NOAA KLM User's Guide

Section 3.2.2

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3.2.2 HIRS/4

3.2.2.1 Instrument Operation

The HIRS/4 instrument provides multispectral data from one visible channel (0.69 m), seven shortwave channels (3.7 to 4.6 m) and twelve longwave channels (6.7 to 15 m) using a single telescope and a rotating filter wheel containing twenty individual spectral filters. An elliptical scan mirror provides cross-track scanning of 56 steps in increments of 1.8 degrees. The mirror steps rapidly (<35 msec), then holds at each position while the optical radiation , passing through the 20 spectral filters, is sampled. This action takes place each 0.1 second. The instantaneous field of view for each channel is approximately 0.7 degrees which, from a spacecraft altitude of 833 km, encompasses a circular area of 10 km at nadir on the earth.

Three detectors are used to sense the optical radiation. A silicon photodiode, at nominal instrument temperature (15 C), detects the visible radiation. An Indium Antimonide detector and Mercury Cadmium Telluride detector (mounted on a passive radiator and operating at 95 K) sense the shortwave and longwave IR radiation, respectively. The shortwave and visible optical paths have a common field stop, while the longwave path has an identical but separate field stop. Size and registration of the optical fields of view in all channels is determined by these stops.

IR calibration of the HIRS/4 is provided by programmed views of two radiometric targets: the warm target mounted on the instrument baseplate, and a view of space. Data from these views provides sensitivity calibrations for each channel at 256 second intervals if commanded. Internally generated electronic signals provide calibration and stability monitoring of the detector amplifier and signal processing electronics.

Data from the instrument (including radiometric and telemetry information) is multiplexed into a single data stream which is controlled by the TIROS Information Processor (TIP) system in the NOAA-N, -P spacecraft. Information from the radiometric channels and voltage telemetry are converted into 13 bit binary data. Radiometric information is processed to utilize the maximum dynamic range of the A/D Converter, such that instrument generated digitizing noise is a small portion of the output signal. Each spectral channel is characterized by a noise equivalent radiance and radiance vs output counts calibration data that may be used to derive atmospheric temperatures and probable errors.

The HIRS/4 instrument is contained within a single package which is mounted on the Instrument Mounting Platform (IMP) of the NOAA-N,-P spacecraft. The cooler radiating surface views space, thereby emitting its heat to provide passive radiant cooling of the shortwave and longwave detectors to the controlled operating temperature of 95 K. The cooler door includes an earth shield which insulates the radiating surfaces of the radiant cooler from earth's thermal radiation. The cooler door is closed during launch and through the initial orbital outgas period. During this procedure, the cooler temperature rises to approximately 300 K. After this outgas period, the door is opened permanently for cooling of the radiator. A door-open outgas procedure can be performed, if indications of contamination become apparent, by applying power to the heaters on both stages of the radiant cooler.

Table 3.2.2.1-1 lists the general characteristics of the HIRS/4 instrument. Table 3.2.2.1-2 lists the spectral channel and sensitivity requirements for the HIRS/4.

Table 3.2.2.1-1. HIRS/4 System Characteristics.
Optical Field of View (FOV) 0.7 degrees all channels
Included Energy 98% within 1.80 degrees
Channel to channel registration LW: <1.5% of Ch. 8 FOV FWHM
SW: <1 % of Ch. 19 FOV FWHM
Earth Scan Angle ±49.5 degrees from nadir
Earth Scan Steps 56 increments of 1.8 degrees
Scan Step and Dwell Time 100 msec total
Scan, Slew and Retrace Intervals 0.8 seconds
Total Scan Line Time 6.4 seconds
Earth Swath Coverage 1127 km
Radiometric Calibration 286 K Internal Blackbody and Space Look
Frequency of Calibration 256 seconds
Dwell Time at Calibration Positions 5.6 seconds at IWT, 4.8 seconds at space
Longwave Channels 12 (6.5 to 15 μm)
Longwave Detector Mercury Cadmium Telluride
Shortwave Channels 7 (3.7 to 4.6 μm)
Shortwave Detector Indium Antimonide
IR Detectors Temperature 95 K
Visible Channel 1 (0.69 m)
Visible Detector Silicon
Visible Detector Temperature Instrument Ambient
Signal Quantizing Levels 8192 (13 bit coding)
Electronic Calibration 32 equal levels (bipolar)
Frequency of Electronic Calibration One level each scan line
Telescope Aperture 15.0 cm (5.9 inches)
Filters Temperature 290 K (with baseplate at 283 K)

Table 3.2.2.1-2. HIRS/4 Spectral Requirements.
Channel # Channel Frequency Half Power Bandwidth
(cm-1)
Maximum Anticipated Scene Temp(K) Specified NEΔN(mW/m2-sr-cm-1) Design Goal
(cm-1) (μm)
LW 1 6685 14.95 3 280 3.00 0.75
2 680 14.71 10 265 0.67 0.25
3 690 14.49 12 240 0.50 0.25
4 703 14.22 16 250 0.31 0.20
5 716 13.97 16 265 0.21 0.20
6 733 13.64 16 280 0.24 0.20
7 749 13.35 16 290 0.20 0.20
8 900 11.11 35 330 0.10 0.10
9 1,030 9.71 25 270 0.15 0.15
10 802 12.47 16 300 0.15 0.10
11 1,365 7.33 40 275 0.20 0.20
12 1,533 6.52 55 255 0.20 0.07
SW 13 2,188 4.57 23 300 0.006 0.002
14 2,210 4.52 23 290 0.003 0.002
15 2,235 4.47 23 280 0.004 0.002
16 2,245 4.45 23 270 0.004 0.002
17 2,420 4.13 28 330 0.002 0.002
18 2,515 4.00 35 340 0.002 0.002
19 2,660 3.76 100 340 0.001 0.001
VIS 20 14,500 0.69 (m) 1000 100% A (albedo) 0.10% A ----

3.2.2.2 System Description

3.2.2.2.1 Introduction

The HIRS/4 is a 20-channel scanning radiometric sounder. It utilizes a stepping mirror to accomplish crosstrack scanning to direct the radiant energy from a 10 km nominal diameter area on the Earth into a 15 cm diameter telescope every tenth of a second. Collected energy is separated by a dichroic beamsplitter into longwave (above 6.5 m) and shortwave (visible to 4.6 m) components. The IR energy then passes through field stops, a rotating filter wheel and onto two cooled IR detectors. In the shortwave path, a second beamsplitter separates and directs the visible energy to a silicon detector which is at instrument ambient temperature.

The scan control and logic circuitry sets the sequence of Earth viewing steps to provide a rapid scan mirror step motion to each of 56 fixed positions for spectral sampling of each respective atmospheric column. The filter wheel rotation is synchronized to this step and hold sequence. Approximately one fourth of the wheel is blank to provide for the time required for each step interval. The filters are thus in position for sampling only after the mirror has reached the hold position. Registration of the optical fields for each channel to a given column of the atmosphere is dependent largely on the alignment of two field stops which are adjusted to reduce residual registration error to less than two percent of the field diameter and to a lesser degree, on spacecraft motion.

Radiant energy is focused onto cooled IR detectors operating at a temperature of 95 K. A longwave (LW) Mercury Cadmium Telluride (HgCdTe) detector and shortwave (SW) Indium Antimonide (InSb) detector are mounted on a two stage passive radiant cooler. This cooler has sufficient reserve cooling capacity to permit active thermal control to maintain the detectors at 95 K. The cooler and its housing are designed to minimize condensation of contaminants on critical components. Windows on the housing and first stage control external thermal input and prevent transport of contaminants to the detectors. Baffles and traps capture condensable contaminants in other areas. A system for heating the patch and first stage provides for initial orbital outgassing and for decontamination later, should it be necessary. The cold first stage windows are heated several degrees above the surrounding area, reducing contaminant condensation on their surfaces.

The HIRS/4 radiant cooler is an improved version of that used in the HIRS/3 instruments. Changes include mounting of the SW Preamplifier on the cooled patch, and adding an aperture of the vacuum housing windows to reduce thermal inputs. The cooler has an operating point of 95 K versus 100K for the HIRS/3 and a control margin of at least 5 K at 95K.

Electronic circuits provide the functions of power conversion, command, telemetry and signal processing. Initial amplification of the signals from the IR detectors is done in two low noise preamplifiers. The visible detector signal uses a separate preamplifier but joins the shortwave just after the shortwave preamplifier. Radiant signals are fed through a base reference, a memory processor, analog multiplexer, and an A/D converter with a 13-bit range. Once converted to digital format, the data are again multiplexed with "housekeeping" telemetry data and provided as a serial data stream at the Digital "A" Output. Data from one scan element, or minor frame are held in one side of a dual path ping-pong memory until called by the TIROS Information Processor (TIP). While one side of the ping-pong memory is transferring data to the TIP, the other side is being loaded real-time with data for the next scan element. The data is sequenced out of the instrument by the TIP data clock.

Table 3.2.2.2.1-1 shows the sensor temperature ranges for the HIRS/4 instrument.

Table 3.2.2.2.1-1. HIRS/4 Sensor Temperature Ranges.
Channel Sensor Location Subcomm Analog(K) Digital "A" (K) Approx. Digital "A" Sensitivity (Counts/K) Nominal @ Operating Temperatures Notes
Patch - full range 90-320 90-320 53 @95K 2
Patch - expanded range   90-150 141 @95K  
Radiator 150-320 150-320 60 @167K 2
Filter Chopper (F/C) Motor 260-320 260-320 71 @300K  
Scan Motor 260-320 260-320 78 @295K 1
Baseplate 260-320 260-320 78 @290K 1
Electronics 260-320 260-320 78 @290K  
Primary Mirror   260-320 78 @290K  
Secondary Mirror   260-320 78 @290K  
Tertiary Telescope   260-320 78 @290K  
Scan Mirror   260-320 78 @290K  
F/C Housing -1 273.15K-333.15K 152 @295K  
-2 273.15K-333.15K 152 @303K  
-3 273.15K-333.15K 152 @295K  
-4 273.15K-333.15K 152 @295K  
Internal Warm Target (IWT) - 1 273.15K-333.15K 152 @290K  
- 2 273.15K-333.15K 152 @290K  
- 3 273.15K-333.15K 152 @290K  
- 4 273.15K-333.15K 152 @290K  
-5 273.15K-333.15K 152 @290K  
Internal Cold Target (ICT) - 1 243.15K-303.15K 152 @273K  
Notes:
1. Housekeeping TLM (full time temperature monitoring on switched +28 vDC TLM BUS)

2. PRT, all others are thermistors.

3.2.2.2.2 Scan System Description

An elliptical scan mirror provides cross-track scanning of 56 steps in increments of 1.8 degrees. The mirror steps rapidly (<35 msec), then holds at each position while the optical radiation, passing through the 20 spectral filters, is sampled. This action takes place each 0.1 second. The instantaneous field of view for each channel is approximately 0.7 degrees which, from a spacecraft altitude of 833 km, encompasses a circular area of 10 km in diameter at nadir on the Earth.

3.2.2.3 Calibration Requirements

IR calibration of the HIRS/4 is provided by programmed views of two radiometric targets: the warm target mounted on the instrument baseplate, and a view of space. Data from these views provides sensitivity calibrations for each channel at 256 second intervals if commanded. Internally generated electronic signals provide calibration and stability monitoring of the detector amplifier and signal processing electronics.

3.2.2.4 HIRS/4 Digital "A" Data

Digital "A" data from the instrument is described in Table 3.2.2.4-1. The TIP clock pulse (C1) and Data Enable pulses determine the time at which this data is called out. The TIP formatter calls out groups of 8-bit words in a sequence that multiplexes HIRS/4 data with that of other instruments on the NOAA-N, P spacecraft. Along with this requirement, the large quantity of instrument data to be transmitted and the use of 13-bit encoding for radiometric data, it was not possible to format the data into 8-bit segments. The HIRS/4 data is therefore provided as a continuous stream composed of 13-bit word lengths but clocked out in 8-bit words by the TIP. During any Minor Frame there are 288 bits of HIRS data which are extracted at an equivalent 8,320 bps rate.

The data format remains the same during the 56 Earth scan element time periods. During retrace, which is an interval of eight Earth scan element time periods, the data format is changed to provide for measurement of the internal electronic calibration and to sample all of the Housekeeping telemetry data.

Scan Element 0 contains the data which describes the scene at the time of viewing the first scan position. The scan positions are described later, but it should be noted that encoder position "1" occurs at the first Earth scan position and hence will be the encoder position noted during element "0". Scan element 55 designates the last Earth scan position. Scan Elements 56-63 occur during the scan mirror retrace during normal Earth scanning. These same element number designations apply also when the scan is commanded to a calibration target during the automatic calibration (Autocal) sequence. Normally, the mirror slewing motion between calibration targets takes place during the normal retrace interval except for the case of slew to the space look position where the motion occurs during scan elements 0 to 7. Therefore, space look during Autocal is only for 48 elements (8 through 55).

In order to determine when radiometric data should not be used, a Valid Data Bit is included in the data stream. This bit is a "1" when all conditions are normal and the radiometric data may be considered good. It will be "0" when the scan system is in a slew mode.

The electronic calibration level advances one level per scan line from 0 to 31, defining the step level measured in each radiometric channel during elements 56 and 57. Since both a positive and negative calibration is made during a scan line, the same level values applies for both.

The Channel 1 Period Monitor measures the time interval of the travel from the Channel 1 to the Channel 2 segments of the filter wheel on each rotation. The reading measures 1.248 MHz clock intervals during that segment, hence defines filter wheel rotation time with a resolution variation of 0.8 microseconds. This is not used in normal system data processing or evaluation but is a powerful diagnostic tool to aid in assessing the filter wheel subsystem health.

With every filter wheel revolution, a block of data is generated. This block, called an element, is 288 bits long. A scan line consists of 56 scene views and 8 retrace elements. Thus, there are 64 (58 + 8) filter wheel revolutions per line and 64 elements per scan line. Each element is numbered 0 through 63 and this 6-bit binary number is included in each element as bit location 20-25.

Filter Sync Designator is a "1" when the filter wheel is in synchronism with the data control timing system. This is diagnostic data not normally used in data collection or processing. If the Filter Sync Designator is "0" the radiometric data is not valid.

Radiant Signal Output is the 13-bit binary level measurement of the signals coming from the various detectors. The first bit is a sign bit ("1" positive, "0" is negative). The remaining twelve data bits are straight binary code in order from the most significant to least significant bit.

Minor Word Parity Check is the last bit of each minor Frame or data element and is inserted to make the total number of "ones" in that data element odd. This permits checking for loss of data integrity between transmission from the instrument and reconstruction on the ground.

Elements 58-61 contain the outputs of the temperature sensors and the ECAL DAC, sampled five times during each element. This approach provides a more accurate measurement of the more critical sensor temperatures.

In Element 62, the data multiplexer connects other voltage and temperature sensors outputs into the A/D converter for one sample each, thereby allowing monitoring of all the major test points in the system.

Element 63 contains the command status, the instrument serial number, the total line number since the last radiometric calibration (in 13-bit natural binary), and a fixed word pattern and fill bits.

Instrument Serial Number is unique for each instrument. The HIRS/4 instruments will be designated 015 and 016.

Command Status is a tabulation of the state of the command relays and it's complete format is contained in Table 3.2.2.4-1.

Table 3.2.2.4-1. HIRS/4 Digital "A" Data Output Format.
Element # Bit # Function Range Counts

0 - 55
(Earth Scan Elements)
1-8 Encoder Position 0 to 200
9-13 Electronic Cal Level 0 to 31
14-19 Channel 1 Period Monitor 0 to 63
20-25 Element Number 0 to 63
26 Filter Sync Designator 0 or 1
27-286 Radiant Signal Output (20 channels x 13 bits) 0 to ±4095
287 Valid Data Bit 0 or 1
288 Minor Word Parity Check 0 or 1
56-63 1-26 Same as above.  
287, 288 Same as above.  
56 27-286 Positive Electronic Calibration 13 bits for each channel (Calib. level advances one of 32 equal levels on succeeding scans)  
57 27-286 Negative Electronic Calibration 13 bits for each channel (Calib. level advances one of 32 equal levels on succeeding scans)  
58 27-91 Internal Warm Target #1, 13 bits x 5 times  
92-156 Internal Warm Target #2, 13 bits x 5 times  
157-221 Internal Warm Target #3, 13 bits x 5 times  
222-286 Internal Warm Target #4, 13 bits x 5 times  
59 27-91 Internal Cold Target #1, 13 bits x 5 times  
92-156 Ground  
157-221 Internal Warm Target #5, 13 bits x 5 times  
222-286 Tertiary Telescope Temp, 13 bits x 5 times  
60 27-91 Filter Housing Temp. #1, 13 bits x 5 times  
92-156 Filter Housing Temp. #2, 13 bits x 5 times  
157-221 Filter Housing Temp. #3, 13 bits x 5 times  
222-286 Filter Housing Temp. #4, 13 bits x 5 times  
61 27-91 Patch Temp Expanded, 13 bits x 5 times  
92-156 First Stage Temp, 13 bits x 5 times  
157-221 Filter Housing Control Power/Temp, 13 bits x 5 times  
222-286 Electronic Calibration DAC, 13 bits x 5 times  
62 27-39 Scan Mirror Temp  
40-52 Primary Telescope Temp  
53-65 Secondary Telescope Temp  
66-78 Baseplate Temp  
79-91 Electronics Temp  
92-104 Patch Temp - Full Range  
105-117 Scan Motor Temp  
118-130 Filter Motor Temp  
131-143 Radiant Cooler Housing Temp  
144-156 Patch Control Power  
157-169 Scan Motor Current  
170-182 Filter Motor Current  
183-195 +15 V DC  
196-208 -15 V DC  
209-221 +7.5 V DC  
222-234 -7.5 V DC  
235-247 +10 V DC  
248-260 +5 VDC  
261-273 Analog Ground  
274-286 Analog Ground  
63 27-39 Line Count  
40 Fill Zero  
41-44 Instrument Serial Number  
45-52 Command Status Command Status Bits
53-57 Fill Zeroes  
58-65 Command Status Command Status Bits
66-78 Binary Code (1,1,1,1,1,0,0,1,0,0,0,1,1) +3875 (base 10)
Fixed word pattern used to establish data stream synchronism with TIP
79-91 +1443
92-104 -1522
105-117 -1882
118-130 -1631
131-143 -1141
144-156 +1125
157-169 +3655
170-182 -2886
183-195 -3044
196-208 -3764
209-221 -3262
222-234 -2283
235-247 -2251
248-260 +3214
261-273 +1676
274-286 +1992
Command Status Bits (from Element 63) 45 Instrument On/Off On=1
46 Scan Motor On/Off On=0
47 Filter Wheel On/Off On=0
48 Electronics On/Off On=1
49 Cooler Heat On/Off On=0
50 Internal Warm Target Position True=0
51 Internal Cold Target Position True=0
52 Space Position True=0
58 Nadir Position True=0
59 Calibration Enable/Disable Enabled=0
60 Cover Release Enable/Disable Enabled=0
61 Cooler Cover Open Yes=1
62 Cooler Cover Closed Yes=1
63 Filter Housing Heat On/Off On=0
64 Patch Temp Control On/Off On=0
65 Filter Motor Power High Normal=1

Amended April 5, 2004

Amended June 23, 2005


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