1、 Rep. ITU-R RS.2068 1 REPORT ITU-R RS.2068 Current and future use of the band near 13.5 GHz by spaceborne active sensors (2006) Page 1 Introduction and background 2 1.1 Introduction. 2 1.2 Background. 2 2 Scatterometers 2 2.1 Use of the band near 13.5 GHz for scatterometers. 2 2.2 Bandwidth requirem
2、ents. 2 2.3 Feasibility of using other bands 3 2.4 Long-term need for operation around 13.5 GHz for scatterometers. 3 3 Altimeters . 4 3.1 Use of the band near 13.5 GHz for altimeters 4 3.2 Bandwidth requirements. 5 3.3 Feasibility of using other bands 5 3.4 Continued need for frequencies around 13.
3、5 GHz for altimeters. 6 4 Precipitation radars (PR) 6 4.1 The use of band near 13.5 GHz by PR . 6 4.2 Bandwidth requirements. 7 4.3 Feasibility of using other bands 7 4.3.1 Measurement dynamic range . 7 4.3.2 Instantaneous field of view (IFOV) . 7 4.3.3 Signal-to-clutter ratio (S/C) 8 4.3.4 Frequenc
4、ies for dual-band radars. 8 4.4 Continued need for operation around 13.5 GHz. 8 5 Summary and conclusions 9 2 Rep. ITU-R RS.2068 1 Introduction and background 1.1 Introduction The purpose of this Report is to address the continued need of the EESS (active) to access frequencies near 13.5 GHz, the ba
5、ndwidth requirements, and the scientific feasibility of performing the same measurements in bands other than the band near 13.5 GHz. These requirements will be addressed from the viewpoint of the three major instruments that make use of the band: scatterometers; altimeters; and precipitation radars.
6、 1.2 Background The World Radiocommunication Conference 2003 (WRC-03) made many changes to the allocations in the 13.75-14 GHz band. Prior to WRC-97, several bands were allocated on a secondary basis to the EESS and the space research service for use by radiolocation stations (i.e. spaceborne active
7、 sensors) installed on spacecraft. One of these bands was the 13.4-14 GHz band. WRC-97 decided to allocate the 13.25-13.75 GHz band to the EESS (active) and space research (active) service on a primary basis as a result of the various allocation decisions taken at the Conference with regard to activ
8、e sensors. However, WRC-97 also saw the need to maintain the 13.75-14 GHz portion of the previous secondary allocation for use by several active sensor instruments that were currently in orbit or were planned and built as their characteristics could not be changed. These provisions were set forth in
9、 the Radio Regulations with termination dates of 1 January 2000 and 1 January 2001 for various sensor instruments. 2 Scatterometers 2.1 Use of the band near 13.5 GHz for scatterometers Scatterometers are radar type devices that measure the near surface vector winds over the oceans. Wind data are cri
10、tical to determination of regional weather patterns and global climate. No other instrument can provide all weather measurements of the global vector winds. At the present time, good capability for acquisition of weather data exists over land, but not over the oceans where our only knowledge of surf
11、ace winds comes from infrequent, and sometimes inaccurate, reports from ships. Since approximately two-thirds of the Earths surface is covered by oceans, data from scatterometers will play a key role in understanding and predicting complex global weather patterns, ocean circulation, and climate syst
12、ems. Two scatterometers that were developed in the United States of America are the NSCAT (NASA scatterometer) that was launched in 1996 on Japans Advanced Earth Observing Satellite (ADEOS) and the SeaWinds scatterometer which was launched in 1999 on NASAs QuikScat satellite and in 2002 on Japans AD
13、EOS-II satellite as part of the Earth Observing System (EOS). NSCAT was designed to operate at a center frequency of 13.995 GHz. SeaWinds is a derivative of NSCAT and uses many of the same components, however the center frequency was changed to 13.4 GHz. 2.2 Bandwidth requirements Existing scatterom
14、eter designs near 13.5 GHz use a fixed-frequency, continuous wave pulse to probe the sea surface. The transmitted frequency spectrum is narrow due to the low pulse repetition rate (62 Hz) and large pulse width (5 ms). When the frequency stability of the transmitter and Doppler shifts of frequency ar
15、e included, the required radio-frequency bandwidth for present-day scatterometers is 1 MHz. Rep. ITU-R RS.2068 3 Future scatterometers may use spread spectrum modulation in order to obtain more precise definition of the surface cell where wind measurements are being taken. The bandwidth requirement
16、for these future instruments could be higher than 1 MHz. 2.3 Feasibility of using other bands Scatterometer measurements, and the derived knowledge about wind vectors, are based on microwave scattering effects over water-surface capillary waves. Measurements at wavelengths comparable to that of the
17、capillary waves caused by water-surface wind interaction is necessary in order to achieve the sensitivity required to measure wind speeds and directions for winds having velocities as low as 3 m/s. Measurements of winds with such velocity are needed to satisfy the requirements for determination of v
18、ariation in weather and climate. The wavelength in the band near 13.5 GHz is commensurate with the dimensions of the capillary waves produced by low speed winds with the result that the scatterometer is highly sensitive to local winds, especially low wind speeds. At the same time, a scatterometer op
19、erating in the band near 13.5 GHz exhibits low sensitivity to non-wind effects such as swells and surface film/surface tension. Possible alternative bands to the band near 13.5 GHz have been considered. The two bands closest to 13.5 GHz that are currently available to the Earth exploration-satellite
20、 service (active) are the 9.5-9.8 GHz and 17.2-17.3 GHz bands. Neither the 9.5-9.8 GHz band nor the 17.2-17.3 GHz band is as desirable for use by scatterometers as the band near 13.5 GHz. This is a consequence of there not being a large body of data on radar scattering from the ocean surface at freq
21、uencies other than 13.5 GHz where the Seasat scatterometer operated and 5.3 GHz where the ERS-1 scatterometer operated. Moving to a new band would require redeveloping the algorithm that relates the radar return to the wind speed and direction. The algorithm developed for the 5.3 GHz band required a
22、 number of aircraft and tower experiments before launch and more than six months of refinements after the launch of ERS-1. An effort to develop a new algorithm would result in an interruption of the data flow to the science community for the period that is required to gain confidence in the new algo
23、rithm. A frequency change will result in some loss of the continuity of the long-term data set for the same reason. Scatterometers operating near 13.5 GHz have higher sensitivity to low wind speeds than scatterometers operating near 5.3 GHz. The low speed wind vectors are important to the studies of
24、 the variability of ocean currents. At frequencies above 13.5 GHz, atmospheric attenuation due to water content (e.g. cloud cover and rain) becomes more variable. At 17.2 GHz, it is probably possible to operate a wind scatterometer, however, operating at a frequency of 17.2 GHz or greater would resu
25、lt in degraded performance since the scatterometer would be more sensitive to atmospheric water content and surface film/surface tension effects. At frequencies above 20 GHz, the variability of the atmospheric attenuation would render the instrument useless without other means of measuring the atmos
26、pheric variability. Another factor that makes continued use of the band near 13.5 GHz for scatterometry important is the large amount of data that has been acquired at this frequency over the past 15-20 years. The Seasat scatterometer and the NASA aircraft scatterometer both operated at this frequen
27、cy, as well as the NSCAT. Continued use of this band for future scatterometers will allow more meaningful cross-comparison of data sets acquired in the future with those from the past. A broader database acquired by instruments operating with similar parameters can be expected to produce a more accu
28、rate scientific model. 2.4 Long-term need for operation around 13.5 GHz for scatterometers There is a long-term requirement to operate spaceborne scatterometers in the band near 13.5 GHz. Scatterometer measurements will be used in operational systems to derive wind speed and wind direction data. The
29、se data will be used to measure and predict weather, ocean circulation, and climate, all key factors in management of the environment. As discussed above, only in the band 4 Rep. ITU-R RS.2068 near 13.5 GHz can the required measurement sensitivity be achieved. In addition, only in this band is there
30、 an existing database acquired over a period of 15-20 years that can contribute to the value of future scatterometer data interpretation. The NSCAT scatterometer was constructed to operate at 13.995 GHz Protection of NSCAT operations until the year 2000 was ensured by a regulatory provision, which w
31、as suppressed by WRC-03. On the other hand, the SeaWinds scatterometer was only in a developmental stage at the time of WRC-97 and its frequency was changed to 13.4 GHz in order to preclude frequency-sharing constraints with respect to the fixed-satellite service. Likewise, any other new scatteromet
32、ers developed for this frequency range should operate below 13.75 GHz. It is projected that a 100 MHz bandwidth will be needed for future scatterometers in order to improve measurements through the use of alternative modulation techniques. Based on the results of WRC-03, the 13.75-14 GHz band can st
33、ill be used for scatterometers on a secondary basis, but this use would not be protected from interference by the primary allocated services in this band. 3 Altimeters 3.1 Use of the band near 13.5 GHz for altimeters A spaceborne radar altimeter is a downward-looking pulsed-radar system mounted on a
34、n orbiting spacecraft. They are primarily ocean remote sensing instruments, although there is some interest in the tracking data that they acquire over land and ice surfaces, as implemented on the ERS altimeters. Current and planned radar altimeter missions are designed to meet over ocean requiremen
35、ts; land/ice tracking is a secondary data product. Altimeters are used to measure range from the satellite to the ocean surface. This very precise height measurement, when combined with very precise orbit determination and corrections for other media effects, provides very accurate global maps of th
36、e ocean topography. From this knowledge of topography, the location, speed, and direction of ocean currents worldwide can be calculated. This provides an understanding of ocean circulation and its time variability that is crucial to understanding the Earths climate change. Altimeter data can also pr
37、ovide measurements of surface-significant wave height (ocean waves), and backscatter at nadir from which wind speed (but not the wind vector) can be determined. The meteorological forecasting community is interested in the above measurements from any operational system. Several spaceborne radar alti
38、meters are currently operating in the allocated band near 13.5 GHz such as JASON, ERS and ENVISAT. Radar altimeters are now an operational tool for earth/ocean/air sciences and, as such, will continue to be launched and used long into the future. The band near 13.5 GHz was chosen long ago based on s
39、uch considerations as an allocation for radars on spacecraft, wide allocated bandwidth, science objectives, hardware availability, and compatibility with the radiolocation service. The first spaceborne altimeter to use this band was the Skylab S-193 experiment in the early 1970s; since then, there h
40、ave been many altimeters using this band (GEOS-C, Skylab, GEOSAT, TOPEX-POSEIDON, JASON, ERS-1, ERS-2 and ENVISAT). This use represents a considerable investment in hardware design, hardware development, missions operations, data reduction, software design, scientific analysis, modelling and databas
41、e construction. A very large database has been obtained from these altimeters that allow the proper interpretation of current and future altimeter data. These data are very sensitive to the hardware transmission frequencies. A change in operating frequency could negate the applicability of a substan
42、tial amount of that existing database. Also, a significant amount of hardware for both inflight use and ground use has been developed in both the United States of America and in Europe that will support future missions. Much of this hardware is designed to operate within the band near 13.5 GHz. Base
43、d on Rep. ITU-R RS.2068 5 the above, the need for altimeters working within this frequency band will extend well into the future. It should be noted that the JASON altimeter alone will operate on multiple satellites through at least 2018. 3.2 Bandwidth requirements The bandwidths being employed by c
44、urrent and planned altimeters are of the order of 320 MHz (for JASON) to 330 MHz (for ERS). As in any radar system, the precision of the altimeters height (range) measurement is dependent on the bandwidth used. The JASON altimeter uses pulse compression (chirp full de-ramp stretch) to achieve its fi
45、ne precision. In JASON, the 320 MHz allows for an effective compressed pulse width of 3.125 ns (46.5 cm basic resolution) before further processing and averaging is done. Ultimately, the precision on the channel near 13.5 GHz is less than 3 cm. Several studies have been carried out in the ITU-R that
46、 examined the need to extend the bandwidth for altimeters to as much as 600 MHz. These studies examined other effects on the accuracy of height measurement including EM-bias, sea-state bias, ionospheric effect, tropospheric effect and orbit determination. It has been concluded that these effects are
47、 large enough at the present time to dominate the error budget for the height measurement. A decrease in the 2 to 3 cm height uncertainty achieved by the TOPEX altimeter would not significantly change the total error. Therefore, the bandwidth of 320-330 MHz used by current altimeters will be adequat
48、e for missions that are currently envisioned. In the future, if systematic errors can be significantly reduced by modelling, new instruments, etc. then increasing the bandwidth to as much as 600 MHz may be desirable. Also, in the future, components for altimeters with such wide bandwidths may become
49、 much more available and affordable. There are potential changes to the basic design of altimeters that may produce a need for wider bandwidths: multibeam altimeters, scanning altimeters, and synthetic aperture altimeters fall into this category. Another design that would require a wider bandwidth is for an altimeter that would decorrelate its along-track measurement by use of frequency agility or frequency hopping. Several such designs have been studied in concept but are not currently supported by any flight project. Another reason that bandwidths greater than 320-330 MHz may be needed
