The useful part of the electromagnetic spectrum is shown in the upper part of this figure. Obviously, it covers many decades in frequency (or wavelength).
The lowest frequencies (longest wavelengths) constitute the radio spectrum. Parts of the radio spectrum are used for radar and passive detection.
Above the radio-frequency spectrum lies the infrared spectrum, followed by the visible range, which is quite narrow. Multispectral scanners are operated in the visible and infrared regions of the spectrum, and are used extensively as remote sensing tools for a wide variety of applications. Above the visible spectrum lies the ultraviolet spectrum and, overlapping it, the X-ray spectrum. Finally, at the highest frequencies are the gamma rays, which are sometimes used in remote sensing; for example, in the determination of the presence of moisture due to the absorption of gamma rays by moisture.
The lower part of the figure illustrates the microwave portion of the spectrum. The portion shown extends from 0.3 to 100 GHz.
Frequencies down to 0.1 Hz are used in magnetotelluric sensing of the structure of the Earth, and frequencies in the range between 0.1 Hz and 1 kHz sometimes are used both for communication with submarines (at least this is a proposed use) and for certain kinds of sensing of the ionosphere and the Earth's crust.
These frequencies certainly are far from the microwave range. Letter designations are shown for decade regions of the spectrum above the frequency chart. These designations have been adopted internationally by the International Telecommunication Union (ITU).
The very-low-frequency (VLF) region from 3 to 30 kHz is used for both submarine communication and for the Omega navigation system. The Omega system might be considered to be a form of radar - but for use in position location, not for remote sensing.
The low-frequency (LF) region, from 30 to 300 kHz, is used for some forms of communication, and for the Loran C position-location system. At the high end of this range are some radio beacons and weather broadcast stations used in air navigation, although in most areas of the world these are being phased out.
The medium-frequency (MF) region from 300 to 3000 kHz contains the standard broadcast band from 500 to 1500 kHz, with some marine communications remaining below the band and various communication services above it. The original Loran A system also was just above the broadcast band at about 1.8 MHz. This, too, is a form of radar system, but it is being phased out.
The high-frequency (HF) from 3 to 30 MHz is used primarily for long-distance communication and short-wave broadcasting over long distances, because this is the region most affected by reflections from the ionosphere and least affected by absorption in the ionosphere. Because of the use of ionosphere reflection in this region, some radar systems are operated in the HF region. One application is an ionospherically reflected long-distance radar for measuring properties of ocean waves from a shore station.
The very-high-frequency (VHF) region from 30 to 300 MHz is used primarily for television and FM broadcasting over line-of-sight distances and also for communication with aircraft and other vehicles. However, some radars intended for remote sensing have been built in this frequency range, although none are used operationally. Some of the early radio-astronomy work also was done in this range, but radiometers for observing the Earth have not ordinarily operated at such long wavelengths because of the difficulty of getting narrow antenna beams with reasonable-size antennas.
The ultra-high-frequency (UHF) region from 300 to 3000 MHz is extensively populated with radars, although part of it is used for television broadcasting and for mobile communications with aircraft and surface vehicles. The radars in this region of the spectrum are normally used for aircraft detection and tracking, but the lower frequency imaging radars such as that on Seasat and the JPL and ERIM experimental SARs also are found in this frequency range.
Microwave radiometers are often found at 1.665 GHz, where nitric oxide (NO) has a resonance. Extensive radio-astronomy research is done using these resonances, and the availability of a channel clear of transmitter radiation is essential. The passive microwave radiometers thus can take advantage of this radio-astronomy frequency allocation.
The super-high-frequency (SHF) ranges from 3 to 30 GHz is used for most of the remote sensing systems, but has many other applications as well. The remote sensing radars are concentrated in the region between 9 and 10 GHz and around 14 to 16 GHz.
Satellite communications use bands near 4 and 6 GHz and between 11 and 13 GHz as well as some higher frequencies. Point-to-point radio communications and various kinds of ground-based radar and ship radar are scattered throughout the range, as are aircraft navigation systems. Because of a water-vapour absorption near 22 GHz (see this figure), that part of the SHF region near 22 GHz is used almost exclusively for radiometric observations of the atmosphere. Additionally, remote sensing radiometers operate at several points within the SHF range, primarily within the radio-astronomy allocations centred at 4.995, 10.69, 15.375, and 19.35 GHz.
Most of the extremely-high-frequency (EHF) range from 30 to 300 GHz is used less extensively, although the atmospheric-window region between 30 and 40 GHz is rather widely used and applications in the neighbourhood of 90 to 100 GHz are increasing.
Because of the strong oxygen absorption in the neighbourhood of 60 GHz, frequencies in the 40-70 GHz region are not used by active systems. However, multifrequency radiometers operating in the 50-60 GHz range are used for retrieving the atmospheric temperature profiles from radiometric observations.
Radars are operated for remote sensing in the 32-36 GHz region, and some military imaging radars are around 95 GHz. Radio-astronomy bands exist at 31.4, 37, and 89 GHz, and these are, of course, used by microwave radiometers for remote sensing as well.
The microwave spectrum itself is illustrated in this table. No firm definition exists for the microwave region, but a reasonable convention is that it extends throughout the internationally designed UHF, SHF, and EHF bands from 0.3 to 300 GHz (1 m to 1 mm in wavelength). Numerous schemes of letter designation for bands in the microwave region exist, and they are indicated in the figure.
Radars may be found in all of the bands, with the possible exception of the Q- and V-bands, with most remote sensing radars at K-band or lower frequencies. Frequency allocations are made on an international basis at periodic but infrequent World Administrative Radio Conferences, which classify radars as "radiolocation stations". Several of the radiolocation allocations of the 1979 WARC list radar for Earth observation as a secondary service to other radars, and some permit such use as a primary service. This table lists these allocations along with some selected non-remote sensing allocations.
Sharing between radar remote sensing systems and other radars is usually not permitted. Thus, the designer of a remote sensing radar system cannot simply choose an optimum frequency and use it.