Images archived in KuDA are derived from either the NOAA or the Dept of Defense polar orbiting weather satellites. The NOAA series includes images from four satellites which were transmitting data during 1991 (NOAA-9, 10, 11, 12). Each of these satellites carries a sensor called the Advanced Very High Resolution Radiometer (AVHRR), which is one source of images for KuDA. This sensor has five channels at approximately 0.63, 0.86, 3.7, 10.5, and 12 microns. Spatial resolution is either 1.1 km (local area coverage) or 4 km (global area coverage). KuDA originally archived only the 1.1 km images, but has recently begun archiving the 4 km images as well. The Department of Defense DMSP (Defense Meteorological Satellite Program) had three satellites transmitting during 1991 (F8, F9, and F10). These satellites carry a sensor called the Operational Line Scanner (OLS), which is the other source of images for KuDA. The OLS is a two-channel sensor with a broadband visible channel (0.4 -1.1 microns) and an IR channel (10 microns). The spatial resolution of the visible channel is 0.5 km, while the IR channel is 2.5 km. Further information about these satellites can be obtained in the following documents: NOAA Polar Orbiter Data Users Guide, July 1991, Katherine Kidwell (editor), National Environmental Satellite, Data, and Information Service, (301)763-8400 The Defense Meteorological Satellite Program Review, Goyette, et al., Preprints Fifth Conference on Satellite Meteorology and Oceanography, American Meteorological Society, Boston, p. 455-458. The remainder of this document describes the processing we have done prior to archival of the images and the format of the data files. ------------------------------------------------------------------- ---------------------- NOTES ---------------------- ------------------------------------------------------------------- PRE-PROCESSING: -------------- All NOAA AVHRR and DMSP OLS image files in the Kuwait Data Archive have undergone pre-processing. This consists of (1) hand navigation of the image using the Sea Space Terascan software (see ABOUT HAND NAVIGATION below), (2) calibration of the AVHRR IR data to brightness temperatures, (3) possible "patching" of missing data lines -- not done very often, and (4) interpolation to a lat-long grid. The images were all reinterpolated to a Cartesian lat-long grid. The corner points of our geographical analysis area are: NE Corner: 57 deg 15.0 min E, 33 deg 33.0 min N ? (was 30.0) SW Corner: 43 deg 45.0 min E, 21 deg 30.0 min N Within this domain the NOAA AVHRR data has been processed to a 1200 x 1200 grid, and the DMSP OLS data (BOTH visible and IR) has been processed to a 2400 x 2400 grid. This is a "rectangular" map projection, essentially a simple lat-long grid, but with the grid spacings adjusted so that the relative grid increments correspond to equal spacings in km at the center of the image (roughly 550 m for the dmsp data, and 1 or 1.1 km for the noaa data). NOTE: The sensor resolution for the dmsp data is typically 500 m (at nadir) for the visible ols data, but only 2.5 km for the IR data. The sensor resolution varies as a function of the distance from nadir. In both cases, however, the data has been gridded (or over-gridded) to the same 500 m grid. The center coordinate is at the center of the four adjacent pixels (since there are an even number of samples). The nominal grid spacing in both the E-W direction (longitude difference between adjacent pixels) and in the N-S direction (latitude difference between adjacent pixels) is given below. The grid spacing for the NOAA data is: 13.50 degrees / (1200-1 grid intervals) = 0.011259 degrees of longitude 12.05 degrees / (1200-1 grid intervals) = 0.010050 degrees of latitude The grid spacing for the DMSP data is: 0.004023 degrees of latitude 0.005627 degrees of longitude These can be converted to kilometer intervals using any earth model or geoid of choice. Using the Smithsonian Meteorological Tables (Tables 162 and 163) for the International Ellipsoid of 1924, the grid spacings are nominally: NOAA: N-S "y" interval 1.113 km at 20-21 degrees N 1.114 km at 27-28 degrees N 1.115 km at 32-33 degrees N E-W "x" interval 1.171 km at 21 degrees N 1.118 km at 27 degrees N 1.052 km at 33 degrees N DMSP: N-S "y" interval 0.556 km at 20-21 degrees N 0.557 km at 27-28 degrees N 0.557 km at 32-33 degrees N E-W "x" interval 0.585 km at 21 degrees N 0.558 km at 27 degrees N 0.526 km at 33 degrees N Users are cautioned that in general, the inaccuracies in the orbital elements, satellite reference time standards, roll-pitch-yaw estimates, sensor alignment, and the interactive hand navigation steps are likely to be much more significant than inaccuracies in an earth model. The following Fortran subroutine provided by SeaSpace may be used to compute Cartesian coordinates (model unknown). > Earth radius is 6378.145 km > > C ll_xyz.f 1.2 1/30/90 > SUBROUTINE ll_xyz(rlat, rlon, x, y, z) > C > C Converts geodetic latitude,longitude to Cartesian > C coordinates. (Earth radius at equator assumed = 1) > C > C Input > C rlat - double precision, latitude (radians) > C rlon - double precision, longitude (radians) > C > C Output > C x,y,z - double precision, (x,y,z) coordinates > C > DOUBLE PRECISION rlat, rlon, x, y, z > C > DOUBLE PRECISION flat, flat2, temp, denom > PARAMETER (flat = 1.D0/298.25D0) > PARAMETER (flat2 = 2.*flat - flat**2) > C > temp = DSIN(rlat) > denom = DSQRT(1.D0 - flat2*temp**2) > z = ((1.D0 - flat)**2)*temp/denom > temp = DCOS(rlat) > x = temp*DCOS(rlon)/denom > y = temp*DSIN(rlon)/denom > C > RETURN > END DATA FORMATS: ------------ The data on the MSS has been kept in its native TDF (i.e. Sea Space) format. This is essentially a variety of CDF. The format consists of a 644 byte header, the data itself, and then a trailer (all in a single file). The data can be accessed by simply skiping the header, reading the data (all channels sequential), and ignoring the CDF stuff at the end. This is essentially what textract.exe does, but one channel at a time. The NOAA data is stored as "short int" variables (2 bytes or 16 bits) per pixel, in units of albedo x 100 and/or brightness temp x 100. Given a 1200 x 1200 array, this means that the channel 1 data will occupy 2,880,000 bytes (2 x 1200 x 1200 = 2,880,000), followed by channel 2 data, 3, 4, and 5 (for a total of 14,400,000 bytes). The overall file sizes will be a bit bigger, since you have to add in the 644 byte header and the variable length TDF trailer. The same pattern applies to the DMSP data, with the exception that the data is stored as "byte" variables (8 bits). The high resolution visible data only maintains 64 gray levels (with a variable gain, hence not calibrated rigorously) in the data, but is still stored in byte format. The low resolution IR data uses all 256 levels available to it, and can be converted to brightness temperature via the relation: T = (I - 176.69)/2.125 where T is in degrees C and I is the (integer) byte value. For the DMSP files, this means that the file sizes will be 11,520,000 (2400 x 2400 x 2 = 11,520,000) and a bit more counting the header and TDF trailer. QUESTION: -------- How are the data stored? Does the file go across (W longitude->E longitude) sequentially then to the next N latitude? Basically, I need to know what the first data point is and what the last one is. ANSWER: ------ The data should go from left to right and top to bottom as you look at a displayed image. That means that the first data point is at the NW corner. The northern-most horizontal row (i.e. constant latitude) comes first, then the next row (immediately below, that is south of, the first one) is next and so on. This means that the data from the SE corner is last. ------------------------------------------------------------------- --------------------- ABOUT RESOLUTION ---------------------- ------------------------------------------------------------------- QUESTION: -------- Why is the IR data LOW RESOLUTION when it uses 256 levels, and the visible OLS data HIGH RESOLUTION when it uses 64 levels? Shouldn't this be reversed? ANSWER: ------ High/low resolution is used in the context of sensor SPATIAL resolution. The 64 gray-level data has a ground resolution of 500 meters, while the 256 level IR data has a reduced resolution of 2.5 kilometers. The number of data bits is in an inverse relationship because of two factors (1) data transmission rates -- the 500 m resolution data generates a lot of data to transmit and just couldn't keep up with more data bits per pixel, and (2) the more rapidly scanning sensor taking the 500 m resolution data just doesn't have the dwell time to justify more significant digits. ------------------------------------------------------------------- ------------------ ABOUT HAND NAVIGATION -------------------- ------------------------------------------------------------------- The nominal image positioning using the transmitted position, time, and attitude values is often in error by as much as 10 km. The "hand" navigation step involves displaying the image in "satellite-sensor" coordinates (lines and samples), with a geometrical transformation of a coast-line data base as an overlay on the image. In an iterative series of steps, the reported time and attitude parameters can be adjusted to improve the registration of the image relative to the coast-line data base. There are many different ways to do this step. In general, however, the final result is clearly better than the unadjusted registration, with accuracy often quoted as good to 1 (or more honestly 1-2) km. Not all areas of the image will be registered "perfectly". Cloud cover, sensor quality, and position of the satellite track will all affect the quality of the navigation. After the image is navigated, it is regridded into the "standard" lat-long grid used in the data base. This permits scientists not interested in the preliminary processing steps, or not having the appropiate software, to have access to the "full" resolution multi-spectral data sets for all available imagery.