## 2 Historical Data
### Active Microwave Scatterometer  
Using the same wavelengths as passive microwave and synthetic aperture radar instruments, active scatterometry sends out microwave pulses and measures the returned backscatter at multiple angles. As with other microwave sensors, the returned signal depends on two properties: surface roughness and electrical properties. Originally, scatterometers were designed to measure winds over the open ocean, as derived from surface roughness. However, because scatterometer signals can penetrate some surfaces, they give bulk, as well as surficial information, which is a function of land/sea ice cover. Therefore, they can also be used for land and ice studies, as well.  
Scatterometers have relatively low spatial resolution (\(\sim\)25 km to 50 km), but they make up for this with frequent, rapid global coverage in all light and weather conditions. Scatterometers have a range of polar applications. Examples include surface wind field measurements (e.g., [59]), surface melt dynamics (e.g., [60, 61]), seasonal melt-freeze transition (e.g., [62]), Greenland ice facies characterization (e.g., [63]) and multi-year sea ice classification (e.g., [64]).  
The first satellite scatterometer, SASS (Seasat-A Satellite Scatterometer System), was launched in 1978 aboard the ill-fated Seasat. Since that baseline, further satellites from Europe, the USA, Japan and India have continued the record, including the Advanced Microwave Instrument (AMI) on ERS-1 and ERS-2, the NASA Scatterometer (NSCAT) on ADEOS-I/Midori, SeaWinds on QuikSCAT and ADEOS-II/Midori 2, the Advanced Scatterometer (ASCAT) on MetOp-A and MetOp-B and the Oceansat-2 Scatterometer (OSCAT) on Oceansat-2 (see Figure 4) [65]. Scatterometer sensors vary in the frequency used (C-band or Ku-band), spatial resolution (\(\sim\)25 km to 50 km), antenna azimuth orientations, polarizations, beam resolution, swath width, incidence angle, orbit and coverage. Further scatterometer comparisons are available from NSIDC ([[http://nsidc.org/data/docs/daac/scatterometer_instrument.gd.html](http://nsidc.org/data/docs/daac/scatterometer_instrument.gd.html)]([http://nsidc.org/data/docs/daac/scatterometer_instrument.gd.html](http://nsidc.org/data/docs/daac/scatterometer_instrument.gd.html))) and the NASA Scatterometer Climate Record Pathfinder (SCP) at Brigham Young University ([[http://www.scp.byu.edu/data.html](http://www.scp.byu.edu/data.html)]([http://www.scp.byu.edu/data.html](http://www.scp.byu.edu/data.html))).  
Figure 4: Historical and ongoing satellite active microwave scatterometer sensors (and platforms) with freely available data, including the frequency band for each sensor. SASS, Seasat-A Satellite Scatterometer System; NSCAT, NASA Scatterometer; ASCAT, Advanced Scatterometer; OSCAT, Oceansat-2 Scatterometer.  
The SCP develops and distributes a variety of scatterometer data products (including backscatter and brightness temperature) and tools designed to support climate studies. SCP typically provides data as enhanced resolution products, improving the \(\sim\)50 km raw data to a scale of \(\sim\)2.5 km to 5 km in some cases. The NASA Physical Oceanography Distributed Active Archive Center (PODAAC, [[http://podaac.jpl.nasa.gov/datasetlist?ids=Measurement&values=Sea](http://podaac.jpl.nasa.gov/datasetlist?ids=Measurement&values=Sea)]([http://podaac.jpl.nasa.gov/datasetlist?ids=Measurement&values=Sea](http://podaac.jpl.nasa.gov/datasetlist?ids=Measurement&values=Sea)) Ice) also hosts many overlapping and similar active microwave scatterometer sea ice products. In addition, derived products (e.g., iceberg tracking database, sea ice motion and sea ice age) are included in Section 5 (see Table 2 and [[http://www.scp.byu.edu/derived.html](http://www.scp.byu.edu/derived.html)]([http://www.scp.byu.edu/derived.html](http://www.scp.byu.edu/derived.html))).