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 fulfil 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 technologies

  Applications 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.

Lidar Working Group

To be Determined

Optical Instrument Working Group

Mission & Vision: The IFT-TC Passive Optical Instruments Working Group provides a forum for international collaboration on advanced space-based passive sensing techniques including high resolution multispectral imaging, hyperspectral imaging, and polarimetric imaging from x-ray to the far infrared wavelengths. The focus of the working group is on new technologies and concepts in sensor system design, optics, structures, filters, gratings, interferometers, focal plane arrays, electronics, processors, and radiometric calibration. Our goal is to bring together diverse ideas and capabilities and match them with new measurement needs and mission requirements, thereby improving scientific value for Earth, space, and planetary missions, enhancing the welfare of our global community.

Trends in Instrumentation: Several trends in instrument technology are evident and are offered below:

Technology Challenges: The need for improved passive optical sensing drives a broad set of instrument technology needs, ranging from optical designs to detectors, materials to components and manufacturing techniques, algorithms to models. By sharing ideas via the Passive Optical Instruments Working Group, members can help establish solutions to these technology challenges.

Market Pull: Advances in optical technologies happen when scientific/mission needs are identified and connected to the potential of each technological opportunity. This working group will help monitor market trends and facilitate linking those trends to innovative ideas in optical instrumentation.

Global Navigation Satellite System (GNSS) Working Group

The purpose of the Global Navigation Satellite System 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.