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Two procedures are presented here as follows:
A] Procedure to minimize occurance of NOAA gridding errors.
B] Procedure to correct NOAA gridding errors if they occur.
A] Procedure to minimize occurance of NOAA gridding errors.
Requirements:
1] Accurate system date/time
2] up-to-date satellite orbital elements(NASA2LINE or TBUS)
3] system location is approximately correct(with 5 degrees or so lat/long)
4] correct satellite is associated with the image.
A.1] SATMOD system time/date is correct, including Windows Timezone if system time is set to local time. The target accuracy is 1 or 2 seconds alignment with UTC/GMT/ZULU. Be sure to check the LOCAL and UTC time/date as displayed at the top of the WTCAP control dialog. If the intent is to set the system time to GMT/UTC, then be sure to specify the Windows Timezone option "(GMT)Monrovia,Casablanca" or else the daylight savings offset will confuse the issue by one hour, and LOCAL time will not be the same as UTC time. You can check/set the Windows time/date by right-clicking on the time in the Windows Taskbar and selecting "Adjust Date/Time..." If you have access to a GPS with NMEA0183 output, you can connect the NMEA0183 output to an available COM port on the system and use the GPS0183.EXE utility to automatically parse GPS time from appropriate NMEA0183 sentences. This will keep the system time aligned with GPS time. You can specify an offset between GPS time and UTC to correct for any slight differences in these timebases.
A1.2] Up-to-date satellite orbital elements. These can be either NASA2LINE or TBUS format. NASA2LINE format elements are recommended, and should be updated approximately once every two weeks for accurate gridding. The latest NASA2LINE element data can be obtained via Internet from the following URL: http://celestrak.com/NORAD/elements/weather.txt Alternately, TBUS PART IV format data can be used. There are two methods of updating the element data; manual and import. To manually enter the data, for example, from a printed hardcopy of the data, use the function WtView:Misc:Sat Elements...:Edit. The EPOCHS.EXE utility will be run, which will allow you to manually enter the satellite data. Select the appropriate format (NASA 2-LINE or TBUS) by using the 'F3' function key. Then, press the '?' to display an appropriate help screen, identifying where on the NASA2LINE or TBUS data to find the appropriate element data. Use the 'PgUp' 'PgDn' keyts to page through the 4 pages of 4 satellites each to locate the current NOAA satellites being updated. (Currently, only NOAA 12, NOAA 14 and NOAA 15 should be active. All other NOAA satellites should be 'UNDEFINED') To import the data from a text file, use the function WtView:Misc:Sat Elements...:Import, and select either NASA2Line or TBUS format. A standard Windows File Open dialog will appear. Browse to the location of the file containing the satellite data and open it. A utility (SATDAT.EXE) will be run which will search the selected file for appropriately formatted data, matched up to any currently defined satellite named in the EPOCHS maintained database of up to 16 active satellites. You can verify that the data was imported by checking the 'Last Update' date in EPOCHS, via WtView:Misc:Sat Elements...:Edit.
A1.3] System location is approximately correct. The accuracy of this does not directly impact the gridding accuracy, but determines the gross ability of the software to associate a given image with a particular NOAA satellite. +/- 5 degrees of accuracy is usually sufficient to allow the software to pick the right satellite. You can enter the system location (latitude/longitude) in any one of three ways.
A1.3.1] Directly, using WtView:Misc:Position:Edit. Just fill in the dialog and press OK.
A1.3.2] Directly, using WtView:Misc:Sat Elements...:Edit, then use 'F2' and 'F4' function keys in the EPOCHS editor.
A1.3.3] Indirectly, by allowing the GPS0183.EXE utility to parse location from a GPS NMEA0183 talker.
A1.4] Correct satellite is associated with the image. This means, for example, that 'NOAA 14' elements are used to grid an image acquired from 'NOAA 14.' The APT signal does not have a satellite identifier embedded in it. Therefore, the software must use indirect means of associating a set of elements with a particular image. The algorithm used is as follows: use the NOAA satellite on the same frequency as acquired (137.50 or 137.62Mhz) that is 'closest' to the station location subpoint at the time of AOS(acquisition of signal). This is why station location, time of day, and satellite elements must be accurate. In addition, this requires that NOAA satellites that are orbiting but not transmitting be 'UNDEFINED' in the EPOCHS database, or else it is possible that a particular NOAA satellite, for example, NOAA 11, that is still orbiting but not transmitting APT, might actually be closer than NOAA 14 at the time of AOS from NOAA 14, so if NOAA 11 is defined in the EPOCHS database, the software will guess wrong and assign the NOAA 11 elements to the image acquired from NOAA 14. Because the algorithm is based on a simple distance to subpoint at AOS, and because the two satellites might actually be in drastically different locations or even different nodes(ascending/descending), the resulting incorrectly applied elements can result in gross gridding errors. When gridding is sometimes grossly in error, yet sometimes is accurate, this is the most likely cause of the gridding failure. There is a method of recovering from this error, as discussed in section B. The currently ACTIVE NOAA satellites are: NOAA 14, and NOAA 15; NOAA 12 has been temporarily disabled because it is in co-visibility with NOAA 15 on the same frequency, but will be re-enabled sometime in 2000 when their orbits once again separate. Prior to NOAA 14, all 'odd' series NOAAs transmitted on 137.62Mhz (NOAA 9, NOAA 11, NOAA 13), and all 'even' series NOAAs transmitted APT on 137.50 MHz. This changed with the loss of NOAA 13. NOAA 14 was put up in its place, and transmits on the 'odd' frequency 137.62 MHz. NOAA 15 was the next in series, and transmits on 137.50 Mhz. Thus, the current NOAA status is: NOAA 12 OFF 137.50MHz NOAA 14 ON 137.62 NOAA 15 ON 137.50 You should remove any other NOAA satellite in the current EPOCHS database by changing its name to UNDEFINED using WtView:Misc:Sat Elements...:Edit. Or, in the case of NOAA 12, since it will eventually be re-enabled, just change its name to XNOAA 12, and it will not be included in the NOAA satellite group.
B] Procedure to correct NOAA gridding errors if they occur.
There are two types of gridding errors that can be corrected:
1] Gross errors due to incorrect satellite elements being applied.
2] Small gridding errors due to out of date elements or small system time errors.
B.1] Gross Gridding Errors due to incorrect satallite elements applied. This can often be corrected by manually selecting the correct satellite. To do this, use the WtView:Grid:Navigate:APT:Satellite, and click on an alternate NOAA satellite. The elements for the selected satellite will be associated with the current APT image.
B.2] Small Gridding Errors. These can be due to small errors in system time, or out of date elements. Small errors in system time will normally evidence themselves as mostly North/South offsets in the overlays(ie, in the direction of subpoint travel, corresponding to the offset in system time vs actual UTC.) NOAA satellites orbit the Earth approximately once every 101 minutes, so a 1 second offset in time corresponds to approximately 4 miles of subpoint offset. An error of 1 minute corresponds to about 250 miles. The 'minute markers' in the APT Space Data column(white column with black stripes on IR, black column with white stripes on VISIBLE)correspond to 1 minute offsets. This is why it is necessary to maintain accurate system time. To adjust for these types of errors, it is possibly to bodily shift the overlays on the image to align with some ground truth point. To do this, us e WtView:Grid:Navigate:APT:Position. An instruction message box will appear. Click LEFT on the grid overlay at a known grount truth point(coast/island feature)that is visible in the bitmap. Then, move the mouse cursor and CLICK RIGHT on the point on the bitmap to which you would like to bodily shift the overlay. This shift will become a part of the file image header. You can apply this shift as many times as you want; the resulting shift will be the cumulative sum of all such shifts.