Geodetic and Atmosphere Future Missions

In the past decades, it has been shown that atomic quantum sensors are a newly emerging technology that can be used for measuring the Earth’s gravity field. There are two ways of making use of that technology: One is a gravity gradiometer concept and the other is in a low-low satellite-to-satellite ranging concept. Whereas classical accelerometers typically suffer from high noise at low frequencies, Cold Atom Interferometers are highly accurate over the entire frequency range. Recent results in different labs show that it is possible to build a reliable system for using atom interferometry on vehicles and for space applications. These last developments prove that this technology, which is already suitable on ground vehicles, is competitive with classic inertial sensors. Some developments already worked in zero-g environment in the drop tower facility in Bremen, Germany, or in a 0g plane and are the state-of-the-art of compact setups aiming for spaceborne platform.

We recently proposed a concept using cold atom interferometers for measuring all diagonal elements of the gravity gradient tensor and the full spacecraft angular velocity in order to achieve better performance than the GOCE gradiometer over a larger part of the spectrum, with the ultimate goals of determining the fine structures in the gravity field better than today. This concept relies on a high common mode rejection, which relaxes the drag free control, and benefits from a long interaction time with the free falling clouds of atoms due to the micro gravity environment in space as opposed to the 1-g environment on-ground.

This instrument allows reaching sensitivity of 4.5 mE/Hz^1/2, with the promise of a flat noise power spectral density also at low frequency, and a very high accuracy on rotation rates (below 35 prad/s/Hz^1/2). Thanks to high common mode rejection, drag free constraints will be relaxed compare to GOCE mission and non-gravitational forces will be measured by combining the different measurements from the cold atom interferometers.

An other concept is also being studied in the frame of NGGM, which relies on the hybridization between quantum and classical techniques to improve the performance of accelerometers. This could be achieved as it is realized in frequency measurements where quartz oscillators are phase locked on atomic or optical clocks. This technique could correct the spectrally colored noise of the electrostatic accelerometers in the lower frequencies.

In both cases, estimation of the Earth gravity field model from the instruments has to be evaluated taking into account different system parameters such as attitude control, altitude of the satellite, time duration of the mission, etc.

Miniaturization, lower consumptions and upgrading Technical Readiness Level are the key engineering challenges that have to be faced for these space quantum technologies.

Over six years observation of GOSAT (alias “IBUKI”) launched on January 23, 2009, have made the importance and availability of the measurements of the greenhouse gases from space points to us.

On the other hand, while GOSAT has accomplished the accuracy targets which is highest ever for any observations from space with involvement of the information of the boundary layer, the accuracies were not enough for the political and scientific use. So the scientists had required to improve the accuracy, and it was desired to improve the measurement accuracies. Through the GOSAT operation, we learned a lot of things on the instrument, software, processing algorithm and operation, and we have recognized what we should improve in the following mission. To elucidate the carbon cycle more precisely, our experiences regarding observation performances as well as hardware design were reflected on the mission requirements on GOSAT-2.

The principal requirements are measuring the CO₂ concentration with the accuracy of 0.5 ppm at 500km and 2,000km mesh spatial resolution over the land and ocean, respectively and 1 month average, estimating the net flux with the error of 100% and measuring the Carbon Monoxide as the correlated matter of the anthropogenic emission of the greenhouse gases to study the detection of that.

Through the preliminary design, the requirements for mission instruments hardware including software and satellite system had been defined.

To improve the measurement accuracy, the signal to noise ratio of the TANSO-FTS-2 which is the Fourier transform spectrometer observing the greenhouse gases will be increased by the extension of the aperture size and cooling the after optics as well as the thermal detectors. And to increase the number of the useful data, GOSAT-2 will equip the function to avoid the clouds using the images obtained by the monitor camera in FTS.

To observe the carbon monoxide, spectral range of the 2.0μm band will be extended to 2.3μm.

The pointing angle in the along track direction will be extend from 20 degrees of GOSAT to 40 degrees to expand the observation area over the ocean where the sun glint is observed. This will make it possible to extend the observation area over the ocean from plus/minus 20 degrees centered on the sub-solar point to plus/minus 40 degrees, and contribute to the global observation including the ocean.

The cloud and aerosol imager, that is CAI, data on GOSAT have been used to compensate the FTS data for the aerosol and to detect clouds. The one on GOSAT-2, TANSO-CAI-2, will be enhanced the aerosols measurements abilities to detect the flow of the PM2.5 which is the critical issues not only in Japan but also in the world. New observation channels are added and the viewing angles will be changed to plus and minus 20 degrees from nadir of GOSAT.

In addition, the GSD of the 1.6μm bands is two-thirds of the one of GOSAT.

To increase the observation angle of each point, the satellite recurrent cycle will be increased from 3 days to 6 days.

The critical design had been completed in 2015, and manufacturing of the flight model was started. The current status and details of the mission instruments and satellite system design will be shown in this presentation as well as the observation results of the first GOSAT.

We propose a conically scanning space-borne broad-swath Dopplerised 94GHz to provide global measurements of winds, rainfall and cloud ice water content using the radar returns from cloud and precipitation particles.  The observations will have 50km horizontal and 1km vertical resolution with several visits every day at European latitudes. Windstorms are the single most damaging meteorological phenomenon in Europe with high losses also resulting from flooding.   The loss of life in tropical cyclones and hurricanes is decreasing due to improved forecasts and warnings.  Assimilation of the wind observations from this satellite into forecast models should improve their skill further leading to better predictions of timing and location so that mitigation activities can be better focussed.

 The 94GHz (3mm) frequency is needed to achieve a 1km vertical resolution as the 500m pulse approaches the earth’s surface at angles around 45deg. The 2.9 by 1.8m elliptical antenna rotates once every seven seconds sweeping out a broad swath on the ground. Two configurations are being considered with swaths of 800 and 1800km; the broader swath having more frequent visits but with lower sensitivity detecting fewer clouds at a reduced spatial resolution.  Studies of this trade off with NWP forecast models are in progress.  The satellite will have the same 94GHz transmitter that has operated flawlessly on Cloudsat since its launch in 2006.  The Doppler velocity will be derived using the returns from cloud particles from a pair of two closely spaced pulses polarized in the horizontal (H) and vertical (V) so that the folding velocity is larger than the highest wind speeds.  The H and V pulses must be propagate, scatter and be received independently; ground based observations funded by CEOI are being analyzed to quantify how often depolarizing cloud targets and multiple scattering lead to cross talk between the H and V returns, and how such occasions can be identified and any errors in the inferred velocity corrected.  A short separation of the pulses is also needed to minimize the blind zone close to the surface where observations will not be possible; when the first pulse impacts on the depolarizing surface it will obliterate the return from the following pulse.   ESA funded aircraft observations over the next twelve months of ocean and land surface returns at high incidence angles will help to quantify the depth of this blind layer.

The GRACE satellite mission has been launched more than a decade ago. Although it was designed for a nominal mission lifetime of 5 years, it still provides valuable science data. An eventual systems failure and, thus, mission termination is expected any time soon, though. Despite a relative low spatial and temporal resolution, the monthly gravity fields have proved an invaluable and novel parameter set in several geoscience disciplines, allowing new research venues in the study of Global Change phenomena.

The enormous success of the mission has driven the need for continuation of monitoring mass changes in the Earth system. Indeed, a GRACE Follow-On (GFO) mission has been approved for launch in August 2017. Like its predecessor it will consist of two satellites flying inline with inter-satellite K-Band ranging as the main gravitational sensor.

To guarantee the continuation of such successful gravity field time series, ESA has embarked several years ago on a long term strategy for future gravity field satellite missions, both in terms of technology development and to service a growing user community.

The recent ESA-funded SC4MGV study has addressed aliasing as one of the key limitations for future gravity field satellite missions. The study’s strategy to reduce aliasing is implemented as a three-staged approach: (i) optimization of double inline pair satellite constellations in a so-called Bender formation, (ii) development of a de-aliasing gravity field retrieval method, and (iii) post-processing. In this presentation, we are going to show the main results of the study in these three stages. We specially show how a good choice of Bender formation reduces aliasing effect to a great extent. We also present the ability of Wiese de-aliasing approach and our selected post-processing method for aliasing reduction and filtering noise. At the end, we discuss the main outputs of the project as well as the new open questions which should be addressed in the future studies.

We report on the on-going development of a low cost sounder for the upper atmosphere (LOCUS).  The scientific rationale of LOCUS is to observe terahertz frequency (THz) atomic and molecular transitions to trace and monitor important chemical species in the Mesosphere (55 - 90 km) and lower thermosphere (90 - 120 km), a region known as the MLT.

Observations show that in some places the mesosphere is cooling up to 30 times faster than the troposphere is warming. Although not fully understood, this can partly be attributed to processes which are linked to climate change either directly (increased greenhouse gas concentrations in the middle atmosphere, stratospheric ozone depletion) or indirectly (changes in meridional circulation patterns, gravity wave spectrum). Despite the uncertainty, it therefore provides a highly geared indicator of global climate change. The MLT is also the region where radiative cooling of the atmosphere to space takes place (through CO₂ and NO) and therefore is a key region for assessing climate forcing. In addition to being a proxy for climate change, MLT chemistry and composition is also strongly influenced by the precipitation of electrically charged particles from the solar wind, making it an ideal region to study and monitor solar weather (i.e. through auroral forcing in NO).

Given its importance, it is thus surprising that the chemistry of the MLT has not been well studied to date by space-based platforms.  The reason for this is that the best probes of the important chemical species (atomic O, OH, HO₂, NO etc.) lie in the THz range for the MLT and the corresponding detection technology is relatively immature and perceived as expensive to deploy in comparison to other wavebands. Atomic Oxygen (O) in particular is the predominant constituent of the upper atmosphere and as such a key element of almost every chemical process. Despite this, atomic oxygen has never before been measured directly by remote sensing with global coverage. However, recent technical advances in heterodyne receivers, especially in the area of Quantum Cascade Lasers (QCLs) used as local oscillators and planar Schottky frequency mixers and high-speed digital signal processing, mean that payload resource demands (mass power and volume) are being significantly reduced and are evolving towards compliance with smaller space borne missions.

In the past years we have developed the LOCUS mission concept, which will measure the species O, OH, O₃, O₂, HO₂, H₂O, NO, CO and CO₂ over an altitude range of 50 km to 150 km. LOCUS will achieve this with a combination of four THz receivers, supplemented by infrared channels for the lower altitudes. Of the four THz channels, the two high frequency channels (4.7 THz for O; 3.5 THz for OH and HO₂) will use QCL pumped Schottky detectors, where as the two lower channels (1.15 THz for NO, CO, O₃ and H2O; 774 GHz for O₂) will use conventional Schottky mixers. The infrared channels contribute pressure, temperature and altitude profiles (from CO₂), vertical mixing ratio profiles of O₃ (9.6 μm), and OH volume emission rate (2.0 μm). With these measurements the O content can alternatively be inferred by solving atmospheric chemistry equations.

The technology for LOCUS is being developed in the UK through the CEOI-ST. In 2014 a comprehensive satellite mission concept was designed via ESA’s in orbit demonstration (IOD) programme. Since then, work has been focussing on increasing the technology readiness levels (TRL) levels of the novel receivers, space coolers and wideband spectrometers. In parallel, the scientific readiness levels (SRL) are being addressed in order to place the LOCUS missions as a potential candidate for the 9^th Earth Explorer call. We present the latest design of the LOCUS mission, and report on recent activities of building and operating an elegant breadboard receiver at 1.15 THz to demonstrate the novel technologies, and increase their TRL.