NOAA KLM User's Guide

Section 3.2.1

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3.2.1 HIRS/3 Instrument Operation

The High Resolution Infrared Radiation Sounder (HIRS/3) is a discrete stepping, line-scan instrument designed to measure scene radiance in 20 spectral bands to permit the calculation of the vertical temperature profile from Earth's surface to about 40 km.

Multispectral data from one visible channel (0.69 micrometers), seven shortwave channels (3.7 to 4.6 micrometers) and twelve longwave channels (6.5 to 15 micrometers) are obtained from a single telescope and a rotating filter wheel containing twenty individual filters. An elliptical scan mirror provides cross-track scanning of 56 increments of 1.8 degrees. The mirror steps rapidly (<35 msec), then holds at each position while the 20 filter segments are sampled. This action takes place each 100 msec. The instantaneous FOV for each channel is approximately 1.4 degrees in the visible and shortwave IR and 1.3 degrees in the longwave IR band which, from an altitude of 833 kilometers, encompasses an area of 20.3 kilometers and 18.9 kilometers in diameter, respectively, at nadir on the Earth.

Three detectors are used to sense the radiation. A silicon photodiode at the instrument temperature (nominally 15 degrees C) detects the visible energy. An Indium Antimonide detector and a Mercury Cadmium Telluride detector (mounted on a passive radiator and operating at 100K) sense the shortwave and longwave IR energy. The shortwave and visible optical paths have a common field stop, while the longwave path has an identical but separate field stop. Registration of the fields of view in all channels is largely determined by these stops with secondary effects from detector position.

IR Calibration of the HIRS/3 is provided by programmed views of two radiometric targets: a warm target mounted to the instrument base and a view of space. Data from these views provides sensitivity calibrations for each channel every 40 lines (256 seconds), if commanded. Internally generated electronic signals provide calibration and stability monitoring of the amplifier and readout electronics.

Data from the instrument is multiplexed into a single data stream controlled by the TIP system of the spacecraft. Information from the radiometric channels and voltage telemetry are converted to 13-bit binary data. Radiometric information is processed to produce the maximum dynamic range such that instrument and digitizing noises are a small portion of the signal output. Each channel is characterized by a noise equivalent radiance (NEΔN) and a set of calibration data that may be used to derive atmospheric temperatures and probable errors.

The HIRS/3 instrument is a single package mounted on the Instrument Mounting Platform (IMP) of the NOAA KLM spacecraft. Thermal blankets cover most outer surfaces other than that of the radiating panel and door area. The cooler radiating surface views space, emitting its heat to provide passive radiant cooling of the shortwave and longwave detectors to the stabilized 100K operating temperature. An Earth shield on the cooler door assembly insulates the door from Earth direction thermal input. The door is closed during launch and through the initial orbital outgas period (at least 14 days). After this outgas period, the door is opened permanently for cooling of the radiator. If there are indications of contaminate accumulation subsequently, a door-open outgas procedure can be performed by applying power to the heaters located on both stages of the radiant cooler. During this procedure, the cooler temperature rises to approximately 300K.

Table lists the system characteristics of the HIRS/3 Instrument. Table lists the spectral channel and sensitivity requirements for the HIRS/3. See Appendix J.2 for specific scan parameters and patterns of the HIRS/3 instrument.

Table HIRS/3 System Characteristics.
Characteristic Value
Optical Field of View 1.4 degrees VIS/SW IR
1.3 degrees LW IR
Included Energy 98% within 1.80 degrees
Channel to channel registration LW: <1.5% of 1.8 degrees Step size
SW: <1% of Ch. 19 FOV FWHM
Earth Scan Angle ±49.5 degrees from nadir
Earth Scan Steps 56 increments of 1.8 degrees
Step and Dwell Time 100 msec total
Retrace Steps 8
Total Scan plus Retrace Time 6.4 seconds
Earth Swath coverage 1127 km
Earth Field Coverage:
       (SW IR) + VIS
       (LW IR)
20.3 km (1.4 degrees IFOV) at nadir
18.9 km (1.3 degrees IFOV) at nadir
Radiometric Calibration 290 K IWT Blackbody and Space Look
Frequency of Radiometric Calibration 256 seconds, typical
Dwell Time at Calibration Position 5.6 seconds at IWT, 4.8 seconds at space
Longwave Channels 12 (6.5 to 15 micrometers)
Longwave Detector Mercury Cadmium Telluride
Shortwave Channels 7 (3.7 to 4.6 micrometers)
Shortwave Detector Indium Antimonide
IR Detectors Temperature 100 K
Visible Channel 1 (0.69 micrometers)
Visible Detector Silicon
Visible Detector Temperature Ambient
Signal Quantizing Levels 8192 (13 bit coding)
Electronic Calibration 32 equal levels each polarity
Frequency of Electronic Calibration One level each scan line
Telescope Aperture 15.0 cm (5.9 in)
Filter Housing Temperature 290 K (with baseplate at 283 K)

Table HIRS/3 Spectral Characteristics.
Half Power
Noise Equivalent
Delta Radiance
1 669 14.95 3 3.00
2 680 14.71 10 0.67
3 690 14.49 12 0.50
4 703 14.22 16 0.31
5 716 13.97 16 0.21
6 733 13.64 16 0.24
7 749 13.35 16 0.20
8 900 11.11 35 0.10
9 1,030 9.71 25 0.15
10 802 12.47 16 0.15
11 1.365 7.33 40 0.20
12 1,533 6.52 55 0.20
13 2,188 4.57 23 0.006
14 2,210 4.52 23 0.003
15 2,235 4.47 23 0.004
16 2,245 4.45 23 0.004
17 2,420 4.13 28 0.002
18 2,515 4.00 35 0.002
19 2,660 3.76 100 0.001
20 (see Note 1) 14,500 0.690 1,000 0.10% albedo
1. Visible Channel System Description Introduction

The HIRS/3 is a 20-channel scanning radiometric sounder utilizing a stepping mirror to accomplish cross-track scanning, directing the radiant energy from the Earth to a single, 15.24 cm (6-inch) diameter telescope assembly every tenth of a second. Collected energy is separated by a beamsplitter into longwave (above 6.5 micrometers) and shortwave (visible to 4.6 micrometers), passed through field stops and through a rotating filter wheel to cooled detectors. In the shortwave path, a second beamsplitter separates the visible channel to a silicon detector.

The scan logic and control set the sequence of Earth viewing steps to provide a rapid scan mirror step motion to 56 fixed positions for spectral sampling of each respective air column. The filter wheel rotation is synchronized to this step and hold sequence, with approximately one-third of the wheel blank to accommodate each step interval and with the filters positioned for sampling only after the mirror has reached the hold position. Registration of the optical fields for each channel to a given column of air is dependent to some degree on spacecraft motion and on the alignment of two field stops, which can be adjusted to reduce registration error to less than 1 percent of the field diameter.

The characteristics of the instrument temperature sensors are shown in Table, where it may be noted that calibration sources are measured very precisely.

Table HIRS/3 Sensor Temperature Ranges.
Channel Sensor Location Subcomm Analog (K) Digital "A" (K) Approx. Digital "A" Sensitivity (Counts/K) Nominal @ Operating Temperature
Patch - full range 90-320 90-320 53 @100K
Patch - expanded range   90-150 141 @100K
Radiator 150-320 150-320 60 @170K
Filter Wheel (F/W) Motor 260-320 260-320 71 @300K
Scan Motor 260-320
(see Note 1)
260-320 78 @295K
Baseplate 260-320
(see Note 1)
260-320 78 @290K
Electronics 260-320 260-320 78 @290K
Primary Mirror   260-320 78 @290K
Secondary Mirror   260-320 78 @290K
Scan Mirror   260-320 78 @290K
F/W Housing (HSG) - 1   273.15K-333.15K 152 @295 K
F/W Housing (HSG) - 2   273.15K-333.15K 152 @303K
F/W Housing (HSG) - 3   273.15K-333.15K 152 @295 K
F/W Housing (HSG) - 4   273.15K-333.15K 152 @295 K
Internal Warm Target (IWT) - 1   273.15K-333.15K 152 @290 K
Internal Warm Target (IWT) - 2   273.15K-333.15K 152 @290K
Internal Warm Target (IWT) - 3   273.15K-333.15K 152 @290 K
Internal Warm Target (IWT) - 4   273.15K-333.15K 152 @290 K
Internal Cold Target (ICT) - 1   243.15K-303.15K 152 @273 K
Internal Cold Target (ICT) - 2   243.15K-303.15K 152 @273K
Internal Cold Target (ICT) - 3   243.15K-303.15K 152 @273 K
Internal Cold Target (ICT) - 4   243.15K-303.15K 152 @273 K
1. Housekeeping TLM (full time temperature monitoring on switched +28 VDC BUS)

Radiant energy is focused on cooled detectors operating at a near optimum temperature of 100K. A Mercury Cadmium Telluride detector and an Indium Antimonide detector are mounted on a two-stage radiant cooler. This assembly is large enough to have reserve cooling capacity, permitting active thermal control to maintain the detectors fixed at 100K. A system of heating the patch and first stage is provided for initial outgassing and for decontamination later if it should be required.

Electronic circuits provide the functions of power conversion, command, telemetry, and signal processing. Amplification of the inherently weak signals from the detectors is done by low noise amplifiers. Radiant signals are fed through a base reference and memory processor, multiplexed, and A/D converted by a 13-bit range system. Once converted to a digital format, the data are again multiplexed with HIRS/3 "housekeeping" data and provided as a serial data stream at the digital A output. Data from HIRS/3 are held in memory until called by the TIP request signals and clocked out of the instrument by the TIP clock.

Repetitive inclusion of electronic calibration signals and the periodic command to scan to space and one internal blackbody provides the system with a complete set of data collection, calibration, and control that permits reliable operation in orbit. Scan System Description

The heart of the scan system is a stepper motor with 200 steps per rotation. Assembled to the shaft of the stepper motor are (1) a scan mirror mounted with the mirror surface plane at 45 degrees with respect to the axis of the motor shaft, (2) a DC torque motor, (3) a DC tachometer, (4) a shaft position encoder, and (5) a two contact slip ring assembly. The DC tachometer is used to derive rate information which is used to control a DC torque motor to aid stopping of the scan rotation as the scan settles into a track position or into a step position.

The drive control signals for the stepper motor are generated by a step generator which has input control features for retrotorque control, step/slew control and rotational direction control (up/down). Output from a scan regulator with power level control is applied to the motor driver to provide stepping torque levels as required.

The Scan Control logic utilizes timing signals and position commands in conjunction with position information derived from a position encoder to monitor and control performance of the system. Connections to a thermal sensor on the scan mirror are provided via two leads from the mirror through the hollow motor shaft to a slip ring assembly at the opposite end of the shaft. Calibration Requirements

The Calibration Enable command enables the radiometric calibration control logic. Calibration is activated by the calibration start pulse (CSP) which occurs every 256 seconds. The CSP initiates a sequence of scans that provide radiometric calibration of the instrument. When the Calibration Disable command is in effect, the CSP will be ignored and the instrument will continue its normal scan sequence. HIRS/3 Digital A Data

Digital A data is clocked into the spacecraft TIP whenever the "A1" Data Enable Pulse is presented to the instrument. A full set of HIRS/3 operational data, including command status monitors, housekeeping information and radiance data of the 20 channels, is contained in the Digital A output. The HIRS/3 data repeats every 6.4 seconds as described below. The 6.4 second period contains 64 elements.

Digital A output is divided into "Elements" of 288 bits. One element is phased to fit into a TIP minor frame. The TIP will allocate 36 8-bit words to accommodate the HIRS/3 element. These TIP words will be grouped by two in fixed locations throughout the 100 millisecond minor frame.

Sixty-four elements make up each scan. The formats for the elements repeat eery 6.4 seconds and correspond to the particular parts of the scan. Element number 0 through 55 are earth scan data (target data during the calibration sequence). Elements 56 through 63 are associated with retrace or when the mirror is slewing between positions. The elements are divided as follows:

Bits 1-26

Two 13 bit words have the same function in all 64 elements. The function assembled in these words are described in Table

Table Functions of Bits 1-26 in the HIRS/3 Elements.
Word # Bits Function Range (Decimal)
1 1-8 Scan Encoder Position 0 to 199
9-13 Electronic Cal Level Indicator 0 to 31
2 1 - 6 Channel 1 Period Monitor 0 to 63
7-12 Element Number 0 to 63
13 Filter Sync Designator n/a

Bits 27-286

This group of bits are divided into 20 13-bit words. The word functions are dependent on element number. These functions are given in Table Except for the two status words in Element 63, all words are quantity where bit 1 is the sign bit and bits 2 through 13 are amplitude (0 to 4095). Bit 2 is the most significant bit (MSB) of quantity. The sign bit is: logic "1" is + and logic "0" is -.

Bits 287 and 288

In the same manner as for bits 1 through 26, these two bits have the same function in all 64 elements as shown in Table

Table Functions of Bits 287-288 in the HIRS/3 Elements.
Bits Function Remarks
287 Valid Data Bit logic "1" Valid Data
logic "0" Ignore Radiometric Data
288 Odd Bit Parity n/a

One HIRS/3 word is contained in two contiguous 8-bit TIP words. These two words occur in 1.899 msec. Eighteen pairs, each consisting of two Minor Frames TIP words, are required per minor frame. These eighteen words make up one HIRS/3 Element. The Digital A output is a serial output of 288 bits per element. The first bit (Bit 1) occurs in the MSB of TIP word 14.

Tables and contain the HIRS/3 Digital A Radiometric and housekeeping functions, and the Digital A Status Telemetry, respectively.

Table HIRS/3 Digital A Radiometric and Housekeeping Functions.
Element # Bit # Function Remarks
0 - 55 27-39 Radiometric Channel No. 1 (668.5 cm-1) 0 counts radiance from scene equal radiance from Filter Wheel (FW). Minus (-) values are colder than FW. Plus (+) values are warmer than FW.
40-52 Radiometric Channel No. 17 (2,420 cm-1) 0 counts offset from FW radiance. Plus and minus are hotter and cooler than offset.
53-65 Radiometric Channel No. 2 (680 cm-1) No offset.
66-78 Radiometric Channel No. 3 (690 cm-1) No offset.
79-91 Radiometric Channel No. 13 (2,188 cm-1) Offset.
92-104 Radiometric Channel No. 4 (703 cm-1) No offset.
105-117 Radiometric Channel No. 18 (2515 cm-1) Offset.
118-130 Radiometric Channel No. 11 (1365 cm-1) No offset.
131-143 Radiometric Channel No. 19 (2,660 cm-1) Offset.
144-156 Radiometric Channel No. 17 (749 cm-1) No offset.
157-169 Radiometric Channel No. 8 (900 cm-1) No offset.
170-182 Radiometric Channel No. 20 Visible (14,500 cm-1) Black is minus. White is plus.
183-195 Radiometric Channel No. 10 (802 cm-1) No offset.
196-208 Radiometric Channel No. 14 (2,210 cm-1) Offset.
209-221 Radiometric Channel No. 6 (749 cm-1) No offset.
222-234 Radiometric Channel No. 5 (716 cm-1) No offset.
235-247 Radiometric Channel No. 15 (2,235 cm-1) Offset.
248-260 Radiometric Channel No. 12 (1,533 cm-1) No offset.
261-273 Radiometric Channel No. 16 (2,245 cm-1) Offset.
274-286 Radiometric Channel No. 9 (1,030 cm-1) Offset.
56 27-286 Positive Electronics Calibration applied to 20 radiometric channels. Calibration level advances one of the 32 equal level steps on successive scans. The offset and gain of each channel will influence the amplitude of the signal. The calibration level applied to the (continued) electronic channels is indicated by bits 9 through 13 of the Element.
57 27-286 Negative Electronics Calibration applied to 20 radiometric channels. Calibration level advances one of the 32 equal level steps on successive scans. The offset and gain of each channel will influence the amplitude of the signal. The calibration level applied to the (continued) electronic channels is indicated by bits 9 through 13 of the Element.
58 27-91 Internal Warm Target Temperature Sensor #1 Value repeated 5 times. Range 273 to 333 K.
92-156 Temperature Sensor #2 Value repeated 5 times. Range 273 to 333 K.
157-221 Temperature Sensor #3 Value repeated 5 times. Range 273 to 333 K.
222-286 Temperature Sensor #4 Value repeated 5 times. Range 273 to 333 K.
59 27-91 Internal Cold Target Temperature Sensor #1 Value repeated 5 times. Range 243 to 303 K.
92-156 Temperature Sensor #2 Value repeated 5 times. Range 243 to 303 K.
157-221 Temperature Sensor #3 Value repeated 5 times. Range 243 to 303 K.
222-286 Temperature Sensor #4 Value repeated 5 times. Range 243 to 303 K.
60 27-91 Filter Wheel Housing Temperature Sensor #1 Value repeated 5 times. Range 273 to 333K.
92-156 Temperature Sensor #2 Value repeated 5 times. Range 243 to 303 K.
157-221 Temperature Sensor #3 Value repeated 5 times. Range 243 to 303 K.
221-286 Temperature Sensor #4 Value repeated 5 times. Range 243 to 303 K.
61 27-91 Patch Temperature Expanded Scale Value repeated 5 times. Range 90 to 150 K.
92-156 First Stage Radiator Temperature Sensor Value repeated 5 times. Range 150 to 320 K.
157-221 Filter Wheel Housing Heater Current Value repeated 5 times. Range 0 to 500 ma.
222-286 Electronic Calibration Digital to Analog Converter Value repeated 5 times. Range 0 to 4 V.
62 27-39 Scan Mirror Temperature Range 260 to 320 K
40-52 Primary Telescope Temperature Range 260 to 320 K
53-65 Secondary Telescope Temperature Range 260 to 320 K
66-78 HIRS/3 Baseplate Temperature Range 260 to 320 K
79-91 HIRS/3 Electronics Temperature Range 260 to 320 K
92-104 Patch Temperature - Full Range Range 90 to 320 K
105-117 Scan Motor Temperature Range 260 to 320 K
118-130 Filter Wheel Motor Temperature Range 260 to 320 K
131-143 Cooler Housing Temperature Range 260 to 320 K
144-156 Patch Control Power Range 0 to 80 mw
157-169 Scan Motor Current Range 0.65 to 1.0 ma
170-182 Filter Motor Current Range 100 to 300 ma
183-195 +15 VDC Range 15 ± 0.2 V
196-208 -15 VDC Range -15 ± 0.2 V
209-221 +7.5 VDC Range +7.5 ± 0.05V
222-234 -7.5 VDC Range -7.5 ± 0.05V
235-247 +10 VDC Range 10 ± 0.2V
248-260 +5 VDC Range 5 ±0.2V
261-273 Analog Ground Range 1 count
274-286 Analog Ground Range ± 1 count
63 27-39 Line Counter (Gives the number of lines from the last auto calibration sequence) 0 to 8191 (There is no sign bit used in the line counter). Reset to 0 counts is only when counter overflows.
40-52 First Status Word First 5 bits are instrument serial number (no sign bit). The remaining bits indicate status as shown in Table
53-65 Second Status Word First 5 bits are zero fill. The remaining bits indicate status as shown in Table
66-78 Data Verification Binary Code Binary code is:
(1 1 1 1 0 0 1 0 0 0 1 1)
Equivalent base10 value: +3875
79-91 Base 10 value: +1443
92-104 Base 10 value: -1522
105-117 Base 10 value: -1882
118-130 Base 10 value: -1631
131-143 Base 10 value: -1141
144-156 Base 10 value: +1125
157-169 Base 10 value: +3655
170-182 Base 10 value: -2886
183-195 Base 10 value: -3044
196-208 Base 10 value: -3764
209-221 Base 10 value: -3262
222-234 Base 10 value: -2283
235-247 Base 10 value: -2251
248-260 Base 10 value: +3214
261-273 Base 10 value: +1676
274-286 Base 10 value: +1992
Table HIRS/3 Digital A Status Telemetry.
Element # Bit # Function Logic State
First Status Word
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
Second Status Word
63 58 Nadir position True=0
59 Calibration Enable/Disable Enabled=0
60 Cooler Door Release Enable/Disable Enabled=0
61 Cooler Door Open Yes=1
62 Cooler Door Closed Yes=1
63 Filter Housing Heat ON/OFF ON=0
64 Patch Temperature Control ON/OFF ON=0
65 Filter Motor Power HIGH Normal=1

Amended April 5, 2004

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