ASAR APP AND APM IMAGE QUALITY

Peter Meadows & Patricia Wright

BAE SYSTEMS Advanced Technology Centre, West Hanningfield Road,Great Baddow, Chelmsford, Essex, CM2 8HN, United Kingdom

Email:peter.meadows@baesystems.com, patricia.wright@baesystems.com

ABSTRACT

This paper gives details of the image quality analysis of ASAR alternating polarisation mode APP and APM productsperformed during the ASAR validation phase. Initially, a description of the analysis methodology and performanceassessment tools, image properties and example APP and APM products is given followed by the image qualityanalysis. This analysis includes format verification, visual inspection of the imagery, impulse response functionmeasurements, cross-polarisation ratio, channel co-registration, equivalent number of looks and radiometric resolutiondetermination, analysis of azimuth ambiguities, localisation accuracy, a preliminary radiometric calibration includingthe derivation of preliminary calibration constants and noise equivalent radar cross-section measurements.

1 INTRODUCTION

During the 3 month Envisat validation phase period after the end of the commissioning phase, further analysis of ASARproducts has been carried out by the ASAR calibration and validation team [1]. Two of the ASAR alternatingpolarisation products are the precision product (APP) and the medium resolution product (APM) - both are groundrange, multi-look, detected imagery with two polarisations (HH & VV, HH & HV or VV & VH). The APP is of highresolution (∼30m) with 12.5m pixel size and approximately 100km azimuth extent while the APM is of mediumresolution (∼150m) with 75m pixel size and an azimuth extent of up to several thousand km. This paper gives details ofthe quality assessment and preliminary radiometric calibration of APP and APM products.

The orbit numbers of ten APP products analysed are shown in Table 1 - these were of either The Netherlands for theASAR transponders or of Resolute, Canada for one of the Radarsat transponders. The ten APM products analysed werefrom the same orbit numbers as the APP products. The table also gives the polarisation combinations analysed. Allthese products were processed with v3.03 of the ESA PF-ASAR processor. Note that no IS1 swath data was availablefor analysis. In addition, APP data acquired from orbits 2076 and 3586 have been used in Fig. 1, Fig 2 and Fig 8(a).

Table I. Orbit numbers of ESA PF-ASAR v3.03 APP and APM products (blue orbit numbers denote imageryof The Netherlands while the purple numbers denote imagery of Resolute, Canada).

All the products listed in Table I were acquired with a pulse bandwidth of 16MHz for all swaths (for data acquiredbefore 17th October 2002, only IS1 and IS2 have a pulse bandwidth of 16MHz). However, all data in Table I wereprocessed with 1 range look which had the same bandwidth as for pre 17th October 2002 acquired data. This leads to aconsistent range spatial resolution irrespective of acquisition date. Future version of the PF-ASAR processor will have 2range looks covering the full 16MHz bandwidth where each look has the same bandwidth as at present.

__________________________________________________________________________________________________________Proc. of Envisat Validation Workshop, Frascati, Italy, 9 – 13 December 2002 (ESA SP-531, August 2003)

2 ANALYSIS METHODOLOGY AND TOOLS

The APP and APM product format verification was performed using the EnviView software [2] and the SAR ProductControl Software (SARCON) [3]. EnviView was used to examine all the product header parameters while SARCONwas used for examination of a sub-set of header parameters and visualisation of the image data. Image analysis wasalso performed using SARCON (point target and calibration modules) and this followed the procedures outlined in theESA document on Quality Measurement Definitions for ASAR Products [4].

Note that as no calibration constants had been derived prior to the processing of the APP and APM products, all theproduct headers included a default calibration constant of 1 (0dB).

3 PROPERTIES OF IMP PRODUCTS

The APP product is an alternating polarisation mode ground range detected image with two polarisations. Theproperties of APP products are listed below:

• Ground range detected alternating polarisation imagery

• Dual polarisation (HH & VV, HH & HV or VV & VH)

• Elevation antenna pattern and range spreading loss corrections applied

• Size up to 300Mbytes with 2 byte (16 bit) amplitude pixel values

• Swath widths of 100km (IS1) to 56km (IS7) with azimuth extents of ~100km

• Azimuth spatial resolution of 27.6m (2 looks of ~250Hz each)

• Range spatial resolution from 21m (IS2 far range) to 37m (IS1 near range) and ~26m for IS3 to IS7

• 12.5m by 12.5m pixels (hence under-sampling for spatial resolutions less than 25m).

The APM product is a medium resolution alternating polarisation mode ground range detected image with twopolarisations. The properties of the APM products are listed below:

• Ground range detected medium resolution alternating polarisation imagery

• Dual polarisation (HH & VV, HH & HV or VV & VH)

• Elevation antenna pattern and range spreading loss corrections applied

• Size up to few Mbytes with 2 byte (16 bit) amplitude pixel values

• Swath widths of 100km (IS1) to 56km (IS7) with azimuth extents of ~100km up to 4000km

• Azimuth spatial resolution of 135m (10 looks of ~50Hz each)

• Range spatial resolution from 109m (IS1 far range) to 163m (IS1 near range), 115m to 150m for IS2 and ~130m forIS3 to IS7

• 75m by 75m pixels (hence under-sampling for spatial resolutions less than 150m).

4 EXAMPLE APP PRODUCTS

Below are three example APP products. Fig. 1 shows an APP product of Ottawa, Canada - Fig.1(a) shows the fullscene while Fig. 1(b) shows more detail around the city of Ottawa. This scene and those shown in Figs. 2 and 3 arefalse colour representations derived from the two polarisations of the APP product. Two of the RGB colours areassigned to each of the polarisations with the third colour being the difference between the two polarisations. Fig. 2shows an APP product of Resolute, Canada while Fig. 3 shows The Netherlands and the region around Amsterdam.

Fig. 1(a). ASAR APP product of Ottawa, Canada from 24th July 2002, IS2 swath and HV & HH polarisations.

Fig. 1(b). ASAR APP product of Ottawa, Canada from 24th July 2002, IS2 swath and HV & HH polarisations.

Fig. 2. ASAR APP product of Resolute, Canada, from 6th November 2002, IS2 swath and HV & HH polarisations.

Fig. 3(a). ASAR APP product of The Netherlands, from 6th November 2002, IS2 swath and HV & HH polarisations.

Fig. 3(b). ASAR APP product of The Netherlands, from 6th November 2002, IS2 swath and HV & HH polarisations.

5 EXAMPLE APM PRODUCTS

Below are two example APM products. Fig. 4(a) shows the HH and HV polarisation images for an APM product to thenorth of The Netherlands. Note that the HV image shows many more point targets (ships etc) than the HH image and inaddition, an amplitude modulation in range over the ocean. A comparison of the HV range profile with that expectedfor an image with just noise (i.e. no ocean backscatter) shows that the range variation is predominantly due to noise (seeFig. 4(b)). Note that the shape of the expected noise profile is determined by the antenna pattern of each swath. Fig. 5shows a false colour composite an APM product starting over The Netherlands.

Fig. 4(a). ASAR APM product to the north of The Netherlands from 9th November 2002, IS3 swath and HH (left)and HV (right) polarisations.

Fig. 4(b). ASAR APM HV Sigma0 profile from 9th November 2002, IS3 swath - the curve is the expected profilefor an image with just noise (i.e. no ocean backscatter). The calibration constant used to derive Sigma0 is fromTable IV.

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Fig. 5. ASAR APM product from 12th November 2002, IS3 swath and VV & VH polarisations.

6 FORMAT VERIFICATION

No problems were identified with the APP and APM product formats or with the header parameters in a selection of theproducts analysed.

7 VISUAL INSPECTION

Visual inspection of all post orbit 3661 APP and APM products showed no artefacts or any other problems (an on-boardpatch solving an alternating polarisation image problem at sampling window start time (SWST) changes was uploadedon orbit 3661(11th November 2002)).

8 IMPULSE RESPONSE FUNCTION MEASUREMENTS

The quality of the ASAR APP products has been assessed via impulse response function (IRF) measurements of theESA ASAR transponders deployed within The Netherlands and the Radarsat transponders located within Canada. Fig. 6shows a selection of these point targets (all sub-images 1.6km by 1.6km). The IRF measurements made are:-

• the spatial resolution (3dB widths of IRF) in azimuth (x) and range (y),

• the integrated sidelobe ratio (ratio of energy in the sidelobes within a box 20x by 20y to the energy in the mainlobe(2x by 2y)),

• the peak sidelobe ratio (ratio of the intensity of the most intense peak outside the main lobe within a box 10x by 10yto the energy in the mainlobe) and

• the spurious sidelobe ratio (ratio of the intensity of the most intense peak outside a 10x by 10y box and within a box20x by 20y to the energy in the mainlobe).

Note that for the Radarsat transponders shown in Fig. 6, the IRF in the co-polarisation (HH) looks fainter than in thecross-polarisation (HV) - this is just due to the brighter background of the co-polarisation image (the measured radarcross-section from both polarisations are almost identical).

Fig. 6. ESA transponders in The Netherlands (Aalsmeer, Edam, Swifterbant, Zwolle) and two of the Radarsattransponders in Canada (Frederiction, Resolute).

Table II gives the ASAR APP IRF measurements. This table also includes the theoretical values, the requirements andan acceptable limit on the measured values [5]. Table II and Fig. 7 show that the APP spatial resolutions, the integratedsidelobe ratio (ISLR) and spurious sidelobe ratio (SSLR) all compare well with their theoretical values while the peaksidelobe ratio (PSLR) is slightly higher than the theoretical value.

Parameter Measured Theoretical Requirement Limit

Azimuth Resolution 28.09±1.97m 27.6m 30m +10%

Range Resolution See Fig. 7 See Fig. 7 < 38m (IS1) &< 30m (IS2 -7)

+10%

ISLR -12.87±1.44dB -12.4dB <-12dB +5dB

PSLR -19.07±0.94dB -21.2dB < -20dB +5dB

SSLR -25.98±2.20dB < -25dB

Table II. ASAR APP impulse response function measurements.

Fig. 7. APP range resolution and the theoretical range resolution profiles for swaths IS1 to IS7 (note that the IS1swath extends to 22.2° and that the IS2 swath starts at 19.2°).

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Fig. 8 shows two example azimuth and range profiles through the peak of the APP IRF’s. As the Radarsat transpondercan receive and transmit in H and V polarisation simultaneously, the transponder IRF will always be visible in any APPpolarisation, hence the four slices shown in Fig. 8(a). As the ASAR transponders cannot receive or transmit in H and Vsimultaneously, the transponder will only be visible in one of the possible polarisation combinations. There is noundersampling in range for the Radarsat transponder example, so the sidelobe structure is different from the ASARtransponder example where there is undersampling in range (there is no azimuth undersamping for APP products).

Fig. 8(a). Example APP Radarsat transponder azimuth andrange IRF profiles (for i = 15.81°, azimuth resolution =27.49m, range resolution = 34.70m).

Fig. 8(b). Example APP ASAR transponder azimuth andrange IRF profiles (for i = 23.85°, azimuth resolution =27.83m, range resolution = 24.14m).

The quality of the ASAR APM products has been assessed in a similar manner to the APP products - Fig. 9 shows aselection of ASAR and Radarsat transponders (all sub-images 4.8km by 4.8km).

Fig. 9. ESA transponders in The Netherlands (Aalsmeer, Edam, Swifterbant, Zwolle) and one of the Radarsattransponders in Canada (Resolute).

Table III gives the ASAR APM IRF measurements. This table also includes the theoretical values, the requirementsand an acceptable limit on the measured values [5]. Table III and Fig. 10 show that the APM spatial resolutions and the

integrated sidelobe ratio (ISLR) compare well with their theoretical values, whereas the measured peak sidelobe ratio(PSLR) is higher than the theoretical value and the requirement (this is due to their being undersampling in azimuth andat almost all ground ranges). The spurious sidelobe ratio (SSLR) is slightly higher than the requirement.

Parameter Measured Theoretical Requirement Limit

Azimuth Resolution 143.9±5.9m ∼135m +10%

Range Resolution See Fig. 10 See Fig. 10 +10%

ISLR -12.37±1.55dB -12.4dB <-12dB +5dB

PSLR -16.62±2.57dB -21.2dB < -20dB +5dB

SSLR -21.34±2.55dB < -25dB

Table III. ASAR APM impulse response function measurements.

Fig. 10. APM range resolution and the theoretical range resolution profiles for swaths IS1 to IS7 (note that themeasurements are made to an accuracy of 1/8 pixel (9.4m)).

Fig. 11(a). Example APM ASAR transponder azimuth andrange IRF profiles (for i = 25.36°, azimuth resolution =145.9m, range resolution = 122.1m).

Fig. 11(b). Example APM Radarsat transponder azimuthand range IRF profiles (for i = 44.22°, azimuth resolution= 144.0m (HV) & 143.7m (HH), range resolution =132.0m (HV) & 133.9 (HH)).

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Fig. 11 shows two example azimuth and range profiles through the peak of the APM IRF’s - one for an ASARtransponder and the other for a Radarsat transponder. As both the azimuth and range spatial resolutions are less than150m, undersampling occurs.

9 CROSS-POLARISATION RATIO

As the ASAR transponders operate in only one polarisation combination at a time, there should, ideally, be nothingvisible at the location of the transponder IRF in any other polarisation combination. Fig. 12 shows three exampleimages at the location of the ASAR transponder IRF in the other polarisation combination from an operational ASARtransponder. At the center of each image a faint point target can be seen - the appearance of these point targets could becaused by the transponder itself and/or the ASAR instrument. Note that the cross-polarisation point targets are moreprominent in the HV polarisation than the co-polarisations due to the reduced background radar cross-section. Based on17 measurements, the average APP cross-polarisation ratio is -32.1±4.2dB (the ASAR transponder cross-polarisationratio requirement is < -35dB).

Aalsmeer (HH) Edam (HV) Swifterbant (HH)Fig. 12. Example ASAR transponder cross-polarisation point targets from APP products (in the middle of each 1.6kmby 1.6km sub-image).

10 CHANNEL CO-REGISTRATION

The co-registration between the two channels can be calculated from the difference in the location of a point target IRFpeak in both polarisation channels. As the Radarsat transponders give strong IRF’s in all polarisations, they are ideal forthe measurement of co-registration. From the set of data shown in Table I, just two measurements were possible forAPP and APM products:

• APP products: 0.0m and 0.0m

• APM products: 9.4m and 0.0m.

These limited set of measurements show no mis-registration between the two channels for APP products and a smallAPM product mis-registration (note that the APM measurements are made to an accuracy of 1/8 pixel (9.4m)).

11 EQUIVALENT NUMBER OF LOOKS AND RADIOMETRIC RESOLUTION

The APP and APM equivalent number of looks and radiometric resolution have been derived using uniform distributedtargets (the radiometric resolution is a measure of the ability to distinguish between uniform distributed targets withdifferent radar cross-sections). The APP results (based on 4 measurements) are:

• Mean equivalent number of looks: 1.99±0.05 (c.f. >1.9 theoretical value, -10% limit)

• Mean radiometric resolution: 2.32±0.03dB (c.f. <2.37dB theoretical value).

The APM results (based on 16 measurements) are:

• Mean equivalent number of looks: 56.1±16.8 (c.f.>30 theoretical value, -10% limit)

• Mean radiometric resolution: 0.56±0.09dB (c.f.<0.7dB theoretical value).

These results compare well with the theoretical values.

12 AZIMUTH AMBIGUITIES

As Doppler frequencies can only be distinguishedmodulo the pulse repetition frequency (PRF), azimuthambiguities occur within the azimuth antenna patternsidelobes. Measurement requires either a very brightpoint target or a bright point target with a lowambiguity background radar cross-section. For ASARimagery, azimuth ambiguities have been derived usingthe ESA transponders (see Fig. 13).

The average APP ambiguity ratio is:

• -27.9±2.6dB

The average APM ambiguity ratio is:

• -28.7±1.9dB

The requirement is -25dB while the worst caseprediction is -27.9dB [5]. Note that the ASARtransponder ambiguity ratio meets the requirement andis similar to the predicted value.

Fig. 13. APM Azimuth ambiguities (top and bottomcenter) and the ASAR Edam transponder (22nd

November 2002, swath IS7, VH polarisation).

13 LOCALISATION ACCURACY

The localisation of ASAR APP products has been assessed by comparing the measured and predicted positions of theESA ASAR transponders deployed in The Netherlands. The predicted positions of the transponders are based on APPimage header parameters, the known latitude & longitude of the transponders and their time delay. No correction hasbeen performed for the terrain height of the transponders compared to the ellipsoid used in the processing of the APPproducts (the ASAR transponders were selected as they have a small terrain height and hence a small rangedisplacement). Localisation measurements of APP products are:

• mean range displacement: -3.8±26.3m

• mean azimuth displacement: -12.9±60.1m

• mean total displacement: 57.8±30.9m.

Fig. 14 shows firstly the azimuth and range displacements and secondly the total displacement as a function ofincidence angle for the individual measurements. There are no obvious localisation trends in azimuth, range or withincidence angle (or indeed with ascending or descending passes). Note that the worse case predicted localisationaccuracy is ∼75m in azimuth and between ∼125m for IS1 and ∼50m for IS7 in range (the total displacement

requirement is < 900m) [5]. The above results show good azimuth, range and total localisation accuracy values whichare comparable to those found for ERS using the same technique [6].

Fig. 14. ASAR APP image localisation accuracy.

14 PRELIMINARY RADIOMETRIC CALIBRATION

ASAR transponder radar cross-section measurements have been used to derive a preliminary radiometric calibration, K,for APP and APM product for swaths IS2 to IS7. As the APP products have been processed with two different productscaling factors, two K values are derived, one applicable for swaths IS1 to IS4 and the other for swaths IS5 to IS7. Fig.15 shows the APP ASAR transponder relative rcs measurements while the derived calibration constants are:

• APP K for swaths IS2 to IS4: 57.05±0.51dB

• APP K for swaths IS5 to IS7: 60.59±0.47dB.

As the PF-ASAR v3.03 APP products used to derived these K values have been processed with image mode nominalchirp powers rather than alternating polarisation mode nominal chirp powers (see Fig. 16), a correction has been appliedto the APP radar cross-section measurements. This correction is particularly important for swaths IS5 and IS7 - in factthe ASAR transponder IRF’s for these swaths were saturated and so these measurements have not been included inderiving the above K values. The standard deviation for these K values is the standard deviation of the relative rcsmeasurements and is a measure of the stability of the ASAR instrument.

Fig. 15. APP corrected relative radar cross-section measurement from the ASARtransponders (the purple box measurements are from saturated IRF’s).

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Fig. 16. AP nominal chirp powers (horizontal lines) and measured AP chirp powers fordata given in Table I.

For APM products, one product scaling factor was used for swaths IS2 to IS7 and so one K value is derived (this is alsoapplicable for swath IS1). As with the APP measurements, a correction for incorrect nominal chirp powers used in thev3.03 products has been included. Fig. 17 shows the APM ASAR transponder relative rcs measurements while thederived calibration constant is:

• APM K for swaths IS2 to IS7: 69.47±0.50dB.

This K values was derived using transponder IRF’s with acceptable integrated sidelobe ratios (due to the larger pixelsize of the APM imagery, more bright point targets appear within the area used to derived the ISLR than is the case forAPP imagery). Note that the APM derived stability value is similar to that derived from APP imagery. The initial APPand APM stability results are encouraging but more work is required before definitive calibration constants and stabilityresults can be derived.

Fig. 17. APM corrected relative radar cross-section measurement from the ASARtransponders.

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From a user point of view, the correction for the use of image mode nominal chirp powers in v3.03 products is notapplicable. Instead, the correction can be incorporated into the calibration constant. A consequence of this is that eachAPP and APM swath will have a different K value. Table IV gives these K values (these have been derived by usingthe above K values and removing the nominal chirp correction).

Swath APP K (dB) APM K (dB)IS1 57.15 69.57IS2 57.35 69.77IS3 57.28 69.70IS4 56.85 69.27IS5 64.04 72.92IS6 60.19 69.07IS7 65.52 74.40

Table IV. APP and APM K values (without the nominal chirpcorrection)

15 NOISE EQUIVALENT RADAR CROSS-SECTION

The upper limit to the noise equivalent radar cross-section (NESigma0) of an image can be estimated by measuring theradar cross-section of low intensity regions (usually ocean, open water or lakes). Using APP calibration constant valuesgiven in Table IV, Fig.18 shows APP HV polarisation NESigma0 measurements. Also shown are the predictedNESigma0 profiles for each of the swath (the two curves for each swath are for around orbit variation). It can be seenthat the minimum NESigma0 estimates compare well with the predictions and that the shape of the noise across eachswath follows that of the predicted curve.

Fig. 18. ASAR APP NESigma0 estimates (points) and predictions (curves).

16 SUMMARY

The image quality analysis of a selection of PF-ASAR v3.03 Envisat ASAR APP and APM products for swaths IS2 toIS7 has shown:

• No format problems identified

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• No visualisation problems found with any post orbit 3661 products

• APP azimuth & range resolutions and ISLR, PSLR & SSLR acceptable. Some range under-sampling (for resolutionsless than 25m)

• APM azimuth & range resolutions and ISLR acceptable. PSLR & SSLR outside expected range due to APM under-sampling in both azimuth and at almost all ground ranges (for resolutions less than 150m)

• An acceptable APP cross-polarisation ratio

• Sub-pixel AP channel co-registration

• APP and APM equivalent number of looks and radiometric resolution acceptable

• APP and APM azimuth ambiguities acceptable

• Good APP localisation accuracy outside the expected values

• Preliminary APP and APM calibration constants have been derived - more work is required before definitive Kvalues can be calculated

• Noise equivalent radar cross-sections are lower than the predicted NESigma0.

17 REFERENCES

[1] Zink, M., ASAR Calibration Review Introduction, Envisat Commissioning Review, ESTEC, The Netherlands,10-11 September 2002.

[2] Levrini, G. & Brooker, G., EnviView: a gateway to access the Envisat Data Products, Earth ObservationQuarterly, 68, pp 8-11, January 2001.

[3] Meadows, P.J., Hounam, D., Rye, A.J., Rosich, B., Börner, T., Closa, J., Schättler, B., Smith, P.J. & Zink, M.,SAR Product Control Software, CEOS SAR Workshop, London, 24-26 September 2002.

[4] ASAR Cal/Val Team, Quality Measurements Definition for ASAR Level 1 Products, Issue1, March 2002.

[5] ASAR Cal/Val Team, ASAR Cal/Val Implementation Plan, Issue 1, March 2002.

[6] Meadows, P.J., The ERS-2 SAR performance: another further update, CEOS SAR Workshop, London, 24-26September 2002.