NOAA KLM User's Guide Section 3.4

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3.4 Advanced Microwave Sounding Unit-B (AMSU-B)

3.4.1 Instrument Operation

The Advanced Microwave Sounding Unit-B (AMSU-B) is a 5 channel microwave radiometer. The purpose of the instrument is to receive and measure radiation from a number of different layers of the atmosphere in order to obtain global data on humidity profiles. It works in conjunction with the AMSU-A instruments to provide a 20 channel microwave radiometer.

The microwave characteristics of the atmosphere are provided in Figure 3.4.1-1. AMSU-B covers channels 16 through 20. The highest channels: 18, 19 and 20, span the strongly opaque water vapor absorption line at 183 GHz and provide data on the atmosphere's humidity level. Channels 16 and 17, at 89 GHz and 150 GHz, respectively, enable deeper penetration through the atmosphere to the Earth's surface.

AMSU-B is a cross-track, line scanned instrument designed to measure scene radiances in 5 channels. At each channel frequency, the antenna beamwidth is a constant 1.1 degrees (at the half power point). Ninety contiguous scene resolution cells are sampled in a continuous fashion, each scan covering 50 degrees on each side of the subsatellite path. These scan patterns and geometric resolution translate to a 16.3 km diameter cell at nadir at a nominal altitude of 850 km. See Appendix J.3 for specific scan parameters and patterns for AMSU-B.

The AMSU-B instrument consists of a scanning parabolic reflector antenna which is rotated once every 8/3 seconds and focuses incoming radiation into a quasi-optic system which then separates the frequencies of interest into three separate feed horns of the receiver assembly. The receiver subsystem provides further demultiplexing of the 183 GHz signal in order to selectively acquire three defined double sided bands around the 183 GHz signal. The passbands for all five channels are shown in Table 3.4.1-1. The center frequencies for channels 18, 19, and 20 are 183.31 ±1.00 GHz, 183.31 ± 3.00 GHz, and 183.31 ± 7.00 GHz, respectively. Further information regarding the AMSU-B channels can be found in Section 3.3.2.1

The reflector is rotated in a profiled scan span, in order to provide a constant velocity Earth scan, a low speed scan across an internal calibration source, a low speed scan towards deep space and a scan repeat cycle time that does not allow for constant velocity rotation. The direction of the scan motion is from the sunside (+Z) to nadir (+X) to the antisunside (-Z). During earthscan, motion is continuous with an angular velocity constant to within ±2%. The deep space and internal blackbody reference views permit a two-point, in-flight calibration. The entire scan profile is achieved through microprocessor control.

Three different data streams are processed and delivered by the instrument to either the spacecraft's AMSU Instrument Processor (AIP) or the TIROS Information Processor (TIP). Digital data, which consist of earth view pixel data, housekeeping data, and space and blackbody view data, is clocked into the AIP. The TIP samples both digital "B" and analog telemetry. The digital "B" telemetry contains instrument status monitors used to verify commands, and the analog telemetry provides health and safety monitoring of AMSU-B.

3.4.2 System Description

3.4.2.1 General

The overall purpose of AMSU-B is to receive and measure radiation from a number of different layers within the atmosphere in order to obtain global data on humidity profiles.

The radiation from the atmosphere is viewed by an offset parabolic reflector inclined to the axis of rotation. The antenna is rotated by means of a scanning mechanism which is capable of slewing the antenna between hot and cold calibration references and scanning the antenna to view a swath across the atmosphere. Radiation from the antenna is propagated via a fixed subreflector to a quasi-optic feed where it is divided into 89, 150 and 183.3 GHz bands and then to a Microwave Receiver where it is downconverted, amplified and detected. The detected signals are amplified, sampled and averaged under processor control to provide an integration time of 18 msec in each channel. The digital signals are then buffered before being relayed to the satellite bus.

The AMSU-B consists of the following:

• Microwave and Antenna Subsystem (MASS) comprising:
  • Main Reflector/Shroud
  • Subreflector
  • Quasi-Optic Feed
  • Microwave Receiver
  • Calibration Target
• Scanning Mechanism
• Electrical Subsystem comprising:
  • Processing Electronics Unit (PEU)
  • Motor Drive Electronics (MDE) and Inductosyn Control Electronics (ICE)
  • Power Supply Unit (PSU)
  • Electrical Harness
• Structure Subsystem
• Thermal Control Subsystem
• Software

A general configuration view of AMSU-B is shown in Figure 3.4.2.1-1.

3.4.2.2 Microwave Receiver

The function of the Receiver Subsystem is to down-convert and detect the radiation from the Quasi-Optics to produce output voltages proportional to the detected RF power. The Receiver has three input channels and five output channels. Channel 16 uses a fundamental mixer with a center frequency of 89 GHz. Channel 17 uses a sub-harmonic mixer with a center frequency of 150 GHz. Channels 18, 19 and 20 are down-converted in one sub-harmonic mixer with a center frequency of 183.3 GHz. The IF output of the mixer is split and detected to produce the three channel outputs. The AMSU-B channel characteristics are shown in Figure 3.4.1-1. The receiver utilizes Gunn diode oscillators which are cavity stabilized and are of the reflection type. The diodes are Germanium mesa construction especially developed for AMSU frequencies.

The temperatures of certain critical components of the Receiver Sub-system are sensed by AD590 temperature/current transducers for Analog Telemetry or by Platinum Resistance Thermistors (PRTs) for Digital Housekeeping Data.

3.4.2.3 Calibration Target

The radiometer is continually calibrated in flight with the aid of a cold reference view to space and a "warm" reference consisting of an ambient temperature Calibration Target. The "warm" reference is internal to the instrument. It consists of the following:

• microwave absorber providing quasi-blackbody radiation at microwave frequencies,
• a magnesium structured core,
• platinum resistance sensors to measure the temperature of the magnesium core accurately,
• an outer containment shroud of aluminum.

The Calibration Target presents a stable, high absorptivity, or quasi-blackbody load at approximately 290K (at nominal instrument temperature) into the instrument Main Reflector and is sized to fill the aperture in the Main Reflector/Shroud. The Calibration Target is viewed once during each scan of the instrument to provide a warm calibration reference. Gain is then computed from FOV counts taken at each of the two views and used to determine a radiance value for each channel. Apparent scene temperatures are finally calculated from the radiance values.

An integral shroud is fitted to match the main reflector/shroud, so that, as the Main Reflector is rotated, extraneous radiation impinging on the target is minimized.

Temperature control of the Calibration Target is passive and is designed to minimize temperature gradients within the target. Target temperature is sensed by seven Platinum Resistance Thermistors (PRTs) whose outputs are conditioned and digitized by the PEU and form part of the digital data telemetry.

3.4.2.4 Systems Requirements

A summary of the AMSU-B systems requirements is given in Table 3.4.2.4-1.

Amended October 17, 2002