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
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
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
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
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
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
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.
Keywords: ESA European
Space Agency - Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,