NOAA KLM User's Guide

Section 3.9

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3.9 Microwave Humidity Sounder (MHS)

3.9.1 Instrument Operation

The Microwave Humidity Sounder (MHS) is a self-calibrating microwave radiometer, observing the Earth with a field of view of ±50 degrees across nadir, in five frequency channels of the millimeter-wave band (89-190 GHz). MHS, together with the complementary AMSU-A instruments, provides the operational microwave sounding capability for the NOAA-N, -P meteorological satellites.

Channels at 157 GHz and around the 183 GHz water vapor absorption line provide a humidity profile sounding capability, while the 89 GHz channel provides information on surface temperature and emissivity (in conjunction with AMSU-A channels) and detects cloud and precipitation contaminated pixels. The MHS instrument represents an improvement to the AMSU-B radiometer on board previous NOAA satellites, while providing continuity to its data. Two MHS instruments are planned to fly on NOAA-N and -P satellites, and three on the METOP satellite series of the European Polar System (EPS).

MHS is a cross-track, line-scanned instrument. Ninety contiguous scene resolution cells are sampled in a continuous scan, covering 49.44444... degrees on each side of the sub-satellite path, with an antenna beam width of 1.11111... degrees at half power point. These scan patterns and geometric resolution translate to a 17-km diameter cell at nadir from the 870 km nominal orbital altitude.

A parabolic mirror is rotated to sample the Earth scene at 90 equidistant angular positions, at a rate of three scans every eight seconds, and at the same time to provide reference measurements against two calibration sources, i.e. an on-board blackbody target and a view on free space. The radiation is then conveyed on four feeds at 89, 157, 183 and 190 GHz via a "quasi-optical" arrangement of lenses, dichroic plates and a polarizing beam splitter. The instrument channels and pass-band characteristics are as shown in Table 3.9.1-1 and Figure 3.9.1-1.

Figure 3.9.1-1. MHS Channels and Passband Arrangement
Figure showing MHS channels and passband arrangement
Table 3.9.1-1. MHS Channels and Passband Characteristics.
Channel (See Note 1) Central Frequency (GHz) No of Passbands RF Bandwidth
(MHz) (See Note 2)
T (K) (See Note 3) Polarization (See Note 4)
H1 89.0 1 2800 0.22 V
H2 157.0 1 2800 0.34 V
H3 183.311 ± 1.0 2 2 x 500 0.51 H
H4 183.311 ± 3.0 2 2 x 1000 0.40 H
H5 190.311 1 2200 0.46 V
1. The five MHS channels provide data continuity with AMSU-B channels 16 to 20, with some minor changes in frequency allocation and polarization, and improved performance.
2. The quoted values for the maximum bandwidths are double-sideband values and represent the maximum permissible bandwidths at the 3 dB points.
3. Ground measured values for the first flight model (NOAA-N).
4. The V and H polarizations correspond respectively to electrical fields normal or parallel to the ground track at nadir, both rotating by an angle equal to the scan angle for off-nadir directions.

The microwave signal at the output of each of the four feeds is down-converted to Intermediate Frequency (IF) using a super-heterodyne receiver. Each receiver consists of a mixer with its associated local oscillator, an amplifier, and an IF filtering and video detection chain. The channels H3 and H4, sharing the same receiver, are separated by a diplexer and dedicated filters at IF level. Thus, five baseband signals are generated.

These signals, after low-pass filtering and removal of the DC component, are digitized and then further averaged over each pixel integration period. The final pixel result is formatted into a packet along with the calibration data, to constitute the scientific data to be used on the ground.

Radiance for the Earth views is derived from the measured counts and the calibration coefficients inferred from the internal black body and space view data. The black body temperature is accurately measured by a set of Platinum Resistance Thermometers (PRT). Moreover, the direction of the space view for cold reference can be adjusted after launch to optimize calibration with respect to spurious radiation sources.

The scientific data generated by MHS are sent to the spacecraft via the MHS Interface Unit (MIU) on the Science Data (SD) bus using Consultative Committee for Space Data Systems (CCSDS) packet formats. Instrument command, control and telemetry functions are also achieved through the MIU, using a separate Command/Telemetry (CT) bus, also using CCSDS packet formats.

The MHS instrument has a high degree of internal redundancy, which is managed by external commands under ground control.

3.9.2 System Description General Configuration

The MHS instrument mechanical configuration is shown in Figure

Figure MHS General Assembly.
Figure showing MHS general assembly.

The MHS instrument consists of three major assemblies:

All three major assemblies are mounted on a common structural baseplate, assuring the mechanical interface to the satellite. The instrument baseplate also hosts the following minor assemblies:

Electrical interface connectors are on the outer envelope of the Electronics Equipment.

The MHS instrument operates at ambient temperature and does not require cooling systems. Thermal control is operated passively, using a combination of thermal insulation and radiative surfaces, the latter specifically used for dissipating the power of the receiver and the electronics equipment. Heaters are provided to speed up the warm-up time of the instrument from switch-on. A thermostat-driven survival heater network is also provided to maintain safe temperatures when the instrument is switched off. Scanning Concept

A closed-loop mechanism, based on brushless motors and contactless position sensors, is used to rotate the parabolic mirror every 2.67 seconds, to provide the Earth scanning and calibration functions. Figure shows the scanning concept.

At each rotation the Earth field of view is scanned at constant speed, then the mirror is directed towards cold space and the internal target before starting the following scan. The scan velocity profile is optimized for maximum radiometric sensitivity. A counter-rotating flywheel is used to compensate momentum induced on the satellite by variations of the scan velocity. The reflector is provided with a co-rotating shroud, to minimize unwanted thermal radiation.

The scanning system is also capable of keeping the reflector indefinitely fixed to a desired position, on ground command, for special calibration purposes during the on-orbit verification phase, and return on command to nominal scan mode.

Figure MHS Scanning Principle.
Figure showing the MHS scanning principle Calibration Concept

MHS data calibration is based on measurements of two reference targets, namely the instrument cold space view (cold reference) and the On Board Calibration Target (OBCT) which is the hot reference. Both targets are sampled at every scan rotation. In order to reduce measurement noise on calibration data, four samples at each position are taken during each scan and averaged over several scans.

The OBCT is a black body whose temperature is left floating in thermal equilibrium with its environment. It consists of an array of pyramids coated with radio frequency absorbing material, having a uniform temperature over its surface. This temperature is accurately measured by a set of Platinum Resistance Thermometers (PRT) located in various regions of the OBCT and used for on-ground processing of the radiometric data.

The accuracy of calibration mainly relies on the precision of PRTs and on the knowledge of unwanted thermal radiation sources affecting the measurement on the cold space view. In order to improve this knowledge, a special scanning mode is activated (during the on-orbit verification phase, shortly after launch) to characterize the in-flight radiative contributions. The position of the cold space view can be adjusted after launch to optimize the calibration process with respect to those disturbances.

The calibration coefficients in terms of gain and offset are calculated, appended to the raw data and used for further processing on ground (i.e. radiance, brightness temperature, etc). Non-linearity of the instrument transfer function is only measured on ground. Modes of Operation

The MHS Instrument operation modes are as shown in Figure

In the Launch mode the instrument is totally unpowered and its temperatures are not controlled. Transition to Off Mode occurs when the Survival Power is applied. The Survival Heaters controlled by a thermostat network are used to maintain equipment temperatures within their non-operating safe limits. In all other modes, the instrument temperatures are maintained by the thermal dissipation occurring in the electrical equipment, with the aid of operational heaters under control of the instrument. The application of Survival Power, and thus the transition from Launch to Off mode, is expected on NOAA-N and NOAA-P just a few hours after launch, so that dangerously low temperatures are not reached because of thermal inertia of the instrument. For safety reasons, it has been recommended never to switch off the Survival Power Bus, although no current is drawn from it during nominal operation.

The Power-On mode is entered when power is applied to both the Main and Pulse Load busses. The Electronics Equipment (EE) is switched on, initializing the on-board processing software (MINOS) and establishing command and telemetry links between the MHS and the satellite. The health and status of MHS will then be provided in the form of Housekeeping data. The unpowered equipment (Receiver and Scan Mechanism) is warmed, through the use of operational heaters, to reach their minimum switch-on temperatures.

Figure MHS Operating Modes.

Figure showing MHS operating modes.

In the Warm-up mode the instrument equipment is fully powered and thermal transition to the nominal operating temperature occurs thanks to the internal power dissipation, with the help of operational heaters for the Scan Mechanism. The Stand-by mode is achieved when the MHS Instrument is ready to perform its nominal scan operations. All equipment are at full performance temperatures and the circuits are powered. The Reflector, after an initialization cycle, is set stationary and pointing at the OBCT, under closed loop control.

The Scan Mode is the normal operating mode of the MHS Instrument. All equipment is active and science data are produced as output. The Fixed View mode is nominally operated only during the initial on-orbit verification. The same activities as Scan Mode are taking place except that the Reflector is not performing the nominal scan but is pointing at a fixed position. Radiometric data are collected at pixel times equivalent to the Earth, Space and OBCT view periods of Scan Mode and placed in the corresponding positions of the Science Data Package.

The Safeing mode is entered on specific external command. The instrument is placed in a safe configuration in readiness for the removal of power. The Reflector is parked at the OBCT to prevent sun illumination in case of a change in the platform attitude.

The Fault mode, Overcurrent mode and Suspend mode are automatically entered whenever the MHS electronics detects an anomaly in the instrument behaviour. The configuration is changed to minimize the chance of permanent damage to any part of the MHS Instrument. A Self-Test mode is provided to allow specific functions of the MHS Instrument to be exercised and tested for the purposes of detailed MHS Instrument health monitoring and diagnostics in case of anomaly.

Amended February 18, 2004

Amended July 14, 2004

Amended August 27, 2004

Amended September 14, 2005

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