NOAA KLM User's Guide

Section 2.3

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2.3 Navigation And Earth Location Processing Within NOAA

The Information Processing Division (IPD) of the Office of Satellite Data Processing and Distribution (OSDPD) of NESDIS supports the instrument data processing for all the operational polar orbiting satellites.. To provide accurate satellite navigation data, the IPD 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 COWELL numerical integrator (using Stormer-Cowell formulas) 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 Earth location data provided in the Level 1b process is produced by the Advanced Earth Location Data System (AELDS). This system was initially implemented on September 8, 1992 in the AVHRR Level 1b process and on September 7, 1994 in the TOVS Level 1b process. It is an on-line Earth location process utilizing the scan line timecodes to produce Earth location. 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 are available in Appendix I. 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 also provide the following for each scan point of a specific instrument:

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:

  1. Navigation: When the UEF containing predicted satellite position and velocity data is generated, the radius vector is compared to that generated using the elements for the previous seven days of data (delta-R). Generally, these differences remain less than one kilometer for at least 7 to 10 days.
  2. 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 reset and the actual option can be turned on or off. This tolerance check gives the reassurance that the Earth location algorithm is behaving correctly.
  3. 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 AVHRR instrument raw 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 provided greater insight into the magnitude of Earth location errors as well as the source of some of the errors. To increase the data accuracy, IPD has enhanced the on-line Earth location process (AELDS) to include fixed attitude corrections and TIP clock corrections.

Fixed attitude corrections include corrections for errors such as instrument mounting errors and constant observed errors. An algorithm has been integrated into the Earth location process that accounts for these errors. At any time during the process, these attitude corrections may be turned on or off.

TIP clock error corrections were added to the AELDS Earth location process. The Satellite Operations Control Center (SOCC) maintains the on-board clocks for the NOAA POES satellites. They monitor the accuracy of the clocks and make adjustments whenever needed. The SOCC clock adjustments are made for two reasons:

  1. The clock has drifted outside the tolerance level and is over corrected to compensate for the error. The clock error is then allowed to drift back through zero until it again exceeds the tolerance.
  2. 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 is utilized to correct the Earth location data using the instrument scan times. It is made available to the user community in the following home page (URL: http://psbsgi1.nesdis.noaa.gov:8080/PSB/NAVIGATION/navpage.html).

2.3.1 Updates to the TBUS Bulletin

The TBUS bulletin is a major source of orbital data for direct readout users. Although, the UEF is used in creating all four parts of the TBUS bulletin, IPD's focus is on Part IV of the bulletin. More information on the TBUS bulletin can be obtained from Section 5.1 and Appendix A. IPD, in conjunction with SOCC, has outlined steps to make Part IV more user friendly and to increase the accuracy of the data (the Brouwer Mean elements). Action has been taken in the following areas:

2.3.2 Navigation Data on the Internet

The IPD navigation area has established a home page on the Internet under the "Navigation/Earth Location" (URL: http://psbsgi1.nesdis.noaa.gov:8080/IPD/IPD.html). Note that the final character in "psbsgil" is the number "one". This page has links to the following existing pages:

The following entries are a part of the navigation page:

Figure 2.3.2-3. Sample NOAA-14 Equator Crossing Data.
N REVNUM EQ.XING.TIME (MJD) Year Month Day Hour Min Second EQ.XING LONGITUDE
1 6893 0.502049988306D+05 1996 5 1 23 58 18.961 208.8628
2 6894 0.502050697122D+05 1996 5 2 1 40 23.132 183.3453
3 6895 0.502051405938D+05 1996 5 2 3 22 27.303 157.8277
4 6896 0.502052114754D+05 1996 5 2 5 4 31.474 132.3106
5 6897 0.502052823570D+05 1996 5 2 6 46 35.646 106.7937
6 6898 0.502053532386D+05 1996 5 2 8 28 39.817 81.2773
7 6899 0.502054241202D+05 1996 5 2 10 10 43.988 55.7614
8 6900 0.502054950018D+05 1996 5 2 11 52 48.159 30.2455
9 6901 0.502055658835D+05 1996 5 2 13 34 52.331 4.7292
10 6902 0.502056367651D+05 1996 5 2 15 16 56.502 339.2124
11 6903 0.502057076467D+05 1996 5 2 16 59 0.673 313.6947
12 6904 0.502057785283D+05 1996 5 2 18 41 4.844 288.1778
13 6905 0.502058494099D+05 1996 5 2 20 23 9.015 262.6632
14 6906 0.502059202915D+05 1996 5 2 22 5 13.187 237.1478
15 6907 0.502059911731D+05 1996 5 2 23 47 17.358 211.6310
16 6908 0.502060620547D+05 1996 5 3 1 29 21.529 186.1136
17 6909 0.502061329363D+05 1996 5 3 3 11 25.700 160.5959
18 6910 0.502062038180D+05 1996 5 3 4 53 29.872 135.0787
19 6911 0.502062746996D+05 1996 5 3 6 35 34.043 109.5618
20 6912 0.502063455812D+05 1996 5 3 8 17 38.214 84.0453
21 6913 0.502064164628D+05 1996 5 3 9 59 42.385 58.5293

Figure 2.3.2-4. Sample NOAA-14 TIP Clock Error Database.
Spacecraft ID# (NOAA-14) Effective Time Clock Error Millisec I5 Drift Rate Millisec/Day F7.3 Comments(Optional)
Year/MM/DD HH:MM:SS.SSS
23455 1996/03/19 23:59:00.000 -035 +7.000 None
23455 1995/12/31 23:59:59.000 -200 +5.000 -1.0 Leap Second Adjustment
23455 1995/08/01 23:59:00.000 -560 +5.000 None
23455 1995/01/01 23:59:00.000 -560 +5.000 None
23455 1994/01/01 00:01:00.000 000 0.000 None

The IPD updates or enhancements to the navigation and Earth location process are on going. For example, an Earth location error database is planned for Level 1b data users. This database will contain Earth location error information based on actual error measurements taken from AVHRR imagery. The format or contents of this file has not been determined at this time. Every attempt will be made to keep the current systems compatible with both the prior satellite series and the future satellite series. As new areas are addressed, the user will see improvements in the navigation product. Comments, suggestions, or inquiries regarding the Navigation page may be sent to this email address: navigate@nesdis.noaa.gov.


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