Last saved by Jennifer A Hanafin, 02/11/2018

The Geostationary Earth Radiation Budget experiment

(GERB)

J. E. Harries, J. E. Russell, S. Kellock, J.A. Hanafin,J. Rufus, H. Brindley, J. Futyan, R. Wrigley, J. Ashmall, G. Matthews, J. Mueller, R. Mossavati, A. Last

Blackett Laboratory, ImperialCollegeLondon, UK

E. Sawyer, D. Parker, M. Caldwell, P.M.Allan, A.Smith, M.J.Bates, B.Coan, B.C.Stewart, D.R.Lepine, G.W.Hall, L.A.Cornwall, D.R.Corney, M.J.Ricketts, B.Wilson, J.A.Abolins, D. Smart, R. Cutler

Rutherford Appleton Laboratory, UK

S. Dewitte, N. Clerbaux, L. Gonzalez, A. Ipe, C.

Bertrand, A. Joukoff, D. Crommelynck

Royal Meteorological Institute, Brussels, Belgium

N. Nelms, D. T. Llewellyn-Jones, G. Butcher

University of Leicester, UK

G. L. Smith, Z. P. Szewczyk

NASA Langley Research Center, Virginia, USA

A. Slingo, R. P. Allen

Environmental Systems Science Centre, University of Reading, UK

M. Ringer

Met Office, Hadley Centre for Climate Prediction and Research

Abstract

This paper reports on the arrival of a new satellite sensor, the Geostationary Earth Radiation Budget (GERB) experiment. GERB is designed to make the first measurements of the Earth’s radiation budget from geostationary orbit. Measurements at high absolute accuracy of the reflected sunlight from the Earth, and the thermal radiation emitted by the Earth are made every 15 minutes: with knowledge of the incoming solar constant, this allows the calculation of the absorbed solar and emitted thermal radiation, that is the two components of the Earth’s energy budget. GERB-1 is flying on the first European Meteosat Second Generation satellite, MSG-1, and is positioned over the equator at 3.5ºW. The paper presents an overview of the project, and includes details of the instrument design and operation; of the pre-flight and in-flight calibration; of the performance after the first year in orbit; and of the initial data and some early scientific studies carries out using the new data. We look forward to a decade or more of observations from GERB, as subsequent models fly on the MSG satellite series up to about 2015, and to the advent of similar measurements from all operational geostationary slots in the future.

1. Introduction

Changes to our Earth’s climate are a real possibility, as greenhouse gases accumulate in the atmosphere, and as the temperature at the Earth’s surface shows a very rapid rise in the past two decades, compared with the past two millennia. Ascribing these known changes to specific mechanisms is, however, a very challenging problem [1]. Not only is greenhouse gas forcing of the climate causing increasing temperatures [2,3], but, more importantly, we do not fully understand the complex feedback processes which can amplify or dampen these increases [4]. Some of the most powerful, yet most poorly understood, of these feedbacks are due to cloud and aerosol particles [5,6]. The aim of the new experiment we describe below is to provide new observations to help in the understanding of the forcing and feedback mechanisms of climate.

The approach has to be to test and improve the best computer models of climate by using accurate observations of the system, following the tried and tested methodology at the base of all quantitative science. In order to improve our observations, a team of European scientists and engineers, led by ImperialCollege, and managed technically by the Rutherford Appleton Laboratory, has developed a new instrument, the first ever to make measurements of how the components of the so-called Earth Radiation Budget (ERB) vary with time as seen from the geostationary orbit. The instrument is called GERB (Geostationary Earth Radiation Budget experiment), and it is flying on the first of a new series of European operational meteorological satellites called Meteosat Second Generation, close to a position above the point where the equator crosses the Greenwich meridian.

GERB is providing accurate, well sampled measurements of the reflected sunlight from the Earth, and the thermal infrared radiation emitted by the planet. Since we know from other measurements the energy of sunlight falling on the Earth, this gives us the ‘in’ and the ‘out’ of the Earth’s energy system. Knowing how this energy balance varies in time will allow us to study the all-important feedback mechanisms mentioned above.

Four GERB instruments have been designed and built by European industry and RAL, calibrated in a modern facility at Imperial, and GERB-1 was launched aboard MSG-1 in August 2002. After an extensive and meticulous checking out, GERB started operating in December 2002. apart from down-time caused by satellite or operational limitations, GERB has been operating since. These four instruments will fly on the MSG series, and will provide over a decade of vital new observations of our Earth.

The project team responsible for the instrument design and development, the calibration, the operations, and the data system have produced this overview paper. The efforts of the GERB International Science Team have also contributed greatly to the development and use of GERB already.

In this paper, we will describe the instrument design and operation; this is followed by sections on the ground segment, the instrument calibration, and on initial data and data validation.

2. Instrument principles and design

2.1 Overview

The GERB instrument views the earth from the MSG platform, which rotates at 100rpm. A de-spin mirror counteracts this rotation, directing a shuttered frozen beam of incoming radiation onto a linear 256 pixel detector array via the instrument optical system. The detector array is aligned north-south, and the mirror pointing direction moves by one pixel in the east-west direction at every rotation, building up a 256x282 pixel image, or scan, in approximately three minutes. The detector array is sensitive to radiation at wavelengths greater than 0.32m, and every alternate scan is measured through a quartz shortwave (SW) filter, which has a 4.0m cut-off, so that only the shorter wavelengths are detected. The required longwave (LW) measurement is obtained by subtraction of this SW channel from the ‘TOTAL’ channel during ground processing. As the same telescope and detector are used to make measurements in the two spectral bands, individual measurements can be physically co-registered, but cannot be made temporally coincident. These ‘Level 0’ SW and LW radiance scans are geolocated, rectified, converted to fluxes and binned or averaged in ground processing, as described in section 3.

The GERB instrument consists of two units: the Instrument Optics Unit (IOU) and the Instrument Electronics Unit (IEU), both manufactured at RAL. The IOU measures 450mm x 200mm x 200mm and contains the imaging optics, detector system, de-spin mirror mechanism and quartz filter mechanism, along with two on-board calibration targets: the black body and the short wavelength calibration monitor. The IEU receives detector data, formats it and passes it on to the spacecraft data handling system for transmission to ground. It also provides regulated power to all the subsystems, thermal control of the IOU, command and data interfaces and instrument health monitoring and control. Monitoring and operation of the GERB instrument and scheduling of GERB observations is handled by the GERB Operations Team based at ImperialCollege.

Two calibration sources are incorporated into the IOU, a black body for long wave calibration and a sun illuminated integrating sphere for short wave calibration. The detector collects data when viewing the Earth, Calibration Monitor and Black Body, in sequence, each revolution of the spacecraft. The data capture period for each source is 40 milliseconds.

2.2 Optical subsystem

2.2.1 Telescope assembly

The size of the earth disc as viewed from geostationary orbit is18 degrees in diameter, a relatively wide-angle range for a camera to image in a single exposure. But in mirror-based imaging optics, a wide field of view in just one direction is possible, by using spherical and conic shaped mirrors, with the mirror axes all lying in one plane, but the mirrors arranged off-axis to fold the beam without obscuring it. This three-mirror anastigmatic (TMA) configuration allows a wide FOV in the direction normal to the plane of the mirror axes. The field of view (FOV)of the very wide-angle imaging system used in GERB is very restricted in the other direction and the consequent image distortion maps a linear detector array to a slightly curved array of pixels on the earth. This optical distortion is modelled and removed in the image rectification process.

Weather imagers operating in the optical regime from geostationary (GEO) orbit normally require large apertures, ~0.5 metres, for signal strength and diffraction-limited imaging requirements. However GERB can operate with just a 2cm aperture, as the signal to noise ratio is increased by both the broadband nature of the measurements, and the lower spatial resolution of the GERB system. The small focal length and aperture allow the GERB requirements to be met in a relatively small instrument (figure 2.2).

In ERB instruments the polarisation of the incoming light is a possible source of errors. Particularly in the sunlight channel, the light can in some observation and sun-angle geometries become strongly polarised, with unknown orientation. To minimise the uncertainty due to lack of knowledge of incoming polarisation, it is required that the instrument response is itself insensitive to polarisation of incoming light. This becomes an issue in the wide-angle GERB optics because all of the telescope mirror-folds are in one plane, so that any polarisation effect of the coating on the mirror is magnified through the system. As the polarisation effect of the coating is unique to the coating type, this effect can be compensated for by adding an extra fold mirror, of the same coating, but with the plane of incidence made normal to that of the of the othertelescope mirrors. This mirror has the added advantage of making the whole instrument more compact.The GERB telescope was manufactured by A.M.O.S.(AMOS) in Belgium.

2.2.2 De-Spin Mirror mechanism

The De-Spin Mirror (DSM) mechanism is a plane double-sided mirror mechanism counter-spinning at 50 rpm in the opposite sense to the 100 rpm spinning spacecraft. The mechanism is driven by a synchronous, two-phase, high output motor with positional feedback provided by an Inductosyn rotary transducer.

The design of this mechanism is simple and rugged to ensure long reliable life in the MSG environment. The most challenging feature of this environment is the 16-18g acceleration caused by the spinning spacecraft, which can result in rapid wear of the cage holding the bearings that support the mechanism shaft and mirror. To prevent this, lead film lubricant was used on the bearings, and debris traps to contain the wear products produced by the bearings were incorporated in the design. Early in the project, a series of tests were carried out in a centrifuge to simulate the inflight environmentin conjunction with the European Space Tribology Centre Long duration tests in the centrifuge indicate that a life in excess of 7 years can be expected. TheDSMwas manufactured by Alenia Difesa – Officini Galileo, Italy.

2.2.3 Quartz filter mechanism

The Quartz Filter (QF) Mechanism is a compact, insertion mechanism which performs the switch between total and short wavelength measurement. This is implemented by rotating a turntable which incorporates a filter and shutter through the optical path between the de-spin mirror and the telescope. The shutter protects the detectors from directsun illumination during early orbit phases, when GERB is unpowered and not synchronised to the MSG spin, and near eclipse conditions, when the sun’s declination brings it within the FOV of the instrument. The QF mechanism was also manufactured by Alenia Difesa – Officini Galileo, Italy

2.3 Detector subsystem

The focal plane assembly (FPA) components include detector, four Application Specific Integrated Circuits (ASIC) and support electronicsThe detector (see table 1 for parameters)is a thermoelectric linear array custom built by Honeywell, Inc. with a reticulated 20 m thick goldblack coating to improve spectral response.. It is a silicon micro-machined structure with 256 pixels mounted on a silicon substrate which provides connection fan-out before mounting on a gold-plated ceramic circuit board. This board also holds four ASICs, each with 64 parallel signal processing channels. Each channel comprises an input amplifier and a sigma-delta modulator to perform digitisation. The output of the sigma-delta modulator feeds into a decimation filter to reduce the data rate and outputs from the 64 decimated channels are shifted into a serial register for data output. Operation from the GERB spacecraft clock results in a parallel (256 channels) sample time of 1.098 ms. The individual FPA sub-circuits are mounted onto a molybdenum backplate and wire bonded together. An aluminium plate covers the whole assembly with a single aperture for the detector.

Number of pixels / 256
Pixel size / 45 m x 55 m
Pixel pitch / 55 m
Fill factor / 82%
Impedance / ~1200 
Responsivity / 300 V/W
Time constant / ~4 ms
Noise source / Johnson noise of resistance

Table 1: Detector parameters

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2.4Electronic subsystems

2.4.1 Front End Electronics

The front end electronics (FEE) is built around an Analog Devices ADSP-2111 digital signal processor which allows connection to the ASIC serial output. The purpose of the FEE is to digitally integrate and process the output from the FPA during the 40 ms observation time. 36 samples are taken during this time (36 x 1.098 ~ 39.5 ms) which are then convolved with a filter optimised for a typical pixel time constant. A second filter provides a correction term for any variation between actual and typical pixel time constant. The filtering provides high-frequency noise reduction and removal of detector memory effects, essential for an integration time of order only ten pixel time constants. The FEE also monitors the warm blackbody temperature, solar diffuser calibration diodes and housekeeping parameters. The FPA was manufactured by Leicester University, UK.

2.4.2Instrument Electronics Unit

The IEU was built by RAL and controls GERB operation by means of its processor, using table-driven software. The processor handles instrument science data (samples of blackbody, earth-view and calibration monitor) and housekeeping parameters (voltages, temperatures and status), combining them into a data packet of 3800 bytes which is generated every 600mS (one spacecraft rotation). In addition to the data handling functions, there are circuits which control the DSM using a brushless DC motor and the QF mechanism using a stepper motor. Four separate DC/DC converters provide regulated power supplies to the IEU and IOU. The operational heaters, which maintain the optical assembly at a constant programmable temperature, nominally 20°C, are also controlled by the IEU. Parameters can be adjusted when required to make changes to scanning modes, instrument thermal control and limit-checking.

2.5 Calibration targets

2.5.1 Black body

The blackbody is a cavity mounted inside the IOU, and is viewed by the detector pixels for 40ms at each rotation. The interior is treated with suitable black to provide an emissivity extremely close to 1. The temperature is not actively controlled, but a fixed power is applied and the temperature is allowed to stabilise at about 20ºC above the ambient temperature of the IOU and is continually monitored using thermistors. A second heater provides redundancy and allows the blackbody to be heated to higher temperatures for detector linearity measurements Temperature is measured at several points and to an accuracy of 5 mK which is sufficient to determine the LW blackbody radiance used to calibrate the GERB Earth pixel measurements. The black body was manufactured by AEA technology, UK.

2.5.2 Calibration monitor

The SW calibration monitor is an integrating sphere, manufactured by RAL, which receives sunlight through one aperture at certain times of the day. Its internal surface is a diffusing reflective material and it includes a set of internal detectors, providing a known, uniform and SW spectrally matched beam which is output through another port viewed by the detector via the DSM at each rotation. The output varies with the time of year due to the relative position of the sun, but it is repeatable. The calibration monitor was manufactured by RAL.

3.Ground Segment

3.1 Organisation of near-real time distributed GERB ground segment

The Near-Real Time (NRT) GERB ground segment is distributed between several institutions, as illustrated in Figure 3.1. EUMETSAT provides the primary ground station for the MSG satellite, handling all communications including transmission of commands to the GERB instrument and reception of GERB raw data. The RAL GERB Ground Segment Processing System (GGSPS) deals with NRT processing of GERB raw data to Level 1.5, and forwards these products to the RMIB GERB Processing system (RGP). The RGP is responsible for processing of the Level 1.5 product to Level 2 fluxes, using additional information from the SEVIRI products distributed by EUMETSAT. A long-term archive of Level 2 data is maintained by the GGSPS.

3.2 Data processing steps

3.2.1 Data Ingest