radiation that has been emitted by other objects but does not emit any of its own. (A SAR sensor, by contrast, is active.) SSMR and SSMI radiometers were flown on the Nimbus program and DMSP series of satellites. Because clouds are translucent in the microwave regime, especially at lower frequencies, microwave radiometers are quite weather insensitive. Since most microwave radiometers operate along a polar orbit with a broad, sweeping scan, full ice maps of the polar regions where the swaths are largely overlapping can usually be obtained within one day. This frequency and reliability comes at the cost of a poor resolution: the angular field of view of an antenna is directly proportional to the wavelength and inversely proportional to the effective aperture area. Thus we need a large deflector dish to compensate for a low frequency . Most ice concentration algorithms based on microwave radiometry are predicated on the dual observation that: 1. different surface types have different, strongly clustered, microwave signatures and 2. the radiometric signature at the instrument head is a linear combination of that of the different surface types, with the weights taking on the values of the relative concentrations. If we form a vector space from each of the instrument channels in which all but one of the signatures of the different surface types are linearly independent, then it is straightforward to solve for the relative concentrations: where is the radiometric signature at the instrument head (normally measured as a brightness temperature), is the signature of the nominal background surface type (normally water), is the signature of the ith surface type while Ci are the relative concentrations. Every operational ice concentration algorithm is predicated on this principle or a slight variation. The NASA team algorithm, for instance, works by taking the difference of two channels and dividing by their sum. This makes the retrieval slightly nonlinear, but with the advantage that the influence of temperature is mitigated. This is because brightness temperature varies roughly linearly with physical temperature when all other things are equal—see emissivity—and because the sea ice emissivity at different microwave channels is strongly correlated. As the equation suggests, concentrations of multiple ice types can potentially be detected, with NASA team distinguishing between first-year and multi-year ice (see image above). Accuracies of sea ice concentration derived from passive microwave sensors may be expected to be on the order of 5\% (absolute). A number