Remote sensing at microwave wavelengths (generally 30cm to 0.1cm wavelength or 1 GHz to a few hundred GHz) is effective because of:
The technology used in passive microwave remote sensing was developed in radio astronomy. Passive sensors receive emitted or reflected radiation. In the early 1960s Mariner spacecraft used passive microwave sensors. Many satellites have since used this technology (Murphy et al 1987, p6). More recently, the Microwave Sounding Unit (MSU), flown as part of the TIROS Operational Vertical Sounder (TOVS) instrument package on successive satellites in the NOAA series, uses four channels (50.3, 53.74, 54.96, and 57.97 GHz) around an oxygen resonance band for derivation of vertical temperature profiles. The microwave sounding measurements complement infrared (IR) measurements primarily because unlike the IR measurements, the microwave data are useful in cloudy or partly cloudy areas. An upgraded version of MSU called the Advanced Microwave Sounding Unit (AMSU) will be flown operationally beginning in the early 1990s. It has twenty channels; one each at 21 and 37 GHz, twelve channels between 50 to 60, two at 90, one each at 160 and 183 GHz. It will be the main operational meteorological sounder, for derivation of vertical temperature and moisture profiles in clear and cloudy conditions, and during day and night time.
There are other passive microwave instruments, many of which detect microwave radiation from the earth's surface. Overall, the passive sensors enable measurements to be made of:
Active microwave remote sensing began with radar. There are many active microwave sensors (pulses of radiation are emitted and information is derived from the characteristics of the backscattered signal) including synthetic aperture radar operating at L-band (23cm), C-band (6cm) and X-band (3cm), wind scatterometers and altimeters. The success of this type of technology for meteorological or oceanographic applications was clearly demonstrated with the short-lived SEASAT research satellite in 1978. New active microwave sensors will be flown beginning in 1990 and throughout the 1990s. These will be used for deriving:
The potential of microwave sensors for "all-weather" remote sensing may be
illustrated by considering the microwave absorption of the atmosphere as shown below. The
coordinate is zenith opacity, so that regions of very high opacity represent very strong
absorption lines while low opacity regions are windows in which the atmosphere does not
significantly attenuate microwave radiation. The frequencies at which microwave
measurements are made or will be made in the near future are indicated at the top of the
figure. These frequencies are 1.4, 6, 10, 18, 21, 37, 55, 90, 157 (planned) and 183 GHz
(planned).
Figure 22: Atmospheric microwave absorption. Strong absorption features appear as peaks. The three curves show absorption in a dry atmosphere, in the same atmosphere with 20 kg/m2 of added water vapour, and with both water vapour and 0.2 kg/m2 of stratus cloud added. Valleys are microwave windows. From Murphy et al (1987), p1.
In some cases, for example at the 55 GHz oxygen feature in the figure, sensor channels with narrow bandwidths are centred on, within, and close to the feature, and are used to measure radiation which arises from different levels in the atmosphere. Radiation reaching the sensor which is at the central wavelength of the absorption band arises in the upper atmosphere. Conversely radiation from the continuum near the wings of the absorption band, arises in the lower atmosphere, because the density of emitting molecules is higher there and atmospheric attenuation is minimal.
There are three curves in the figure. The bottom curve represents microwave absorption for a dry standard atmosphere. The middle curve is for the same atmosphere with 2 g/cm^2 water vapour added. The top curve shows the effect of adding a further 20 mg/cm^2 of liquid water to represent a typical (non-precipitating) stratus cloud. The atmosphere is relatively transparent away from absorption features, and there is little effect due to cloud. The main absorption features, which show up as peaks are:
Measurements at the peaks or within the bands are traditionally used to derived information about the absorbing species (e.g. total water content derived from 22 GHz data, or corrections to other data for water vapour attenuation) or about the vertical atmospheric temperature profile if the absorbing species (e.g. oxygen) is fairly evenly distributed (e.g. 50 to 70 GHz measurements made by MSU and used for temperature profiles). These are examples only and there are many other applications.
Measurements through microwave atmospheric windows allow the sensor to "see" down to the earth's surface for derivation of sea ice cover, snow cover, soil moisture, sea surface temperature, imagery, etc. Longer wavelength microwaves are emitted or reflected by objects in a manner determined by their bulk dielectric properties including, for example, moisture content of snow, grain size of snow, leaf shape and size for vegetation, etc. As a result, microwave sensors allow one to infer bulk properties of matter in a way which complements the information obtained from visible or infrared sensors, which tend to be and indirect measure of surface or skin properties of matter.
Dr D C Griersmith
