There are numerous bands of spectrum used in weather forecasting. For an in-depth look at the complexity of the spectrum needs, see this guide for spectrum used for meteorology from the World Meteorological Association and the ITU (warning: highly technical document). It goes into depth about the various bands of frequency that are used for various weather gathering purposes.
The current controversy involves the use of spectrum at 23.8 GHz. It turns out this frequency has the characteristic that it is absorbed by water vapor. This makes it valuable for meteorological purposes since it can be used by devices in satellites called sounders to measure the different levels of water vapor in the air. This is one of the most valuable tools in the weather data gathering system, particularly over oceans where there are few other measuring devices.
The sounders work by emitting the 23.8 GHz spectrum and measuring the return signals, working similarly to radar. The process of measuring water vapor is extremely sensitive to interference because the return signals to the sounders are extremely faint. The weather community is worried that even a little bit of interference will kill the utility of this valuable tool.
In May 2019 the FCC raised over $2.7 billion through the auction of spectrum in the 24 GHz and 28 GHz bands, including spectrum sitting directly adjacent to the 23.8 GHz band. Before the auction, the administrator of NASA warned the FCC that leakage from the newly auctioned spectrum could degrade the use of the 23.8 GHz spectrum. NOAA (the National Oceanic and Atmospheric Administration) told Congress the same thing. NOAA said that a 30% degradation in the accuracy of the sounders could worsen the ability to predict where hurricanes will land by two or three days – something that would have a huge negative dollar cost.
It’s a convenient fiction in the wireless world that radios stay within the exact frequency bands they are supposed to use. However, in real life radios often stray out-of-band for various reasons and cause interference in adjacent frequency bands. This happens up and down the radio spectrum, but in this case, scientists say that even a little interference could make it difficult or impossible to read the faint signals that are read by the sounders to measure water vapor.
Both NASA and NOAA have proposed that the FCC lower the chances of interference by lowering the power level and the ‘noise’ that comes from cross-band interference. They asked for a limit of -42 decibel watts of noise for nearby spectrum bands while the FCC is recommending -20 decibel watts. The lower the decibel watts number, the less the interference. The World Radiocommunications Conference has a current recommendation of -33 decibel watts, which is scheduled to lower to -39 decibel watts in 2027.
The carriers that bought the spectrum, through filings made by the CTIA, say that the frequencies would be a lot less valuable to them if they have to lower power to meet the noise levels recommended by NASA and NOAA, and the FCC is siding with the carriers.
This is just the first of many frequency battles we’re going to see as the thirst for more 5G spectrum invades spectrum that has been used for scientific or military purposes. The FCC often tries to mitigate interference by moving existing spectrum users to some different frequency band in order to accommodate the best use of spectrum. However, in this case, the weather satellites must use 23.8 GHz because that’s where nature has set the interference with water vapor.
It’s hard not to side with the weather scientists. Everybody, including the carriers, will suffer great harm if the ability to predict hurricanes is degraded. When it comes to something as vital as being able to predict hurricanes, we need to use common sense and caution rather than give the 5G companies every possible slice of available spectrum. It’s not hard to predict that the carriers will fight hard to keep this spectrum even if there is too much interference. Unfortunately, the current FCC is granting the carriers everything on their wish list – expect more of this in 2020.