This section describes the instrument, data characteristics, and magnetic tape formats of the High Resolution Infrared Radiation Sounder/2 (HIRS/2). Section 4.1.1 contains a description of the instrument and data characteristics. Section 4.1.2 contains the tape formats for full data set copies, 16-bit unpacked format, and selective extract subsets.

Instrument Description and Data Characteristics

The HIRS/2 instrument measures incident radiation primarily in the infrared region of the spectrum including both longwave (15 micrometers) and shortwave (4.3 micrometers) regions (see Table 4.0-1). The IFOV of the HIRS/2 channels are stepped across the satellite track by use of a rotating mirror. This cross-track scan, combined with the satellite's motion in orbit, will provide coverage of a major portion of the Earth's surface. The width of the crosstrack scan is 99 degrees or 2240 km and consists of 56 steps. The mirror is stepped from home position in 55 steps of 1.8 degrees. At the end of the scan (at position 56) the mirror rapidly returns to the home position and repeats the scanning pattern. Each scan takes 6.4 seconds to complete (100 milliseconds per step) and there are 42 km between IFOVs along the sub-orbital track. The optical FOV is 1.25 degrees which gives a ground IFOV of 17.4 km diameter at the nadir. At the end of the scan, the ground IFOV is 58.5 km cross-track by 29.9 km along-track.

HIRS/2 can be commanded to automatically enter a calibration mode every 256 seconds. When the instrument is in the calibration mode, the mirror (starting from the beginning of a scan line) rapidly slews to a space view and samples all channels for the equivalent time of one complete scan line of 56 scan steps. Next, the mirror is moved to a position where it views a cold calibration target and data is taken for the equivalent of 56 scan steps. The mirror is then stepped to view an internal warm target for another 56 scan steps.

Upon completion of the calibration mode, the mirror continues its motion to the home position where it begins normal Earth scan. The total calibration sequence is equivalent to three scan lines (no Earth data are obtained during this period). The analog data output from the HIRS/2 sensor is digitized onboard the satellite at a rate of 2880 bits per second. At this rate, there are 288 bits per step (step time = 100 milliseconds) which includes all 20 channels. The data is digitized to 13-bits precision.

Table 4.1.1-1 contains typical values of the HIRS/2 spectral characteristics and noise equivalent differential radiance (NEΔN's in units of mW/(m2-sr-cm-1)). There will be some variation in the parameters from one HIRS/2 instrument to another.

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 4.1.2.1 and 4.1.2.2 contain formats for HIRS/2 full data set copies and HIRS/2 selective extract subsets, respectively. Section 4.1.2.1 also includes an explanation of the 16-bit unpacked data format for HIRS/2 data.

Full Data Set Copies

Each HIRS/2 data set normally contains an individual satellite recorder playback in chronological order. Each record of HIRS/2 data contains the data for one HIRS/2 scan. Each record is written in binary and contains 4253 bytes (4256 bytes prior to January 1, 1995) in the format contained in Table 4.1.2.1-1.

The scan line number is from 1 to n. In the case of data gaps which cover one or more complete scans, the scan line number will be incremented to compensate for the data gap.

The time code consists of the year, Julian day, and UTC time of day in milliseconds. The year is contained in the first 7 bits of the first two bytes, the 9-bit day of year is right-justified in the first two bytes and the 27-bit millisecond UTC time of day is right-justified in the last four bytes. All other bits are zero. The time code has the same format for all Level 1b data sets.

The scan quality information is contained in four bytes. The first two bytes contain processing detected conditions, the third byte contains DACS quality indicators, and the last byte is spare. The scan quality indicators contain a summary of the quality of the 64 TIP minor frames which make up a HIRS/2 scan. The quality bytes are defined in Table 4.1.2.1-2.

The Earth location delta is the time difference between the scan time code and the time code associated with the Earth location data appended to this record. The value is in milliseconds and is right-justified in the four bytes.

The twenty HIRS/2 channels occur in the Level 1b data record in the following order: 1, 17, 2, 3, 13, 4, 18, 11, 19, 7, 8, 20, 10, 14, 6, 5, 15, 12, 16, and 9.

The HIRS/2 calibration coefficients are contained in three groups. The first group of 240 bytes contains the three manually calibrated coefficients for each of the 20 channels stored in descending order. The second group of 240 bytes contain the three auto-calibrated coefficients for each of the 20 channels stored in descending order. The third group of 240 bytes contains the normalization coefficients by channel used in auto calibration stored in ascending order. Each value is stored in four bytes and contains a scaled number of fractional bits. The scaling factor for the coefficients in ascending order is 222, 230, 244, respectively, and fields for non-computed values are set to zero. The order of the coefficients is shown in Table 4.1.2.1-3.

The sets of coefficients for the HIRS/2 channels are present in the same sequence in which the channels are present in the data record (ascending/descending refers to sequence of the three terms of each coefficient).

Calibration intercepts whose absolute values exceed 512 are truncated when they are scaled and inserted into the Level 1b database by the program DEFERT. For most channels of most TIROS-N series satellites instruments, this presents no problem because the absolute values of their intercepts are less than 512. However, Channels 1 and 2 of the NOAA-12 HIRS/2 and Channel 1 of the HIRS/2 instruments on NOAA-6, -7, -8, -10, -11, -13 and -14 are exceptions. In these channels, the absolute values of the intercepts exceed 512, and the scaled intercept values in the Level 1b database are truncated. Consequently, the intercepts for these channels are incorrect when they are descaled by users of the Level 1b data. Users may recover the correct values by adjusting the descaled values using the following procedures:

NOAA-12 Channel 1:

If the absolute value of the descaled intercept is less than 200, increase the absolute value by 2048 (and maintain the original sign). For example, if the descaled intercept is -11, the correct intercept is -2059.

If the absolute value of the descaled intercept is greater than or equal to 200, increase the absolute value by 1536 (and maintain the original sign). For example, if the descaled intercept is -511, the correct intercept is -2047.

NOAA-12 Channel 2:
and NOAA-6, -7, -8, -10, -11, -13 and -14 Channel 1:

If the absolute value of the descaled intercept is less than 200, increase the absolute value by 512 (and maintain the original sign). For example, if the descaled intercept is -38, the correct intercept is -550; if the descaled intercept is 95, the correct intercept is 607.

If the absolute value of the descaled intercept is greater than or equal to 200, make no adjustment.

Also note that the HIRS/2 instrument can have negative numbers for the calibration coefficients. The HIRS/2 auto coefficient 2 will be negative for all channels except Channel 20. Typical values for auto coefficient 2 range from approximately -0.5 for Channel 1 to approximately -0.5x10-3 for Channel 19. The auto coefficient 3 can be positive or negative depending on the channel. Values usually range from several hundred for Channel 1 to approximately 1 for Channel 19.

See Section 4.5 on how to use the HIRS/2 calibration coefficients.

The height and Local Zenith Angle of the scan is contained in four bytes. The height is stored in the first two bytes in kilometers. The Local Zenith Angle is stored in the last two bytes in 128ths of a degree. The Local Zenith Angle is the angle from the observed point on the Earth to the satellite as measured from the local vertical. The Local Zenith Angle given here is for the fields of view at the edge of scan. The Earth location data consists of latitude and longitude pairs for each of the 56 field of views (FOV) in the scan. The latitude and longitude values are stored in two-byte fields in 128ths of a degree. The order of the data is: FOV 1 latitude, FOV 1 longitude, FOV 2 latitude, FOV 2 longitude, etc.

Each TIP minor frame contains 286 bits of HIRS/2 data packed into 36 bytes. The 286 bits are made up of 22 13-bit words. The 22 13-bit words will be repacked into 44 bytes according to the following scheme.

(a) The first two 13-bit words will be packed left-justified into the first four bytes of the 44-byte group. The least significant six bits of the four bytes will be zero filled. These two words describe the encoder position, electronic calibration level, Channel 1 period monitor, element number, and filter sync designator.

(b) The last 20 13-bit words will be repacked into 20 halfwords (right justified), and if negative, will be stored in 2's complement form. (Note: 2's complement of a number can be found by forming its bit complement (1's complement); that is, every 1 must be changed to a 0 and every 0 must be changed to a 1; and then add the number1.) These 20 words contain the radiant signal output for the 20 HIRS/2 channels for the first 56 minor frames. Minor frames 56-63 also contain these 20 words as described in Table 4.1.2.1-4.

The 64-byte group will be packed sequentially in the scan record. The content of the 286 bits found in each of the 64 TIP minor frames within the HIRS/2 scan is found in Table 4.1.2.1-4.

The 64 bytes of the minor frame quality field contains data quality information for each of the 64 minor frames making up a scan line. The format for each byte is contained in Table 4.1.2.1-5.

The 16-bit unpacked data format for full copy HIRS/2 data has the same format as the "packed" data described above except for the format of the HIRS/2 channel data. Also, the minor frame quality has the same format as described above but it starts on the first halfword (16 bits) boundary following the end of the HIRS/2 channel data.

The channel data itself (contained in halfwords 3-22 when all channels are requested) will be packed in consecutive halfwords for TIP minor frames 0-55. The other eight minor frames (56-63) still retain their 44-byte format. The unpacked data format for full copy HIRS/2 data is shown in Table 4.1.2.1-6. Note that the order of the channels is the same as for the packed format in Section 4.1.2.1. The record for the HIRS/2 full copy unpacked data format contains 3620 bytes.

Selective Extract Subsets

When channels are selected for HIRS/2 data, the format is like that described in Section 4.1.2.1, except that the format of the HIRS/2 channel data changes. Also, the minor frame quality has the same format described in Section 4.1.2.1 but it starts on the first halfword (16 bits, half of a 32-bit word) boundary following the end of the HIRS/2 channel data.

The channel data itself (contained in halfwords 3-22 when all channels are requested) will be packed in consecutive halfwords for TIP minor frames 0-55 (corresponding to the 56 FOVs per scan). The other eight minor frames (56-63) will still retain their 44-byte format. Note: The channels will be in ascending order when packed into the record. Table 4.1.2.2-1 shows the structure of the HIRS channel data when two channels (1 and 2) are selected.

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