This section describes the data characteristics and magnetic tape formats of Local Area Coverage (LAC)/High Resolution Picture Transmission (HRPT) data. Section 3.2.1 contains the data characteristics and Section 3.2.2 contains the tape formats available. The tape formats include the full data set copy, selective extract subsets, and the 16-bit unpacked format which is available for all channels. NESDIS has enhanced the Level 1b format for LAC/HRPT and the current format (for data collected after November 15, 1994) are included in this section. Previous formats are included in Appendices K and L.

Data Characteristics

The AVHRR data are digitized to 10-bit precision. The digitized data are both transmitted from the satellite in real-time as High Resolution Picture Transmission (HRPT) data, and selectively recorded on board the satellite for subsequent playback as Local Area Coverage (LAC) data. A maximum of ten minutes of LAC data may be recorded per orbit.

In the event that a user would want SOCC to schedule an AVHRR LAC orbit over a specific area (out of direct readout range of Wallops Island or Fairbanks CDA's), the procedures and requirements are contained in Section 1.3.

Magnetic Tape Formats

The data set format for full data set copies (all channels) is different from the format for selective extract subsets (selected channels). Sections 3.2.2.1 and 3.2.2.2 contain formats for LAC/HRPT full data set copies and selective extract subsets, respectively. Section 3.2.2.1 also includes an explanation of the 16-bit unpacked data format.

Full Data Set Copies

Each HRPT data set contains the HRPT data from one CDA contact. Each LAC data set contains an individual satellite recorder playback (10 minutes of recorded HRPT data). The data within each data set are in chronological order with one scan contained in two physical records. The records are written in binary and contain 7,400 bytes in the format shown in Table 3.2.2.1-1. This table includes the enhancements made on November 15, 1994.

The content and order of the LAC/HRPT records is identical to the GAC record described in Section 3.1.2.1, with the exception of three fields. The three exceptions are the Solar Zenith Angles, Earth location, and LAC/HRPT video data. Also, note that the calibration coefficients are ordered Channels 1, 2, 3, 4, and 5, the same as GAC data. See Section 3.1.2.1 for translation of the time code.

There are 2,048 points in a LAC/HRPT scan line. The Solar Zenith Angle and Earth location data (latitude and longitude) are sampled every 40 points starting at point 25 (25, 65, 105,..., 1945, 1985, 2025). There are 51 possible Zenith Angles and Earth location values for each scan line. Each Solar Zenith Angle requires one byte and is stored as degrees x 2. The latitude and longitude values are each stored in two-byte fields in 128ths of a degree (0 to 180E positive, 0 to 180W negative). See Section 2.0.2 for explanation of negative binary integers in two's-complement notation.

The LAC/HRPT video data contain five values (one for each channel) for each of the 2,048 points in a scan (i.e., 2,048 points x 5 channels = 10,240 samples). The data are packed as three (10-bit) words in four bytes, right-justified. The last four-byte group contains two leading zero bits, one (10-bit) word, and 20 trailing zero bits. The first two bits of each four-byte group are zero. The LAC/HRPT video data is ordered scan point 1 (Channel 1, 2, 3, 4, 5), scan point 2 (Channel 1, 2, 3, 4, 5), etc. The video data are stored in binary. See Figure 2.2.1-1 for the arrangement of the LAC/HRPT channels (packed format).

The 16-bit unpacked format for full copy LAC/HRPT data has the same format as the "packed" data described above, except for the video data. The video data are unpacked into two 16-bit words in every four bytes. The data values for each channel are contained in the ten least significant bits and the six most significant bits are zero filled. The format for unpacked LAC/HRPT video data (for all five channels) is contained in Table 3.2.2.1-2.

Note that one scan of unpacked LAC/HRPT data is contained in two 10,464 byte records. (The decision was made to divide into two records because a 20,928 byte record may have been too large for some smaller computers to handle.)

Selective Extract Subsets

This section describes the data set format for LAC/HRPT data sets which have been produced using channel selection (16-bit format) or by choosing the 8-bit format offered by SAA. Section 3.2.2.2.1 describes the 16-bit format and Section 3.2.2.2.2 describes the 8-bit format for LAC/HRPT data. SSB is aware of a problem which affects the format of both the 8-bit and the 16-bit (unpacked) data for the Level 1b AVHRR. The problem involves the omission of 22 bytes of extra precision information containing the Solar Zenith angle and clock drift delta in each record. These bytes were added to the Level 1b data record (on October 24, 1992) after the extraction software was written, and were therefore not included. If users of the 8- or 16-bit format data want this extra precision data, they are only available if the user requests packed format output from the SAA's online archive of Level 1b data sets.

16-bit format

When channels are selected for LAC/HRPT data, the format is identical to that described in Section 3.2.2.1 except for the LAC/HRPT video data. For the LAC/HRPT video data, the selected channels are packed in consecutive halfwords (16 bits). The data values for each channel selected are contained in the ten least significant bits and the remaining six most significant bits are zero filled. If one channel is selected, the LAC/HRPT video data have the format shown in Table 3.2.2.2.1-1.

Note that all LAC/HRPT data set scans require two records regardless of the number of channels selected. For an example of two channel selection for LAC/HRPT data, use the GAC two-channel format in Section 3.1.2.2 as a guide.

8-bit format

When the 8-bit format is chosen (from SAA), the normal 10-bit video data are truncated to 8-bits. (The 2 least significant bits of data are dropped.) The format is identical to that described in Section 3.2.2.1 except for the LAC/HRPT video data. In the LAC/HRPT video data, the data are packed in consecutive bytes (four data points per 32-bit word). The LAC/HRPT video data have the format shown in Table 3.2.2.2.2-1.

Similarly, a user may select channels and request them in the 8-bit format. The output record length information for LAC/HRPT is included in Table 3.2.2.2.2-2.

Source: http://www.ncdc.noaa.gov