1、 Rec. ITU-R RS.577-7 1 RECOMMENDATION ITU-R RS.577-7 Frequency bands and required bandwidths used for spaceborne active sensors operating in the Earth exploration-satellite (active) and space research (active) services (1982-1986-1990-1994-1995-1997-2006-2009) Scope In this Recommendation frequency
2、bands and bandwidths for five basic types of spaceborne active sensors are given. Although the discussion in Annex 1 mainly concentrates on Earth observation, it is generally believed that the measurement techniques are equally valid on other planets. Therefore, this Recommendation covers both Earth
3、 exploration-satellite (active) and space research (active) services. The ITU Radiocommunication Assembly, considering a) that spaceborne active microwave sensors can provide unique information on physical properties of the Earth and other planets; b) that the sensing of different physical propertie
4、s requires the use of different frequencies; c) that the spatial resolution of the measurement determines the required bandwidth; d) that simultaneous measurements at a number of frequencies are often needed to distinguish between the various properties; e) that sharing is generally feasible between
5、 spaceborne active microwave sensors operating in the Earth exploration-satellite (active) and space research (active) services and terrestrial radars operating in the radiolocation service, recommends 1 that frequency bands and required bandwidths for spaceborne active sensing should be in accordan
6、ce with Annex 1; 2 that the frequency bands and bandwidths given in Table 1 should be used for active sensing measurements of the Earth for: soil moisture; vegetation mapping; snow distribution, depth and water content; geological mapping; land use mapping; ice boundaries, depth, type and age; ocean
7、 wave structure; ocean wind speed and direction; 2 Rec. ITU-R RS.577-7 mapping of ocean circulation (currents and eddies); oil spills; geodetic mapping; rain rates; cloud height and extent; surface pressure; measurement of biomass in tropical forests; etc. TABLE 1 Application bandwidths Frequency ba
8、nd as allocated in Article 5 of the Radio Regulations Scatterometer Altimeter Imager Precipitation radar Cloud profile radar 432-438 MHz 6 MHz 1 215-1 300 MHz 5-500 kHz 20-85 MHz 3 100-3 300 MHz 200 MHz 20-200 MHz 5 250-5 570 MHz 5-500 kHz 320 MHz 20-320 MHz 8 550-8 650 MHz 5-500 kHz 100 MHz 20-100
9、MHz 9 300-9 900 MHz(1) 5-500 kHz 300 MHz 20-600 MHz 13.25-13.75 GHz 5-500 kHz 500 MHz 0.6-14 MHz 17.2-17.3 GHz 5-500 kHz 0.6-14 MHz 24.05-24.25 GHz 0.6-14 MHz 35.5-36 GHz 5-500 kHz 500 MHz 0.6-14 MHz 78-79 GHz 0.3-10 MHz 94-94.1 GHz 0.3-10 MHz 133.5-134 GHz 0.3-10 MHz 237.9-238 GHz 0.3-10 MHz (1)See
10、 the relevant decision of WRC-07. Rec. ITU-R RS.577-7 3 Annex 1 Factors related to determination of frequency bands and required bandwidths used for spaceborne active sensing 1 Introduction Active sensors differ from passive sensors in that they illuminate the object under observation and respond to
11、 the reflected energy. There are 5 basic types of active sensors: scatterometers; altimeters; imagers (synthetic aperture radars); precipitation radars; cloud profile radars. Radar scatterometers are useful for determining the roughness of large objects. When operating at frequencies higher than 300
12、 MHz, the scatterometer measures the amount of backscatter from the surface roughness in broad categories ranging from smooth to very rough. At frequencies around 200 MHz, reflectivity depends upon the dielectric constant of the object; at lower frequencies, reflectivity depends primarily upon elect
13、rical conductivity. These lower frequencies can be used to penetrate the surface of the Earth to detect sub-surface structures. Radar altimetry has yielded three possible operation concepts for practical systems. One of these techniques is based upon the use of a very narrow beamwidth (2 mrad) and a
14、 very short transmitted pulse (2 ns). Timing of the round-trip delay of the transmitted pulse leading edge is used to provide altitude information. A technique that is similar to the short pulse system is the pulse compression technique. A short impulse pulse generates a longer frequency modulated p
15、ulse and the return, which has a wide bandwidth, is compressed back to a short pulse which is then leading edge detected. The third technique requires moderate antenna size and spacecraft stabilization, with radar return from the nadir point obtained by a time-gating technique. In this system, altit
16、ude information is extracted by measuring the centroid of the early portion of the radar waveform rather than the leading edge of a very short pulse. Radar imaging systems are employed to produce high resolution images required by users in such fields as geology, oceanography and agriculture. To ach
17、ieve reasonable resolution from space, synthetic aperture focused radars will be employed for many applications as they have resolutions independent of range. In the area of meteorology scanning Doppler radars may also be employed. Knowledge of the global rainfall and cloud distributions is required
18、 to understand and predict global climate change. Microwave sensors have a clear advantage over visible/infrared sensors in that they have the capability to penetrate the cloud cover, thereby providing direct information of rain and cloud volume. Active sensors are especially important because they
19、are the only instruments which provide vertical rain and cloud structures, and therefore are essential to study large-scale atmospheric circulation and the radiation budget. Moreover, the active sensors can provide quantitative rain and cloud information independent of the microwave emission propert
20、ies of background surfaces. 4 Rec. ITU-R RS.577-7 Active remote sensing in the microwave region offers several advantages over visible region sensors and passive microwave sensors. Besides being uniquely sensitive to several land/ocean/atmosphere variables (e.g. plant moisture and cloud height), act
21、ive sensing can, for instance, penetrate the surface and vegetation, operate on an all-weather, day/night basis, attain high spatial resolution (synthetic aperture radar (SAR) enhance features by changing the illumination angle, and operate over broad spectral ranges independently of emissions from
22、narrow-band phenomena. Active sensors illuminate the object under observation and respond to reflected energy. In order to gather information concerning the Earths surface from space, the transmitted signal must traverse the atmosphere twice. As a result the electromagnetic absorption and scattering
23、 properties of the atmosphere play an important role in determining the spectral regions suitable for active remote sensors. Severe atmospheric attenuation is confined to the shorter wavelengths, and for this reason, active sensors usually operate below the 60 GHz oxygen absorption region and also a
24、void the spectral region near the 22 GHz water vapour line. Electromagnetic scattering by precipitation and clouds can present a more serious problem than atmospheric absorption. Echoes from water droplets increase with droplet diameter and decrease with increasing wavelength. Thus, at longer wavele
25、ngths clouds give little echo, but precipitation can give somewhat stronger echoes because of the larger particle diameters of the rain drops. Several aspects of active sensor research, particularly as it relates to the choice of frequencies for measuring Earth-oriented variables from a space platfo
26、rm, are presented below. It should be noted in determining optimal frequencies that, due to the broad frequency response range of various phenomena of interest, there is often a need for simultaneous measurements at several frequencies so that contributions of the radar return from different sources
27、 can be separated. The radar return from any surface is a function of radar frequency, surface roughness, surface dielectric properties, angle of incidence and aspect, and sub-surface microstructure. In each of the applications listed, the energy reflected back to a radar sensor is strongly affected
28、 by at least one backscattering mechanism related to the measured phenomenon. In general, these are: oceanic roughness (used in the study of ocean structure and winds over sea surfaces); O2absorption (used in determining surface pressure over oceans); and surface roughness and dielectric constant va
29、riations (used in studies of ice, snow and land parameters). 2 Active sensing of ocean and ocean winds Oceanic active sensor studies are dominated by wave structure determination, sea-surface wind measurements and ocean current investigations. Generally the reflected microwave energy is due to ocean
30、 roughness: specifically, the radar return is a function of diffraction effects from both large gravity waves and small capillary, surface-tension ripples riding on large-scale waves, and foam. The amount of reflected radiation due to each of these effects observed by an active sensor depends on the
31、 sea state and the particular active measurement technique. Work at several frequencies within the 3-30 GHz range has shown that the effects of large gravity waves dominate at near-normal incidence and those of capillary waves at incidence angles greater than 20. Thus to sense sea roughness (a funct
32、ion of the very breeze-dependent ripples) and the size and direction of long-lived gravity waves (coarse sea structure), a two-way component concept is used. In the study of ocean surface winds (important in weather prediction models), the underlying principle is that ocean roughness is a gauge by w
33、hich wind variables can be inferred, since the small roughness elements which convey the transfer of momentum from the wind to the sea are in at least near equilibrium with the wind. Rec. ITU-R RS.577-7 5 Using variable frequencies, polarizations and incidence angles, investigators can infer details
34、 of ocean surface wind, significant wave height and mean square wave slopes, an accomplishment beyond the capabilities of passive sensing. Experiments have shown that good wind speed sensitivity is obtained at frequencies near 14 GHz and that there is a reduced sensitivity to wind speed at 1.3 GHz.
35、SARs have shown promise in coarse ocean structure measurements (average significant wave height). One design employs four frequency bands in the 1-10 GHz range and three polarizations with wide-swath and multiple-incidence angle capabilities. Ocean oil slicks suppress short-wavelength ocean waves an
36、d therefore the slick area can be discriminated from the surrounding clean surface by microwave imaging radars. Altimeters have been used successfully from a number of satellites over the worlds oceans. For oceanographic studies, an altimeter system having an overall range measurement precision bett
37、er than 2 cm is required. To achieve 2 cm precision will require removal of the range errors due to ionospheric electron content which cause errors as great as 22 cm at 13.5 GHz. A two-frequency altimeter system can eliminate the range uncertainty due to the ionosphere. A two-frequency altimeter sys
38、tem can also provide accurate measurements of continuous swaths of the ionospheric electron content, measurements which are not available today over large regions of the Earths oceans. A region of the spectrum, separated by more than an octave from the 13.25-13.75 GHz band, would be a suitable choic
39、e for the second frequency. The second frequency could be selected around 5 GHz, with the main frequency remaining near 14 GHz. It is thought that in the longer term, higher frequencies around 35 GHz will also be used. It can thus be seen that several frequencies have proven useful for the remote ac
40、tive sensing of ocean-wave structure. Due to high wind speed dynamic range and the relative absence of atmospheric effects, wind speed measurement technology has converged on the 10-15 GHz region. 3 Active sensing of ice-covered surfaces Investigations indicate that the following types of ice variab
41、les are amenable in varying degrees to active microwave sensing: ice-type (young, old, etc.), surface roughness, concentration, floe size and number, water openings, drift, surface topology, pressure characteristics, thickness and changes in nature and in distribution of types. Based on these studie
42、s, a frequency between 3-30 GHz appears to be the best for determining sea-ice types. A radar frequency in the range 0.3-3 GHz is useful in resolving ambiguities resulting from measurements of thin ice, especially when utilized in conjunction with radars between 3-30 GHz. Higher frequencies are also
43、 under consideration. The most important spaceborne active microwave sensors for sea-ice application are the SAR, radar altimeter and radar scatterometer. Satellite research on sea-ice has been primarily carried out by SAR at 1.3 GHz. Airborne synthetic aperture radar imagery (1.3 and 9.6 GHz) has s
44、hown that in some cases, including sea-ice mapping, the higher frequency channel is preferable. Although the interpretability of sea-ice imagery does improve with higher frequencies, there is no question of the usefulness of the product at 1.3 GHz. Altimeters have been used to map sea-ice parameters
45、 and the height of the Greenland ice-cap. Currently, remote sensing of the Earth from space is generally limited to a thin superficial layer, while many issues related to climate, Earth resources or risk monitoring require information over greater depths. Radars operating at frequencies near 435 MHz
46、 offer, under certain circumstances, the possibility of imaging through ice layers down to the bed(rock), which could be over 4 km depth. To model global ice sheet dynamics and mass balance (accumulation of snow and losses 6 Rec. ITU-R RS.577-7 through melt and iceberg calving) over longer periods (
47、100 to a few 100 000 years), it is essential to have a complete coverage of the Antarctic ice sheet with observations of homogeneous quality, which can be best accomplished using a spaceborne platform employing such as a nadir ice sounder. 4 Meteorological and climatological observations The knowled
48、ge gained in ground-based and airborne measurement of rainfall, storm features and pressure fields in weather prediction models has also been extended to spaceborne systems. The techniques are based upon changes in clear atmosphere refractive index due to rain-related features or differential reflec
49、tivities of multi-frequency echoes. Studies carried out with orthogonally polarized radars and multiple, narrow-beam coverage at several frequencies between 2 and 37.5 GHz have been able to measure precipitation rate, intensity, spatial distribution, drop size and surface pressure over oceans and wind movements within storms. There are several factors constraining frequency choices. A combination of bands must be chosen to match minimum sensitivity and spatial resolution, yet not be swamped by ground echo at needed viewing angles.
