## 2 Historical Data
### Passive Microwave  
Passive microwave data, as the name implies, involve the detection and measurement of thermal radiation in the microwave region of the electromagnetic spectrum (typically 5-90 GHz) [45]. The fundamental variable that is measured is the brightness temperature. The absorption and emission of radiation by the Earth's atmosphere can be significant, even dominant, in the microwave spectrum, and some instruments are optimized for measuring properties of the atmosphere itself. Others are optimized for measurement of the Earth's surface, and these are the focus of this section.  
The brightness temperature [46] is simply the product of the absolute temperature of the emitting material and a dimensionless property called its emissivity, which can take values between zero and one. Most passive microwave radiometers are multispectral instruments, usually with separate channels for different polarization states. These multivariate measurements allow the possibility of deriving a number of different geophysical variables from the data, but note that only some channels are used for surface imaging, while others are used for atmospheric sounding. One of the main applications of passive microwave data is to measure sea and land surface temperatures [47, 48]. However, they also have other important applications that are particularly relevant to the cryosphere. These include the measurement of sea ice extent and concentration [49] and various parametersdescribing snow cover, both on land [50] and on sea ice [51,52]. A major advantage of passive microwave data is the fact that they can be collected at night and (except at the highest frequencies) through cloud. A major disadvantage is that the spatial resolution of the data (5 km to 50 km) is poor compared with visible-near infrared systems or SAR.  
Although terrestrial passive microwave radiometry has been collected from space since the late 1960s (and earlier from Venus, since the Mariner-2 Venus fly-by mission carried a microwave radiometer [53]), the first instruments to provide readily available data that are usefully compatible with those collected today were carried on Nimbus-5, launched in 1972 (see Figure 3). The ESMR (Electrically-Scanned Microwave Radiometer), which collected data from 1972 to 1976, operated at a single frequency of 19.35 GHz, but electrical scanning of the beam gave it an exceptionally wide swath of around 3000 km. The NEMS (Nimbus-E Microwave Spectrometer) was a multichannel instrument, but it viewed only at nadir, with a 185-km footprint, and operated from 1972 to 1973. A longer-lasting ESMR instrument (1975-1983) was carried on-board Nimbus-6, this time operating at 37 GHz (less useful for surface observations). Since 1978, with the launch of Nimbus 7 and the short-lived Seasat, both of which carried the SMMR (Scanning Multichannel Microwave Radiometer) instrument, there has been continuous capability to provide spaceborne multichannel microwave imagery of the Earth's surface. SMMR was a 10-channel instrument, operating at frequencies between 6.6 and 37 GHz and achieving a spatial resolution of around 150 km to 30 km, respectively. Gridded data products, including sea ice properties [54, 55, 56, 57] and snow water equivalent [58], are available for the polar regions (and in some cases, the whole world) from NSIDC ([[http://nsidc.org/data/seaice/pm.html](http://nsidc.org/data/seaice/pm.html)]([http://nsidc.org/data/seaice/pm.html](http://nsidc.org/data/seaice/pm.html))).  
Figure 3: Historical and ongoing satellite passive microwave missions (and sensor names) with some freely available data, including the number of surface-imaging channels available for each sensor. NEMS, Nimbus-E Microwave Spectrometer; ESMR, Electrically-Scanned Microwave Radiometer; SMMR, Scanning Multichannel Microwave Radiometer; AMSR-E, Advanced Microwave Sounding Radiometer-Earth Observing System. DMSP, Defense Meteorological Satellite Program; SSM/I, Special Sensor Microwave Imager; SSMIS, Special Sensor Microwave Imager Sounder.