MetOp-SG (MetOp-Second Generation Program)
The current MetOp series of EUMETSAT has three identical satellites. Launched in 2006, MetOp-A is Europe's first polar-orbiting mission dedicated to operational meteorology, and will be followed by MetOp-B in 2012 and MetOp-C in 2016. - These first three MetOp satellites guarantee the continuous delivery of high-quality data for medium- and long-term weather forecasting and for climate monitoring until at least 2020.
The EPS-SG (EUMETSAT Polar System-Second Generation) Phase System baseline has been agreed by Council in June 2010 and is based on a Two-Satellite In-orbit Configuration. 1)
In 2011, ESA, in partnership with EUMETSAT, embarked upon the initial steps (industrial studies, Phase A) towards developing concepts for the next generation of MetOp -SG satellites. While the three satellites in the first series are identical, the current concept for MetOp-SG, also known as 'Post-EPS', will comprise two different satellites orbiting as a pair. It is envisaged that each satellite will carry a different, but complementary, instrument package. 2) 3)
In November 2012, the EUMETSAT Council successfully concluded the approval process for the EPS-SG (EUMETSAT Polar System Second Generation) Preparatory Program with all 26 Member States having now firmly committed themselves. 4)
MetOp-SG encompasses the objective of obtaining consistent, long-term collection of remotely sensed data of uniform quality for operational services for meteorology and climate monitoring state analysis, forecasting and operational service provision, in the context of the EUMETSAT’s EPS-SG system.
The MetOp-SG program is being implemented in collaboration with EUMETSAT. ESA develops the prototype MetOp-SG satellites (including associated instruments) and procures, on behalf of EUMETSAT, the recurrent satellites (and associated instruments). EUMETSAT is responsible for the overall mission, funds the recurrent satellites, develops the ground segment, procures the launch and LEOP services and performs the satellites operations. The corresponding EUMETSAT Program is termed EPS-SG (EUMETSAT Polar System – Second Generation).
A two-satellite architecture has been selected for MetOp-SG by ESA and EUMETSAT, namely MetOp-SG- A and -B, flying in the same sun-synchronous orbit. Unlike the current MetOp system of identical satellites operating in a relay, the MetOp-SG system envisages a pair of different satellites, each carrying a different but complementary suite of instruments. This will comprise a mix of instruments offering data continuity with improved performance and new instruments to meet the evolving demands of the meteorological community.
Altogether, the new MetOp-SG system concept features six satellites; the launch of the first one is planned for 2020. The overall system lifetime is 21 years, with each satellite designed to exceed an eight and a half year lifetime.
Note: A description of the spacecraft will be provided when available.
Since 2006, the European contribution to operational meteorological observations from polar orbit has been provided by the first generation of Meteorological Operational (MetOp) satellites. The MetOp-SG (MetOp Second Generation) series of satellites will provide continuity and enhancement of these observations in the timeframe of 2020 to 2040.
• METimage (Meteorological Imager), to provide information on clouds, cloud cover, land surface properties, sea, ice and land surface temperatures, etc. The METimage instrument is provided by DLR.
• IASI-NG (Infrared Atmospheric Sounding Interferometer–New Generation), to provide atmospheric temperature and humidity profiles, as well as monitor ozone and various trace gases. IASI-NG is provided by CNES.
• MWS (MicroWave Sounder), to provide atmospheric temperature and humidity profiles
• SCA (Scatterometer), to provide ocean surface wind vectors and land surface soil moisture
• RO (Radio Occultation sounder), to provide atmospheric temperature and humidity profiles, as well as information about the ionosphere
• UVNS/S5 (Ultra-Violet /Visible/Near Infrared/Short Wave Infrared spectrometer -Sentinel-5) instrument, to monitor various trace gases, air quality and support climate monitoring
• MWI (MicroWave Imager), to provide precipitation monitoring as well as sea ice extent information
• ICI (Ice and Cloud Imager), to measure cloud ice water path, properties and altitude
• 3MI (Multi-viewing, Multi-channel, Multi-polarization Imager), to provide information on atmospheric aerosols
• DCS (Data Collection System) Argos-4, for the collection and transmission of observations and data from surface, buoy, ship, balloon or airborne data collection platforms. DCS is provided by NOAA and CNES.
ESA is responsible for design/development of the following 6 instruments within the MetOp-SG program: MWS, SCA, RO, MWI, ICI and 3MI. The Sentinel-5 instrument (UVNS/S5) is developed by ESA under the Copernicus Program, formerly GSC (GMES Space Component) program. The other instruments are provided through EUMETSAT under cooperation agreements with its partners DLR, CNES and NOAA and will be provided as Customer Furnished Items to the MetOp-SG contractor.
ICI (Ice Cloud Imager)
The ICI instrument is a millimeter and sub-millimeter wave conically scanning radiometer, providing a total number of 13 channels (including dual polarization channels). ICI is designed to monitor the exchange mechanisms in Earth's upper troposphere and lower stratosphere, and focusing in particular on the remote sensing of high altitude ice clouds using several heterodyne receiver channels centered at 183 GHz, 243 GHz, 325 GHz, 448 GHz and 664 GHz, with two window channels (243 GHz and 664 GHz) measured at both V and H polarization. In recent years, several ESA funded activities have been aimed at further advancing the maturity of sub-millimeter wave Schottky technology in Europe in order to fulfil the ICI stringent requirements (Ref. 14). 9) 10) 11)
The ICI instrument onboard MetOp-SG will include several receiver channels that covers part of the millimeter and part of the sub-millimeter wave range, at center frequencies of 183 GHz, 243 GHz, 325 GHz, 448 GHz and 664 GHz. A lot of results have already been obtained up to 325 GHz, as discrete devices can be used in this frequency range without major limitation or performance degradation compared to integrated MMIC devices. At 448 GHz, it is expected that both discrete and integrated mixer devices will give similar performance. The decision factor might come from repeatability issues during mounting and eventually stress tests. A schematic of a 448 GHz receiver front-end is shown in Figure 1 on the left hand side. It includes a medium power 224 GHz doubler, a 448 GHz sub-harmonic mixer and an IF Low Noise Amplifier (LNA).
Figure 1: Left hand side: schematic of a 448 GHz receiver front-end including a 224 GHz medium power doubler and a 448 GHz sub-harmonic mixer with IF LNA. Right hand side: schematic of a 664 GHz receiver front-end including a 166 GHz high power doubler, a 332 GHz medium power doubler, and a 664 GHz sub-harmonic mixer with IF LNA (image credit: ESA)
Table 2: Specification of the ICI channels
To acquire measurements on a wide swath, the instrument will rotate continuously about an axis parallel to the local spacecraft vertical with an active portion of the scan of ~±65º centered in the forward direction of the spacecraft; the antenna system will view an Earth scene with a nearly constant incidence angle of about 53º.
While the rotating part has to be external to the platform, the fixed part can be accommodated either internally or externally to the platform. The advantage of having the fixed part placed inside the platform is the reduction of instrument envelope that makes it easier to manage the ICI payload accommodation.
The moving part is rotated about the axis of the instrument by a coaxially mounted motor and it includes the reflector and feed horns that are mounted on a "drum" which contains the receivers, a digital processing unit and a power supply unit.
Figure 2: Mechanical layout of ICI radiometer (image credit: ICI consortium)
Contrary to traditional conical scanners, ICI will be mounted on the nadir side of the spacecraft. The 183 GHz channel selection overlaps with the one of MWI, allowing a cross-calibration between the two instruments.
Table 3: Some parameters of the ICI instrument 12)
MWI (MicroWave Imager):
MWI is a conically scanning passive radiometric imager (antenna size of ~ 75 cm) which rotates continuously about an axis parallel to the local spacecraft vertical with an active portion of the scan of > ±65º centered on the fore (or afterward) direction of the spacecraft. The antenna system will view an Earth scene with a nearly constant incidence angle ,or OZA (Observation Zenith Angle), of about 53º. 13)
The instrument is providing a total number of 26 channels (including dual polarization channels). The MWI frequency coverage is from 18 GHz up to 183 GHz. Table 4 shows the MWI channels, utilization purpose and NEΔT. All MWI channels up to 89 GHz are measured with both V- and H polarizations. Channels above 89 GHz are measured at V polarization only (Ref. 14).
Table 4: Summary of the MWI channel specification
The main objective of the MWI is to measure precipitation. In addition, MWI provides measurements of cloud products, water vapor and temperature profiles and surface imagery. MWI supports Numerical Weather Prediction at regional and global scales. MWI has a moderate antenna size providing on-ground footprints ranging from 50 km down to 10 km, depending on frequency. The smallest antenna footprint defines the minimum rotation cycle, and footprint overlap is present also for highest frequencies due to the high rotation rate (45 rpm). This guarantees a good image quality and even Nyquist spatial sampling at the lowest frequency channels.
The relatively small antenna of MWI does not require deployment mechanism and enables a complete sun shield around the instrument to protect from sun-intrusions.
Additional features of MWI include RFI (Radio Frequency Interference) mitigation at the lowest frequency channel and internal calibration targets to complement traditional external targets. RFI mitigation is done by dividing the measured signal spatially, spectrally and temporally and detecting non-Gaussian (man-made) signals and removing them from the data. Internal calibration targets will be implemented at the lowest frequency channels, providing improvement of orbital stability of the instrument.
Instrument: The MWI instrument consists of a fixed part and a rotating part. The moving part is rotated about the axis of the instrument by a coaxially mounted motor and it includes the reflector and feed horns that are mounted on a "drum" which contains the receivers, a digital processing unit and a power supply unit.
All data, commands, timing and telemetry signals, and power, pass through the rotating part to the fixed part via the PSTD (Power and Signal Transfer Device).
The fixed part of the instrument comprises the assembly of scan mechanism (which locates the scan motor, an angular position encoder, mechanical unbalance sensors, various electronics including the control of scan and balance functions), and allocates the units interfacing with the platform (power supply connected to the platform power bus and a control/processing unit which links respectively with the platform command & telemetry bus and the platform science data bus).
A fixed rod through the scan assembly, emerging above the rotating drum upper plane, is providing off axis mechanical support in a fixed position for the two calibration targets that consist of a small reflector and a hot blackbody load, which once per rotation illuminates the feed horns obscuring them from the view of the main reflector.
The calibration targets, a CCR (Cold sky Calibration Reflector) and the on-board HL (Hot Load),are accommodated in such a way that the CCR is constantly pointing to the deep space on the opposite side of the sun direction. The CCR and the HL calibration devices are viewed once every rotation cycle.
Figure 3: Mechanical layout of MWI radiometer (image credit: MWI consortium)
The structural design and the adopted materials of the rotating assembly shall be such to optimize the stiffness to mass ratio and minimize the in orbit mechanical and thermal distortions.
Figure 4: Scan geometry of the MWI instrument (image credit: EUMETSAT, Ref. 7)
MWS (Microwave Sounding Mission):
MWS is conventional cross-track scanning microwave radiometer with a total number of 24 channels from 23 GHz up to 230 GHz. The instrument provides measurements of temperature and humidity (water vapor) profiles and total liquid water columns. These are key parameters for Numerical Weather Microwave soundings, which greatly enhance the ability of the various NMS (National Meteorological Services) to initialize global and regional NWP models with realistic information on temperature and moisture. The frequent availability of detailed temperature and moisture soundings would also contribute to fulfil other key requirements common to NWC (Now Casting) and very short range weather forecasting at regional scales. 14)
All MWS measurements are performed with a single polarization (QV or QH). The instrument will have a 40 km footprint at the lowest frequencies leading to an antenna size of ~35 cm. The footprint at highest frequency channels will be 17 km. The sampling distance on the ground is defined by the highest frequency channels. MWS has a non-uniform scanning profile, which maximizes the scene viewing time. A quasi-optical system is used to collocate all channels into one “main beam”. All channels are required to have the same pointing accuracy within 0.1°. The MWS total scan angle will be around ± 49° with respect to nadir with a total pointing knowledge of 0.25°.
Figure 5: MWS scan angles and sampling interval (image credit: ESA)
The instrument design is a single unit, single antenna concept. The large frequency range required infers a stringent requirement to the MWS quasi-optical network, which must be able to handle this frequency range. The first electrical demonstrator of the quasi-optical network has been built and measured losses are typically below 0.3 dB for all channels. Table 5 shows the predicted NEΔT values for each channel in nominal conditions at BOL (Beginning of Life). The predicted radiometric accuracy (RSS) is below 1 K for all channels, the inter-channel accuracy is better than 0.5 K and the inter-pixel accuracy <0.3 K (at 280 K reference scene). An orbital stability of better than 0.2 K is targeted. The beam efficiency is between 95% and 99%, depending on channel.
Table 5: Specification of the MWS channels, utilization and predicted NEΔT
SCA (Scatterometer Mission):
The Scatterometer (SCA) is one of the high priority payload instruments to provide vector surface wind observations over ocean, which constitute an important input to the NWP (Numerical Weather Prediction) as well as valuable information for tracking of extreme weather events. The secondary products derived from the scatterometer data are: 15)
- Land surface soil moisture
- Leaf area index
- Snow water equivalent
- Snow cover
- Sea-ice type
- Sea-ice extent.
Instrument requirements: The SCA instrument is a real-aperture, pulsed imaging radar with six fixed fan beam-antennas. In this configuration, the principal elevation planes of the SCA antenna beams are oriented at 45º (Fore-left), 90º (Mid-left), 135º (Aft-left), 225º (Aft-right), 270º (Mid-right) and 315º (Fore-right) with respect to the flight direction, similar to MetOp’s ASCAT (Figure 6).
Each of the SCA beams shall acquire a continuous image of the normalized (per-unit-surface) radar backscatter coefficient of the ocean surface, called σο over a swath. Both sides of the subsatellite track are imaged each with three azimuth views, with an unavoidable observation gap below the satellite. A large number of independent looks are summed in range and azimuth (multi-looking), for each azimuth view, in order to achieve the specified radiometric resolution of the σο estimate on each measurement pixel.
The three σο measurements (σο triplet) are uniquely related to the 10 m vector wind through the GMF (Geophysical Model Function. The wind inversion is based on a search for minimum distances between the measured σο triplet and all the backscatter model solutions lying on the GMF surface, taking into account instrumental and geophysical noise sources . Due to measurement noise, multiple solutions are usually found (wind ambiguities), which have to be filtered out using the background wind information provided by a NWP model (ambiguity removal).
Figure 6: ASCAT (Advanced Scatterometer) measurement geometry on MetOp (left) versus SCA measurement geometry on MetOp-SG (right), image credit: ESA
As compared to ASCAT, SCA shall have a smaller nadir gap by reducing the minimum incidence angle from 25º (ASCAT) to 20º. The main technical requirements of SCA are reported in Table 6 and compared to the ones of ASCAT. The major improvements to be brought by SCA with respect to ASCAT are the spatial resolution of 25 km x 25 km, the radiometric stability of ≤ 0.1 dB and the addition of VH polarization measurements on the mid beams.
Table 6: Main technical requirements of SCA versus ASCAT
Instrument design: The SCA instrument has 6 antennas, 3 on both sides of the satellite ground-track. All antennas emit in vertical polarization. The 4 side antennas receive only vertically polarized signals, whereas the 2 mid antennas receive both V and H-polarized signals. For the mid antennas, two different concepts are under investigation: simultaneous / non- simultaneous reception of V and H-pol signals. The antennas consist of slotted waveguide arrays, connected through waveguides to the beam-switching matrix. For the Fore-/Aft-antenna assemblies, rotating RF-joints are required for enabling deployment.
The SCA antennas are considered a key component of the instrument, as they have direct impact on performance figures like e.g. radiometric stability. Their stability is therefore considered of utmost importance. The design of the SCA antennas is based on a aluminum support structure and RF elements are also made of aluminum.
A high level SCA block diagram is depicted in Figure 7. The baseband radar pulse is stored in the digital memory read-out and followed by the DAC (Digital-to-Analog Converter). The analog pulse is then up-converted to the carrier frequency by a quadrature mixer. The HPA (High Power Amplifier) is driven by a high voltage EPC (Electronics Power Conditioner). The HPA feeds the six antennas sequentially through the beam-switching matrix. The receive signal is amplified by the LNA (Low Noise Amplifier) and down-converted to the in-phase (I) and quadrature phase (Q) baseband signals. The digitized I and Q baseband signals are downlinked and further processed on ground.
An internal calibration loop measures the transmit pulses at the output of the HPA and that of the beam-switching matrix. The calibration pulses are also injected at the input of the beam-switching matrix and measured at the input of the LNA. Those measurements enable gain characterization of the transmit- and receive-chains, as well as losses of the components in the radar front-end. The necessity of measuring the pulses at the input ports of the antennas is a subject of further analysis in relation to meeting the radiometric stability requirement.
The instrument also measures the thermal noise in the absence of radar echo for determining the background noise level. After the noise estimation on ground, noise subtraction is performed for determining the unbiased ocean surface radar cross-section.
Figure 7: SCA block diagram for the case of simultaneous acquisition of V and H-polarized signals from the mid antennas (image credit: ESA)
Two possible implementation configurations have been studied for the SCA instrument. These configurations are characterized by the presence of 6 slotted wave guide array antennas. Table 7 summarizes the instrument budgets. The ranges correspond to the budgets of the two concepts.
Table 7: SCA instrument budgets
The use of a short, chirp-modulated transmit pulse was assumed for the design optimization. The optimization of the pulse length, taking into account the feasibility of the high power amplifier, has been subject of trade-offs in Phase A.
Instrument performance: The GMF (Geophysical Model Function) is an empirically derived function that relates backscatter measurements to surface wind vectors and viewing geometries in the form of σο= GMF (incidence angle, azimuth angle, wind vector). For C-band VV simulations, the project uses the CMOD5 model for ocean backscatter, which is valid for incidence angles ranging from 18 to 58º. For VH simulations, the project uses an empirical model function derived from the last RADARSAT-2 mission and from the NOAA SFMR (Stepped Frequency Microwave Radiometer) flight campaigns in VH polarization over hurricanes. These campaigns confirmed a linear tendency of σοVH with the wind-speed (as depicted in Figure 8) and a low sensitivity to both incidence and azimuth angles.
Figure 8: RADARSAT-2 versus NOAA SFMR (left); VH-pol Geophysical Model Function (right), image credit: ESA
External calibration approach: The SCA timing is based on the use of transmission pulses which are much shorter than those of ASCAT (600 µs against 8.5 ms and 10.8 ms respectively). For sake of clarity, the SCA TX/RX timing is shown in Figure 9. The essential difference is that the transmit pulse length is now shorter than the noise measurement window. Therefore reception of a transponder response can, in principle, be accommodated within the noise measurement window. This window is ideally suited for calibration reception, as it is not contaminated by clutter. If a time delaying transponder is set-up to inject its signal into the noise measurement slot, then a transparent calibration method can be implemented.
For the transparent calibration, it is only necessary that the ground processing is aware of the presence of the transponder echoes within the noise window. Since SCA does not use an on-board processing, it is not necessary that the on-board software is aware of the presence of calibration signals. In the ground processor, the noise data packets containing transponder echoes will be separated from the nominal noise data stream and processed separately. The calibration processing is mostly identical to the ASCAT calibration processing, with the exception that signal demodulation and timing has to be adapted to SCA. Transponder echoes are present in the noise window for about 10 s per over flight. This is a small time interval when compared with 150 s noise integration time employed in the nominal ground processing. Therefore, there is no specific need to compensate for the lost noise measurements.
With the external calibration being transparent to the space segment, it is possible to continuously acquire calibration data and decide when new sets of calibration data need to be injected into the level 1b processor. Explicit calibration campaigns would no longer be needed. In principle, more than one transponder echo (up to three) could be received at the same time, when the transponders are aligned along the direction of an antenna footprint. As it can be seen from Figure 9, the duration of the noise window can easily be extended without impact on the overall timing concept if a larger margin for timing tolerances is desired.
Figure 9: SCA instrument timing (image credit: ESA)
RO (Radio Occultation sounding mission):
The main objective of the RO mission is to provide measurements of refractivity profiles in the troposphere and the lower stratosphere with a good vertical resolution and high accuracy. Refractivity profiles are then used for retrieving atmospheric temperature and humidity profiles as well information on surface pressure. High quality and global observations of atmospheric temperature and humidity profiles are of great importance for real time assimilation on NWP (Numerical Weather Prediction) and climate monitoring. 16)
A secondary objective of the RO mission is the retrieval of depth of the planetary boundary layer and the height (and structure) of the tropopause. Additionally, ionospheric TEC (Total Electron Content) and electron density profiles can be retrieved.
The RO instrument is equivalently called the GNSS (Global Navigation Satellite System) RO instrument, since it exploits the L-band radio-navigation GNSS signals to extract the required information on the Earth atmosphere.
The RO instrument is a passive instrument measuring the time variation of the excess path length of GNSS signals as they are occulted by the atmosphere. As depicted in Figure 10, the excess path length is the path length difference between the straight line path (between the GNSS satellite and the RO sensor, light blue line in Figure 10) and the actual refracted path (red line in Figure 10) travelled by the signal when passing through the atmosphere (the path is refracted/bended due to vertical refractivity gradients of the atmosphere). The excess path length depends on the refractive index of the atmosphere which is a function of pressure, temperature and humidity. On board the RO sensor, the excess path length is simply obtained by measuring the received GNSS signal carrier phase (equivalent to a signal Doppler shift).
Figure 10: Illustration of the radio occultation geometry (image credit: ESA)
Legend to Figure 10: Shown are the bending angle (α) , the GNSS and LEO side impact parameters (pG and pL), the GNSS and LEO coordinate vectors (RG , RL ), the ray path (solid red line), the SLTA (Straight Line Tangent Altitude), Altitude or Ray Path Tangent Height (in orange), and the satellite side asymptotes of the ray path (dashed).
The RO Level 1b product, starting point for the derivation of instrument requirements, is defined as the geolocated, time-tagged neutral bending angle as a function of impact parameter, available for each occultation.
Main RO instrument requirements are listed in the following:
- Number of occultations per SAT: > 1300/day
- Bending angle accuracy: < 0.5 µrad @ 35 km (1σ)
- Minimum SLTA (Straight Line Tangent Altitude): < -300 km
- Carrier and Code Open-Loop in Dual Frequency
- Open-Loop and Closed-Loop parallel processing
- Altitude range: 0-80 km for atmosphere, 80 km- 500 km for ionosphere
- Tracking of GNSS constellations: GPS, Galileo, and optionally GLONASS, BeiDou.
Table 8: GNSS CDMA open-access signals for RO mission
Instrument description: The RO instrument is mainly composed of 3 antennas (zenith, velocity and anti-velocity antennas), of a central electronics unit, and, if needed, depending on satellite configuration, by external LNA/filtering units for system noise figure minimization. The velocity and anti-velocity antennas are, respectively, looking in the velocity and anti-velocity satellite directions for tracking of rising and setting occultations. The central electronic unit is the core of the instrument. It performs signal filtering, down-conversion, signal processing, storage and instrument control tasks. The electronic unit is able to track new high-rate GPS, Galileo, GLONASS and Beidou signals as well as pilot signals. The signals are tracked by means of classical closed-loops, but also, in parallel, by carrier and code open-loops in order to deal with multipath conditions occurring in the lower troposphere. Tracking parameters such as sampling rate, integration time, loops bandwidth, are configurable anytime during the mission in order to guarantee maximum signal processing flexibility. The instrument operates both at L1 and L5 GNSS center frequencies in order to support the on-ground processing for correction of frequency-dependent ionospheric effects.
Predicted performance: The upcoming deployment of new GNSS constellations (i.e. Galileo, Beidou-Compass) and the improvement of associated signal characteristics make the MetOp-SG Radio Occultation sounding mission very attractive, considering the success of the current mission on board MetOp.
As an example, the coverage will improve up to three times thanks to the exploitation of Galileo and GLONASS (or Beidou/Compass) systems. With a single satellite it will be possible to achieve more than 1300 occultations per day. - In addition, the characteristics of novel signals such as the pilot signals, will guarantee higher quality observations on the lower troposphere regions. Novel design of open-loop acquisition schemes will also improve the quality and robustness of observations on the low troposphere. Other improvements come from the use of dual frequency channels with wide separation (i.e. L1 and L5), leading to better correction of the ionosphere.
Figure 11 shows the expected evolution of GNSS systems deployment, according to latest available public information.
Figure 11: Expected time-evolution of GNSS satellites (GPS, Galileo, Compass, GLONASS), image credit: ESA
All these features will be such that the radio occultation instrument will almost double the bending angle accuracy performance with respect first generation and will provide up to three times better Earth spatial-temporal sampling.
Table 9: Radio occultation instrument performance summary and comparison with GRAS
3MI (Multi-viewing, Multi-channel, Multi-polarization Imaging mission):
The primary 3MI mission objective is to provide aerosol characterization for climate monitoring, NWP (Numerical Weather Prediction), atmospheric chemistry and air quality. High quality aerosol imagery ,delivered by the 3MI mission, will facilitate the measurement of all essential aerosol parameters for climate records, such as aerosol optical depths, particle types and sizes, refractive index, sphericity and height index. When used as constrains to the models, these products will be used to provide improved AQI (Air Quality Index) and Aerosol Load Masses for different particles sizes (Ref. 8). 17)
The measurement of surface albedo as well as improved cloud characterization are 3MI mission’s secondary objectives. The first will be facilitated via the observation of the surface BRDF (Bidirectional Reflectance and Distribution Function), made possible by the unique multi-angular measurement concept adopted. Similarly, while METimage will provide information on most cloud properties, the multi-viewing and multi-polarization measurements delivered by the 3MI mission will allow for accurate characterization of the extension, optical depth, particle size as well as asphericity factor and crystal orientation of cirrus clouds.
While currently, aerosol and cirrus parameters are mostly used in GCMs ( General Circulation Models) for climate simulation and prediction, utilization of these parameters is becoming increasingly important in operational NWP as the representation of radiative processes in the atmosphere is a recognized area of deficiency. Hence, the 3MI mission is expected to be of great benefit to both real-time and non-realtime user communities.
Background: Aerosol properties can only be unambiguously determined by instruments that can provide the so called 3M type of measurements, i.e. by instruments offering multi-viewing, multi-channel and multi-polarization capabilities. The only instrument designed specifically to do exactly that is the POLDER instrument. Developed by CNES, POLDER-1 and POLDER-2, respectively, launched in 1996 and 2002 on board the Japanese satellites ADEOS (Advanced Earth Observation Satellite) and ADEOS-2, were short-lived. POLDER-3 followed in 2004 on board the PARASOL satellite (A-train) and still delivers its measurements today (2012), having the highest aerosol retrieval capability of all other passive instruments currently in space. The APS (Aerosol Polarimetric Sensor), which was designed to provide even higher polarimetric accuracy and a larger number of viewing angles, unfortunately failed during launch of the Glory satellite in March 2011.
3MI is an evolution of the POLDER-3 / PARASOL instrument. It will therefore provide similar type of measurements (multi-angle, multi-wavelength and multi-polarization) nevertheless with an improved spatial resolution (4 km at nadir) and coverage, and over an extended spectral range (400 -2100 nm).
The 3MI measurement concept: In order to facilitate the ‘multi-viewing’ type of measurements, 3MI adopts a similar to the POLDER instrument concept, upon which overlapping 2D images on the surface of Earth are recorded consecutively at regular points along the orbit [called along-track (ALT) acquisition points], thus providing the means to sense the TOA (Top of Atmosphere) radiance at different OZAs (Observation Zenith Angles) for each target.
Figure 12: The 3MI measurement concept – the example shows two images recorded in two along-track acquisition points (image credit: ESA)
The various spectral channels as well as the polarization state of the incoming to the instrument signal is then recorded by means of a rotating filter wheel, which accommodates all necessary filters and polarizers. All channels are acquired (in any given ALT acquisition) in less than 7 s during a single filter wheel rotation, while the distance between two consecutive ALT acquisitions is in the order of 22 seconds.
The 3MI instrument is providing a number of spectral channels spanning over an extended (compared to that of POLDER) spectral range, from 410 nm to 2130 nm. To achieve this, 3MI features two optical modules (VNIR and SWIR), each made of a dedicated telescope and focal plane assembly, both sharing a single filter wheel hosting all necessary spectral filters and polarizers. In the current instrument baseline, the two modules feature different ALT FOVs which has an impact both on the number of OZAs acquired per target and also on the OZA sampling range achieved for the two different groups of channels.
The VNIR module features a FOV of ±50.2º x ±50.2º (ACT x ALT, respectively), while the 3MI SWIR module features a FOV of ±50.2 º x ±30º (ACT x ALT, respectively). Due to this difference and given the timings mentioned earlier, this leads to approximately 14 and 6 OZAs views for each target within the swath of the instrument for the VNIR and SWIR groups, respectively. The angular sampling range will also vary accordingly between the two modules. It is possible, however, to increase the number of angular samples in the SWIR module by implementing the so-called angular ‘oversampling’, whereby additional images are obtained, only for the SWIR channels, using one of the extra rotations of the filter wheel in-between the two nominal ALT acquisitions (during which the instrument would otherwise remain idle). These extra samples, however, cannot be co-registered with the VNIR ones.
One direct consequence of the adopted measurement concept is that, during any given pass of the satellite, a set of views is collected, which are nevertheless different for different targets on Earth. It is also obvious that, for any given target, the angular sampling step is also different as well as irregular. Figure demonstrates this, reporting the OZAs recorded for various targets on the sub-satellite track (from the VNIR module), between two sub-satellite points corresponding to two consecutive ALT acquisition points. The horizontal axis reports the normalized distance of a given target between those two points. It can also been seen from this figure that the angular sampling interval among the different angular samples collected for each target is not constant either (varies from about 5 deg to approximately 12 deg) and also different for different targets. Still, this irregular sampling is thought to facilitate improved accuracy of the aerosol specific retrieval algorithms.
Figure 13: OZA (Observation Zenith Angle) sampling for targets on the subsatellite track of 3MI (assumed ALT FOV = 50.2º), image credit: ESA
Figure 14 shows the footprint of the VNIR module of 3MI when the satellite crosses the equator in the day part of the orbit, indicating the range of OZAs accessible by the instrument for the different targets on Earth in a single image acquisition.
Figure 14: Footprint of the 3MI VNIR channels; the colors indicate the recorded observational zenith angle at any given point within the FOV of the instrument (image credit: ESA)
3MI key performances:
• 3MI swath: 3MI delivers a minimum swath of 2200 km, which is considerably larger of that of POLDER-3 on PARASOL. This is the result of its increased ACT (Across Track) FOV (± 50.2° for 3MI compares with ±42.6° for POLDER-3), as well as the higher orbiting altitude of MetOp-SG (approx. 820 km of MetOp-SG compares with approx. 705 km of the A-train /PARASOL orbit). Figure 15 shows the footprints of the two instruments for cross-comparison.
Figure 15: Comparison of the footprint and swath of the 3MI (VNIR module) and the POLDER-3 instrument on PARASOL (image credit: ESA)
• Illumination geometry and scattering angle: The illumination geometry is of great interest too for the retrieval of the final 3MI products. Of particular importance is the so called Scattering Angle (SCA), defined as the π supplement of the angle formed by the direction of sun illumination and the satellite viewing direction (OZA) in the target reference frame. Figure 16 shows the accessible scattering angles by the nadir view of the instrument, (i.e. the one obtained using the central detector line), both for POLDER-3 and 3MI, indicating the differences which are the result of the two different orbits.
Figure 16: Scattering Angle for POLDER-3 and 3MI (only the nadir view of the instruments is shown), image credit: ESA
• Spatial sampling: The SSD (Spatial Sampling Distance) of 3MI in the along track and across track directions is a maximum of 4 km at nadir and varies slightly with the variation of altitude along the orbit. This compares with approximately 6 km of the POLDER-3 instrument. For off-nadir points, this figure increases slightly due to the curvature of the Earth and the mapping law of the instrument. For 3MI, the mapping law is an “f.tan(THETA)” law, which leads to a relative mild increase of the SSD across the swath. Figure 17, demonstrates the SSD variation for all samples within an 3MI image acquired above the equator. The color scale gives the average (mean ACT- and ALT-) SSD.
Figure 17: 3MI mean SSD variation across the FOV of the instrument (samples shown only for visualization purposes), image credit: ESA
• 3MI spectral channels: The 3MI spectral bands are shown in Table 10. For comparison this table contains also the POLDER-3 spectral bands as well as the bands of 3MI at the beginning and the end of the Phase A studies. This last table is the current 3MI baseline. Red color is used to indicate the main differences. The extension of the spectral coverage of 3MI towards both sides of the spectrum is evident from this table.
Table 10: Spectral bands of 3MI
• Polarization sensitivity: Assuming the measurement of a stable, spatially uniform and linearly polarized scene, the polarization sensitivity of the instrument is defined as PS = (Smax-Smin)/(Smax+Smin), where Smax and Smin are the maximum and minimum sample values, respectively obtained when the polarization is gradually rotated over 180º. Table11 shows the predicted values for this key instrument performance parameter for samples acquired with an OZA ≤ 60° and for samples acquired with an OZA> 60°. Figure 18 gives a typical example of the behavior of polarization sensitivity across the FOV of the instrument, for both a polarized and a non-polarized channel.
Table 11: The 3MI spectral bands acquired at various OZA channels
Figure 18: Typical example of polarization sensitivity performances for a polarized (left) and a non-polarized (right) 3MI channel (image credit: ESA)
• Instrument radiometric resolution and calibration: As with POLDER, 3MI does not feature an on-board calibration system. Its absolute radiometric calibration cannot be subsequently guaranteed at instrument level, but will be subject to the accuracy of the vicarious calibration campaigns to be performed during the lifetime of the instrument. As a result, the output product of 3MI at instrument level (conventionally noted as Level 1b1), will be subject to absolute radiometric scaling and correction during the actual operations. Regarding radiometric resolution, a SNR (Signal-to-Noise Ratio) of 200 is specified for a range of TOA (Top-of-Atmosphere) radiances between Lref and Lmax, where the ratio of Lmax/Lref is approximately 11 for all specified spectral bands.
3MI instrument design:
The 3MI instrument concept which was adopted following the feasibility phase (Phase A) is a very similar one to the one proposed at the end of the previous Phase 0 studies, and, as already mentioned, draws largely on the design heritage of the POLDER instrument. Figure 19 depicts this concept.
Figure 19: The 3MI instrument design concept (figures are from the Phase 0 studies), image credit: ESA
Currently, two modules are foreseen for 3MI. The two modules are similar in form and function and are meant to cover, respectively, the two different specified spectral domains of the instrument (VNIR & SWIR). The channel (910 nm) of 3MI is to be included in both modules. Each module features a wide FOV dioptric telescope (minimum ± 57° diagonal). The optical design is telecentric and deploys several optical elements, some of which include aspheric surfaces. High performance / low polarization anti-reflection (AR) coatings are deployed on all surfaces to meet the stringent straylight and polarization sensitivity requirements of the instrument.
The measurement of the different spectral channels and polarizations is performed sequentially in time and facilitated by a rotating filter wheel performing one full revolution in less than 7 s each time and which is placed between the optical heads and the focal plane assemblies. The wheel features two concentric rings each accommodating the filter slots for the VNIR and SWIR modules respectively. One filter wheel serves both modules.
Two-dimensional large format detectors are used on all modules. Charged Coupled Devices (CCDs) and photovoltaic HgCdTe detectors hybridized on top of a CMOS ROIC (Read-Out Integrated Circuit) are proposed for the VNIR and SWIR modules respectively. In order to reduce the dark current, the SWIR FPA is cooled down to 180 K-190 K via means of passive cooling (radiator), and its temperature stabilized using controlled heater circuits.
METimage (Meteorological Imager)
METimage, formerly known as VII (Visible and Infrared Imager), is an advanced multispectral medium-resolution imaging radiometer for meteorological applications to be integrated onto the MetOp-SG satellite series, formerly known as EPS-SG (EUMETSAT Polar System –Second Generation), which is planned to be operational in 2021.
Germany intents to provide the first flight unit of METimage as in-kind contribution to the MetOp-SG program of EUMETSAT. DLR (German Aerospace Center) awarded a study contract to Jena-Optronik in December 2008. Initial concept studies for METimage infrared detectors were already conducted in the timeframe 2007-2009 by AIM Infrarotmodule GmbH, Heilbronn. In November 2012, DLR awarded to Jena-Optronik a Phase-B2 design contract for METimage, funded by two German federal government ministries, BMWi (Bundeswirtschaftsministerium - Ministry of Economics and Technology) and BMVBS (Bundesverkehrsministerium - Ministry of Transport, Building and Urban Affairs). 18) 19) 20)
METimage is regarded a successor of the AVHRR-3 (Advanced Very High Resolution Radiometer) of NOAA flown on the current EPS/MetOp satellite series, and is the European counterpart of the VIIRS instrument on the USA new-generation weather satellite series (NPP and JPSS). 21) 22) 23)
The primary objectives for METimage of the MetOp-SG mission are to provide high quality imagery data for global and regional NWP (Numerical Weather Prediction), NWC (Now-Casting), and climate monitoring. To meet these objectives, METimage has to provide high resolution cloud products, aerosol products, sea- and ice surface temperature and others. Other mission objectives are to support the Post-EPS sounders by providing geolocation and cloud characterization, and to provide data continuity of other key imagers in support of long-term climate records. From these objectives the requirements for the METimage mission have been derived, among them:
- Global coverage within 12 hours
- Large number of spectral bands and wide spectral range
- Demanding figures for dynamic range and signal-to-noise ratio (SNR)
- Polarization insensitivity of 5% for the reflective solar bands (443 nm to 3 µm) and 11% for the thermal emission bands (3 - 13.35 µm)
- Spatial resolution of 500 m and 250 m for selected channels
- Coregistration of all spectral bands: spatial > 87%, temporal < 1 second.
The main drivers for the METimage instrument design result directly from the mission requirements. To meet 12-hour global coverage from an orbit of 817 km altitude, a large swath of 2800 km, corresponding to a cross-track scan angle of ± 55° about nadir, must be observed by the instrument. To cover the large FOV (Field of View), METimage is based on a mechanical scanner telescope (i.e., whiskbroom type), using reflective optics to cover the wide spectral range required to implement the 20 channels on two focal planes.
The products to be derived from the METimage mission are:
- Cloud observations including microphysical analysis
- Water-vapor imagery
- Aerosol observations
- Polar AMVs (Atmospheric Motion Vectors)
- Earth surface albedo
- Cryosphere (snow observations, sea and land ice imagery)
- Fire observations
- Surface temperature (land and sea).
Optics: The rotating telescope scanner design consists of a three-mirror anastigmat, rotating at constant speed about an axis parallel to the line-of-sight, followed by a HAM (Half-Angle-Mirror), a plane mirror rotating at half of the telescope speed. Such a system provides a de-rotated image in the focal plane. Calibration sources are mounted at scan angles outside the ±55° Earth view (Figures 20 and 21). Compared to a two-mirror telescope, the three-mirror anastigmat design provides a high image quality over a larger FOV, allowing implementation of more detectors on the focal plane. With the in-field separation of spectral bands (see below), a large instantaneous FOV is essential for the number of spectral bands that can be implemented. In addition, the large FOV allows increasing the number of detectors per spectral band, leading to a larger footprint on ground. This allows a reduced rotation speed of the telescope and thus increases the integration time per pixel, resulting in improved radiometric performance.
Figure 20: Schematic view of the METimage rotating telescope (image credit: DLR, Jena-Optronik)
Legend to Figure 20: On-axis view of the rotating telescope. Downward facing is the large ±55° Earth view (nadir at center); the calibration sources are mounted at scan angles outside the Earth view. The telescope is rotating at constant speed, the calibration sources are sampled during each rotation.
Figure 21: Schematic view of the rotating telescope (image credit: DLR, Jena-Optronik)
Legend to Figure 21: Along axis view of the rotating telescope. At the left side of the telescope is the half-angle-mirror, rotating at half the telescope speed, producing a de-rotated image in the focal plane.
Following the main optics is a compact, passively cooled secondary optics, providing both an aperture and a field stop, to minimize the impact of stray light and radiative input to the thermal channels. A beam splitter directs the visible and infrared parts of the incoming radiation to one of the two respective FPAs (Focal Plane Assemblies). The IR focal plane is actively cooled to cryogenic temperatures. Each focal plane accommodates detectors and filters for 10 or 11 spectral bands.
A common problem for most scanning systems is different incident angles on optical surfaces, which result in scan-angle dependent polarization properties. In practice, the polarization dependence will also result in a wavelength dependent change of mirror reflectivity. This effect directly adds to the radiometric error budget for all channels, especially when the calibration sources are at different scan angles than the earth view. For the thermal emissive bands there is an additional error contribution resulting from the angle dependent thermal emission of the HAM. It is therefore important for the optics not only to keep the polarization within the values specified in the polarization requirement, but also to keep the difference between different scan angles as small as possible to maintain radiometric accuracy and homogeneity.
FPA design: The spectral separation of individual spectral bands is done by in-field separation, where detectors for different spectral bands are located side-by-side in the focal plane. Due to the cross-track scanning, the image moves sequentially over all spectral bands on a focal plane (Figure 22). For each spectral band there is a row of detectors in along-track direction, so as for a whiskbroom scanner, the instrument records a number of image lines simultaneously during each scan.
Filters are mounted directly in front of the detectors and provide the required spectral shape and resolution. The number of spectral bands that can be accommodated on a focal plane is limited by the usable FOV and the mechanical size of a detector row and filter assembly. The usable FOV is not only limited by the image quality of the optics, but also by the co-registration budget: co-registration errors increase with the distance of detectors within a focal plane (image quality, rotation rate errors, alignment errors, thermal effects, satellite ground speed, and others). With the current design, about 10 or 11 channels can be implemented on each focal plane. The exact number depends on the radiometric requirements and performance of the selected channel combination: some spectral bands will need TDI (Time-Delayed Integration) to achieve the required SNR, thus using up more space on the focal plane.
Figure 22: FPA: In-field separation of spectral channels (image credit: DLR, Jena-Optronik)
Detectors: Detectors are another crucial element in the imaging radiometer. The quality of the detection chain, consisting of detector plus read-out electronics, is decisive for the radiometric accuracy. To achieve the radiometric requirements, METimage will use detectors made from different semiconductor materials. As the spectral sensitivity is dependent on the material, it has to be matched to the target wavelength range. This is especially true in the infrared region. The long wavelength IR channels are the design driver of focal plane temperature and cooling needs. State of the art ROIC (Read-Out Integrated Circuits) with integration stages and amplifiers will be mounted directly on the focal plane arrays.
Calibration: The calibration for the thermal emissive bands is based on a two-point calibration, measuring "zero" by using a cold space view and a high temperature by looking at a high precision blackbody. The blackbody is operated close to instrument temperature, reducing errors due to non-perfect blackness and ageing effects of the black coating. Both calibration targets are scanned during every revolution of the telescope and calibrate all detectors on the cold focal plane. The blackbody can be heated to verify the linearity of the thermal emissive bands detection chains on a regular basis. Blackbody and the primary cold space view are located at opposite positions of the telescope (Figure 20) and are seen under the same scan angle in order to minimize scan-angle dependent effects. A second cold space view can be implemented at a different scan angle to monitor the scan-angle effects and temperature variations of the HAM.
The reflective solar bands are calibrated using the blackbody as optical zero and a solar diffuser as bright source. While the blackbody provides a suitable zero measurement during every revolution of the telescope, the solar diffuser is illuminated only for short time during each orbit (about once every 100 minutes) by the sun, so that a full calibration of the solar channels is only done once per orbit. A common problem with solar diffusers is their ageing under exposure to UV radiation. Even though the planned diffuser for METimage is exposed only for short times and shows relative little ageing under UV exposure, the overall degradation during eight years mission lifetime accumulates to much more than the required 1% lifetime stability. A diffuser monitor device that is well protected from UV radiation is used to calibrate the main diffuser on a regular basis so that the degradation of the main diffuser can be monitored and corrected.
Mechanical instrument design: The instrument consists of an optical head and detached electronics box. Figure 23 shows the optical head. The mechanical configuration is strongly influenced by the different FOV requirements for operational measurements, of the calibration sources and radiators for thermal conditioning. The need for high thermal stability is reflected in a structure that is well shielded. It is as far as possible closed to external radiation intake, which also benefits the stray-light suppression.
Figure 23: Preliminary view of the METimage instrument (image credit: DLR, Jena-Optronik)
Legend to Figure 23: The front of the picture shows the large operational view (110° earth-view). The right side points toward deep space and shows the radiators and the deep space view for calibration. The baffle for the solar diffuser is visible on the left side of the instrument.
While the necessity for a well adapted optical design is easy to perceive, the intricacies of the mechanical design may not be so obvious. However, the accuracy and stability of the mechanical structure supporting the rotating telescope and the half angle mirror is crucial for core performances related to line-of-sight stability. The requirements for relative stability of subassemblies can be as low as a few arcsec. Figure 24 shows the schematic block diagram of METimage.
Figure 24: Block diagram of METimage, containing the basic building blocks, i.e. the scanner, which produces the optical image, the secondary optics, and the focal planes (image credit: DLR, Jena-Optronik)
Table 13: Overview of some METimage system parameters (Ref. 23)
IASI-NG (Infrared Atmospheric Sounding Interferometer–New Generation)
• To assure the continuity of IASI for NWP, atmospheric chemistry and climate applications.
• To improve the characterization of the lower part of the troposphere, the UT/LS region and, more generally, of the full atmospheric column.
• To improve the precision of the retrievals and to allow the detection of new species.
The IASI-NG instrument requirements call for:
- 16921 spectral channels between 645 and 2760 cm-1 (15.5 - 3.63 µm)
- with a spectral resolution of 0.25 cm-1 after apodization (0.50 cm-1 for IASI)
- a reduction of the radiometric noise by at least a factor of 2 as compared to IASI on MetOp (factor of 2 on the spectral resolution, sampling and the radiometric noise).
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24) Vincent Guidard, URL: http://www.eumetsat.int/groups/pps/documents/document/pdf_peps_ucw3_13.pdf
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.