ANALYSIS OF ASAR POLARISATION SIGNATURES FROM URBAN AREAS (AO-434) Dan Johan Weydahl and Richard Olsen Norwegian Defence Research Establishment (FFI), P.O. Box 25, NO-2027 Kjeller, NORWAY, Email: dan-johan.weydahl@ffi.no ABSTRACT This project focuses on ASAR backscatter change from urban categories as a function of polarisation and incidence angle. Results show more bright scattering points from manmade objects in like-pol than in cross-pol. ASAR AP data acquired with several incidence angles may be used with success to obtain more knowledge about structural properties of certain man-made objects. 1. INTRODUCTION Satellite synthetic aperture radar (SAR) is an interesting sensor for mapping urban areas and manmade objects. Now, SAR backscatter from urban type objects may change significantly as a function of the radar viewing geometry (e.g. aspect angle and incidence angle). For satellite SAR, ascending/descending satellite pass directions mainly govern aspect angle differences, while incidence angle differences can be achieved using a SAR system with a steerable antenna. RADARSAT-1 gives the opportunity to acquire SAR data using a wide range of incidence angles (20-59 ). Results from an urban area shows that RADARSAT-1 images acquired with different incidence angles can give complimentary information about man-made objects [1]. The objective of ESA AO-434 ( Analysis of ASAR polarisation signatures from urban areas using multiple incidence angles ) is to extend this research by also taking into account different polarisations. The data source in this respect is ENVISAT ASAR. 2. DATA SET AND METHODOLOGY Many ASAR alternating polarisation (AP) data sets were ordered over the Oslo region in Norway from July 2003 to March 2004 as part of ESA AO-434. The AP data were processed to single-look-complex (APS) at ESA PAF s. The APS data are calibrated in the sense that an absolute calibration factor (K) is Proc. of the 2004 Envisat & ERS Symposium, Salzburg, Austria 6-10 September 2004 (ESA SP-572, April 2005) estimated for every product type. This value is found in the accompanying ASAR image header. However, the APS data are not corrected for range-spreading-loss or elevation-antenna-pattern-gain. On the World-Wide- Web, ESA has published auxiliary data files that can be used to estimate the elevation-antenna-pattern-gain. Together with data found in the ASAR header, both the range and antenna pattern were corrected for at FFI using procedures described in the ESA technical notes [2] and [3]. Sigma-nought backscatter values were then estimated for several land surface cover types using the calibrated APS data. A block averaging was performed on the ASAR APS data for several test areas. An averaging window of 16x48 or 32x96 pixels (in range and azimuth respectively) was used. This leads to a more or less square area (in meters) being investigated. Results are shown in Table 1 and Table 2 in the next section where backscatter response for the various polarisation modes are given as a function of increasing incidence angle. 3. RESULTS 3.1 Industrial Table 1 a) shows the ASAR backscatter estimated from an industrial area where a large chemical plant (Dynea ASA) is located. A lot of pipes and metal tanks are present. The like-pol channels (both HH and VV) are always around 10 db above the cross-pol channel. See also area no.1 in the ASAR image in Fig.1. This result seems to be independent of incidence angle variations, or radar aspect angle. The cardinal direction effect (see [4] and [5]) is therefore not very noticeable for this industrial site. We may therefore deduce that this area mainly are made up of two kind of man-made objects: Objects with a lot of corners or complex structures that very likely will give a strong SAR backscatter regardless of radar viewing directions. Simple spherical objects (also including pipes and tanks) that are more or less omnidirectional in nature.
3.2 City centre Lillestrøm city centre consists of concrete office houses, shops, parking areas and residential houses in wood (see area no.2 in Fig.1). Results in Table 1 b) show neither extreme differences between the like- and cross-pol channels, nor stable inter-channel differences as for the chemical plant in Table 1 a). From this, we may deduce that the city area is a mixture of simple objects, complex objects and objects that are influenced by the cardinal direction effect. 3.3 Residential A residential area in Lillestrøm is investigated in particular. The houses are mostly built in wood, and the streets are laid out in a regular pattern. Table 1 c) shows the average backscatter from this area. Ascending mode HH backscatter decreases with increasing incidence angle. At the same time, the crosspol HV is fairly stable around 18.5 db. The AP mode channel difference (last column in table) is between 10.5 and 16.6 db for ascending pass data. This is decreasing to a difference of 6 db for descending pass. This residential area is clearly influenced by the cardinal direction effect (see area no.3 in Fig.1) but not in the extent as some industrial areas holding large warehouses with metal roof or walls (see the evaluation in section 3.4). 3.4 Large buildings Trandum is an Army site with some large rectangular buildings. The ASAR backscatter in Table 1 d) shows a high degree of influence from the cardinal direction effect, but also incidence angle variations. The area is shown in the ASAR image in Fig.2. We notice that an incidence angle change of 6.5 degrees (August IS4 to July IS6) leads to a 9.7 db difference in HH-backscatter. A change in both incidence angle and aspect angle (ascending August IS4 to descending August IS7) is producing a difference of as much as 14.4 db in HH. A remarkable observation in this context is that the cross-pol channels only vary by 5.3 db when evaluating the same three acquisitions (HH: IS4, IS6 and IS7). From Table 1 d) we may also suggest that the presence of the cardinal direction effect will give a large spread of backscatter differences between like-pol and crosspol channels when operating with different radar observation angles. Take IS7 in July and August as an example: the like- cross-pol difference (right most column in table) changed by 9 db (from 17 to 8 db) when changing the observation aspect angle from ascending to descending pass. This is quite contrary to the stable situation noticed for the chemical plant in Table 1 a). 3.5 Coniferous forest A coniferous forest area north of Tien Lake in Fet is evaluated and results given in Table 2 a). Here, the likpol backscatter gives less variation (only 1.5 db) than the cross-pol (6.1 db) for the different radar viewing angles. There are only small differences from HH to VV backscatter. 3.6 Agricultural field Agricultural fields are evaluated in Table 2 b). The ASAR data is given in square no.5 in Fig.1. The HH lik-pol backscatter gives a variation of 2.3 db, while HH cross-pol. gives only 1.8 db variations for the same set of radar acquisition angles. In other words: Fields show larger like-pol variations as a function of incidence angle, than coniferous forest. Fields show smaller cross-pol variations as a function of inc. angle, than coniferous forest. 3.7 Grass land A golf area is having large grass patches. The ASAR data is given in square no.4 in Fig.1. Results are given in Table 2 c) and shows that HH and VV polarisation give very similar backscatter from a grass-covered area. From the present data set, it is not possible to evaluate backscatter changes as a function of incidence angle variations. The cross-pol backscatter is low. 3.8 Asphalt covered ground Results from the asphalt-covered aircraft parking area next to the main terminal building at Gardermoen Airport Oslo, is shown in Table 2 d). As expected, the backscatter is independent of aspect angle differences. We notice that the like-pol backscatter is quite low, but not as low as for the category lake, see Table 2 e). The cross-pol response seems to decrease with larger incidence angle. This needs to be investigated further, especially as the cross-pol backscatter is approaching the system noise level, leading to possible inaccuracies in the ackscatter estimate.
there are only small differences between the like-pol and cross-pol channels, but this can change considerable if a strong wind is blowing on the lake. Generally speaking, the backscatter from this lake is very low, almost into the system noise region for both like-pol and cross-pol. Fig. 1. ASAR AP colour composite images (Red=HH, Green=HV, Blue=HH) over Lillestrøm city area. The marked squares show the location of the following test areas: 1=Industry area, 2=City centre, 3=Residential area, 4=Golf area, 5=Agricultural field. 3.9 Lake surface Table 2 e) shows results from ASAR backscatter evaluated over Maridalen lake outside Oslo. In our case, Fig. 2. ASAR AP colour composite images (Red=HH, Green=HV, Blue=HH) over Gardermoen Airport. The marked squares show the location of the Trandum test area holding a large building complex (see section 3.4).
Table 1. ENVISAT ASAR AP backscatter values from various land surface covers and object types. Green colour is ascending and yellow is descending satellite pass. a) Industry area with tanks/ pipes (Dynea) b) Lillestrøm city centre c) Residential area d) Large building complex (Trandum) Aug, IS4 33.9-0.9-11.9 11.0 Dec, IS4 34.0-13.3-3.1 10.2 July, IS6 40.5-1.5-12.0 10.5 Aug, IS7 43.7 2.6-7.9 10.5 July, IS7 44.6 0.7-9.4 10.1 Aug, IS4 33.9-6.1-14.9 8.8 Dec, IS4 34.0-12.7-8.0 4.7 July, IS6 40.5-7.7-12.9 5.2 Aug, IS7 43.8-3.2-12.6 9.4 July, IS7 44.3-6.6-13.1 6.5 Angl( deg) Aug, IS4 33.9-1.9-18.5 16.6 Dec, IS4 34.0-19.6-6.2 13.4 July, IS6 40.6-7.4-18.9 11.5 Aug, IS7 43.7-10.5-16.5 6.0 July, IS7 44.3-8.0-18.5 10.5 Dec, IS4 34.3-18.4 2.6 21.0 Aug, IS4 34.5 9.3-16.7 26.0 July, IS6 41.0-0.4-17.2 16.8 Aug, IS7 43.8-5.1-13.1 8.0 Dec, IS7 43.8-14.8-5.3 9.4 July, IS7 44.9 1.5-15.5 17.0
Table 2. ENVISAT ASAR AP backscatter values from various land surface covers and object types. Green colour is ascending and yellow is descending satellite pass. a) Dec, IS4 34.1-18.0-12.0 6.0 Aug, IS4 34.1-10.6-14.8 4.2 July, IS6 40.8-11.3-16.7 5.4 Aug, IS7 42.7-11.7-17.3 5.6 Forest (Garderåsen) July, IS7 44.7-10.5-15.5 5.0 b) Agriculture (Jølsen) Dec, IS4 33.9-22.5-12.7 9.8 Aug, IS4 34.0-13.2-19.1 5.9 July, IS6 40.8-13.2-20.9 7.7 Aug, IS7 43.7-10.9-20.1 9.2 July, IS7 44.5-12.9-20.5 7.6 c) Grass (golf area) Dec, IS4 33.9-15.9-21.4 5.5 Aug, IS4 34.3-24.5-17.6 6.9 July, IS6 40.5-17.9-23.2 5.3 Aug, IS7 43.7-16.3-21.7 5.4 July, IS7 44.5-17.6-23.6 6.0 d) Asphalt Aug, IS4 34.3-19.3-23.7 4.4 covered Dec, IS4 34.4-25.0-20.5 4.5 (aircraft July, IS6 40.9-20.3-24.0 3.7 parking) Aug, IS7 43.7-18.9-27.0 8.1 area July, IS7 44.9-20.5-25.7 5.2 e) Lake (Mari-dalsvatn) Aug, IS4 32.5-22.9-26.2 3.3 Dec, IS4 32.8-26.9-20.0 6.9 July, IS6 39.6-24.7-26.7 2.0 July, IS7 43.6-25.0-28.7 3.7 Aug, IS7 44.7-23.1-27.3 4.2
4. CONCLUSIONS Omni-directional scattering objects show stable backscatter difference properties (around 10 db) between the like-pol and cross-pol ASAR AP mode channels, regardless of radar incidence or aspect angles. Buildings prone to the cardinal effect seem to give large inter-channel backscatter variations when comparing the ASAR AP data sets (HH versus HV). The cardinal effect does not seem to have a noticeable effect on the AP mode cross-pol channel. ASAR like-pol and cross-pol backscatter will vary as a function of radar viewing angle for certain man-made objects. This may be used to set up rules in a classification procedure. 6. REFERENCES 1. Weydahl D. J., Backscatter changes of urban features using multiple incidence angle RADARSAT images, Can. J. Remote Sensing, Vol. 28, 782-793, 2002. 2. Absolute Calibration of ASAR Level 1 Products Generated with PF-ASAR, ESA Technical Note, revision 4, January 2004. 3. ASAR Product Handbook, ESA, revision 20 August 2002. 4. Levine, D., Radargrammetry. New York: McGraw-Hill Book Co, 1960. 5. Hardaway G. and Gustafson G. C., Cardinal effect on SEASAT images of urban areas. Photogrammetric Engineering and Remote Sensing, Vol. 48, 399-404, 1982. Fields show larger like-pol variation as a function of incidence angle, than coniferous forest. Fields show smaller cross-pol variations as a function of incidence angle, than coniferous forest. It seems to be a trend that shorter vegetation will give lower cross-pol signatures: forest > fields > grass The AP inter-channel difference between copol and cross-pol is more stable for forest areas than for agriculture when evaluating ASAR images acquired in different swaths throughout the growing/harvest season. 5. AKNOWLEDGEMENT The ENVISAT ASAR data were ordered as part of ESA AO-434.