NOAA KLM User's Guide

Section 3.3

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3.3 Advanced Microwave Sounding Unit-A (AMSU-A)

3.3.1 Instrument Operation

The Advanced Microwave Sounding Unit-A (AMSU-A) system is implemented in two separate modules: the AMSU-A1 and AMSU-A2. The AMSU-A is a multi-channel microwave radiometer that will be used for measuring global atmospheric temperature profiles and will provide information on atmospheric water in all of its forms (with the exception of small ice particles, which are transparent at microwave frequencies) from the NOAA KLM spacecraft.

AMSU-A is a cross-track, line-scanned instrument designed to measure scene radiances in 15 discrete frequency channels which permit the calculation of the vertical temperature profile from about 3 millibars (45 km) pressure height to the Earth's surface. At each channel frequency, the antenna beamwidth is a constant 3.3 degrees (at the half power point). Thirty contiguous scene resolution cells are sampled in a stepped-scan fashion every eight seconds, each scan covering 50 degrees on each side of the subsatellite path. These scan patterns and geometric resolution translate to a 50 km diameter cell at nadir and a 2,343 km swath width from the 833 km nominal orbital altitude.

3.3.2 System Description

3.3.2.1 General

Hardware for the two lowest frequencies is located in one module (AMSU-A2) and that for the remaining thirteen frequencies in the second module (AMSU-A1). This arrangement puts the two lower atmospheric moisture viewing channels into one module and the oxygen absorption channels into a second common module to ensure commonality of viewing angle independent of any module and/or spacecraft misalignment due to structural or thermal distortions. The AMSU-A1's concept of multiplexing thirteen frequencies in this second module is provided by a two-antenna system. This multiplexing approach provides minimum front-end RF loss and a constant 3.3 degree antenna beamwidth with greater than 95 percent beam efficiency. The radiometer characteristics of the AMSU-A channels are summarized in Table 3.3.2.1-1.

Table 3.3.2.1-1. Channel Characteristics and Specifications of AMSU-A
Chan. # Channel Frequency (MHz) # bands Nominal Bandwidth (MHz) Nominal Beamwidth (degrees) NEΔT (K) (Spec.) Polarization at nadir (See Note 1) Instrument Component
1 23,800 1 270 3.3 0.30 V A2
2 31,400 1 180 3.3 0.30 V A2
3 50,300 1 180 3.3 0.40 V A1-2
4 52,800 1 400 3.3 0.25 V A1-2
5 53596±115 2 170 3.3 0.25 H A1-2
6 54,400 1 400 3.3 0.25 H A1-1
7 54,940 1 400 3.3 0.25 V A1-1
8 55,500 1 330 3.3 0.25 H A1-2
9 f0=57,290.344 1 330 3.3 0.25 H A1-1
10 f0±217 2 78 3.3 0.40 H A1-1
11 f0±322.2±48 4 36 3.3 0.40 H A1-1
12 f0±322.2±22 4 16 3.3 0.60 H A1-1
13 f0±322.2±10 4 8 3.3 0.80 H A1-1
14 f0±322.2±4.5 4 3 3.3 1.20 H A1-1
15 89,000 1 <6,000 3.3 0.50 V A1-1
Notes:

1. H indicates horizontal and V indicates vertical polarization.

AMSU-A1 consists of 12 V-band channels (3 through 14) and one W-band channel (15) and associated circuitry. This module provides a complete and accurate vertical temperature profile of the atmosphere from the Earth's surface to a height of approximately 45 km.

AMSU-A2 contains the two lower frequencies (K-band channel 1 and Ka-band channel 2), and the associated scanning, calibration, processing, and power control hardware and circuitry. This module is used to study atmospheric water in all of its forms with the exception of small ice particles.

AMSU-A is configured in the following major subsystems: Antenna/Drive/Calibration Subsystem; Receiver Subsystem; Signal Processor Subsystem and Structural/Thermal Subsystem.

The AMSU microwave radiometers are heterodyne receivers, where the received radio frequency (RF) is downconverted to a lower intermediate frequency (IF). The relationship between the RF and IF signals is shown in Figure 3.3.2.1-1.

Figure 3.3.2.1-1. Frequency translation of a broadband signal in heterodyne reception.

Frequency translation of a broadband signal in heterodyne reception

Given the IF filter band edge frequencies, f1 and f2, and the central frequency, f0, the frequency response of a channel can be fully characterized (assuming unity response for all channels).

For the AMSU instruments, there are three types of channels:

Single passband channels can be defined as those whose bandwidth span the channel central frequency, as shown in Figure 3.3.2.1-2. Typically for these channels, stopbands are specified to reduce the effects of local oscillator noise (source of the central frequency signal). The frequency range for these channels can be expressed as:

f sub 0 - SHW - HW to f sub 0 - SHW(3.3.2.1-1)

and

f sub 0 + SHW to f sub 0 + SHW + HW(3.3.2.1-2)

to exclude the stopband frequencies. Comparison to Figure 3.3.2.1-1 would give:

f sub 1 is identical to SHW and f sub 2 is identical to SHW + HW(3.3.2.1-3)

For those AMSU channels that have relatively narrow stopbands (channels1-4, 6-9; around 18MHz) the difference between calculating Planck radiances using the frequency response shown in equations 3.3.2.1-1 and 3.3.2.1-2 and a simple f0 - HW -> f0 + HW is a fraction of a thousandth of a degree. Note for AMSU channels 15-17, the stopband widths are about 800-1000 MHz and should always be considered.

Figure 3.3.2.1-2. Schematic illustration of a single passband channel. AMSU-A channels 1-4, 6-9, and 15; and AMSU-B channels 16 and 17 are considered single passband channels.

Schematic illustration of a single passband channel

Double sideband channels are shown schematically in Figure 3.3.2.1-3. These channels are also referred to as folded passbands with the lower frequency sideband referred to as the lower sideband and the higher frequency sideband being the upper sideband. For these channels the frequency response of the lower sideband can be expressed as:

f sub 0 -df - HW to f sub 0 - df + HW(3.3.2.1-4)

For the upper sideband, it can be expressed as:

f sub 0 + df - HW to f sub 0 + df + HW(3.3.2.1-5)

Comparison to Figure 3.3.2.1-1 would give,

f sub 1 is identical to df - HW and f sub 2 is identical to df + HW(3.3.2.1-6)

where df is the offset to the center of the sideband.

Figure 3.3.2.1-3. Schematic illustration of a double passband channel. AMSU-A channels 5 and 10; and AMSU-B channels 18-20 are considered double passband channels.

Schematic illustration of a double passband channel

Quadruple sideband channels are shown schematically in Figure 3.3.2.1-4. They resemble two double sideband "subchannels" on either side of the channel central frequency (Note: "subchannels" is not a common or rigorous term and is used here for explanation only). For the lower "subchannels," the frequency response can be expressed as:

f sub 0 - df sub 1 - df sub 2 -UHW to f sub 0 -df sub 1 - df sub 2 + UHW and f sub 0 -df sub 1 + df sub 2 - LHW to f sub 0 - df sub 1 + df sub 2 + LHW(3.3.2.1-7)

and for the upper "subchannels," the frequency response can be expressed as:

f sub 0 + df sub 1 - df sub 2 -LHW to f sub 0 + df sub 1 - df sub 2 + LHW and f sub 0 + df sub 1 + df sub 2 - UHW to f sub 0 + df sub 1 + df sub 2 + UHW(3.3.2.1-8)

Comparison to Figure 3.3.2.1-1 gives four IF values. Two for the lower sideband:

f sub 1 is identical to df sub 1 - df sub 2 -LHW and f sub 2 is identical to df sub 1 - df sub 2 + LHW(3.3.2.1-9)

and two for the upper sideband:

f sub 3 is identical to df sub 1 + df sub 2 -UHW and f sub 4 is identical to df sub 1 + df sub 2 + UHW(3.3.2.1-10)

where df1 and df2 are the offsets to the center of the sidebands.

Figure 3.3.2.1-4. Schematic illustration of a quadruple sideband channel. AMSU-A channels 11-14 are quadruple sideband channels.

Schematic illustration of a quadruple sideband channel

3.3.2.2 Antenna/Drive/Calibration System

The Antenna/Drive/Calibration subsystem consists of a conical corrugated horn-fed shrouded reflector, multiplexer, closed-loop antenna scan drive assembly and closed path calibration assembly. The shrouded reflector is rotated once every scan line (8 sec) for:

During the rotation cycle, the shroud prevents solar reflections from interacting with the warm load and also ensures maximum coupling of the source radiation to the antenna feed. A complete end-to-end in-flight calibration is achieved in a through-the-antenna method, which provides maximum in-flight calibration accuracy. The accuracy of the warm calibration load brightness temperature is better than ±0.2K. The closed loop antenna scan drive provides beam pointing accuracy within ±0.2 degrees. A resolver in the antenna drive assembly provides antenna beam position information.

The AMSU-A1 modules contain two antenna assemblies: designated A1-1 and A1-2. Antenna A1-1 is located the farthest from Earth in-flight and provides inputs to channels 6 and 7 and channels 9 through 15. A1-2 is located closest to Earth in-flight and provides input to channels 3 through 5 and 8. Table 3.3.2.2-1 shows the number of PRTs contained in each of the AMSU-A modules. Figures 3.3.2.2-1 and 3.3.2.2-2 show actual photos from NASA for AMSU-A1 and AMSU-A2.

Table 3.3.2.2-1. The number of PRTs in each AMSU-A module.
Instrument Number of PRTs
AMSU-A1 A1-1 5 (see Note 1)
A1-2 5 (see Note 1)
AMSU-A2 7 (see Note 1)
Note:
1. The number of PRTs specified for each calibration target in the AMSU-A instrument is four. Five were provided for AMSU-A1 and seven for AMSU-A2. The NOAA-K AMSU-A1-1 has one of the five PRTs inoperable.

The antenna beamwidth of all AMSU-A channels is 3.3 degrees. The beamwidth is defined as the half-power points beamwidth (HPBW). The beamwidth in any plane containing the main beam axis (electrical boresight axis) is within ±10% of the 3.3 degree value. Beamwidth variation from channel to channel is smaller than 10% of the specified beamwidth value.

The polarization angle changes as a function of the scan angle. The polarization angle is defined as the magnitude of the angle between the electric field vector of the incoming radiation and a line which is the intersection of a plane perpendicular to the propagation direction of the incoming radiation and a plane tangent to the earth surface. Scan angle is defined as the angle between nadir and the antenna electrical boresight direction. Vertical polarization is defined as having the polarized vector in the sun-nadir plane when the beam is pointing at nadir. Horizontal polarization is defined as having the polarization vector in the velocity-nadir plane when the beam is pointing at nadir.

Photograph of AMSU-A1 instrument

Photograph of AMSU-A2 instrument

Each channel of the AMSU-A is considered to form a beam. All main beam axes of the AMSU-A are coincidental, i.e., they are pointing in the same direction at the same time for any given beam position. The AMSU-A beams have cross-track scanning. All beams scan in a plane perpendicular to the spacecraft orbital velocity vector. The sense of the scan is counter clockwise as one looks along the spacecraft orbital velocity direction, namely, the antenna scans from the sun direction through nadir to the cold space direction.

The scanning profile of AMSU-A is a "step" scan type. The instrument's FOV rotates to a data collection position, stops, collects data, and then moves to the next collection position, stops, collects data, etc. The instrument starts at each earth position 1, then goes sequentially to earth position 30, then to the cold calibration view position, and then to the warm load view position. After the warm load view, the instrument goes back to earth position 1 and the cycle begins again. See Appendix J.3 for specific scan parameters and patterns of the AMSU-A instrument.

The AMSU-A beams scan the earth viewing sector a total of 96.66 degrees (±48.33 degrees from nadir) on beam centers. There is a total of 30 beam positions (30 resolution cells on the earth's surface), which are called cell numbers 1 through 30, from sun to antisun. There are 15 cells on either side of nadir. The beam center position of each cell is separated from the adjacent cell along the scan direction by 3.33 degrees (there is a noncumulative step tolerance of ±0.04 degrees).

3.3.3 Calibration Requirements

There are no specific on-orbit calibration procedures for AMSU-A. The instrument is automatically calibrated each data cycle by measuring both warm and cold calibration targets.

Amended July 20, 2001

Amended October 17, 2002


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