## 2 Historical Data
### Synthetic Aperture Radar  
Synthetic Aperture Radar (SAR) is a method of active microwave imaging that uses the echo returns from many radar pulses to create a single image. Because it is an active microwave system, the image shows radar backscatter rather than optical reflectance or emitted radiation; a simplistic way to think of the image is it being related to surface roughness. SAR technology has many research and monitoring applications in the polar regions (e.g., iceberg tracking, glacier and ice sheet velocity measurements, sea ice mapping, lake ice mapping, _etc._), and especially valuable is the capability to image through cloud cover and during long periods of darkness during polar winters. As such, polar regions have been historically well served by SAR satellites. There is a history reaching back multiple decades of synthetic aperture radar (SAR) data being collected by the European Space Agency (ESA), as well as subsequent Canadian and Japanese launches (see Figure 2). Early efforts were funded and operated by government funding, directly or indirectly through space agencies. SAR acquisition plans were dictated primarily by science programs, national monitoring requirements and direct commercial/non-commercial requirements. This often led to patchy and inconsistent coverage. Details are available through data archive search tools, which (as referenced below) are often mission-specific. The earliest orbital SAR data ever gathered are from the Seasat mission. Operational for just over 100 days in 1978 before a catastrophic failure, Seasat's SAR collected L-band imagery with 25-m spatial resolution and a \(\sim\)100-km swath width. Although coverage was limited in spatial extent by available ground stations, Seasat SAR imagery nevertheless has many applications in Arctic North America, Arctic Europe and adjacent seas. Full details and data are available from the Alaska Satellite Facility ([[https://www.asf.alaska.edu/seasat/](https://www.asf.alaska.edu/seasat/)]([https://www.asf.alaska.edu/seasat/](https://www.asf.alaska.edu/seasat/))) [44]. There has been a historical trend towards an increasing number of SAR imaging modes. Early satellites had only one imaging mode, _i.e._, fixed extent, resolution and polarization. Historical SAR developments largely focused on the development of C-band sensors. The experimental SIR-C/X-SAR instruments delivered C-, L- and X-band, multipolarization data and was flown on the space shuttle for two missions in 1994; more information on image products is available at ([[http://edc2.usgs.gov/sir-c/sir-c.php](http://edc2.usgs.gov/sir-c/sir-c.php)]([http://edc2.usgs.gov/sir-c/sir-c.php](http://edc2.usgs.gov/sir-c/sir-c.php))). More recent satellites have a larger number of imaging modes, ranging from wide-area lower resolution modes (used, for example, for operational sea ice monitoring) to smaller-area higher resolution modes (used for targeted acquisitions); these satellites have been in the L-band and, more recently, X-band. Some sensors also have modes focused on interferometric methods, for example for measuring glacier flow velocity and other change detection studies. In addition, there is a parallel trend away from single polarization data, to the collection of dual-polarization and then full quad-polarization data. This has facilitated the development of SAR polarimetry applications. Figure 2: Historical, ongoing and future spaceborne SAR missions with some freely available data (based on input from the International Polar Year Space Task Group). More recently, SAR satellites have been operated by commercial companies, which will license imagery to pay for the instrument itself, launch and operation. This has led to a perception of high costs for accessing commercial SAR acquisitions, although there are many communities who still have access to these data freely or at low cost (e.g., Alaska Satellite Facility, [[https://www.asf.alaska.edu/](https://www.asf.alaska.edu/)]([https://www.asf.alaska.edu/](https://www.asf.alaska.edu/))).