Active Microwave – Radar and SAR Working Group
The goal of the Active Microwave – Radar and SAR Working Group is to promote the advancement of active microwave remote sensing instruments and instrument technology.
If spaceborne Synthetic Aperture Radar (SAR) sensors were categorized according to their operational flexibility, four generations could be identified. The first sensors were simple turn on/off sensors with a single mode of operation (fixed antenna beam, bandwidth, pulse repetition frequency etc.) Later, sensors were developed which could be operated in multiple modes, such as StripMap, ScanSAR or SpotLight; however the macro-based commanding was basically restricted to selecting a specific mode without the flexibility to alter individual instrument settings. Current SAR sensors -third generation- offer a greater flexibility in commanding nearly each individual parameter of the instrument. This can be understood as providing the basic building blocks to construct any operation mode in combination with any possible instrument setting. However, current sensors still do not offer a satisfactory solution to the “fundamental limitation of SAR sensors” which can be summarized as the incapability to simultaneously provide high (azimuth) resolution and wide coverage (swath), instead they offer the flexibility to choose a compromise between high resolution or wide coverage. As a result, even state-of-the-art systems can not fulfill the heterogeneous user demand on products at the required performance level.
Intensive research is on-going for a new, “fourth” generation of Smart Multi Aperture Radar Techniques (SMART). The main innovative characteristic of this forthcoming generation of SAR systems is the use of multiple elevation and/or azimuth channels combined with Digital Beam-Forming (DBF) capability. This allows for the synthesis of multiple or dynamic digital receiver beams. In general SMART operational modes produce one or more digital receiver beams, each covering an unambiguous range segment on the ground. Utilizing the Digital Beam-Forming (DBF) property of SMART sensors, these receiver beams are actually formed on the recorded echo signal without the necessity for analogue beam-forming capabilities. These beams may additionally be scanned in elevation direction to follow the echo on the ground.
In the most general sense SMART sensors allow a relaxation of the system design parameters, by increasing the degrees of freedom. Specifically, for a given geometric resolution this results in systems with wider swath, higher signal-to-noise ratio and lower ambiguity-to-signal ratio, all being key requirements on SAR systems, which enable further improvement of the performance; for example an increased swath width can be used to reduce the revisit time and by this enable frequent observations. Equipped with digital receive channels, SMART sensors do not require phase and amplitude control of the received signals, this yields an RF hardware free of transmit/receive modules and complex control and calibration units. Instead, SMART will push the development of onboard digital signal processing capable of handling multiple channels of high data rate. An additional advantage of multi-azimuth (along-track) channel SAR is, that it inherently enables Ground Moving Target Indication (GMTI) capabilities; thus it is expected SMART radar sensors will open a new era in civil spaceborne SAR GMTI provided that the necessary processing algorithms are developed.
One limitation of monostatic pulsed SAR systems which is not overcome by SMART is the maximum echo window length. This is restricted by the fact, that during the transmit instances the radar cannot simultaneously receive ground echoes. Thus for a given Pulse Repetiton Interval (PRI) the transmit duty cycle limits the duration during which ground echoes can be received. This either imposes a restriction on the maximum swath width (single swath operation mode) or causes gaps in the coverage (multi-swath mode). Reducing the duty cycle mitigates this effect; however for a fixed peak power this is at the expense of the reduced average transmit power. Recent development in semiconductor technology, specifically Gallium Nitride (GaN), allows handling significantly higher amount of peak power and be this prove to be an attractive solution to increase the echo window length without sacrificing average transmit power and by this SNR.
The above discussion motivates three interrelated issues which are taken as focus points for the activity of the Working Group on Active Microwave – Radar and SAR
- Digital Beam-Forming
- Gallium Nitride Technology,
- Spaceborne GMTI Signal Processing
Other topics in active microwave remote sensing may be pursued based on community input. We solicit suggestions and comments. The working group intends to organize special informal workshops and special sessions at IGARSS.
Microwave Radiometer Working Group
The Microwave Radiometer Working Group (MRWG) addresses issues related to passive remote sensing in the microwave, millimeter wave and sub-millimeter wave portions of the electromagnetic spectrum. This includes all aspects of this area of remote sensing technology including in particular:
- Trends in instrumentation and new instrument and calibration concepts
- Specific technology challenges
- Disruptive technologies for passive microwave observations
- Technology push, and
- Market pull from both scientific needs and commercial applications
The desire of the MRWG members is to use this website as a place to both provide information to the community on these topics as well as gathering input from the community as a whole. This includes offering members of the general remote sensing community the opportunity to highlight relevant developments. This also includes the compilation of white papers summarizing the views of the MRWG members in these areas. To that end, the MRWG has identified the following key trends and issues within the passive microwave remote sensing field:
- Radio-frequency interference (RFI) – detection and mitigation methods
- RFI is a significant issue in many microwave bands and is expected to increase in the near future. Methods are being developed to detect and remove RFI to enable interference-free remote sensing in the affected bands. These methods need to be validated and implemented – key implementation issues still exist, particularly for space-based missions.
- Digital radiometers – digital back-ends, correlators, and spectrometers
The desire for on-board digital signal processing is pushing the passive microwave field towards the use of digital radiometry. This includes digital spectrometers for the detection and removal of RFI and for sounding trace gases in the Earth and in planetary atmospheres, and digital correlators for use in the digital beam forming and in polarimetric radiometry. These applications require low mass, low power, and high-performance digital solutions that can operate in high radiation environments.
- High-frequency, mm and sub-mm technologiesApplications such as the sounding of trace gases and the remote sensing of cloud ice require observations in the 100s of GHz to THz range. For these applications, low power, low noise receiver technologies are required and a number of competing technologies are being developed.
- Radiometer calibration including: noise diodes, cold-Field-Effect Transistors (cold-FETs) and external targets.
- Remote sensing observations in general require sources of stable, accurate calibration. The current set of calibration standards on, for example, space-borne passive microwave instruments have a number of important limitations. New methods of calibrating radiometers, such as vicarious calibration via stable external targets, and new internal calibration methods such as the use of noise diodes and cold-fet calibration targets, are being developed to address these issues.
- Low power, low mass, miniaturized radiometers including MMIC/MIC (Monolithic Microwave Integrated Circuits/Microwave Integrate Circuit) technology
- Nearly all applications of passive microwave technology, and in particular space-borne applications, benefit from low power, low mass, miniaturized receivers. Aside from the obvious benefits of reduced payload accommodation requirements, miniaturization reduces thermal gradients thereby improving receiver calibration. Developments in MMIC/MIC technology have enabled significant reductions in receiver mass, power, and volume and developments in the area will further enable the use of these microwave radiometers on future remote sensing missions.
- Aperture technologies including: synthetic thinned aperture, phased arrays, large deployable apertures
- A number of current and future applications of passive microwave radiometry require new aperture technologies. These include applications such as the desire to make passive microwave observations from geosynchronous orbit and the desire to make low frequency observations from low Earth orbit, where apertures using conventional technologies to provide the required spatial resolution would be prohibitively large.
- Also included is the desire to make high resolution observations from small platforms such as UAVs (Unmanned Aerial Vehicles). The fundamental issue is the desire for small beam-widths from an aperture that can fit within a small volume. Synthetic thinned apertures, phased arrays, and large deployable apertures are all examples of technologies that address this issue.
In the future, this web page will include white papers addressing the key developments in the above areas. Additional information in these specific areas as well as any other relevant area, including specific developments from members of the general community, is welcomed and encouraged. Any input can be sent via email to the WG leads. At a later time, a discussion forum will be opened, which can also be used to communicate this information.
Active Optical and Lidar Working Group
The objective of Active Optical & Lidar working group is to address the challenges related to active optical remote sensing as well as promote the advancement of Active Optical & Lidar remote sensing instruments not only for ground but airborne and satellite applications. With a number of instruments already operational or pending launch within the coming years, many of their original technological issues have been resolved, still the long term reliability of key active components and the survival on harsh space environment requires additional efforts and investments. Some focal areas that summarizes the key topics for the working group of Active Optical & Lidar instruments are:
- Continuation of work in the domain of long-lived/ high power UV lasers and optics, especially contamination and optical damage
- Research to improve the reliability of lasers and high power optics operated in vacuum
- Space-qualification of tuneable lasers and optics to support trace gas lidars operating in the 1-5 µm region
- Space-qualification of higher efficiency lasers such fiber lasers and amplifiers. Radiation hardening is an area of particular concern
- General power scaling of space-qualified lasers with a focus on improved efficiency and thermal tolerances
- Improved high gain, low dark noise and low NEP space qualified/qualifiable array/detector at all wavelengths, and in particular in IR bandwidth
These areas are not restrictive in any way and additional topics may be pursued based on community inputs. Furthermore, the generic goal of instrument flexibility and multi-functionality is gaining momentum in this WG with the broadening of active optical applications (optical telecom, navigation etc.) In the future, this web page shall include white papers addressing the key developments in the above mentioned topics. Feel free to provide any input via email to the WG leads to include additional topics for consideration.
GNSS and Signals of Opportunity Working Group
The purpose of the GNSS and Signals of Opportunity Working Group is to cover all non-navigational uses of GNSS signals. These can be broadly divided into two distinct groups: one is the use of GNSS signals received after passing through the limb of the Earth’s atmosphere i.e. occultation, the other is the use of GNSS signals reflected off the Earth’s surface from which properties of that surface may be determined, also known as GNSS-R.
With the steady increase in sources of GNSS signals over the coming years, these two techniques are also set to increase in their use and understanding. The working group will therefore act as a forum for exchange and possible cooperation in these fields with respect to the instrument designs, processing and interpretation of results.
On the one hand occultation is already established as an Earth Observation instrument (e.g. GRAS on board Metop), on the other, GNSS-R is very much in its infancy and is likely to develop significantly in the next few years. For both, the increase in numbers of navigation systems (GPS, Galileo, Glonass, Beidou), numbers of satellites, numbers of different frequencies (including possible extension into other bands such as C-band), increase of power transmitted and increase of bandwidth, will all add to the capabilities required of future GNSS instruments.
These developments in GNSS can be of great benefit to the remote sensing community since they will not only allow far higher coverage than can currently be achieved with GPS alone, but also improved signal quality and hence enhancing the quality remotely of the remotely the technology for future instruments. For instance, for certain applications, e.g. mesoscale ocean altimetry, many high-gain antenna beams will be required of the instrument in order to be able to cover all the glistening zones of the GNSS signals. At the same time, the beam patterns must not only be agile but also well known and very stable, placing significant demands on the antenna design and the beam forming network behind it. Another point is that the raw data rate for all these signal sources is extremely high necessitating on-board processing by which very significant data rate reductions can be realized.
As the use of GNSS signals for remote sensing gains acceptance in the various scientific user communities, demands on the instrument are very likely to change as are the ways of processing and interpreting the data. The GNSS WG will therefore provide a focal point for innovation and excellence in future instrumentation.
Remote Sensing Instruments and Technologies for Small Satellites
In the past, the preferred architecture for most spaceborne Earth remote sensing missions was a single large spacecraft platform containing a sophisticated suite of instruments. Following the evolution of the computer from room-sized to pocket-sized, technology has paved the way for a similar shift in satellites.
Three distinct advantages arise from going ‘small’ to compensate for the loss in mass, power and volume. First, small satellites allow for cheap access to space. By flying as secondary payloads and utilizing excess capacity, launch costs can be reduced by an order of magnitude or more. Second, small satellites allow for rapid development and lower costs through use of common parts/frameworks. Third, small satellites allow for a more relaxed risk posture due to the significantly lower cost, and the capability to distribute the risk to multiple platforms.
This working group was created to track the various remote sensing instruments and technologies that are being developed for this quickly evolving field. Specifically the needs of various science measurements, add a distinct set of requirements to the instrumentation. A goal of the SmallSat Working Group is therefore to:
1) Address these diverse requirements and assesses how they might be met by small satellites
2) Identify the needed core technologies to enable and facilitate small satellite mission development
3) Bridge the gap between small satellite and instrumentation technologists and remote sensing mission planners
The broad membership of the SmallSat Working Group can aid in this process, merging the new experience base from the University developments, with the space agencies worldwide. Care must be taken incorporating these practices, as they may be at odds with traditional space qualification grade missions. The co-chairs invite readers who are interested in contributing to contact them for membership details.