This section contains more specific information on the AVHRR instrument. Section 3.1 describes the GAC data characteristics and the tape formats available (full copy, selective extracts, and unpacked format). Similarly, Section 3.2 contains specific information about LAC/HRPT data. Section 3.3 describes the calibration procedures for AVHRR data (both visible and thermal). NESDIS has instituted several improvements to the original Level 1b format. The current enhancement and resulting data format is detailed in this section, with previous enhancements detailed in Appendices K and L.
The Advanced Very High Resolution Radiometer (AVHRR) represents an improvement over the VHRR sensor flown aboard the ITOS series of operational satellites (the last of which was-NOAA-5). The AVHRR is a cross-track scanning system similar to the VHRR, but features four or five spectral channels, compared to just two for the VHRR. The AVHRR flown aboard TIROS-N, NOAA-6, NOAA-8, and NOAA-10 has four channels, and the AVHRR aboard NOAA-7, NOAA-9, NOAA-11, NOAA-12 and NOAA-13 has five channels. Subsequent satellites in the series will have five. Provision has been made for five channels in the data format for all satellites. Channel 5 contains a repeat of Channel 4 data, when only four different channels are available.
The spectral band widths (in micrometers) of the AVHRR channels for the TIROS-N series and those proposed for the remaining spacecraft are shown in Table 3.0.1-1. In addition, the Instantaneous Field of View (IFOV) in milliradians is included for each channel in Table 3.0.1-1. The spectral response functions for each satellite are contained in the figures in Section 1.4.
|5||Channel 4 repeated||Channel 4 repeated||11.5-12.5||11.4-12.4||1.30|
The IFOV of each channel is approximately 1.4 milliradians leading to a resolution at the satellite subpoint of 1.1 km for a nominal altitude of 833 km. The scanning rate of the AVHRR is 360 scans per minute. The time within each scan line of AVHRR data represents IFOV #1.
The analog data output from the sensors is digitized on board the satellite at a rate of 39,936 samples per second per channel. Each sample step corresponds to an angle of scanner rotation of 0.95 milliradians. At this sampling rate, there are 1.362 samples per IFOV. A total of 2048 samples will be obtained per channel per Earth scan, which will span an angle of ±55.4 degrees from the nadir (subpoint view).
The IR channels are calibrated in-flight using a view of a stable blackbody and space as a reference. No in-flight visible channel calibration is performed (although the spaceview is available as one reference point). Although these will vary from instrument to instrument, the design goals for the IR channels were an NEΔT (Noise Equivalent differential Temperature) of 0.12 K (@ 300 K) and a S/N (signal to noise ratio) of 3:1 @ 0.5% albedo.
Users should be aware that AVHRR Channel 3 data on each TIROS-N series spacecraft have been very noisy due to a spacecraft problem and may be unusable, especially when the satellite is in daylight.
As a result of the design of the AVHRR scanning system, the normal operating mode of the satellite calls for direct transmission to Earth (continuously in real-time) of AVHRR data. This direct transmission is called HRPT, for High Resolution Picture Transmission. In addition to the HRPT mode, about ten minutes of data may be selectively recorded on each of two recorders on board the satellite for later playback. These recorded data are referred to as LAC (Local Area Coverage) data. LAC data may be recorded over any portion of the world as selected by NOAA/NESDIS and played back on the same orbit as recorded or during a subsequent orbit. LAC and HRPT data have identical formats.
The full resolution data is also processed on board the satellite into GAC (Global Area Coverage) data which is recorded only for readout by CDA stations. GAC data contains only one out of three original AVHRR lines and the data volume and resolution are further reduced by starting with the third sample along the scan line, averaging the next four samples, and skipping the next sample. The sequence of average four, skip one is continued to the end of the scan line.