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GeoCommunity GeoImaging Feature

Turning the Scientifically Possible into the Operationally Practical: RADARSAT-2 Polarimetry Applications

By Gordon C. STAPLES and John HORNSBY, RADARASAT International (2002)

RADARSAT-2, planned for a late 2003 launch, is an advanced SAR satellite. Key features of RADARSAT-2 are high resolution (3 m), polarimetric modes, enhanced ground system providing rapid satellite tasking and near-real time data processing, improved image location accuracy, and on-board solid state recorders. The focus of this paper is on the RADARSAT-2 polarimetric capability. RADARSAT-2 offers three polarimetric modes: (1) selective polarization (dual pol) providing one co-pol channel (HH or VV) and the corresponding cross-pol channel (HV); (2) high resolution (3 m) single pol channel (HH or VV); and (3) a fully polarimetric mode (quad pol) providing both amplitude and phase. The fully polarimetric mode is significant since RADARSAT-2 is the first commercial satellite to offer this mode. The commercial focus of the RADARSAT-2 mission dictates the development of operational applications, and ultimately the extraction of information from the SAR data. This paper concludes with an assessment of operational practicality of the RADARSAT-2 polarimetric modes.


RADARSAT-2, the second in a series of Canadian spaceborne Synthetic Aperture Radar (SAR) satellites, is being built by MacDonald Dettwiler, Richmond, Canada. RADARSAT-2 builds on the heritage of the RADARSAT-1 SAR satellite, which was launched in November 1995 and continues to perform extremely well (Srivastava et al, 2001). The RADARSAT-2 sensor is planned for launch in late 2003. Plans are underway to build RADARSAT-3, and operate RADARSAT-2/3 in a tandem mission. The RADARSAT-2/3 tandem mission will focus on interferometric applications, particularly for the generation of Digital Elevation Models (DEMs).

RADARSAT-2 will be a single sensor satellite, and will have on-board a C-band SAR (5.405 GHz). Of note is the fact that while both RADARSAT-2 and RADARSAT-3 will operate at the same frequency, the RADARSAT-1 SAR frequency is marginally different (5.3 GHz). The RADARSAT-2 orbit parameters will be the same as RADARSAT-1 thus allowing co-registration of RADARSAT-1 and RADARSAT-2 images. Radiometric and geometric calibration will also be implemented permitting correlation of time series data for applications such as long-term change detection (Luscombe and Thomson, 2001).

RADARSAT-2 retains the same capability as RADARSAT-1 (Luscombe, 2001), but has advanced SAR-based Earth Observation (EO) by offering enhanced features including:
  • 3 m high resolution mode
  • Selective and Polarimetry modes
  • Enhanced ground system
  • Routine left and right looking capability
  • Increased geometric accuracy
  • On-board solid state recorders

While each of the enhanced RADARSAT-2 features are deserving of individual discussion, the focus of this paper is on the polarimetry modes and what these modes will bring to the end-users in terms of better information for SAR-based applications. The next section will outline the RADARSAT-2 polarimetry modes and provide definitions as adopted by the RADARSAT-2 program. Section 3 will discuss the applicability of polarimetry for a variety of applications, and finally Section 4 discusses how polarimetry data can be exploited to meet user needs.

II. RADARSAT-2 POLARIMETRY MODES

The intent here is not to outline polarimetry theory, but to present the concepts in an intuitive manner so that those not familiar with polarimetry can understand the benefits of polarimetry and the information available in polarimetry data. Many articles are available that discuss polarimetry theory, applications, and provide excellent background information (CCRS, 2001; Ulaby and Elachi, 1990). Notwithstanding the inherent complexity of polarimetry, polarimetry in its simplest terms refers to the orientation of the radar wave relative the Earth’s surface and the phase information between polarization components.

RADARSAT-1 is horizontally polarized meaning the radar wave (the electric component of radar wave) is horizontal to the Earth’s surface. In contrast, the ERS SAR sensor was vertically polarized, implying the radar wave was vertical to the Earth’s surface. Spaceborne SAR sensors such as RADARSAT-2, ENVISAT, and the Shuttle Imaging Radar have the capability to send and receive data in both horizontal (HH) and vertical (VV) polarizations. Both the HH and VV polarization configurations are referred to as co-pol modes. A second mode, the cross- pol mode, combines horizontal send with vertical receive (HV) or vice-versa (VH). Polarization configurations are shown in Fig, 1. As a rule, the law of reciprocity applies and HV ? VH (Ulaby and Elachi, 1990). The co-pol and the cross-pol modes are amplitude data, and in the format familiar to SAR-data users (e.g. RADARSAT-1 Path Image product).

A unique feature of RADARSAT-2 is the availability of polarimetry data, meaning that both the amplitude and the phase information are available. The amplitude information is the same as discussed above, but the phase information is likely new to most SAR users and rather non-intuitive. In its simplest term, phase can be thought of as the travel time for the SAR signal: the travel time is the two-way time between the sensors and the Earth, and includes any propagation delays as a result of surface or volume scattering. It is the propagation delays and the scattering properties of the HH and VV polarization configurations that make polarimetry data so powerful. In effect, SAR interferometry exploited the phase information, not by the time delay between, for example, the HH and VV modes at the same time (RADARSAT-2 case), but the time delay between the HH and HH mode at different times (RADARSAT-1 case).

The RADARSAT-2 program has adopted conventional terms to define the polarimetry modes, which are consistent with accepted definitions. The two polarimetry terms are Selective Polarization and Polarimetry. Selective Polarization implies the availability of only amplitude data. For example, amplitude data may be HH, VV, or HV imagery. Other terms for Selective Polarization include dual, alternating, and multi-polarization. The second term, Polarimetry implies the availability of both amplitude and interchannel phase information. The amplitude information is the same as the Selective Polarization case, but adds phase information, such as the co-phase term (i.e. ?HH-VV). Other terms include quadrature polarimetry (“quad-pol”) and fully polarimetric. The RADARSAT-2 modes are shown in Table 1.



Fig. 1. RADARSAT-1, ERS, and RADARSAT-2 polarization configurations.




Table 1. RADARSAT-2 modes. Beam mode name, swath width, swath coverage, and nominal resolution.

III. RADARSAT-2 POLARIMETRY APPLICATIONS

RADARSAT-2 will offer a rich source of data for a variety of applications. As with any spaceborne sensor, there are trade-offs, which for SAR sensors are mainly spatial coverage and resolution. Typically high resolution is available at the expense of reduced spatial coverage, and increased spatial coverage at the expense of coarser resolution. These trade-offs will apply to RADARSAT-2, although mitigated by the availability of left and right looking SAR.

The availability of polarimetry data adds additional imaging choice, with trade-offs dependent on application needs. Referring to Table 1., the polarimetric trade-offs are reduced for the RADARSAT-1 Selective Polarization modes, since there is no impact on swath width or resolution as a function of polarization choice. By default, data products are one co-pol channel (HH or VV) and the cross-pol channel (HV). The only impact will be on the ground segment due to the extra channel of data which adds to the processing and delivery (if electronic) time.

Depending on application needs, a decision is required between the use of Selective Polarization or Polarimetry modes. The Polarimetry mode offers a choice of resolutions (25 m and 10 m nominal), four channels of data (HH+VV+HV+VH), and both co-pol and cross- pol phase information, subject to the caveats that as aforementioned HV ? VH, and the cross-pol phase information does not provide as much information as the co-pol phase. From an applications perspective, a key parameter of the Polarimetry mode is the 25 km swath. Therefore, the end user must decide, depending on the application requirements, whether the Selective Polarization mode or the Polarimetry mode will provide the information needs.

The utility of the RADARSAT-2 Selective Polarization (SP) mode and Polarimetry (P) mode for a number of applications is shown in Table 2. The assessment was based on application requirements and divided into three categories: Improvement (I), Limited (L), and Not Applicable (NA). Improvement implies consistently better information, Limited implies better information, and Not Applicable implies no better information. This assessment is relative to RADARSAT-1. It is clear from Table 2 that the SP mode has the widest application, with the P somewhat less. For example, it has been shown that the co-pol mode (HH) provides ice type information, and the cross-pol mode (HV) provides ice edge information (Scheuchl et al, 2001).

The use of the Selective Polarization mode for a given application will be better understood following the availability of ENVISAT data, with better understanding of the Polarization mode following the launch if RADARSAT-2 (ENVISAT does not have Polarimetry-like mode). It is important to point out that RADARSAT-2 can be readily configured even after launch. Subject to three main parameters, namely resolution, swath width, and noise- equivalent sigma-0, new modes can be defined to meet users needs (T. Luscombe, pers. comm.)



Table 2. Use of RADARSAT-2 polarization modes by application. Selective Polarization (SP), Polarimetry (P), Improvement (I), Limited (L), Not Applicable (NA).

IV EXPOITING POLARIMETRY TO MEET USER NEEDS

Polarimetry, while intrinsically challenging from a scientific perspective, is daunting to the end user. The commercial focus of the RADARSAT-2 mission dictates the development of operational applications, and ultimately the extraction of information from the SAR data. Polarimetry represents a steep learning curve for even the radar knowledgeable user, and perhaps a step-function for many new users.

To turn the scientifically possible into the operationally possible, a number of approaches are suggested.

* Much of the polarimetry reference material has been written from the engineering perspective, but many end users are geoscientists, so polarimetry theory, explained from the geoscience perspective is required;

* Polarimetry “tool kits” that provide sample data sets and functionality for simple analysis. Of note is the observation that through NASA’s Jet Propulsion Laboratory, a polarimetry toolkit called Sigma-0 is available. A similar toolkit, called the Polarimetry Work Station is available from the Canada Centre for Remote Sensing;

* Provide guides as the appropriate use of a given polarization mode for a given application. This will have to be done following the launch of RADARSAT-2 and data validation, but can be initiated with ENVISAT data;

* There is a need to combine good R&D directed toward the development of operational applications. This type of initiative has been fostered during the RADARSAT-1 program where R&D institutes such as the Canada Centre for Remote work with both the private and the public sector to develop applications.

REFERENCES

CCRS, Applications Potential for RADARSAT-2, J. van der Sanden and S. Ross (Eds), 117 pp., Ottawa, Canada, 2001. Luscombe, A., and A. Thomson, RADARSAT-2 Calibration: Proposed Targets and Techniques, Proceedings IGARSS’01, Australia, 2001.

Luscombe, A, RADARSAT-2 Product Specification, MacDonald Dettwiler RN-SP-50-9786, January 2001.

Srivastava, S, B. Banik, R. Hawkins,T. Lukowski, K. Murnaghan, RADARSAT-1 Image Quality, Proceedings IGARSS’01, Australia, 2001.

Scheuchl, B., Caves, R., Cumming, I., and G. Staples, Automated Sea Ice Classification using Spaceborne Polarimetric SAR Data, In Proceedings IGARSS’01, Sydney, Australia, July 2001.

Ulaby, F. and C. Elachi (Eds), Radar Polarimetry for Geoscience Applications, Artech House, 364 pp., 1990.

About the Authors

This paper was presented at International Remote Sensing of Environment, Buenos Aires, Argentina, April 2002

Gordon C. STAPLES and John HORNSBY
RADARASAT International
13800 Commerce Parkway
Richmond, B.C., CANADA
Gstaples@rsi.ca

More News & Feature Articles

Nov 15, 2002 Feb 27, 2003


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