often extensive and permanent darkness occurs for many months of the year, they have the potential, therefore, to provide continuous, synoptic observations. Thus passive microwave radiometers (PMRs) provide valuable (and sometimes the only) data over the polar oceans. The first instrument to provide useful data on sea-ice extent was the electrically scanning microwave radiometer (ESMR) launched in 1972, which was later superseded by the special sensor microwave imager, SSM/I. This instrument was first launched in 1987 and is still in operation. The SSM/I data have been used extensively for monitoring sea-ice characteristics and extent in both the Southern and Arctic oceans (as discussed in section 73.4.1).
A problem with PMRs, however, is their relatively poor spatial resolution (typically ca. 40 km). This is due to the fact that spatial resolution is, usually, proportional to wavelength, which is about 104 larger in the microwave compared with the visible. It should be noted, however, that a new generation of PMRs are now in operation.
The advanced scanning microwave radiometer (AMSR) has an improved resolution (by a factor of two) compared with SSM/I and utilizes a greater range and number of frequencies. The first modified instrument (AMSR-E) was launched on the US AQUA satellite in May 2002 and the second onboard the ADEOS II satellite in December 2002 (which subsequently failed in October 2003). Table 73.2 details the frequencies and resolutions of the instrument.
Was this article helpful?