Constellations have proven to be an effective and efficient way to acquire earth science data. By flying together, sensors on all satellites in a constellation take measurements of the same air or land mass at essentially the same time. The sensors form a single “virtual satellite”. The keys to making a constellation effective and efficient are keeping the operations as independent as possible in order to minimize the operational burden and costs. The Earth Science Constellation (ESC) has been successful on all counts and continues to welcome new missions to continue its 18+ year record of coincidental earth science observations. The ESC also serves as a model for future constellation designs. This paper describes the ESC and its evolution from its initial launches in 1999 through the present. The paper also touches on the possibilities for new missions.

The ESC is truly international in scope, involving space agencies from the United States (NASA, USGS, and USAF), France, Japan, and Argentina. The ESC comprises two main components: The Morning Constellation and the Afternoon Constellation (also known as the ‘A-Train’). These constellation names are derived from the time of their equator crossings. All Morning Constellation satellites (currently Landsat-7, Terra, and Landsat-8) cross the equator at points where the local time on the ground is approximately 10:30 a.m. This makes these satellites ideal for observations of land processes. All Afternoon Constellation satellites (currently Aqua, Aura, CALIPSO, CloudSat, GCOM-W1, and OCO-2) cross the equator at points where the local time is approximately 1:30 p.m., which makes these satellites ideal for observations of atmospheric and water processes. Their orbits are referred to as being ‘sun-synchronous’ since orbital precession is used to advantage to maintain the similar lighting conditions throughout the entire life of the missions.

The Morning Constellation missions launched first and maintained relatively large inter-satellite orbital spacings, so little coordination between mission teams has been required. In contrast, the Afternoon Constellation satellites have flown as close as 10 seconds apart, so it became clear at the outset that the missions would need to coordinate their operations to some extent in order to ensure their missions’ safety. The challenge was to find a way that did not place undue burdens on each mission team’s operations. To address this challenge, the Afternoon Constellation formed a Mission Operations Working Group (MOWG) in 2003 with representatives from each science and operations team. The MOWG then established a framework whereby each mission was allocated constellation space for its operations; this space became known as its ‘control box’. A mission can maneuver and move around as much as it likes as long as it stayed within the limits of its control box. Hence there is little need to coordinate with other teams on a daily basis, so the costs and complexities have been minimized. The MOWG also established guidelines and procedures to follow in the event of spacecraft anomalies; procedures that have successfully been followed on more than one occasion.

The most recent Decadal Survey placed high value on continuing constellation science. The ESC has evolved by seeing new missions joining and old missions retiring. Most recently, GCOM-W1, Landsat-8, and OCO-2 joined during 2012-2014. Landsat-9 is set to join in 2020. Each new mission provides new and improved suite of sensors. The new sensors also benefit both from the multitude of other existing on-orbit sensors as well as from the long-term cross-calibrated climate observations from the sensors that preceded them. At the same time, existing missions leave the constellation due to low fuel reserves or aging spacecraft subsystems. For example, CloudSat and CALIPSO left the ESC orbits in 2018, although they plan to continue making science observations at their new lower altitudes. This ESC evolution will continue and this paper will discuss the opportunities for other new missions to join the ESC.

Since more than two decades a main focus of the spaceborne SAR research at
DLR lies on tandem missions and on exploiting interferometric and
tomographic techniques in earth observation. Starting with DLR’s
contribution to the Shuttle Radar Topography Mission in 2000, a first
Digital Elevation Model (DEM) in X-band was generated. Based on this
experience and additional studies on satellite tandems, the bi-static
satellite mission TanDEM-X was proposed in 2003 and became reality and
operational in 2010 [1], [2]. The great success of the mission and the
outstanding quality of the generated global digital elevation model led to a
further elaborated concept, the Tandem-L mission proposal [3]. This
revolutionary mission, expected to launch around 2024, addresses pressing
scientific questions related to dynamic processes on the Earth in the bio-,
geo-, hydro-, and cryosphere. The following paper describes the orbital
characteristics and corresponding observation concepts of the missions in
order to fulfill their scientific goals.

The main mission goal of both missions is the generation of global data
products. In TanDEM-X, this is the digital elevation model with its
unprecedented height accuracy of less than two and four meters over flat and
steep terrain, respectively in terms of the 90% point-to-point error. In
Tandem-L more than 20 products shall be generated and regularly updated,
covering e.g., forest height structure, forest biomass, displacement and
deformation maps of volcanoes and earthquake risk areas, and different types
of DEMs. Especially forest products generated by exploiting tomography and
DEMs exploiting interferometry rely on simultaneous acquisitions from two
slightly different incidence angles. Both missions acquire these data
utilizing the principle of Synthetic Aperture Radar (SAR) from a low-earth
orbit of 514 km and 745 km, respectively. TanDEM-X in X-band enables
acquisitions with a high geometric resolution of less than one meter and the
generation of DEMs at a resolution of 12 m x 12 m. The focus of Tandem-L is
on the frequent, weekly coverage of the Earth with a still sufficient
resolution of seven meters. To achieve this, the Tandem-L system combines a
reflector antenna with a digital beamforming instrument enabling swath
widths of up to 350 km.

The orbital characteristics of these missions are similar in such that both
are based on the successfully proven helix concept [4], [5]. The helix is
realized by separating the eccentricity-inclination vectors. This results in
intersection-free orbits which minimizes the collision risk. As a
consequence of the helix, the distance between both satellites, the
baseline, changes permanently over the orbit. Both, for the acquisition of
DEMs with TanDEM-X as well as e.g., for tomographic acquisitions over forest
or ice with Tandem-L, different data sets with homogeneous accuracies are a
key requirement on the mission. Hence, the distance between the satellites
has to be adapted (over time spans of weeks/repeat cycles) depending on the
requirements on the baseline for the dedicated applications and on the
incidence angles and latitude ranges to be acquired.

The realization of how the distance between both satellites is changed is
performed differently in TanDEM-X and Tandem-L. In the TanDEM-X mission, the
distances are generally in the order of 200 m to 600 m (in dedicated phases
even up to 3600 m). The change of the baseline and hence the formation is
mainly realized by altering the ascending node of one satellite at the
equator and the eccentricity vectors at the poles compared to the other. In
Tandem-L in contrast, baselines of up to 20 km shall be realized demanded by
the higher wavelength of the system. Here, the inclination of the orbits is
differed by a small amount. By this, the ascending node of one satellite at
the equator drifts compared to the other and hence the horizontal baseline
steadily increases (or decreases). With this technique, the amount of fuel
required to change the baseline is much smaller than it would be with
dedicated maneuvers.

The full paper will concentrate in further detail on the relation between
formation and accuracy of the products, the realization of the formation and
its benefits for the different applications.

[1] G. Krieger, M. Zink, M. Bachmann, B. Bräutigam, D. Schulze, M.
Martone, P. Rizzoli, U. Steinbrecher, J. W. Antony, F. de Zan, I. Hajnsek,
K. Papathanassiou, F. Kugler, M. R. Cassola, M. Younis, S. Baumgartner, F.
Lopez-Dekker, P. Prats, A. Moreira: “TanDEM-X: A radar interferometer with
two formation-flying satellites,” in Acta Astronautica, Vol. 89, 2013, pp.
83–98
[2] P. Rizzoli, M. Michele, C. Gonzalez, C. Wecklich, D. Borla Tridon,
B. Bräutigam, M. Bachmann, D. Schulze, T. Fritz, M. Huber, B. Wessel, G.
Krieger, M. Zink, Manfred, A. Moreira: “Generation and performance
assessment of the global TanDEM-X digital elevation model“. ISPRS Journal of
Photogrammetry and Remote Sensing (132), pp. 119-139. EIsevier. ISSN
0924-2716
[3] A. Moreira, G. Krieger, I. Hajnsek, K. Papathanassiou, M. Younis, P.
Lopez-Dekker, S. Huber, M. Villano, M. Pardini, M. Eineder, F. De Zan, A.
Parizzi: “Tandem-L: A Highly Innovative Bistatic SAR Mission for Global
Observation of Dynamic Processes on the Earth's Surface,” in IEEE Geoscience
and Remote Sensing Magazine, Vol. 3, No. 2, 2015, pp. 8-23
[4] H. Fiedler, G. Krieger: "Close Formation Flight of Passive Receiving
Micro-Satellites", International Symposium On Space Flight Dynamics, Paper
1030, Oct. 2004
[5] S. D'Amico, O. Montenbruck: “Proximity Operations of
Formation-Flying Spacecraft Using an Eccentricity/Inclination Vector
Separation”, in Journal of Guidance, Control, and D., Vol. 29, No. 3, 2006,
pp. 554-563

FLEX (FLuorescence EXplorer) is an Earth observation mission currently being developed by ESA to perform quantitative measurements of the solar induce vegetation fluorescence. It will advance our understanding of the functioning of the photosynthetic machinery and the actual health of terrestrial vegetation.
The FLEX payload is a single instrument, called FLORIS, which consists of 2 separate imaging spectrometers. The fluorescence signal measured from space, however, is so faint that additional information is required for an accurate retrieval and interpretation of the vegetation Fluorescence emissions.
Instead of embarking several instruments on one satellite, the FLEX mission concept is based on using existing capabilities provided through operational satellites. The most critical information required from the tandem partner satellite is related to the characterisation of the atmosphere (clouds, aerosols and water vapour) and land-surface characteristics (land-cover type, biophysical parameters and surface temperature). Sentinel-3 can deliver the required auxiliary datasets and is therefore the chosen companion for FLEX.
To this purpose the FLORIS imaging geometry will be nadir pointing to coincide with the nadir views of the Sentinel-3 instruments OLCI and SLSTR. Analysis of the temporal co-registration requirements of FLEX and Sentinel-3 observations revealed that cloud displacement and changes in the atmospheric state require a separation in the range of 6-15 seconds between the two satellites.
The concept of flying in tandem with Sentinel-3 has already been studied during Phase A. More recently the impact of uncertainties in satellite ballistic coefficients on the robustness of the convoy flight has been analysed in a parametric study. That study confirmed that it is feasibility to fly in a passive safe manner, meaning that during nominal operations and in case one or both satellites enter safe mode an active intervention from either side is not mandatory to exclude collision.
Furthermore it is important that FLEX operations do not disturb the one of Sentinel-3. Hence the FLEX operations based on a master-slave concept has been defined and verified on a dedicated Orbit Control Simulator. The simulator provides in-depth understanding of the constellation behavior and allows optimizing the FLEX manoeuvres.
The FLEX-S3 tandem objectives will be explained and an overview of the operations concept will be provided.

With the advent of increased CubeSat and SmallSat constellation deployments by both Government and Commercial entities, there is a need to assess the technology maturity and their impact on scientific research. Since 2012 the NASA Earth Science Technology Office has been running research program focused on technology validation in space, In-Space Validation of Earth Science Technologies (InVEST). This program encourages flying new technologies and new methods on CubeSat platforms. The basic premise of the program is to validate new technologies before they are implimented on CubeSat constellations or small satellites. The technology validated under this program was instrumental in architecture of future CubeSat constellations like TROPICS and TWICE. Recently three CubeSats under this program were launched from International Space Station. Two of these CubeSats have started returning very valuable data on hurricanes and storms. Some of the successful instruments like IceCube has successfully produced the first ever global atmospheric ice map at the 883-GHz band. Similarly RainCube and TEMPEST-D are producing a lot of valuable data from overlapping observations of typhoons and hurricanes.
Since we have graduated from amateur experiments in the universities to building highly capable CubeSats, it’s time to look into possible science applications of these platforms. There is always a question of traceability and calibration of large amount of data acquired from these constellations. Some of these platforms are really compact with no space for any calibration hardware. One of the examples is the use of Landsat as transfer radiometer for optical images of earth imaging CubeSat constellations. We have to look at other options like vicarious methods and other innovative techniques to acquire scientifically meaningful data. This talk will provide an overview of methods and instruments on CubeSats being funded by NASA/ Earth Science Technology Office and efforts underway to address some of the technical issues with the future constellations.

In the last decades, we assisted to the launch of large Synthetic Aperture Radar (SAR) satellites, with masses over 1000 Kg. As much as these systems are highly performant they are also highly expensive, with costs in the order of hundreds of millions of euros. Fostered by a Dutch-led network of academic and industry partners with a broad spectrum of competences on Radar instruments and applications, the project aims at studying and promoting cost-effective concepts with miniaturized Radar satellites. The main concept design envisions the achievement of high-resolution, low-noise and multi-application imagery by a swarm of lightweight (within 100 Kg) instruments flying in a close formation (within a few kilometres distance) and capable to operate in MIMO mode. The use of N satellites in MIMO configuration would lead to an ideal N2 performance boost. Two technical factors are responsible for such benefit: 1) The simultaneous transmission from the N satellites through frequency multiplexing, that allows to achieve N-times larger range bandwidth while distributing the radiating power on the N instruments; 2) The simultaneous reception with N antennas, that opens the way to different enhanced imaging modes, i.e. ATI/GMTI, tomographic and high-resolution processing configurations. This latter in particular will be the focus of the presented work. In the high-resolution mode, a multi-static system with N receiving antennas can be thought of as an equivalent monostatic system with N-times larger PRF. This opens up a possibility to reduce the length of the SAR antenna considerably, thereby miniaturizing the radar satellites, without sacrificing swath or image quality due to ambiguities. Enhanced imaging mode performance requires small baselines (much smaller than the critical apertures) and optimal along track positioning of the antenna phase centres with respect to the pulses. The achievement of such ideal conditions is however hindered by the variations of the relative satellite positions along the orbits and by the uncertainty in their knowledge. In practice, the achievable performance is hence lower and the added value of a new satellite in the constellation will diminish with N. The work will shed light on the design of effective PRF strategies for the multi-static configuration, and highlight an optimal trade-off between number of satellites (and hence mission costs) and the image quality. Both uniform and staggered PRF sampling schemes and different signal reconstruction strategies (in the azimuth frequency and time domain) has been considered. In conclusion, it shall be remarked that although the study is specifically tailored to the S-Band concept, it will entail theoretical and practical findings of general applicability for any SAR formation flying.

Clouds are key elements of the Earth climatic system. However, lot of uncertainties remains about their evolutions and their roles in the context of climate change. Increasing the knowledge of the cloud development and interactions with water vapor and aerosols is then essential.

The French-Israeli C³IEL (Cluster for Climate and Cloud Imaging of Evolution and Lightning) is an innovative spatial mission currently under study that will provide unprecedented new insights to outstanding climate questions. This demonstration mission, mainly focused on convective clouds, aims at characterizing dynamically the clouds and their environment at a high spatial and temporal resolutions of the scales of the individual convective updrafts. The different nano-satellites of the C3IEL mission will carry visible cameras measuring at a spatial resolution of about 20 meters, near-infrared imagers measuring in and near the water vapor absorption bands, optical lightning sensors and photometers. The observational strategy will consist in 10 to 20 multi-angular measurements of a given cloudy scene during the 200 seconds of the overpass at a rate of an observation captured every 10 to 20 seconds. The observations of these space-borne sensors will simultaneously document the vertical cloud development retrieved by a stereoscopic method, the lightning activity and the distribution of water vapor at a high resolution by exploiting the multi-angle measurements for application of tomography methods.

The scientific objectives of the C³IEL mission will be introduced. Then we will discuss the nano-satellite train configuration, the observational strategy and the different sensors of the mission. Finally, we will introduce the observations and products of the C³IEL mission that will give new understanding of the redistribution of the energy and water vapor in the atmosphere, and of the relation between storm vigor and frequency of lightning activity. In addition, using synergistically with measurements of others missions like the Joint Polar Satellite System mission (JPSS) and the geostationary satellites such as the METEOSAT Third Generation (MTG) Imager (MTG-I) and the Sounder (MTG-S) missions, a first use of C³IEL products will allow to disentangle aerosol impacts and cloud updrafts and to study the life cycle of convective clouds.