NOAA AVHRR System Summary
NOAA AVHRR System Summary

This section summarizes the characteristics of the NOAA polar orbiter satellite instruments and properties of the satellite orbit. A detailed description of the NOAA Polar Orbiter instrumentation may be found in NOAA Technical Memoranda NESS 95 and 116, entitled respectively, The TIROS-N/NOAA-A-G Satellite Series, and Modified Version of the TIROS-N/NOAA A-G Satellite Series (NOAA E-J) - Advanced TIROS-N (ATN). These documents are available from SSB.

The satellite system includes the following instrument package:

AVHRR - Advanced Very High Resolution Radiometer, from which is obtained:

  1. HRPT - High Resolution Direct Readout AVHRR
  2. LAC - Recorded HRPT AVHRR
  3. GAC - Reduced Resolution Recorded AVHRR

TOVS - TIROS Operational Vertical Sounder, which includes:

  1. MSU - Microwave Sounding Unit
  2. SSU - Stratospheric Sounding Unit
  3. HIRS/2 - High Resolution Infrared Radiation Sounder/2

The TIROS-N/NOAA A-G (6-10) series of satellites have been modified to add payload capacity without changing the basic environmental mission of the series. Seven additional satellites (NOAA-H, -I, -J, -K, -L, -M -N and -N') were added to the initial procurement to extend the lifetime of the program to the end of the century. Incrementally added to the payload was a Search and Rescue Demonstration System (SAR), an Earth Radiation Budget Experiment (ERBE) flown on NOAA-9 and NOAA-10 only, and an operational Solar Backscatter Ultraviolet Radiometer (SBUV/2) to monitor the distribution of ozone in the atmosphere (flown on NOAA-9, -11, -13 and -14).

The NOAA-E through -N' spacecraft, also called the Advanced TIROS-N (ATN), are similar to the NOAA-A through -D satellites that preceded them with the exception that the Equipment Support Module has been enlarged to allow integration of new payloads. A change from the TIROS-N through NOAA-8 spacecraft is that the spare word locations of the low bit rate data system TIROS Information Processor (TIP) is used for special instruments such as the ERBE and SBUV/2. The SAR system will be independent, utilizing a special frequency for transmission of data to the ground.

As the satellite orbits the Earth, data are both broadcast continually (direct readout mode) and recorded on board for later playback. NOAA/NESDIS operates two Command and Data Acquisition (CDA) stations, one in Wallops Island, Virginia and one in Fairbanks, Alaska (formerly Gilmore Creek before 1984), to receive both recorded and direct readout environmental data from the satellite and send these data to Suitland, Maryland, via satellite relay. However, during two (sometimes three) sequential orbits of the Earth, the satellite remains out of contact with any of these sites.

The NOAA/NESDIS ground stations in Wallops, VA, and Fairbanks, AK receive Direct Readout High Resolution Picture Transmission (HRPT) data. Fairbanks maintains a 90-day rotating pool of these data, while Wallops keeps a 60-day pool. As of January 1, 1997, both CDA stations reduced their rotating pools to 30 days. The data are in Field Station format which is described in Appendix C . These data may be ordered through SSB. The Field Station data do not have appended Earth location and calibration information, and are 8-bit precision rather than 10-bit precision. The amount of HRPT data received during one pass of the satellite over the ground station is limited to the acquisition range of the station. A satellite pass directly over an antenna site will be within view of that antenna (horizon to horizon) for about 15.5 minutes when the satellite is at 833 km and 16 minutes when it is at 870 km. Figures 1.1-1 and 1.1-2 depict the overall HRPT coverage from each of the NESDIS ground stations over the Northern and Southern hemispheres, respectively.

Figure showing HRPT coverage for the Northern Hemisphere
Figure 1.1-2. Overall HRPT coverage for the Southern Hemishpere.
Figure showing HRPT coverage for the Southern Hemisphere

In addition to providing direct readout, the TIROS-N series satellites carry five digital tape recorders, each with a single electronic module and dual tape transport, to record data for subsequent transmission through the CDA to the data processing facility. Each transport has the capacity to record one of the following:

1) 15 minutes (slightly more than a full orbit) of GAC with embedded TIP data. (TIROS Information Processor is the on board computer system that formats the sensor data for transmission. TIP includes TOVS and auxiliary data.)
2) Eleven and one half minutes of HRPT data (called LAC when recorded), or
3) 230 minutes of TIP data only (called stored TIP).

Between October 1978 and April 11, 1985, direct readout HRPT and recorded GAC, LAC, and TIP data were ingested by NESDIS computers and stored temporarily on staging disks used as work space and for interfacing these computers with the NESDIS Terabit Memory (TBM) mass storage system. The ingested data were then retrieved from disk storage on a time-available basis, processed to Level 1b format (which included appending of Earth location and calibration information) and returned to the disks for subsequent transfer to the TBM for NESDIS product processing and the SSB archive.

On April 11, 1985, NESDIS abandoned the TBM system as a means of storing ingested polar orbiter data. Over the years, SSB's hardware complement has changed with advancing technology, but the corresponding media changes remain transparent to the satellite data user.

During the period of conversion from TBM tapes, SSB attempted to transfer as much of the data from TBM tapes as possible (while simultaneously servicing user requests). Priority was given to recovering the GAC data sets and 88% of these were saved. Similarly, 49% of the TOVS and 6% of the LAC/HRPT were saved. Users should contact SSB for the availability of any Level 1b data which were ingested between October 1978 and April 1985.

As mentioned above, Earth location and calibration data are appended to the data as part of the Level 1b processing. The Earth location data are read from CCTs which contain up to 29 hours of information. These data are updated every 24 hours. These Earth location tapes (called GELDS data) were archived by SSB between March 8, 1985 and September 7, 1992. On September 8, 1992, NESDIS/I.D. discontinued generating the GELDS data since the data were redundant with the Level 1b data.

For purposes of clarification, Figure 1.1-3 will be used as the definition of Solar Zenith Angle, Local Zenith Angle, and satellite scan angle. These angles are referred to throughout this guide and have the relationships shown in the figure.

Figure showing definition of solar zenith angle, local zenith angle and satellite scan angle