Weather scientists use a wide variety of frequencies for a number of purposes. In some cases different frequencies are used because their allocations are different, that is, each band is only allowed to be used for certain purposes. In other cases, different bands are used because their propagation characteristics differ, and so each band has technical advantages compared to others.
NOAA operates weather radars primarily in two bands, 2700-3000 MHz and 5600-5650 MHz, although there are occasional operations on other frequencies as well. Wind profilers (another form of weather radar) operate temporarily at 404.37 MHz but are gradually moving to 449 MHz. Other bands are used also; see below. NOAA Weather Radio (NWR) transmits weather forecasts and public safety information at several frequencies in the 162 MHz region. Weather satellites operate at a wide variety of frequencies, both to collect data and to transmit them from the satellite to the ground. Data collection is done from active transmitters on the ground called Data Collection Platforms (DCPs) and sent by radio to weather satellites. Data are also collected by passive sensing, that is, by having the satellite sense radiation from the earth and the lower atmosphere, providing information about winds, the temperatures of the atmosphere and the oceans, and a variety of other matters important to weather forecasting.
NOAA operates two types of weather satellites: geostationary ("geo") and non- geostationary ("non-geo"). Geo satellites appear to stand still when viewed from the earth, while non-geo satellites are seen to circle the earth twice a day, gathering data from all over the planet. Because they maintain a constant view of certain parts of the globe, geo satellites provide a constant flow of data regarding a limited area. NOAA operates two such satellites, called "GOES-East" and "GOES-West". GOES is the acronym for Geostationary Operational Environmental Satellite. The East bird sits over the equator at 75 degrees West longitude and GOES West at 135 degrees.
Graphic of constellation of Geostationary satellites needed to view all continents except Antarctica.
Polar regions are viewed by polar-orbiting satellites.
NOAA's current generation of non-geo satellites is known as the NOAA series, while its earlier models went by the name of TIROS. These non-geo satellites are polar orbiters, each covering any one point on the Earth twice a day. NOAA is now working with the Defense Department to develop a series of satellites which will combine the functions of the NOAA series with those of the spacecraft in the Defense Meteorological Satellite Program (DMSP), reducing the total number of meteorological spacecraft operated by the two agencies.
Passive sensing by satellite is done primarily at frequencies well above 10 GHz. At recent meetings of the World Radiocommunications Conference (WARC -92, WRCs -95 and -97) efforts have been made to generate agreement on the use of certain frequencies especially well suited for passive sensing, by limiting such sensing to these frequencies and removing from these bands other operations which could interfere with sensing operations.
Radiosondes ("weather balloons") have an allocation which extends from 1668.4-1700 MHz. Radio astronomers work from 1668.5-1670 MHz; the U.S. 1993 Omnibus Budget Reconciliation Act took away 1670-1675 MHz effective in 1999 so that it could be auctioned off to the private sector to provide revenue; the range 1690- 1710 MHz (and increasingly, 1684-1690 MHz) is heavily used by weather satellites. NOAA is working to move its radiosonde operations to the 1675-1684 MHz band to avoid conflict with satellites.
Weather satellite downlinks (from space to the ground) are primarily in the bands 137-138 MHz (Automatic Picture Transmissions (APT) from NOAA spacecraft); 1670-1675 MHz transmissions to a few satellite control stations; 1682-1690 MHz GOES Variable (GVAR) broadcasts; 1690-1695 MHz GOES transmissions of Weather facsimile (WEFAX) and other data; and High Resolution Picture Transmissions (HRPT) in the region 1696.5-1710 MHz from the polar orbiters. APT and HRPT are broadcast worldwide as the NOAA satellites pass over any given spot on the Earth. WEFAX and GVAR are broadcast throughout the Western Hemisphere continuously from the GOES satellites. A variety of metsats operated by other countries use many of the same frequencies. Interference is avoided by coordination among the satellite operators, with this being made possible by the fact that only a small number of such satellites exists. Only a small number is needed, since the data gathered by one country are shared with all others at no cost. The 400.15-406 MHz band is used both for METSATS (400.15-403 MHz) and radiosondes (allocated in the entire band).
At recent WRCs, efforts have been made to obtain additional downlink frequencies. Future generations of metsats will have greatly improved instruments capable of providing considerably more and better data. To move these data from space to the ground requires additional bandwidth that is not available under earlier allocations. So, additional allocations have been sought (and some obtained) through the ITU. A few additional frequencies have been obtained for passive satellite sensing, and arrangements made to protect some existing bands, at recent WRCs.
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As noted above, the meteorology bands are a primary target of industry. One reason is clear: cellular, MSS and many other commercial systems which would be best served by having the same spectrum available to them worldwide. This would simplify the design of their satellites and allow use of the same ground terminals around the globe. Coincidentally, the primary meteorological bands (137-138 MHz, 400.15-406 MHz, 1670-1710 MHz and 2700-2900 MHz) are allocated to meteorology worldwide and, if captured by industry, would provide them with exactly the sort of arrangement they would most like to have. The results show in the history of WARC-92 through WRC-2000: the industry is working to obtain additional parts of these bands.
The discussion takes place in terms of band sharing with meteorology, but "sharing" has several meanings in the ITU's International Radio Regulations. One definition includes the removal of present occupants from part of a band so it can be given over to the new users. This form of "sharing" is, in fact, expropriation. Other types of sharing involve limiting the use by both occupants in a way which permits both to use the band. The U.S. has decided that the MSS will share 400.15-401 MHz by limiting its emissions so as not to interfere with the MetSats using the band (time sharing) and with radiosondes by having the radiosondes go somewhere else. The U.S. proposals to WRC-97 involved limiting the power flux density of MSS satellites in 402-405 MHz so as not to interfere with radiosondes (thus mandating the use of spread spectrum by the MSS) and removing radiosondes from 405-406 MHz, making this the exclusive domain of MSS systems. These U.S. proposals failed to be adopted at the WRC.
The foreign proposals seen at WRC-97 respecting the 1675-1683 MHz band would have required the removal of radiosondes from this spectrum. In the United States radiosondes are barred from future use of 1670-1675 MHz by OBRA-93 (discussed above) and would thus have to use the spectrum above 1683, where they would interfere with MetSat downlinks. GVAR (GOES VARiable data) occupies the region 1682-1690 MHz; WEFAX, EMWIN and other data are in 1690-1697 or so, and HRPT from the polar metsats occupies 1697-1710 MHz. Clearly, there is no place for the radiosondes to go that would not cause problems for MetSat receivers. Fortunately, these issues were resolved at WRC-2003, where the MSS was given a new allocation at 1668-1675 MHz with protection of meteorology required in certain countries.
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Those designing MetSat receivers need to bear in mind that the bands will be changing significantly in the next few years. Where in the past they were the exclusive domain of met systems, they may soon be occupied as well by large numbers of commercial satellites. Receivers thus must be capable of functioning properly in the presence of additional signals, some fairly strong, operating in adjacent channels. Selectivity, adjacent channel rejection and resistance to overload will become increasingly important in coming years and designing for minimum cost may no longer produce adequate equipment.
In 137-138 MHz, there have already been complaints of interference from the MSS satellites operating in the band. Receivers having excessive bandwidth are vulnerable to MSS satellites operating in adjacent channels to those used by MetSats.
In 1675-1700 MHz, there have been instances of interference to GVAR and WEFAX receivers caused by radiosondes operating in the adjacent bands. Tests reveal this interference to result from excessive bandwidth and non-linearity in the front end of satellite receivers.
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