Project proposal

These are the main texts of the project proposal for NWO & NSO, without work packages and other administrative data.
Authors: Paco Lopez Dekker, Peter Hoogeboom (Delft University of Technology, Faculty of Civil Engineering and Geosciences, Geoscience and Remote Sensing department) and Rens Swart (Swartvast)
Submitted September 2017, granted December 2017. Published here August 2018.

Summary

For a summary, see the overview page.

Instruments and missions

The project will study breakthrough technologies needed to implement miniaturized radar payloads serving two types of SmallSat missions:

1. Nadir looking miniaturized radar altimeter constellation
2. Miniaturized Synthetic Aperture Radar

Aside from aiming at small systems and platforms, which opens the door to the implementation of large constellations, one of the technological focuses will be on exploring distributed (multistatic) solutions.

Objectives of the intended instruments and missions

We consider two main missions, based on previous work and studies in The Netherlands, i.e. the PanelSAR project, the PRECIP and HydroCAM altimeter/scatterometer studies, and the SAR and microwave market opportunity studies (by Swartvast and HERMESS for NSO):

1. Nadir looking miniaturized radar altimeter constellation. Among active microwave instruments, the radar altimeter is the smallest, lightest and least demanding (in terms of power consumption and data rate) radar payload. However, the coverage of a single altimeter is fundamentally constrained, which results in poor spatial and temporal sampling. There is clear user-case for constellations of (small) altimeter satellites in order to dramatically increase these spatio-temporal sampling. Such constellations would provide observing capabilities at levels that meet emerging requirements of user communities in various fields, i.e. in hydrology and in oceanography/ offshore. In addition to improving the sampling for traditional ocean wave and wind applications, constellations of small altimeters would enable application in coastal areas (which are currently dramatically undersampled) and deliver water levels of inland (fresh) water reservoirs, rivers, and so on. It is the objective of this study to clarify user requirements in relation to operational usage, to identify critical technologies and to prepare for future missions.
2. Miniaturized Synthetic Aperture Radar. Low-SWaP (Size, Weight and Power) radar system is an emerging need due to the payload weight restrictions of small spaceborne systems. In 2010 first ideas about low-SWaP SAR arose in The Netherlands which led to the PanelSAR project. Unfortunately, PanelSAR project stopped in 2015 due to bankruptcy of the main contractor. Today, the need for low-SWaP SAR systems still exists due to the increasing interest to the multistatic-distributed spaceborne mission concepts.The relevant expertise in The Netherlands makes it possible to continue pursuing this idea. Although SAR systems are certainly not the smallest active radio frequency (RF) instruments, there are possibilities for small satellites and formation flying of (distributed) instruments. Again, technological innovations enable new possibilities. It is the objective of this study to clarify user requirements in relation to operational usage, to identify critical technologies and to prepare for future missions.

In the network commonality between critical technologies will be identified and lead to prioritization of key technologies to be studied.

Background

General context

Since its beginning, the space sector has been a domain for space agencies and large corporations acting as large scale integrators (LSI), and smaller stakeholders contributing specialized subsystems. In that context, The Netherlands has been particularly successful in implementing and supplying optical remote sensing instruments for air quality and pollution monitoring, such as GOME, SCIAMACHY and OMI. A strong and homogeneous user community together with a lasting cooperation between prominent technological parties can be identified as key factor in this success.

In the field of Radio Frequency (RF) technology and instruments, where Dutch parties are world players in the areas of antenna technology, micro-electronics, and radar instrumentation, the available expertise has crystalized in any major role in the development of spaceborne microwave remote sensors. Although no single reason can be found for this, it is probably associated to the wide range of applications of RF technologies (telecommunications systems, marine and airspace safety, defense applications, etc.). The implementation of microwave payloads flown in spaceborne missions is typically lead by LSI (in Europe by Thales Alenia Space and Airbus Defence and Space).

In the last couple of decades, we are witnessing the accelerating emergence of what is often referred to as NewSpace[1], with a typical focus on small spacecraft, light weight and agile development cycles, and dramatically reduced costs. A good example of the disruptive nature of this development, and its potential implications in the field of Earth Observation, is the massive constellation of optical imaging CubeSats launched and operated by Planet Labs. In the field of microwave remote sensing, UrtheCast and Capella Space are developing SmallSats based Synthetic Aperture Radar systems.

What we expect for the midterm future of (microwave based) Earth Observation is the consolidation of two types of missions and instruments:

1. Heavy duty missions in the spirit of the Sentinel program, with emphasis on systematic global observations. This sector will remain in hands of LSIs.
2. Commercially driven light-weight missions emphasizing the delivery of high resolution data and/or very short revisit times over specific areas of interest.

This second type provides a new level-field, with lower technological and financial entry barriers, for the development of miniaturized microwave Earth Observation systems and their exploitation. Thus, the overarching purpose of the proposed activity is to bundle the radar-related know-how available in The Netherlands with the goal of becoming a leading player in SmallSat radar-based Earth Observation.

Motivation for active microwave systems (radar)

An important factor in microwave remote sensing is the all-weather/day and night observation capability, which greatly enhances the observation opportunities. This includes the ability to observe through clouds. Microwave remote sensing system include passive (radiometers) and active ones (radar altimeters, Synthetic Aperture Radars, precipitation radars, scatterometers, etc). In the proposed project, we will address only active systems, both attending to the available expertise and to the exploitation potential.

Radar systems have full control over the emitted waves (in frequency and polarization), which allow to make optimal choices for applications. E.g., depending on frequency the transmittance and reflection of media to be penetrated or to be observed will change. The final choice for an optimized frequency however at the same time may limit the application in other fields of interest. This leads to instruments for specific, limited applications, like observation of volumes or surfaces; observation of snow, ice, rain, ocean, water, vegetation, etc.

Currently, two classes of instruments are recognized, imaging and non-imaging. SAR (Synthetic Aperture Radar) is the workhorse for satellite RF imaging. Experience in The Netherlands with this technology goes back to the eighties when the first airborne SAR instruments were developed here. Non-imaging instruments, like radar altimeters and (wind and precipitation) scatterometers are even longer existing in ground-based and airborne applications and have been operationally used in satellites for several decades now. Also, these types of instruments have been widely investigated and used in The Netherlands, both in ground-based and airborne applications.

SAR is not only used as an imager for classification and land use studies, but also for change detection. Especially the development of interferometry as a very precise and quantitative measurement technique for height and height variation of earth surface has augmented the commercial interest and use of SAR. Frequent monitoring of man-made infrastructures of all kinds are very important nowadays. Timely and frequent observation at the frequency and resolution of choice are key factors in the commercial success, also because the detection algorithm relies on tens of images to cope with the underdetermined problem. In addition, for commercial viability, there is a need for low cost observation, which in the current timeframe points to constellations, small satellites, miniaturized instruments, dedicated to a limited set of applications or even just a single application.

Altimeters are used to measure wave height and wind over oceans and for precise absolute heights with respect to the geoid, resulting in information on sea-level rise, ocean currents, eddies, and the El Niño effect. Innovative systems which are currently under development are expected to measure also water levels over inland waterbodies. The challenge is here to designate the waterbody from surrounding uprising landmasses which are at shorter distances and have far higher backscatter. The use of higher frequencies, interferometry and improved waveforms will have to defeat this problem. Continuous inventory of inland fresh water is important to hydrology, for instance for irrigation (water shortage is a threat to increasing demand for food production), flood early warning, drinking water resources, for renewable energy (hydropower) and for logistics (shipping). SWOT (to be launched in 2020) is an example of a new generation altimeter that will measure inland water bodies. The system is very expensive and will have a limited revisit capability. It would be worthwhile to extend its capabilities with a constellation of lower cost altimeters (with less accuracy but higher update rate).

Heritage

PanelSAR was a project for the realization of a small, but capable X-band (3 cm wavelength) satellite SAR for infrastructure monitoring and change detection. This system was to make use of FMICW (Frequency Modulated Interrupted Continuous Wave) technique to optimize the efficiency of the RF instrument. With a 3×1 meter antenna size it approached the lower limit of practical SAR antenna dimensions. The radar was designed with electronic beam steering over limited angular ranges, enough to support the mission requirements. The resolution was in the order of 2-4 meter. The instrument was designed for the repeat pass interferometry technique, to support applications in subsidence, e.g. in infrastructure change monitoring. The design had some specific innovations to reduce development and implementation costs, yet make it scalable (1×1 meter antenna panels that could be combined in sets of 3, 4 or more) and therefore versatile towards applications

The project died with the bankruptcy of SSBV, the main contractor. However, among several parties in the Netherlands there is interest in continuation of small satellite SAR activity, either under Dutch primeship or by teaming with other international smallsat SAR initiatives such as ICEYE from Finland or Cappella from the US. There is confidence that it could be commercially important. Investigation of some critical technologies, which could be used for the existing or for new designs would be relevant. Also, such research could be important in relation to international projects, e.g. through ESA. A further driver is the wide usage of SAR imagery, making its business case possibly the most viable.

HydroCAM (Hydrological Cubesat Altimeter Mission) and PRECIP (a PRECIPitation monitoring constellation) are both TU Delft design studies, in part carried out with students, to demonstrate capabilities of innovative miniaturized FMCW radars with a new generation of non-linear waveforms to overcome certain performance issues of earlier generation radars. The systems have not been designed in detail, nor has any breadboarding taken place. However, in student studies sensor-platform integration, orbit optimization and user aspects have been addressed.

The recent NSO-funded study Markstudie toepassingen microgolf-satellietinstrumentenresulted in an advice to the Instrument Cluster to investigate interferometric altimeter technology on a small satellite constellation. Furthermore, the low reliability of continuity of existing SAR missions was identified as an opportunity for SAR mission development, given the high demand for SAR imagery in terms of coverage and frequency. This fits well with the TU Delft studies and is now accepted as a starting point for the NL-RIA project proposal.

General objectives

As already stated, the proposed activity has two classes of objectives: the first relates to establishing a long-lasting framework for collaboration in the realm of microwave instruments and missions, while the second class relates to the specific instrument and mission concepts that will be investigated in the proposed activity. As a network or community cannot exist without actual and relevant content, the second class of activities is required to get the network going. Thus, this proposal seeks establishing the NL-RIA and bootstrapping its activities with a first project.

The network related objectives are:

• Establishing a community incorporating all Dutch expertise on microwave space instruments and missions, platforms and applications, that will last beyond the scope of the proposed study.
• Defining a common roadmap towards the development of Dutch-lead SmallSat radar EO systems and their exploitation.

Project specific objectives:

• Evaluate and prioritize potential instrument concepts and associated mission scenarios in terms of their user (and business) cases, their technological feasibility, and the relative advantages associated to Dutch technology.
• Derive mission and system requirements based on user needs.
• Identify the main technological bottlenecks (e.g. deployable antennas), investigate possible technical solutions and for each, propose a technological development roadmap.
• For the selected concepts, perform a phase-0 feasibility study, including and end-to-end performance analysis both at instrument or system level and at final product level to verify compliance with the user needs.

Project overview

The project will work towards one or more future small-satellite based radar missions lead by, or at least with critical contributions from, Dutch industry, both from the upstream and/or from the downstream side.

The first stage of the project will define several mission/instrument scenarios taking into consideration both the instrument technologies and the exploitation (and commercialization) expertise present in The Netherlands. Although the current proposal already focuses on radar altimetry (with optionally precipitation monitoring capabilities) and SAR, a wide range of mission concepts can be brought under these two categories.

For the mission scenarios considered, the following will be derived or identified:

• A use-case or exploitation scenario.
• A set of user requirements.
• Critical technologies with a preliminary assessment of Dutch capacities in those critical areas.

Based on these three inputs, one or two mission concepts will be selected for further study. In this second stage of the project, product level performance models or key performance indicators (KPIs) will be derived by the user/exploitation community. For example, KPIs may include measurement accuracy, spatial resolution, temporal sampling, spatial coverage, data availability, etc.

Simultaneously, several studies will be done to address critical technologies, with an emphasis on selecting the most promising technical solution. Critical technologies planned to be addressed are:

• Overall design with an emphasis on the miniaturization of subsystems and the generation of RF power.
• The radar antenna, which needs to be light and compactly stowed for launch in order to be compatible with a small satellite, but needs to be provide an aperture of a few square meter.
• On-board processing technologies.
• And platform solutions and associated performances.

Technology and performance models or KPIs will be brought together by the development of an End-to-end performance model, which end purpose is to evaluate the product-level performance of the proposed mission, considering the expected performances of the different subsystems, and comparing it to the user requirements.

The last stage of the project will involve establishing and documenting technology and exploitation roadmaps to accompany a consolidated mission proposal and instrument concept.

User needs

As we have pre-selected two types of small-radar missions, in the following lines we address the user needs associated to radar altimetry and to SAR separately. As many well-known applications are well served by current missions, emphasis in this section is on user requirements for applications where it seems the technology proposed in this proposal offers best opportunities for scientific, societal or business purposes.

Altimetry

The following applications of data acquired with altimeters can be identified.

• Ocean wind speed, wind direction and wind profile – Although altimetry can be used to derive wind speed, these applications are typically based on scatterometer data. KNMI is a center of expertise.
• Significant wave height – From the backscatter pattern received from the altimeter signal, the significant wave height can be determined. Ground track distance is very large, while a high revisit rate is demanded. The current operational missions are optimally configured for this and a reliable service delivery for the oceans seems to be guaranteed. However, for small seas and near coasts, a demand for improvements exists, analogous to what is described under the following application.
• Tide heights and tidal currents – From the absolute sea height as determined with radar altimetry, tide heights and spectra can be derived, as well as tidal currents. These are very important parameters for shipping, both for planning, economy and safety. For safety of offshore installments, these parameters are important as well. HERMESS is an important service provider of these data, using modelling and detailed hydrographic data. Severe limitations of microwave instruments near the coast also hampers this application:
  • Land causes a much higher backscatter than water. As a result, a microwave instrument in general cannot observe water backscatter immediately near the coast, as saturation occurs. Therefore, the above parameters can in general not be provided, while there is a serious user demand for them. This will be investigated in this project.
  • As near coasts a rather high spatial sampling is required, the usual altimeter data does not suffice, as the ground tracks are several hundred kilometers apart. As is demonstrated by the current altimeter missions, a trade-off must be made between a dense spatial sampling at the cost of revisit frequency, versus a high (exact) revisit rate with a coarse spatial sampling. This can be overcome by employing multiple identical satellites.
  • Near coasts also the very low spatial resolution of an altimeter is a limitation with respect to used needs. Spatial resolution could be improved by using a kind of aperture synthesis in the flying direction and by interferometry in the across-track direction. Interferometry can be performed by utilizing a separation between two antennae on the same platform, although this is not straightforward on the foreseen smallsats, or by combining the signal of two smallsats flying in formation.
• Land-water-boundary for modelling – For several applications, the determination of the boundary between land and water is of much use. For applying a natural boundary condition in models to determine sea currents (as described above), mapping the land-water-boundary is of help. This application suffers from the effects described above.
• Mapping flooding and water surface extent – An application of determining the land-water-boundary with large societal importance is mapping flooding and the extent of water surfaces in general. The same difficulties arise as mentioned above.
• Mapping of inland water bodies – For water management, in particular in less developed countries (in the latter much waterbodies are measured using terrestrial techniques), determining the extent of inland water bodies and their height is important. Applications are water discharge, flooding and draught mapping and modelling, but also energy, as hydropower is a commercial market influenced directly by the water content of reservoirs. Hydrological models will improve. This will benefit hydropower companies, but also flood early warning systems can be more accurate. As stated before, measuring water levels of non-oceanic water bodies with altimetry is a challenge due to saturation and limited spatial coverage, spatial resolution and temporal sampling. With ‘classic’ altimetry only very larges lakes can be measured. Some existing instruments utilize an open loop and a close loop mode, using a global height model, to cope with the saturation and gain problem. This and several other techniques will be investigated.
• Flood modelling – A precise determination of water levels is used to model flooding. This is especially useful in data-scarce regions, where gauging networks are absent or not dense enough to provide the required accuracy.
• Storm surge and hurricane monitoring and measurement – This application requires a precise determination of the water level, with a fairly high resolution and low ground track separation, but in particular with a dense temporal sampling.
• Oceanography: sea height, ocean circulation, eddies and the El Niño effect – Via absolute sea height measurements, based on a very accurate determination of the satellite orbit in which the TU Delft plays a leading role, ocean circulation can be measured. Also eddies and the El Niño effect can be monitored. In general, a high resolution nor a dense coverage are required. However, as eddies are important not only for oceanography but also for shipping and offshore, for their precise mapping, a better coverage in time and space is expected to be advantageous.
• Precipitation and water vapour – New wave modulation technology combines high sensitivity with large dynamic range, leading to an opportunity to combine altimetry and scatterometry for precipitation measurements in one instrument.

Several other applications with minor application of altimetry exist. Some of them are mainly served by using other microwave sensing techniques, such as SAR.

SAR

SAR imagery sees a wide range of applications. Many of them have been developed to established services and several SAR satellites, utilizing different wavelengths, are completely and reliably operational. The advent of the Sentinel-1 mission, e.g., guarantees the delivery of C-band SAR imagery for the decade to come. However, there is a need for more frequent SAR image acquisition and/or better coverage to serve specific user needs. Multiple platform, small and rather low power and cost SAR systems could make a business case for these needs.

• Change detection – An application typically in need for a high update frequency is change detection. A high update frequency can be achieved by a swarm of low-SWaP SAR satellites. Orbital mechanics restrict the coverage and repetition frequency of satellites, making a multiple of satellites the way to go. Applications depend on wavelength, resolution and dynamic range. Many of the specific applications below are in fact change detection applications.
• Infrastructure monitoring – Infrastructure comprises of e.g. roads, rail tracks and pipelines, but also dikes and dams. Oil and gas pipelines in The Netherlands, e.g., are monitored visually from a helicopter every week, which is very expensive. Depending on the type of infrastructure, a very high repetition frequency is demanded and coverage is very specific as opposed to wide or even global.
• Land-water-boundary for modelling – For several applications, the determination of the boundary between land and water is of much use: see above at altimetry.
• Mapping flooding and water surface extent – see above at altimetry.
• Interferometric change detection – classic change detection relies on the amplitude of the SAR imagery. Speckle or a low dynamic range can hamper detection. By using interferometry the whole complex signal (amplitude, phase) can be exploited for detection of changes.
• Soil moisture – Like other microwave sensing techniques as scatterometry and passive radiometry, SAR is used to measure soil moisture. It is not particularly suitable but has the advantage of a high resolution compared to non-imaging techniques. There is a wide demand for soil moisture measurements, as several other applications are derived from it. A reliable service delivery, a high resolution and a frequent coverage are user requirements for these.
• Precision farming – Because of its promising ability of all weather and cloud-independent sensing, SAR imagery is investigated to enhance algorithms for precision farming. However, as SAR imagery has a relatively low resolution, at least for agricultural application in the European countries, and consists of only one band and exhibits the speckle fenomenon, application is far from straightforward. Low-cost high-resolution dedicated (Low SWaP) SAR missions with a high revisit frequency will accelerate and facilitate research and application development.
• Biomass determination, crop monitoring, yield forecasting and forest and deforestation monitoring – This class of applications sees a high demand. In general, as with precision farming, Low SwaP missions serve the need for cost-effective, high resolution and high-revisit imagery, but in general these applications ask for a longer wavelength.
• Parcel mapping – Mapping and updating parcels profits from high resolution, high revisit imagery as low SWaP missions could provide.
• Subsidy control – Idem.
• Oil spill detection – Idem, although operational services exist.
• Motion measurements – Interferometric SAR (InSAR) is an extremely powerful high-precision motion measurement technique, serving needs in infrastructure, building, construction and mining monitoring. Applications also exist in land (slide), dam and dike monitoring and early warning systems. For this type of monitoring, a high resolution and high revisit rate are more important than a high dynamic range, making low SWaP SAR missions very profitable.The persistant scatterer InSAR technique relies on tens of images to cope with the underdetermined problem, making clear the need for cost-effective imagery with a very high revisit rate. Furthermore, the number of natural measuring points increases heavily with shorter wavelengths, making small platforms advantageous.
• Ice coverage and thickness – Several microwave sensing techniques can be applied to detect and map sea ice extent and to measure ice thickness: mainly SAR is used, but also altimetry, passive radiometry and scatterometry. Services are operational for shipping and offshore safety. Ice coverage is also an important climate parameter, as is ice thickness. Very short wavelengths have special applications in ice measurements because of their very superfluous penetration. Smallsats are particularly suitable for short wavelength microwave instrumentation.
• Positioning; weather; atmosphere – Ionospheric signal delay is important for GNSS corrections and atmosphere studies.
• Defence and security – For security-oriented missions in, e.g., reconnaissance, goal detection, mine detection and camouflage recognition, timeliness of acquiring the needed information is an important parameter. Although a global coverage is a must, the area of interest during an incident is limited, so that tasking the satellite to maximize revisit of that area is a must-have. The ground resolution needs to be high enough to detect and classify objects of military interest. This seems an opportunity for Low SWaP missions, with multiple satellites in orbit.

Mission & instrument outline

The project will focus on breakthrough technologies needed for the implementation of miniaturized radar payload. The two types of mission considered have different requirements, with SAR systems being obviously significantly more demanding in most aspects. However, the two types of systems have significant commonalities at several levels:

• They share a general architecture, as illustrated in the block diagram in Figure 1, and could easily share many subsystems.
• The technological challenges to build a miniaturized system, and in most cases the solutions, are also common. For example, in both cases deployable antenna solution will be required.
• In both cases, we aim at investigating multistatic solutions, with relative positioning and system synchronization as main challenges.

Altimeter with precipitation observation capabilities

The main limitation of nadir looking instruments, such as radar altimeters, is that their spatial coverage (and therefore also temporal sampling) is fundamentally limited to the near-specular region around the sub-satellite point. Miniaturization of these type of systems would overcome this limitation by allowing the dense and scalable constellations. Going one fundamental step beyond mere miniaturization, a swarm of N multistatic altimeters, where each system would receive the echoes of the signals transmitted by all other, would not only provide the N sub-satellite points, but potentially a total of  N (N+1) / 2 specular points. This could represent an enormous gain in coverage, although it requires a high degree of beam agility and increases the on-board processing demands.

Altimeters and atmospheric sounding scatterometers have always been considered as different types of  instruments. Altimeters exploit strong surface reflections to measure distance to and roughness (mainly wave height) at the surface, whereas atmospheric scatterometers measure the weak reflections from precipitation and clouds. Both are nadir looking instruments. Since the ranges (distances) of the areas of interest do not overlap in these two applications, one could think in principle of combined functionality. With previous generations of technology however this was not possible, mainly because the strong reflection of the surface interferes with the weaker reflections of the atmosphere. Atmospheric measurements were therefore performed with ultra-short pulse-length, high peak power radars, that could separate the two reflections. Such systems are costly, complex and heavy, quite different from the technology required for altimeters.

Newer digital technology offers the possibility to use non-linear FMCW (Frequency Modulated Continuous Wave) waveforms that combine high sensitivity with large dynamic range for good separation of echoes. The use of CW signals lowers required the peak power, which leads to simpler hardware. The study will investigate the possibility of combining altimetric and scatterometric capabilities in a single system. FMCW concepts work particularly well in a multistatic configuration (in monostatic systems the transmit signal typically needs to be interrupted in order to allow reception of the radar echoes). Furthermore, a bistatic geometry may help separating the surface from the volume return.

Initial back-of-the-envelope calculations performed for the HydroCAM and PRECIP studies of TU Delft indicate compatibility between the system requirements for these two applications. Further analysis and design is required, e.g. on frequency selection, resolution, on-board processing, etc. It is obvious that such a combined instrument usage in a constellation of satellites with abundant coverage in space and time, is attractive and efficient for these two different applications. In NL-RIA the fallback mode in case a combined instrument does not seem feasible is to go for altimetry only. The less complex altimeter could also be a safer starting point for a longer lasting program, with milestones that designate extension, increased complexity and functionality of a constellation.

Small SAR

The main reason for a SAR system to be large, are the antenna dimensions, which depend on the requirement to observe a reasonably large swath without ambiguities at a user defined resolution and image quality. In addition, reducing antenna size directly leads to an increase of the required transmit power, which will lead to increased size and weight of other parts of the satellite SAR system. As already explained PanelSAR was a project that tried to optimize these aspects for a system primarily aimed at infrastructure monitoring. The antenna in PanelSAR was electronically steered, which has clear advantages but also a drawback on power, weight and complexity.

In NL-RIA we intend to continue investigation of small SAR solutions, with new upcoming applications in mind. Not necessarily focusing on active array antenna’s, but also taking other techniques like reflectors and reflect arrays into account. When not or very limited beam steering is required, a passive array antenna with sparse filling of the array aperture has also been proposed for SAR applications. The available expertise in the NL-RIA network can be used to study critical technologies for radar systems. Such studies will reduce the risk of possible future project plans, not only in SAR but also in other systems, since many of the technologies required in radar are common to various instruments.

Small SAR systems seem ideal to be combined into multistatic formation-flying systems. The potential of single-pass interferometry has been showcased by the TanDEM-X. Generalized multistatic SAR systems allowing single-pass tomography, or the exploitation of bistatic scattering mechanisms, should be reasonably expected to become the next major breakthrough in radar remote sensing.

Common building blocks/tech-challenges

Table 1 provides a list summarizing the critical components or technologies that need to be addressed in the proposed study. Technology has to be interpreted in a broad sense here, not only hardware, but also software, signal processing, measurement principles. In NL-RIA we intend to identify these commonalities and make them priorities in the proposed research efforts. An initial, short list which needs extension from the network members follows. This list includes the work topics that are proposed in the project activities and financial sections of this proposal:

Component	Technology	Notes
1-3 meter diameter foldable reflector antenna	Deployable parabolic reflector or reflect-array antenna structures	All instruments, but SAR requires a larger antenna
Platform	Space engineering	Focus on platform requirements and performance (platform technology taken as input)
High efficiency RF power amplifiers	Radar & Micro electronics	All instruments, higher power required for SAR system
Miniaturised radar chips to enable single chip or reduced component count radars	Radar & Micro electronics	All instruments
Multistatic radar aspects	Synchronization aspects, digital beamforming techniques
On board processing hardware and algorithms	Software, signal processingUse of pilot data, e.g. from airborne systems	Altimeters, precipitation radar
Nonlinear FMCW waveform handling and capabilities	Software, signal processing	Altimeters, scatterometers and possibly SAR

Table 1  Critical components and technologies to be addressed by NL-RIA.

Proposed activities

The project is organized in four thematic pillars, as illustrated in Figure 2:

1. Data exploitation: this provides a user-driven exploitation scenario.
2. User requirements: the goal of this task is to collect and justify the user requirements and models to verify that these are met by providing product-level performance models and preliminary experimental validation using pilot data.
3. Mission Scenarios and Analysis: this is the backbone of the project, in which mission scenarios with their corresponding system concepts will be defined and analyzed, establishing the link between user requirements and exploitation scenarios and the required technological developments.
4. Technology assessment: after assessing the technological positioning of Dutch companies or research institutions, possible technical solutions or approaches to implement the most critical sub-system or components will be evaluated. This is one of the central tasks in the proposed study.

Further details of each task are provided in Section 10 of the project proposal document.

Relevance

Improving the positioning of Dutch industry with regard to microwave instrumentation for future spaceborne missions is an explicit priority of NSO, as stated in the call.

Dutch companies and research institutions have leading expertise in various key technologies that are key in the implementation of spaceborne microwave instruments and missions, as well as first-in-class expertise in the exploitation and commercialization of derived products. However, this expertise is not reflected in major contributions to current spaceborne microwave instrument.

We are currently witnessing the rapid emergence of new-space companies aiming at disrupting the space industry by combining the use of small-satellites technologies (which dramatically lower the costs) with lean and agile development strategies. This will open the Earth Observation market, up to now in hands of space agencies and large corporations, to much smaller parties. This new scenario provides a window of opportunity to establish a Dutch presence in this emerging market.

In the end, all space instruments and the data and information derived from it, need to serve societal needs. By doing so, their relevance is self-evident. In this proposal, user needs play an important role in the selection of parameters of the instruments and platforms. Because this project deals with rather low maturity technology and applications (TRL), this is organised by closely cooperating with application developers.

Cooperation

One of the main goals of the project is to build up a knowledge network, fostering active collaboration between partners. In addition, cooperation with external partners will be sought by:

• Inviting external parties to the regular NL-RIA meetings. In particular, three of these meetings are planned as open workshops, where external partners will be invited to attend and also present developments relevant to small radar missions.
• Coordinate efforts within the network to participate in consortia to bid to ESA tenders, or participate in European (e.g. Horizon 2020) projects relevant to objectives of the network. This would have the double function of providing additional funding to achieve the objectives of the network, while requiring collaboration with partners across Europe.