Overview of NESDIS Polar Earth Location Process

When references are made to satellite navigation, the emphasis is on information representing the satellite orbital position, velocity. To provide accurate satellite navigation data, the Engineering Branch of the Mission Operations Division (MOD) receives daily a set of Inertial Osculating Cartesian orbit parameters for each polar satellite from the Air Force or Navy. This orbit vector is used to generate a predicted user ephemeris file (UEF) of orbit vectors spaced one minute apart that cover a 10 day time span. This file is created using a numerical integration of the COWELL equations integrator which maintains the one kilometer accuracy of the initial orbit vector. The UEF is the foundation for all the navigation data produced in MOD. It is utilized to create the TBUS bulletins, the equator crossing information files, the Search and Rescue (SAR) orbit ephemeris files, and level 1B instrument data files. The SAR ephemeris data is provided for use by the U.S. Mission Control Center for Search and Rescue. The level 1B process uses the orbital information in the UEF files to provide Earth located data for the NOAA polar satellite instruments.

The Earth location data provided in the level 1B process is produced by the Advanced Earth Location Data System (AELDS). AELDS is an on-line Earth location process. It takes into account the fixed attitude corrections and the TIP clock corrections. Fixed attitude corrections include corrections for errors such as instrument mounting errors and TIP observed errors. The Earth location algorithm used to produce the latitude and longitude parameters within the AELDS process is available as part of the NOAA-KLM documentation either in a hardcopy form or on the Internet (NOAA-K Users Guide). The AELDS process provides the following parameters for each scan point of a specific instrument:

• Geodetic Latitude
• Longitude
• Solar zenith angles
• Satellite zenith angles
• Solar azimuth angles
• Satellite azimuth angles
• Relative azimuth

In order to insure that the Earth location and navigation information provided by MOD lies within acceptable accuracy limits, quality control (QC) operations are performed during and after generation of the data. At present, three types of checks are used:

• Navigation: When the UEF containing predicted satellite position and velocity data is generated, the radius vector is compared to that generated for the same time by the UEF's from the previous seven days (delta-R). Generally, these differences remain less than one kilometer for at least 7 to 10 days.
• On-line Earth location: An Earth location tolerance check of the satellite subpoint (NADIR) location has been integrated into the AELDS process. The subpoint position is calculated by an independent method and compared with the position generated by AELDS. The acceptable value of the difference can be set or reset and the actual option can be turned on or off. This tolerance check gives reassurance that the Earth location algorithm is behaving correctly.
• Post processing Earth location: Post processing Earth location: An image QC system is used to verify the accuracy of the Earth location data generated using the UEF and appended to the raw instrument data in the level 1B files.

The Satellite Operations Control Center (SOCC) maintains the on-board TIP clocks for all polar satellites. They monitor the accuracy of the clocks and make adjustments whenever needed. The SOCC clock adjustments are made mainly for two reasons:

• The clock has drifted outside the tolerance level and is reset or corrected to over compensate for the error. The clock error is then allowed to drift back through zero until it again exceeds the tolerance.
• When a leap second is needed at the end of December or June, the TIP clock is adjusted depending on the resultant error when combined with the existing clock error.

MOD has set up a clock drift file that contains all SOCC corrections. This file will be utilized to correct the Earth location data using the instrument scan times.

Generally, after the fixed attitude corrections and TIP clock corrections, the earth location error seen in the image data around the satellite subpoint remains within 1 kilometers (specifications for AVHRR are 4-5 kilometers). The error near the limb is expected to be larger, and is often less than 3 kilometers. At any time during the process these attitude corrections may be turned on or off.