When references are made to satellite navigation, the emphasis is on information representing the satellite orbital position, velocity, and orientation. To provide accurate satellite navigation data, the Product Systems Branch of the Information Processing Division (PSB) 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 IPD. 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 replaced a system that generated Earth location data as much as 24 hours before the instrument data was available. The old system tried to match the data start times with the Earth location data times, which produced a timing error. It performed multiple interpolations that introduced errors in the orbit vectors. The introduction of the AELDS process eliminated the need to match data times in that the new system utilizes the timecodes within each scan of data to produce the Earth location data. Hence, AELDS is an on-line Earth location process. It also eliminated the multiple interpolations needed to produce the Earth location data. Now only one interpolation is needed to determine the corresponding satellite orbit vector for the scan position being processed. With the introduction of AELDS the accuracy of the Earth location data in the level 1B file was improved by 50%.

The Earth location algorithm used to produce the latitude and longitude parameters within the AELDS process is available in an appendix to the NOAA Technical Memorandum 107. The algorithm will soon be 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 more than just latitude and longitude information. Given the satellite position and velocity vector and the Greenwich Hour Angle (GHA), AELDS will provide the satellite height and Northbound/Southbound flags. Also, given the scan time, stepping time, stepping angle, and number of positions desired, AELDS can provide the following 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 IPD 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: 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. Generally, the Earth location error seen in the image data around the satellite subpoint remains within 2 to 4 kilometers (specifications for AVHRR are 4-5 kilometers). The error near the limb is expected to be larger, and is often near 10 kilometers.

Utilization of the above image QC techniques has given us greater insight into the magnitude of Earth location errors as well as the source of some of the errors. In the future, our goal is to provide Earth location data with an accuracy of near 1 kilometer at the satellite subpoint. To increase our data accuracy, IPD plans to enhance the on-line Earth location process (AELDS) to include fixed attitude corrections and TIP clock corrections.

Fixed attitude corrections will include corrections for errors such as instrument mounting errors and constant observed errors. An algorithm has been integrated into the Earth location process that will account for these errors. After testing is complete the algorithm will be included in the operational AELDS process. At any time during the process these attitude corrections may be turned on or off.

TIP clock error corrections will be added to the AELDS Earth location process. 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.
  IPD 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. =============================================================================

  Navigation Data Archive

  A navigation data archive has been established under the Climate Services Division of the National Climatic Data Center. To rotrieve any of the data listed below, please click here or contact National Climate Data Center.

  • Equator Crossing Archive: Printouts from SOCC exist from October 1978 (TIROS-N) through the present but are very difficult to use because of their size and fragility.

    IPD began archiving the equator crossing information files on floppy in November 1994. This archive contains the orbit numbers with corresponding equator crossing times and longitudes in clear text format.

  • 2-line orbital elements: The 2-line elements in the NOAA archive were generated by the Air Force (U.S. Space Command - USSC, formerly NORAD). All satellites from NOAA-1 (1970) through September 24, 1991 are included except TIROS-N.

  • 4-line orbital elements: The 4-line elements are generated by the Navy (Navy Space Command - NAVSPACOM) or Air Force (U.S. Space Command - USSC, formerly NORAD) and are transmitted to SOCC around 0000Z each day. IPD uses them to update or reinitialize the User Ephemeris Files each day. The 4-line elements are Inertial Osculating Cartesian elements that are significantly more precise than the 2-line elements.

    These data are available on floppy beginning with June 1, 1992 and extending through the current date.

  • PSCEAR Initialization reports: The PSCEAR Initialization reports contain information from the User Ephemeris File (UEF) header. The PSCEAR reports contain orbital elements for the epoch time in the forms of Osculating Keplerian elements, Osculating Inertial Cartesian elements, and Brouwer mean elements. It also contains the anomalistic and nodal periods, orbit number, plus the first time derivatives of the right ascension of the ascending node, argument of perigee and the mean anomaly.

    These data are available on floppy beginning with June 1, 1992 and extending through the current date.

  • TBUS Bulletin: The TBUS bulletins are available in hardcopy form (sporadic coverage) beginning with March 1, 1983 and extending through February 12, 1993 for most of the Polar-orbiting Operatinal Environmental Satellites (POES).

    TBUS data on floppy (collected by downloading files from NOAA.SIS) extends from August 1, 1994 through July 1995.

    Collection of TBUS on 3480 cartridge began in June 1991 and continues to the present (with some missing data).