Fig. 5. Notably, the difference between an empty cave and the generated from different polarimetric quantities: ks, kd, kh, HH, same cave with a person present is clearly visible at P-band in VV, HV. spite of the rather low range resolution. Likewise, smeared and displaced signatures of the moving test objects are clearly visible. It should be noted here, that the images in Fig. 6 are not all recorded simultaneously, since the system only allowed for up to three frequencies at a time, and P-band only in single- band mode. Hence, the X-, C- and L-band images were recorded simultaneously, while the S- and P-band images were recorded during different passes. A closer look at a smaller area around the blue cottage is shown in Fig. 7, where SDH images from two consecutive passes are shown. Between the passes, several changes were made as marked in the images. As can be seen, even a rather small change like a medium-size person in or out of a snow cave is clearly visible at P-band despite the rather low range Fig. 7. SDH images of a smaller area in the vicinity of the blue cottage for resolution, even without special processing techniques. two consecutive passes. Location of test objects are marked by yellow frames.

Fig. 8. Sphere component difference map Fig. 9. Diplane component difference map Fig. 10. Helix component difference map

Fig. 11. HH component difference map Fig. 12. VV component difference map Fig. 13. HV component difference map

As can be seen, the different polarimetric quantities highlight different scattering characteristics, and as expected, the changes caused by removing trihedral reflectors are more pronounced in the sphere component image than in the diplane component image. However, since the trihedrals were small in terms of wavelength at P-band, significant contributions are also seen in the diplane and helix component images. Notably, the change caused by a person in the upper cave is most clearly visible in the HH image, while barely visible in the others. Fig. 14 shows a series of SAR images of an area near Qeqertarsuaq harbour, where the water was mostly covered by ice at the time of data acquisition. Based on an aerial photo (also shown in Fig. 14) taken from a helicopter on the way to the base on the Disko Island, details of the scene in Fig. 14 could be identified. Thus, as can be seen in the left side of the geocoded aerial photo, a number of boats were lined up along the edge of the ice, where melting had begun. The photo was taken the day before the acquisition of data at X-C-S-L-bands, while the P-band data acquisition took place four days later, when the melting had extended further into the ice-covered region. For comparing with the polarimetric images, Fig. 14 also includes a single-polarization image (VV) at X-band. Evidently, a scene like this contains many scattering contributions with features that cannot be exploited from images based on a single-polarization system. Furthermore, the images at different frequency bands illustrate how different characteristics can be extracted depending on the frequency band. At X-band, for example, the tracks due to snow scooters and dogsleds are clearly visible, while the tracks disappear at the lower frequencies. The images clearly show how the different frequency bands provide complementary information. Fig. 14. SAR images at five frequency bands (based on SDH decomposition) At the higher frequencies, tracks in the surface from snow of scene from Qeqertarsuaq ice-covered harbour area. Also shown is a single- scooters and dogsleds are seen, while the lower frequencies polarization X-band image. Lower left: geocoded aerial photo. Lower right: pick up reflections from objects and structures below the ice- cropped aerial photo before geocoding. and snow-covered surface. V. CONCLUSIONS REFERENCES An advanced SAR technology demonstration campaign [1] E. Krogager, “Results from the DALOEX 2015 campaign with F-SAR (DALOEX 2015) was carried out in Greenland in April/May in Greenland”, Proceedings of EUSAR 2018, Aachen, Germany, 2018. 2015 by the F-SAR system of DLR, Germany. The test [2] A. Reigber, E. Krogager, M. Keller, M. Jäger, I. Hajnsek, R. 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VI. ACKNOWLEDGMENT

The author would like to thank the F-SAR team of DLR for the highly professional and efficient conduct of the test campaigns under the difficult conditions in the Arctic areas. Likewise, the support and guidance from the SAR Technology Department of DLR is highly appreciated, as is the support from the Danish MoD AGFOA experimentation program, from the Joint Arctic Command in Nuuk, and from Air Group West in Kangerlussuaq. Crucial was also the support from SikuAput, Qeqertarsuaq, and the dedicated assistance from the dog leaders and snow scooter drivers, who made the 10 km rides up to the test site on the Disko Island in the snowy and foggy Saturday morning of 9th May 2015 to act as test objects for experiments with this "all-weather, day-and-night sensor system". Finally, sincere thanks are due to Mr. Stig von Platen Rosenmunthe and our student assistant, Ms. Katrine Feld, for invaluable assistance with the handling and processing of the huge amount of data from the DALOEX trials.