MERLIN (Methane Remote Sensing Mission)
MERLIN is a Franco-German collaborative minisatellite climate mission. The primary objective is to obtain spatial and temporal gradients of atmospheric methane (CH4) columns with high precision and unprecedented accuracy on a global scale. Methane (CH4) and carbon dioxide (CO2) both cause global warming, although the impact of methane is 25 times more powerful than the one of carbon dioxide on a timescale of 100 years. Now, at a time when there is much discussion about mankind being directly responsible for the rise in the emission of greenhouse gases, methane emission levels already far outstrip carbon dioxide. Since pre-industrial times, the amount of methane in the atmosphere has more than doubled, whereas the growth in carbon dioxide levels during the same period has been 'only' thirty percent. Methane is after carbon dioxide the strongest anthropogenic greenhouse gas. Methane emissions are caused by human activities as well as natural sources – for example, from rice paddies, animal husbandry, biomass decomposition, landfill sites or energy generation. Natural sources include swamps and marshlands as well as thawing permafrost. Alongside carbon dioxide, methane is one of those gases for which the Kyoto Protocol stipulates that cuts must be achieved. 1) 2) 3)
A major problem in the understanding of CH4 source- and sink-processes is the lack of precise global measurements of atmospheric CH4. Ground based in-situ observations are insufficient because the existing measurement network is too coarse. Source regions of key importance to the global CH4 cycle such as the Arctic permafrost, Boreal forests and Tropical wetlands are difficult to access; hence, they are underrepresented or not sampled at all. Therefore, it is necessary to apply spaceborne measurement techniques in order to obtain global coverage at high precision. Today, GOSAT (Greenhouse Gases Observing Satellite) of JAXA has the ability to measure CH4 from space. The observation strategy is based on measuring spectra of sunlight backscattered by the Earth's surface and atmosphere in the shortwave infrared spectral region. The main problem of these passive methods is that undetected aerosol layers or thin ice clouds produce systematic measurement errors of unknown magnitude, resulting from the complexity of the retrieval algorithms and the limited availability of independent measurements for validation. To counter these limitations, the use of active remote sensing instruments like the IPDA (Integrated Path Differential Absorption) LIDAR, was proposed.
The data that the MERLIN climate satellite will gather from orbit will enable scientists in both countries to draw conclusions about the various different sources of methane emissions. What is the impact of rising levels of energy production? What are the implications when tracts of permafrost release methane as they start to thaw? Above all, what are the implications for our climate?
MERLIN is a joint mission by DLR (German Space Administration) and CNES (French Space Agency), where Germany is developing and building the methane LIDAR ((Light Detection And Ranging) instrument, while France is providing the satellite platform Myriade Evolutions and the mission control. Joint data processing and science activities in France and Germany will be established.
The science activities are led by two Co-Principle-Investigators from the French LMD (Laboratoire de Météorologie Dynamique) of CNRS and the German DLR Institute for Atmospheric Physics, with additional support of several French and German Research Institutes.
The MERLIN mission was initiated in February 2010, when the Franco-German Council of Ministers decided to start a minisatellite mission monitoring the greenhouse gas methane in the atmosphere. Phase 0 of the project started in early 2010. In the second quarter of 2012, MERLIN successfully finished Phase A. A launch is expected in the timeframe of 2016 with a minimum mission life time of 3 years on orbit. 4) 5) 6) 7)
Figure 1: Artist's rendition of the MERLIN minisatellite (image credit: CNES)
CNES is providing the MERLIN platform, a minisatellite of the Myriade platform series, referred to as “Myriade Evolutions”. The main improvements are: a new structure in order to reach 250 kg for the satellite launch mass, an increase of the solar array capacity and the power distribution, a new adapted propulsion system, an improved AOCS (Attitude and Orbital Control Subsystem), an increase of the payload data storage, the obsolescence handling, a compatibility with low flight altitudes (atomic oxygen concerns). 8) 9) 10)
Table 1: Main platform parameters of MERLIN
The Myriade Evolutions platform is designed so as to be compatible with a wide range of missions, including high-resolution optical imaging, and ranging from 500 km to 800 km orbits at any local time. The platform is compatible with most launchers, and in particular with the European VEGA and Soyuz launchers (ASAP-S auxiliary structure for Soyuz). The target lifetime is 5 years.
Figure 2: MERLIN platform (left) and spacecraft (image credit: CNES)
Launch: A launch of MERLIN is planned for 2016. The baseline launcher is the Soyuz vehicle from Kourou; Vega is considered a potential backup.
Orbit: Sun-synchronous near-circular orbit, altitude ~ 506 km, inclination = 97.4º, LTAN (Local Time on Ascending Node) = 6:00 hours or 18:00 hours, repeat cycle = 28 days.
RF communications: The TT&C data are transmitted in S-band at 625 kbit/s. The payload data are downlinked in X-band at 150 Mbit/s.
SOC (Satellite Control Center): Spacecraft control, monitoring, orbit control are performed by the CNES facilities located in Toulouse (France).
Sensor complement: (IPDA LIDAR)
IPDA (Integrated Path Differential-Absorption) LIDAR:
Measurement principle: An IPDA LIDAR uses the laser light scattered back from a surface to obtain measurements of the column content of a specific atmospheric trace gas between instrument and scattering surface. For this, the difference in atmospheric transmission between a laser emission with a wavelength placed at or near the center of a CH4 absorption line (λon) and a reference wavelength (λoff) with significantly less absorption is used. A telescope collects the backscattered photons and focuses them onto the detector. Since the return signals are very weak, it is necessary to accumulate several single measurements of the return signals along the track in order to achieve the required measurement sensitivity. From the ratio of the two return signals, the DAOD (Differential Atmospheric Optical Depth) can be calculated. 11) 12)
The goal of the MERLIN mission is to measure the spatial and temporal gradients of atmospheric CH4 columns with high precision and unprecedented accuracy. The main data product of MERLIN will be column-weighted dry-air mixing ratios of CH4, (XCH4) measured along the subsatellite track according to equation 1:
with the received signal powers Poff and Pon, normalized by the associated ratio of transmitted pulse energies Eon and Eoff. Psurf is the surface pressure at the location where the laser beam hits the ground and WF is the weighting function describing the altitude sensitivity of XCH4.
In order to provide a solid scientific basis for the mission, the scientific performance requirements of were formulated. They are based on the random and systematic error of the instrument (Table 2). The requirements were chosen in such a way that the following quality levels can be reached: to resolve large wetland fluxes, inter-hemisphere gradients, seasonal and annual budgets on continental scale (threshold), to resolve the seasonal and annual budgets on country-scale (breakthrough ), highest Methane flux estimate quality, and the Kyoto protocol like monitoring (goal).
Table 2: Overview of scientific mission requirements
The DLR instrument selected consists of an IPDA LIDAR. The industrial consortium, comprising Astrium GmbH and Kayser-Threde GmbH, is realizing this LIDAR instrument, where Astrium is acting as the prime leader and is contributing the transmitter laser. Kayser-Threde is responsible for the optical receiver system and the signal chain.
Main driver for the actual payload design are the limited payload allocations by the platform. The Myriade Evolutions platform provides a payload allocated power of about 110 W (during eclipse phase) and can carry a payload with a maximum mass of about 95 kg in a volume of 82 cm x 88 cm x 92 cm.
Aside from these platform constraints, the main driver for the instrument design is the necessity to reach the scientific requirements. The RRE (Relative Random Error) is mainly driven by orbit altitude, size of the receiver telescope and the available laser pulse energy. The minimal orbit altitude is limited by the platform (due to atomic oxygen constraints), maximum telescope size due to volume allocations of the launcher and the pulse energy due to power allocations by the platform. Aside from the random error, the RSE (Relative Systematic Error) has also to be taken into account. This error depends mainly on the accuracy and stability of the energy calibration, stability and knowledge of the laser frequency, stability and knowledge of pointing and detector linearity.
The industrial consortium, comprising Astrium GmbH and Kayser-Threde GmbH, is realizing this Lidar instrument, where Astrium is acting as the prime leader and is contributing with the transmitter laser. Kayser-Threde is responsible for the optical receiver system and the signal chain. 13) 14)
Figure 3: Illustration of the InnoSlab concept (left) and the amplifier beam path (right), image credit: MERLIN consortium
Laser: As experiences and problems encountered in other missions like EarthCARE (Earth Clouds, Aerosols and Radiation Explorer) and ADM-Aeolus (Atmospheric Dynamics Mission) demonstrate, the design and realization of spaceborne lasers is rather sophisticated and involves several risks which have to be taken into account. These risks are mainly contributed to stable operation under space conditions and lifetime aspects of components. In contrast to the ALADIN instrument (Atmospheric Laser Doppler LIDAR Instrument) of the ADM-Aeolus mission, the laser used for the MERLIN mission is not a completely new development but deduced from the ESA EQM (Engineering Qualification Model) FULAS (Future Laser System) which is currently under construction. The main parameters of the current MERLIN laser are shown in Table 3.
Table 3: Laser performance parameters
The first step in the laser design was the selection of the appropriate measurement wavelength which depends mainly on these factors: CH4 absorption bands, detector efficiency and eye safety considerations. In the SWIR (Short-Wave Infrared) spectral range, where eye safety considerations are less critical, the CH4 lines are abundant, but the presence of water vapor and carbon dioxide lines drastically constrains the selection. Essentially two atmospheric water vapor transmission windows around 1600 and 2300 nm allow CH4 measurements. The detector performance is significantly better in the 1600 nm region where low-noise photodiodes (InGaAs APDs) are available. Since no directly pumped laser at this wavelength is available up until now, or its technology level is too low for consideration, a laser concept based on a Nd:YAG pumped OPO (Optical Parametric Oscillator) was selected.
The pump laser will consist of a seeded oscillator followed by a single-end pumped slab amplifier. The oscillator design includes an end-pumped rod-crystal as gain medium. The oscillator will be injection seeded and cavity controlled in order to achieve single longitudinal mode operation and to fulfill the stringent requirements on the pulse quality. For the amplifier, the so called InnoSlab concept, developed by the Fraunhofer Institute for Laser Technology, will be used. For the amplifier, the so called InnoSlab concept, developed by the Fraunhofer Institute for Laser Technology, will be used (Figure 3). This concept is ideal for high energy and high efficiency power amplifiers for single mode operation. For MERLIN, the slab crystal is partially end pumped from one end. This gives the advantage that only two faces of the crystal need to be optically polished. The heat dissipates very efficiently and homogeneously over 2 metal heat sinks soldered to the large faces of the crystal. The four optically unused surfaces of the crystal are roughened to optimally prevent internal parasitic laser oscillations.
The four optically unused surfaces of the crystal are roughened to optimally prevent internal parasitic laser oscillations. To operate the InnoSlab laser as an amplifier (right part of Figure 3), the signal beam is folded in a single pass configuration through the crystal. By choosing appropriate mirror radii and signal beam divergence, the beam is widened with every pass, the fluence can be kept constant and remains far away from the damage threshold. By adapting the slab crystal width and the number of passes the InnoSlab amplifier is scalable in its output power. The MERLIN amplifier will be designed to deliver an output pulse energy of about 30 mJ at 1064 nm while the peak fluence will be smaller than 2.5 J/cm2. Simulations showed that this output energy is the best compromise between strain of the pump modules and efficiency of the pumped OPO.
The baseline for the OPO is a 4-mirror 2-crystal setup. The same setup is also implemented in the DLR airborne demonstrator for the DLR Jet HALO (High Altitude and LOng Range Research Aircraft). The crystal will be either KTA (Potassium Titanyle Arsenate) or KTP (Potassium Titanium Oxide Phosphate). Like the oscillator, the OPO will be injection seeded and cavity controlled in order to achieve single longitudinal mode operation.
To realize an efficient and compact spaceborne laser, new mounting technologies like soldered optics will be applied. This allows a compact, precise, stable and glue free mounting of the optical laser components. These mounts are currently developed within a DLR-funded research project. It was possible to design and build solder based mounts for all optical elements within the laser. Temperature cycling tests of soldered mirrors were performed. The results demonstrate, that for critical components like mirror mounts, the tilt deviation at the operation temperature of 20°C is being kept to better than 10 µrad.
Figure 4 shows the MERLIN pressurized Laser housing which includes the entire laser head. The laser itself will be mounted on the laser-plate. To use the available space as efficiently as possible, the oscillator will be mounted on the top side of the laserplate while the amplifier and OPO are mounted on the down-side. The laserplate will be fixed at three points with isostatic mounts to a frame. The housing will be fixed only to the frame. Consequently, the vacuum forces are decoupled from the laserplate itself. Heat generating units will be decoupled thermally from the baseplate and the generated heat will be piped out of the laser housing by mini loop heat pipes.
Figure 4: Illustration of the MERLIN laser housing (image credit: DLR)
Calibration devices: High laser stability and the actual knowledge of the laser frequency are crucial for this kind of instrument; hence, a compact and precise frequency reference is necessary. To minimize the required amount of instrument equipment, it is highly desirable to use the frequency reference not only for absolute frequency monitoring, but also to generate error signals for control of the OPO and the seed lasers. The current approach for MERLIN is to use an air spaced Fizeau interferometer and a CCD detector row for fringe measurements. A verification and also calibration is possible by ground echo calibration (determination of the spectral CH4 line center from scanning its spectral line shape). Preliminary studies show that for the maximum rms deviation of the seed frequencies from their required absolute values, the following values must be reached: the online signal must provide an absolute stability of ~15 MHz rms over lifetime while the offline signal must provide an absolute stability of ~100 MHz rms over the mission lifetime.
An IPDA LIDAR is solely relying on the measurement of relative signal intensities. The intensity of the recorded return signal is a direct function of the emitted laser pulse energy. Even for very good lasers, this variation is in the area of up to 5%. Therefore, it is necessary to monitor the energy of each emitted laser pulse with high accuracy and to correlate with the intensity of the corresponding optical return signals (equation 1).
To generate these internal calibration signals, a small fraction of the emitted laser pulses is extracted and fed directly through an attenuation stage to the instrument detector. Measuring the return and calibration signal with the same detector ensures that variations within the signal chain are eliminated. The challenge here is that because of the very weak return signal (for λon ~1000 photons per shot), the reference signal has to be attenuated by about 14 orders of magnitude.
Receiver: Aside from the laser, the receiver telescope is the most challenging component of MERLIN. Its field of view has to be large enough so that the complete laser ground spot (150 m for 99% of the encircled energy) and additional margin for the satellite and the laser jitter is within this field. The other limiting factor is the size of the detector. For APD (Avalanche Photodiode) detectors, the noise is increasing drastically with the detector area, which means that a detector with a small active area has to be chosen. Furthermore, the overall height of the telescope is strongly limited due to the available space within the launcher fairing (MERLIN baseline: Soyuz ASAP-S inner position). These factors result in a small F-number and thus a complex optical design. Aside from the size, also the mass is a very critical value. To realize such a telescope, serious light-weighting of about 85% has to be applied. Furthermore, the thermal design of the telescope has to be in such a way, that a purely passive thermal control system is sufficient since no extra power for heating is available. The current baseline for MERLIN is a Zerodur off-axis telescope with an aperture of 690 mm and an F-number of 0.65, mounted on an optical bench made of CFRP (Carbon Fiber Reinforced Plastic).
Due to the limited pulse energy and telescope size, the receiver and detector noise are both critical. The detector and the following amplifier stages have to be as noiseless as possible. In the 1650 nm region, InGaAs ADPs seem to be the most promising candidates and are currently the baseline for MERLIN. Also, MCT detectors are very promising candidates. They may offer some interesting features of interest for low light level detection around 1.65 µm due to their low excess noise.
ICU (Instrument Control Unit): The ICU is the main control instance for the whole instrument. Its main tasks include: scheduling of the laser trigger, control of the APD sensor data acquisition and buffering, detector temperature control, payload FDIR (Fault Detection, Isolation and Recovery) support, telecommand decoding and execution, and calculation of the range gate according to the altitude data received from the platform.
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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.