CHRIS DATA PRODUCTS - LATEST ISSUE

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1 CHRIS DATA PRODUCTS - LATEST ISSUE Mike Cutter (1), Lisa Johns (2) (1) Sira Technology Ltd, South Hill, Chislehurst, Kent, BR7 5EH, England. Mike.Cutter@sira.co.uk (2) Sira Technology Ltd, South Hill, Chislehurst, Kent, BR7 5EH, England. Lisa.Johns@sira.co.uk ABSTRACT At the CHRIS/PROBA Workshop held last year in ESRIN a review was presented of various aspects associated with the CHRIS calibration [1]. The review addressed: first, the variation of the dark signal data, secondly, the use of the generic dark-field data sets for correcting analogue electronic offsets and, thirdly, the influence of the platform temperature variations on instrument response and hence radiometric calibration. It was concluded, first, that the errors introduced by gradual increase in dark signal through the mission life were negligible. Secondly, the use of the generic dark image data for correcting analogue offsets is probably acceptable for most land applications, where the scene albedo is relatively high. However, improvements in absolute offset errors may be worth pursuing for coastal scenes where the scene albedos are low, accepting that some images may experience saturation when imaging bright targets. Lastly, the temperature-induced pixel shifts significantly affect radiometric calibration and this effect required modelling within the image processing software. This paper reviews the changes that have been made to the data processing, following on from last year s conclusions, and also addresses other information that has been added to the CHRIS data sets. 1 INTRODUCTION Sira Technology Ltd developed the Compact High Resolution Imaging Spectrometer (CHRIS). This instrument has been designed principally to provide remote sensing data for land applications and aerosol measurements; it is also used for coastal zone monitoring, although this was not a design driver. It is the main instrument payload on the European Space Agency (ESA) small satellite platform PROBA (Project for On-Board Autonomy). At perigee, CHRIS provides a ground sampling distance of 17m, over typical image areas 13km square. It has a spectral range from 400nm to 1050nm, at spectral resolution <11nm. The instrument provides sets of images of selected target areas, at different pointing angles, forming up to 5 images of each target in a single overpass. The small platform offers limited scope for on-board calibration facilities, nevertheless, calibration is provided by a mixture of pre-flight and in-orbit measurements. The calibration data sets are derived from: Pre-flight: In-flight: full aperture radiometric calibration, wavelength characterisation DC offset measurements, wavelength calibration, 2 CALIBRATION PROCESS Each image acquired by CHRIS includes raw image data and DC offset data. Components of DC offset include electronic offsets, dark signal and smear generated during CCD frame transfer. Full-frame dark field data is acquired using images of dark Earth, with the platform in eclipse. The current process of correcting CHRIS images involves subtracting dark signal and other offsets, and dividing the result by response data acquired from pre-launch measurements. Adjustments are made at various stages during the processing to take account of the gain, integration time and number of CCD rows in each spectral band. 3 ANALOGUE OFFSET ERRORS The raw data generated by CHRIS includes analogue offsets. These originate from dark charge introduced in the Charge Coupled Device (CCD) and by the CHRIS analogue electronics. The dark charge can be estimated by imaging the ground in the eclipse period of the orbit. Typically the signal levels are small (approx. 1 ADC count) and do not vary dramatically with time [1], except where proton damage may take place. The offsets introduced by the analogue electronics are significant and do require removal. The analogue offsets can be removed in the image processing procedure by subtracting the pixel data from a specific dark image data set [1]. A specific dark image data set is a dark image, acquired in dark scene conditions, with the same Proc. of the 3rd ESA CHRIS/Proba Workshop, March, ESRIN, Frascati, Italy, (ESA SP-593, June 2005)

2 spectral, spatial and gain configuration as the bright image that is being corrected. In practice the use of specific dark image data for offset correction is quite cumbersome for the CHRIS operations for two reasons. First, the configuration gain is optimised with respect to target latitude to maximise the signal to noise ratio, leading to a large number of different configurations. Secondly, only one dark image can be acquired in a single download session and in practice we would wish to repeat the acquisition of dark image data each month. For the 2004 Science Plan it would have been necessary to acquire 46 specific dark images, taking several days, and probably requiring monthly repeats. This process was considered cumbersome and was not preferred. To minimise the number of configurations that need to be used, a generic dark image data set is acquired and used together with the average bright image offset data that is recorded at each end of a CCD row. The generic dark image data is acquired by recording a dark image of all the bands that are within the total spectrum of interest, i.e. 400 to 1050nm, plus the smear band. This data is then used to construct any band combination required to correct the dark signal offsets in the bright images. The use of the generic dark image approach is convenient for the CHRIS operation. However, it provides the correct offset only if the offsets are constant across the CCD. In practice this is not the case and a small variation is seen. Assuming the analogue levels across the CCD stay within the offset pixel levels recorded at the ends of each recorded CCD row, which is probably a reasonable assumption, then the maximum offset error is half the difference between the averages of the offset levels recorded at the ends of each row. An example of the level of maximum bias errors, as a proportion the scene signal, was presented at the last workshop [1] and is shown here in Fig. 1. In most cases the level of errors are probably acceptable, although Mode 2 errors are higher than those of the other modes. Maximum bias error 1.4% 1.2% 1.0% 0.8% 0.6% 0.4% 0.2% 0.0% Mode 1 - Great Plains (3E6C) Mode 2 - Rame Head (3E3F) Mode 3 - Jornada Mode (3E8A) Mode 4 - Swiss National Park (3DBD) Mode 5 - Railroad Valley (3E62) Band number Fig. 1. Maximum Bias Errors In practice the higher errors seen are due to low scene signals. The Mode 2 image over Rame Head an area mainly of water giving low signals resulting in the high bias error for band 18 of >1%. Analogue Gain Mode 2 - Rame Head (3E3F) Spectral band Fig. 2. Original Mode 2 gain levels The original settings for Mode 2 associated with Rame Head are indicated in Fig. 2. To reduce the errors in Mode 2, which is specifically a coastal band, it was decided to increase all the gains to the maximum level, not only reducing the bias errors but also improving the signal to noise. (NB. The gain settings are achieved by using different amplifier chains and this means that there is little additional noise added as the gain is increased. The noise has been measure at a level below 1 ADC for all gains.) The consequence of increasing the gains is that saturation will occur at lower scene albedos. Previously the maximum signal level was sized for an albedo of 1.1. With the new gain settings saturation will now occur for albedos of approximately 0.25 and 0.5 albedo, corresponding to gain increases of 4 and 2 respectively. 4 CALIBRATION WRT TEMPERATURE The temperature of the CHRIS instrument in-orbit varies due to seasonal variations and platform events.

3 Temperature changes of typically 4 C are recorded, although within an imaging session changes are <0.5 C. The temperature variation of CHRIS produces a shift of the spectrum along the CCD columns, i.e CCD pixels per C. This effect was not compensated in the original CHRIS data sets (pre-version 4). Modifications were made to the CHRIS processing algorithm and a correction curve derived to take account of the instrument temperature. The correction curves for a number of image scenes were generated and used to compare with data supplied by two PIs (Heike Bach, VISA GmbH and Luis Guanter, Univ. of Valencia). Both PIs had undertaken vicarious calibration on the assigned sites (Upper Rhine and Barrax respectively.) The results from the two PIs are presented in Fig. 3 & 4. Correction factor Fig. 3. Correction curve estimated by Heike Bach, from the Upper Rhine validation campaign, compared to the CHRIS processing algorithm correction. Calibration coefficient Correction Hbach Image 3665 Correction L Guanter Results Guanter Image 3598 H Bach Results Wavelength (nm) Spectral bands Fig. 4. Correction curve estimated by Luis Guanter, from the Barrax validation campaign, compared to the CHRIS processing algorithm correction. The results (Fig 3 & 4) show reasonable correspondence between the algorithm and ground validation trials. Correction factor Fig. 5. Correction curve estimated by Luis Guanter, using Barrax validation data from 2003 and 2004 together with the average correction curve estimated using the CHRIS algorithm. Luis Guanter also undertook a comparison of the Barrax results from 2003 and 2004, see Fig. 5. The interesting feature of the curves in Fig. 5 is that the variation between 2003 and 2004 is relatively small. The correction curve indicted is the average calculated correction for the 2003 and 2004 acquired data. Furthermore, these averages are based on the measurements made during pre-launch calibration tests. (It should also be noted that an on-board Solar Calibration Device (SCD), which interrogates a small area of the aperture, showed variations between 2003 and 2004 of between 5-4%. However, this may not be representative of the full aperture and is itself susceptible to contamination.) 5 OBSERVATION ANGLES The multi-angle observation angles made by CHRIS are nominally: +/- 55, +/-36 and 0. These angles are established by appropriate division of the observation time Tobs over each target. The details of the scan procedure is defined below [2]. Tobs C1 C2 C3 h L Guanter (Barrax) results Average 2003 Average 2004 Correction Wavelength (nm) 55 ϕ C4 C5 Fig. 6. Schematic of observation periods.

4 The CHRIS acquisition procedure is based on a total observation time interval Tobs, which is calculated onboard based on the following parameters: φ is defined based on a 55º acquisition cone θ centred on the test site as illustrated in Fig 6. Target longitude and latitude (long, lat) Target height (h) Angular velocity of the PROBA platform ω Tobs defines the beginning of the 1 st scan to the end of the 5 th (and final) scan and is determined by Tobs = φ/ω The estimates of φ and ω are made at a time t=390s prior to the start of the first image acquisition using instantaneous on-board values of the moment, i.e. the result of the filtering of the on-board GPS data. Tobs is frozen 390s prior to the acquisition manoeuvre. The estimation methodology is 1) φ assumes a circular orbit with a fixed distance to the spacecraft equal to the semi-major axis of the orbit. The semi-major axis is the estimated value on-board. To determine the location of the target centre a spherical Earth is assumed with the radius defined by the centre of the Earth to target. The target s geographical latitude and longitude are transformed into geocentric coordinates. The transformation geodetic geocentric accounts properly for the Earth flattening. The altitude is then measured above the WGS84 geoid. 2) ω is estimated from actual PROBA orbit data, i.e. the true angular velocity at t=390s Once Tobs is fixed, the timing of the remaining acquisition manoeuvres is by definition fixed and can be subdivided into: 1) scan time Tsc = 20s within which the imaging is performed (NB. Only 10s is used for imaging). 2) slew period Tsl = 12.5s between each scan 3) margin periods Tmar added to both sides of the scan to damp transients occurring between slew period and the scan period. The total observation period is made up of 5 scans, 4 slew periods and 8 margin periods (the margin periods before the 1 st scan and after the 5 th scan are considered outside Tobs) as illustrated in Fig. 7. The scan times Tscan are evenly distributed across Tobs. Tmar is the only free variable and is chosen so that the total time adds up to Tobs. All Tmars are equal. As Tobs is calculated based on the actual angular rate of the orbit, the centre times C1 to C5 vary slightly over time as function of orbit height and corresponding changes in. Tobs = Tsc+Tmar+ Tsl+Tmar+Tsc+Tmar Tsl+Tmar+Tsc+Tmar Tsl+Tmar+Tsc+Tmar Tsl+Tmar+Tsc 1 st acquisition 2 nd acquisition 3 nd acquisition 4 th acquisition 5 th acquisition Fig. 7. Illustration of the image acquisition sequences including scan time (Tsc), slew period (Tsl) and margin periods (Tmar). The spacecraft is oriented such that the instrument lineof-sight is pointing towards the target at all time. This definition leaves one degree of freedom open; the rotation around the Line-of-sight (LOS). PROBA has adopted a convention to fully define the rotation matrix from the orbital frame (roll-pitch-yaw) to the frame defining the attitude of the spacecraft while imaging. Instead of a general sequence of three rotations, only two are used. First, a pitch rotation is made to bring the pitch-yaw plane onto the target. Then a roll rotation (the new roll) is made to bring the LOS towards the target. This strategy effectively assumes a flat Earth. The projection of the instrument slit on the ground is a straight line perpendicular to the ground track when looking straight down towards nadir. It would still be a line perpendicular to the ground track after the two above rotations if the Earth were flat. However, the Earth sphere effectively distorts the line. The scanning motion is super-imposed on to the above manoeuvre by targeting a moving point on the Earth instead of targeting always the centre of the image. This point is moving over the area to image in a plane parallel to the orbital plane, effectively rotating back and forth around the orbital axis, buried in the Earth. In order to keep the same scanning direction for all 5 images, the rotation axis is also frozen shortly before the beginning of the acquisition and maintained throughout. This compromise keeps the direction of scan constant on Earth at the expense of scanning always exactly parallel to the ground track. The on-board derived GPS data was used by Sira to derive the observation angles, as seen from the target location. (GPS data is available with a frequency of 6 seconds during the acquisition period.)

5 An orbital model was developed using Mathcad and the genfit routine used to fit functions to the GPS data on x, y, z axes, over a period 200s to +200s with respect to the image centre. The data is smoothed and the x,y,z location calculated at the image centre times. In detail: Earth rotation is subtracted from raw GPS data the coordinate system is rotated to targetlatitude to take out orbit inclination x,y,z data are fit to epicyclic functions (approx. values for these functions are computed from historical TLE data orbit altitude, eccentricity and argument of perigee. The TLE data is used to aid convergence) Inclination and Earth rotation are re-instated before using the functions The target location is computed with respect to GPS coordinates using target latitude, longitude and altitude (NB, Geoid shape is included). Finally the azimuth and elevation angles are calculated using conventional trigonometry. Fig. 8 shows the difference between the actual position taken from the GPS data and the modelled data. The differences are less than 10m in all axes. An example of the calculated zenith and azimuth angles are shown in Fig. 9. The above routine for calculating the azimuth and zenith angles is now undertaken routinely. These angles are included in version 4-1 of the CHRIS HDF files. 6 ADDITIONAL CHRIS DATA UPDATES AS well as the changes described above to the CHRIS HDF files, namely radiometric calibration and observation angles, changes have also been made to the files as follows: A quality mask has been added to the HDF data, this includes: o Pixel saturation information o Occurrence of PPU reset errors Negative numbers were present in the radiance data due to the standard application of the processing algorithms on the zero data arising from PPU reset errors this data has now been re-instated to zero to avoid confusion. 7 DOCUMENT CHANGES Fig. 8. Actual position (GPS data) and modelled satellite position for (Lake Constance 22 nd April 2004) top curves and differenced data in the lower curves. All the changes to the HDF data has been recorded in CHRIS Data Format document issue 4-1. The previously recorded data is being updated to version 4-1 (i.e. CHRIS_CL_050315_505F_41.HDF) The CHRIS data sets (approx. 700) acquired prior to 2005 are being up-issued to version CONCLUSIONS This paper has reviewed the latest situation on the CHRIS data product and detailed the changes and updates that have been implemented in 2004/5. In particular the following changes have been made: Fig. 9 Calculated azimuth and zenith angles (Lake Constance 22 nd April 2004) Radiometric calibration o The radiometric calibration procedure now models the effect of small variations in instrument temperature. Furthermore, the results have been compared with ground validation data acquired over Upper Rhine and Barrax.

6 Gain levels o Following the review presented at last years CHRIS-PROBA Workshop [1], of the offset errors introduced by the analogue electronic offsets, it has been decided to increase the gains of the Mode 2 (coastal) to the maximum level. This will reduce not only the bias errors but also improve the signal to noise. However, the consequence is that saturation will occur at low albedo levels. Observation angles o The methodology for setting up the five acquisitions for imaging each target has been described as has the procedure for calculating the azimuth and zenith angles as seen from the position of the target. Miscellaneous updates o Various data artefacts have been highlighted by use of a mask in the HDF files. In addition the non-zero radiance numbers in the HDF files, related to the PPU reset errors, have been re-instated to zero. Documentation and data updates o Finally a new version of HDF files 4-1 has been prepared and is being issued for data acquired from 1 January The previously acquired data is being updated to the version ACKNOWLEDGEMENTS We would personally like to thank Luis Guanter, University of Valencia, and Heike Bach, VISTA GmbH, for supporting the validation activities using data acquired over their respective sites. In addition, we would also like to thank the team at the ESA ground station in Redu for cooperating in the provision of the additional GPS data. Fig. 9. Malindi, Kenya, ( ) Fig. 10. Topana, French Polynesia ( ), 10 REFERENCES 1. M Cutter, Review of Aspects Associated with the CHRIS Calibration, Proc. of the Second Workshop CHRIS/Proba, (ESA Proc. SP-578) April 2004, Frascati, Italy, ISBN M Davidson and P Vuilleumier, Note on CHRIS Acquisition Procedure and Image Geometry, Internal ESTEC Note