NOAA KLM User's Guide

Section 3.1

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3.1 Advanced Very High Resolution Radiometer/3 (AVHRR/3)

3.1.1 Instrument Operation

This section describes the six channel Advanced Very High Resolution Radiometer (AVHRR/3) developed by ITT-A/CD. The AVHRR/3 is used as the meteorological imaging system of the NOAA KLM spacecraft.

The AVHRR/3 is an imaging system in which a small field of view (1.3 milliradians by 1.3 milliradians) is scanned across the earth from one horizon to the other by continuous 360 degree rotation of a flat scanning mirror. The orientation of the scan lines are perpendicular to the spacecraft orbit track and the speed of rotation of the scan mirror is selected so that adjacent scan lines are contiguous at the subsatellite (nadir) position. Complete strip maps of the earth from pole to pole are thus obtained as the spacecraft travels in orbit at an altitude of approximately 833 km (450 n. miles). 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). All six spectral channels of the AVHRR/3 are registered so that they all measure energy from the same spot on the earth at the same time. All six channels are also calibrated so that the signal amplitude in each channel is a measure of the scene radiance. Although the AVHRR/3 has six channels, only five are transmitted to the ground at any one time. The radiometers are designed to operate within specification for a period of three years in orbit.

3.1.2 System Description

3.1.2.1 General

The AVHRR/3 is a six channel scanning radiometer providing three solar channels in the visible-near infrared region and three thermal infrared channels. The AVHRR/3 has two one-micrometer wide channels between 10.3 and 12.5 micrometers. The instrument utilizes a 20.32 cm (8 inch) diameter collecting telescope of the reflective Cassegrain type. Cross-track scanning is accomplished by a continuously rotating mirror directly driven by a motor. The three thermal infrared detectors are cooled to 105 Kelvin (K) by a two-stage passive radiant cooler. A line synchronization signal from the scanner is sent to the spacecraft MIRP which in turn sends data sample pulses back to the AVHRR. Although AVHRR/3 is a six channel radiometer, only five channels are transmitted to the ground at any given time. Channels 3A and 3B cannot operate simultaneously. The data from the six channels are simultaneously sampled at a 40 kHz rate and converted to 10-bit binary form within the instrument. The data samples from each channel are output in a non-continuous burst of 10 space samples, 2048 Earth samples, and 10 internal calibration target samples per scan.

A summary of the AVHRR/3 spectral characteristics and system performance characteristics are given in Tables 3.1.2.1-1 and 3.1.2.1-2, respectively.

Table 3.1.2.1-1. Summary of AVHRR/3 Spectral Channel Characteristics.
Parameter
Ch. 1
Ch. 2 Ch. 3A Ch. 3B Ch. 4 Ch. 5
Spectral Range (μm) 0.58-0.68 .725-1.0 1.58-1.64 3.55-3.93 10.3-11.3 11.5-12.5
Detector type Silicon Silicon InGaAs InSb HgCdTe HgCdTe
Resolution (km) 1.09 1.09 1.09 1.09 1.09 1.09
IFOV (see Note 1) (milliradian) 1.3 sq. 1.3 sq. 1.3 sq. 1.3 sq. 1.3 sq. 1.3 sq.
S/N @ 0.5% albedo ≥9:1 ≥9:1 ≥20:1 - - -
NEdT @ 300K - - - ≤.12K ≤.12K ≤.12K
MTF @ 1.09 km >.30 >.30 >.30 >.30 >.30 >.30
Temperature Range (K) - - - 180 - 335 180 - 335 180 - 335
Note:
1. Tolerance on IFOV values are ±0.2 mr with a ±0.1 mr design goal.

Table 3.1.2.1-2. AVHRR/3 System Performance Characteristics.
Parameter/Characteristic Value
Telescope 8 in. diameter afocal Cassegrain
Scan Motor 360 rpm hysterisis - synchronous
Scan Mirror 8.25 in. x 11.6 in. Elliptical ribbed beryllium
Cooler Two stage radiant cooler controlled @ 105 K
Data Output 10 bit parallel words
Video Sample Rate 40 kHz simultaneous sample of all channels
Output Data Rate 200 k word/sec max
Line Sync Pulse Out 100 microseconds @ 6 pps
Input Clock 0.9984 MHz
Overall Dimensions 31.33 in. X 14.35 in. X 11.5 in.
Weight 73 lbs.
Line to Line Scan Jitter ± 17 microseconds
Scan Sync Drift/24 hours <3.0 microseconds
Calibration Accuracy
Relative 1 NEΔT
Absolute Traceable to NIST
ICT monitor accuracy 0.1 degree C
Power
Orbital Average 27W
Main Supply Bus Voltage Nominal 28±0.56V (16-38V range)
Interface Bus Voltage Nominal 10±0.5V (9-15V range)
Radiated EMI
SAR Bands -150 dBm
Instrument Temperature Range
Operating +10 to +30 degrees C
Storage +5 to +30 degrees C
Survivable -5 to +30 degrees C
Number of Telemetry channels
Analog 22 channels (+0.2V to +5V)
Digital 15 channels (0V = "1", 5V = "0")
Commands
Quantity 30
Amplitude 10 ± 0.7V
Duration 0.5 to 2.0 seconds
A/D Conversion
Quantizing Level 10 bits
Accuracy (15 degrees C) ±1/2 LSB
Differential non-linearity (15 degrees C) ±1/2 LSB
Maximum Error (10 - 30 degrees C) ±1 LSB
Uncompensated Momentum
Yaw Axis 0
Roll Axis 38 in-oz-sec, CCW, (+Y axis)
Pitch Axis 0
Maximum Angular Momentum <0.268 Newton-m-sec
Data Sample Pulse from MIRP
Duration 1.0 microsecond
Rate 39.936 kpps
Amplitude 5.0V
Electronic Calibration
Ramp-Cal (during space view) 1024 levels
Volt-Cal (during earth scene) 3 levels

AVHRR/3 data is sampled only at specified times during the Earth scene and backscan regions of each scan period and not on a continuous basis.

Visible Channels 1, 2 and 3A have dual slope gain characteristics with slope intercepts as shown in Table 3.1.2.1-3.

Table 3.1.2.1-3. AVHRR/3 visible channel gain and intercept characteristics.
1, 2 0-25%
26-100%
0-500
501-1023
3A 12.5%
12.6-100%
0-500
501-1023

The data from each of the five active channels is digitized in the radiometer to a 10-bit word and brought out of the instrument as a 10-bit parallel digital output to the spacecraft MIRP. In addition to these digital signals, the line synchronization signal is also routed to the MIRP on a separate buffer isolated line.

Some differences exist when switching between Channels 3A and 3B. The Channel 3A Select and Channel 3B Select commands are received and stored in a latching relay. A set of contacts in this relay is used to indicate the relay status virtually instantaneously through the digital telemetry. The second set of contacts in the relay switches a logic level on the A/D logic board, which, in turn, is used by the A/D sample and hold board to switch the A/D input between Channel 3A and Channel 3B. This same logic level is also used by the 3A/3B select flag circuitry. Since the command relay is asynchronous, the data output will switch instantaneously between 3A or 3B, even if the scan is in the middle of a line. The select flag circuitry, however, operates differently.

When Channel 3B is selected, the patch temperature data is output every scan line (during the backscan), and every other scan line when 3A is selected. When switching from 3B to 3A, the zero volt marker between lines 5 and 6 indicates that they switched sometime during line 4. Therefore, there is one scan line of uncertainty when switching from 3B to 3A. When switching from 3A to 3B, the presence of patch temperature data between lines 11 and 12 indicates that the switch occurred sometime between lines 9 and 12. Therefore, there are two scan lines of uncertainty when switching from 3A to 3B.

The AVHRR/3 is comprised of five modules which are assembled together into a single unit instrument. These modules are:

An exploded view of the AVHRR/3 is shown in Figure 3.1.2.1-1.

Figure showing AVHRR/3 Exploded View

3.1.2.2 Scanner Module

This module includes the scan motor, the mirror, and the scan motor housing. The scan motor is an 80 pole hysteresis synchronous motor. The motor has two power modes of operation and is normally operated in the high-power mode (~4.5 watts) in orbit in order to minimize scan line jitter. The scanner housing is an integral part of the motor and is made of beryllium. The scan mirror is also made of beryllium and is ~29.5 cm (11.6 inches) across the major axis and 20.96 cm (8.25 inches) across the minor axis. The scan motor rotates the mirror at 360 rpm to produce a contiguous scan of the Earth scene. The line-to-line jitter is less than ±17 microseconds. See Appendix J.1 for specific scan parameters and patterns of the AVHRR/3 instrument.

3.1.2.3 Electronics Module

The electronics module is in two sections, both of which bolt onto the instruments' inboard side panel. The curved box (Reference Figure 3.1.2.1-1) is the motor power supply. Thirty electronic circuit assemblies are used to make up the electrical system of the AVHRR/3. Twenty-two of these are located in the electronics box. The preamplifiers for Channels 1, 2, and 3A are located on the relay optics assembly. The IR Channel 4 and Channel 5 preamplifiers are located on the optics side of the electronics box. Channel 3B preamplifier is located near the 3B detector on the radiant cooler. Channels 4 and 5 preamplifier modules are accessible without the removal of the instrument from the spacecraft.

3.1.2.4 Radiant Cooler Module

The radiant cooler module is made up of four basic assemblies. These are (1) the cooler housing, (2) the first stage radiator, (3) the patch or second stage radiator and (4) the cooler cover. The first stage radiator is configured in such a manner as to shade most of its radiating area from the Earth by the cooler cover when the cover is deployed. A solenoid is used to deploy the cover. Once deployed, the cover cannot be closed. Mounted on the cold (105K) patch are the three thermal infrared detectors. The patch has a 144.5 cm2 (22.4 in2) radiating area.

Multi layer insulation thermally separates the first stage radiator from the housing, and the first stage optical window is thermally isolated and heated several degrees warmer than the 171 K radiator temperature to prevent condensation on it. During nominal operation the patch temperature is temperature controlled to 105K.

3.1.2.5 Optical Subsystem

The optical subsystem consists of two subassemblies, a collecting telescope and a relay optics unit. The telescope is a 20.3 cm (8.0 inch) diameter aperture, reflective Cassegrain of a focal design (collimated output). The relay optics split the telescope exit beam into six discrete spectral bands and focus them onto their respective field stops. The spectral bands are:

The instantaneous field of view is 1.3 by 1.3 milliradians in all channels and is defined by an aperture plate in Channels 1, 2, and 3A and by the detector active areas in Channels 3B, 4, and 5. In addition, the optical subsystem has been designed to meet the total system MTF requirements with the detectors laterally displaced for channel-to-channel registration.

Polarization effects have been minimized in Channels 1, 2, and 3A by passing the optical beam transmitted by the first visible/infrared beamsplitter through a second beamsplitter of the same type which is oriented so as to compensate for polarization introduced by the first beamsplitter.

3.1.2.6 Baseplate Unit

The baseplate unit is the common structure to which all other modules are secured. Dowel pins are used to establish and maintain alignment of the scanner and optics modules. Alignment of the cooler to the optics is established by shims.

3.1.3 MIRP Processing

This section is provided as an overview of the AVHRR/3 data processing that takes place in the Spacecraft Manipulated Information Rate Processor (MIRP). The MIRP sends a data sample pulse to the AVHRR/3 digital output timing logic which results in the sequential transfer of a data word for each radiometric channel. The MIRP processes these data into the following four outputs.

3.1.3.1 Automatic Picture Transmission (APT)

Any two of the six AVHRR/3 channels can be command-selected for processing. This data undergoes the following:

  1. Resolution reduction by using every third scan line of AHVRR data.
  2. Geometric correction to reduce the perspective effect due to the Earth's curvature and the satellite altitude. Details are contained in Section 4.2.
  3. Digital to Analog - The digitally processed APT data are converted to a 2080 Hz bandwidth analog signal, amplitude modulated onto a 2.4 kHz carrier, and bandwidth limited to 4160 Hz in preparation for transmission by the VHF transmitters.

3.1.3.2 Global Area Coverage (GAC)

MIRP produces the GAC output by combining processed AVHRR/3 data with the TIP data. The GAC frame rate is 2 frames per sec; that is, it is one third of the AVHRR/3 frame rate. The GAC processing of the AVHRR/3 data makes the frame rates directly compatible by only using the data from every third AVHRR/3 scan. The MIRP further reduces the data by averaging the value of four adjacent samples and skipping one sample of each channel of AVHRR data across each scan line used. The TIP word and frame rate is directly compatible with the GAC word and frame rate; five TIP minor frames are inserted (at 0.1 seconds per frame) into the GAC frame each 0.5 seconds. Two parity bits are added to the 8-bit TIP word to form the 10-bit GAC word. The GAC output is supplied only to the spacecraft DTR Input Selector Unit of the XSU, and it is, therefore, not available for direct readout users.

3.1.3.3 High Resolution Picture Transmission (HRPT)

MIRP produces the HRPT and the LAC outputs by combining unprocessed AVHRR/3 data with TIP data. The basic frame rate and data rate of the HRPT is compatible with the AVHRR/3. Therefore, only buffering is required to construct the HRPT frame from the AVHRR/3 data. However, since the TIP frame rate is only one third that required to fill the HRPT frame, the TIP data must be repeated three times. This is accomplished by repeating the five TIP minor frames (104 words each) in each of three HRPT frames. As in GAC, two parity bits are added to the 8-bit TIP word to form the 10-bit MIRP word. The HRPT output is supplied to the S-band transmitter input control for real-time transmission.

3.1.3.4 Local Area Cover (LAC)

LAC is, by definition, recorded HRPT; thus, the LAC output is supplied only to the spacecraft DTR input selector for recording.


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