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Q1 - How can we assign the band B1, B2 and B3 of XS to the Red Green Blue colour so that the natural green colour can be reproduced? The transformation formulas used for natural green colours are much more complex. The process is not automatic. First of all, one does a special radiometric enhancement. Then, the process is a polynomial transformation from bands B1, B2, B3 to new R, G, B bands. The coefficients of this transformation depend on the original radiometry fand landscape. The final step is manual adaptation.

Q2 - How can I get the irradiance values measured by the instruments? The conversion of the Spot count values to irradiances is performed as follows: L = (X/A) + B where: L = the equivalent irradiance at the input of the instrument (W * m-2 * sr-1 * micrometer-1) X = the count (0 to 255), A = absolute calibration gain, for the considered spectral band B = absolute calibration offset, for the considered spectral band. A and B can be found in the Header Record of the Leader File (Format CAP) . On a CD-ROM the first byte of the Header Record is the byte number 3961 in the Leader File. Then, within this Header Record A and B are encoded as follow:(in the following the numbers are bytes numbers within the Header Record) A: bytes 1765 to 1772 (AA.AAAAA) in panchromatic, bytes 1765 to 1772 (AA.AAAAA) in multispectral (band 1), bytes 1773 to 1780 (AA.AAAAA) in multispectral (band 2), bytes 1781 to 1788 (AA.AAAAA) in multispectral (band 3), bytes 1789 to 1796 (AA.AAAAA) in multispectral (band 4, for Spot 4), B: bytes 2277 to 2284 (BB.BBBBB) in panchromatic, bytes 2277 to 2284 (BB.BBBBB) in multispectral (band 1), bytes 2285 to 2292 (BB.BBBBB) in multispectral (band 2), bytes 2293 to 2300 (BB.BBBBB) in multispectral (band 3), bytes 2301 to 2308 (BB.BBBBB) in multispectral (band 4, Spot 4), In the DIMAP SPOT Scene format, A is the parameter "PHYSICAL GAIN" and B is the parameter "PHYSICAL BIAS". There are in the "Image Interpretation Spectral Band Info" group.

Q3 - What are the spectral characteristics of the instruments? You will find a table about Normal Solar Equivalent Irradiance and Spectral profiles in the section "Technical Information/The SPOT satellites"  [+] More

Q4 - What is the sampling frequency used by the instruments of Spot to capture the images? The time elapsed between the capture of two consecutive lines (called "line period") is: 3.008 milliseconds in multispectral mode, 1.504 milliseconds in panchromatic (or monospectral) mode. This information is posted in the ancillary data of the SPOT Scene products (see, in section "The CAP format", the document "The SPOT Standard Digital Product Format" p 46, 7.7: Ancillary "ephemeris/attitude, bytes 947-958). or in the DIMAP format (group "Sensor configuration", parameter "LINE PERIOD"  [+] More

Q5 - The ephemeris data, giving the position of the satellite every minute, are encoded with a 0.1 m resolution. Is it indicative of the actual accuracy of the knowledge of the satellite position? On Spot 1, 2 and 3, the encoding of the ephemeris data posted in the Ancillary files of the SPOT Scene products does not reflect the actual accuracy of these data which is a few hundred metres (rms). However, with Spot 4, the use of data coming from the DORIS passenger allows to get an accuracy better than 1 m. Just note that this applies only to the position of the satellite on its orbit; one has to realise that the final location accuracy of the images on the ground depends also on the pointing accuracy of the spacecraft and its instruments (attitude of the spacecraft, mirror pointing angle, etc). This lead to a final location accuracy between 300 and 500 metres (rms).

Q6 - In several papers, the focal length of Spot HRV is 2,087.4 mm. However, it is referred as 1,082 mm in Spot User's Handbook. What is correct? The focal length is 1,082 mm

Q7 - What are the differences in the Dimap format between SPOT 1 - 4 data and SPOT 5 data? The main differences in the Dimap format between SPOT 1 - 4 data and SPOT 5 data concern essentially five points: Time of acquisition Data attitudes Angles Mirror Position 1B Models Satellite_time: (Data_Strip/Satellite_Time) In this group, SPOT 1 - 4 data are described by four keywords: UT_DATE CLOCK_VALUE CLOCK_PERIOD BOARD_TIME On SPOT 5, a supplementary keyword, TAI_TUC, describes data which allow international atomic time to be calculated. Satellite_attitudes: (Data_Strip/Satellite_Attitudes) The attitude data are described by a block keyword on SPOT 1 - 4, and by two block keywords on SPOT 5. On SPOT 1 - 4: Row_attitudes, On SPOT 5: Row_attitudes and Corrected_Attitudes. It is recommended, on SPOT 5, to use the information from the block Corrected_Attitudes which integrate the measures from the star tracker. Instrument_Look_Angles_List: (Data_Strip/Sensor_Configuration/Instrument_Look_Angles_List/Instrument_Look_Angles/Look_Angles_List) The keyword is identical on SPOT 1 - 4 and SPOT 5, while the number of values is greater on SPOT 5: For SPOT 1 - 4, only the look directions of the first and the last detector of the CCD array are supplied, For SPOT 5 a look direction is supplied for each 6,000 (or 12,000) elementary detectors. Mirror_Position: (Data_Strip/Sensor_Configuration/Mirror_Position) In this group, SPOT 1 - 4 data are described by a single keyword: STEP_COUNT. For SPOT 5, three supplementary keywords describe the data: AVERAGE_ENCODER_COUNT Encoder_Counts MCV_Matrix Models: (Data_Strip/Models) The information described by this keyword block allow to convert the geometry of a level 1B SPOT 1 – 4 into the level 1A geometry. This keyword group does not exist on SPOT 5.  [+] More

Q8 - Which image processing software on the market are compatible with the Dimap format used for the SPOT 1 to 4 SPOT Scene products? The image processing software editors received a test data set in September 2004. At present the latest versions of the following software have been validated as compatible: Erdas Imagine(Leica LPS ) Geomatica (PCI ER Mapper Envi ZI Imaging (Solution photogrammétrie Intergraph) Geoimage If your software does not recognize the Dimap format used for the SPOT 1 to 4 SPOT Scene products, and if it is not in the list above, please contact us or contact the software editor directly.  [+] More

Q9 - In what order are the spectral bands in SPOT products displayed? > SPOT spectral bands: The spectral bands in SPOT imagery are: XS1 = green XS2 = red XS3 = near-infrared XS4 = short-wave infrared (SWIR) for SPOT 4 & 5 > RGB display: The RGB display scheme is: 1st band (1) in the extracted raster file displayed in the red channel 2nd band (2) in the green channel 3rd band (3) in the blue channel > TIFF file: To conform to the RGB display scheme, spectral bands in TIFF files are extracted in the following order: 1: XS3 2: XS2 3: XS1 To obtain the following display scheme for SPOT products: - XS3 displayed in red, since it is the 1st spectral band extracted (1) - XS2 in green (2) - XS1 in blue (3) The DIMAP file describes how spectral bands are assigned to channels to obtain this display scheme: <RED_CHANNEL>1</RED_CHANNEL> <GREEN_CHANNEL>2</GREEN_CHANNEL> <BLUE_CHANNEL>3</BLUE_CHANNEL> > BIL files: In BIL files, spectral bands are extracted in the following order: 1: XS1 2: XS2 3: XS3 The DIMAP file describes how spectral bands are assigned to channels to obtain the SPOT product display scheme: <RED_CHANNEL>3</RED_CHANNEL> <GREEN_CHANNEL>2</GREEN_CHANNEL> <BLUE_CHANNEL>1</BLUE_CHANNEL> > SWIR band: As only 3 channels can be displayed in colour at once, the SWIR band is not displayed. To display the SWIR band, you can assign XS4 to the green channel, XS3 to red and XS2 to blue. Be careful not to confuse the names of SPOT spectral bands (XS3, XS2 and XS1) with the order of display channels (RGB).