Will 5G Undermine Weather Prediction?

There have been a number of media stories this week about a major threat to weather prediction:  the sale of electromagnetic spectrum for new 5G cellphone service.   The problem is that some of the wavelengths being auctioned off for 5G are critical for an important class of weather satellites, with 5G signals potentially undermining our ability to forecast the weather.

Currently, 4G cellphone technologies provide roughly 100 megabits per second (100 million bits per second) of communication speed, while the proposed 5G service could achieve 10 gigabits per second (10 billion bits per second).  Downloading movies and animations would be much quicker, with hardwired connections becoming less critical for most uses. 

But to achieve such service one needs a larger communications highway, which means the use of more of the electromagnetic spectrum.   Electromagnetic energy, such as radio, microwaves, and visible light, are characterized by ranges of wavelength and frequency.  The use of these wavelengths is controlled by our government, which can auction off specific frequency/wavelength bands.

  Among the spectrum recently auctioned off by the FCC for 5G is a band of frequencies near 24 GHz (GHz is gigahertz, or a billion cycles per second).   Unfortunately, this is close to 23.8 GHz, a frequency in which water vapor emits microwave radiation and which is used by weather satellites to determine the three-dimension properties of the atmosphere.  And that information is very important for providing the description of the atmosphere that is required for numerical weather prediction.

Why weather satellite information is important for numerical weather prediction

Numerical weather prediction, the foundation of all weather forecasts, depends on securing a comprehensive, three dimensional description of that atmosphere--known as the initialization.  The better this initialization, the better the forecast. 

One of the key reasons why modern numerical weather prediction has gotten so good is that weather satellites now provide 3D data over the entire planet.  Even over remote oceans and the polar regions.  Roughly 95% of the total volume of weather information now comes from weather satellites.

Before weather satellites, radiosondes were the main source of 

weather information above the surface

And the most important source of weather information is from a collection of satellites that contain microwave sounders.  These satellites observe the earth by sensing microwave radiation being emitted by water vapor, liquid water, ice, and the surface. 

The amount of radiation being emitted can be related to temperature.  And different wavelengths/frequencies reveal the conditions at different levels of the atmosphere.   To put it another way, by sensing emissions at various wavelengths, one can secure a profile of temperatures at various levels in the atmosphere.  Kind of like have radiosondes (balloon-launched weather observations) everywhere.  Very valuable information

The Microwave Sounder Unit on the AMSU-A satellite

What is the most valuable of all satellite observations? 

Satellites with microwave sounders like AMSU-A (see below).  That platform ALONE contributed to a 17% reduction in forecast error in the European Center global model (the world's best)

AMSU A looks at the atmosphere in 15 wavelength/frequency bands or channels,  including sensing the atmosphere at wavelengths that the atmospheric water vapor has peaks in emission (see below). 

Channel 1 is at 23.8 GHz.   The problem is that the FCC has sold off 24 GHz, which is very close to 23.6 GHz.   And if the 5G transmitters aren't very high quality, with little spread to neighboring frequencies, they could well interfere with the microwave weather satellites. 

Why?  Because the weather satellite have very, very sensitive receivers because they are trying to sense the weak microwave emissions of atmospheric water vapor.  These sensors could be overwhelmed by the active TRANSMISSION in nearby wavelengths by thousands of 5G cell tower transmissions or other sources.

And the problem is even worse than that.  The FCC is planning to auction off more wavelengths/frequencies, some of which are close to other wavelength/frequency bands used by the weather satellites.

The potential harm to U.S. and worldwide numerical weather prediction by interfering with the 23.8 GHz band is certainly real, but difficult to quantify exactly. 

First, it will depend on the characteristic of the 5G transmitters and to what degree they will contaminate the nearby weather observation bands.

Second, it depends on how many wavelength bands would be affected.

Third, cell phone coverage does not include the entire planet.  One analysis suggests that only 34% of the earth's surface has cell phone coverage, suggesting that roughly 90% of the planet would be clean of interference (71% of the earth's surface is covered by water).  But if plans to establish satellite-based 5G on commercial ships and aviation come to fruition, the problem would be much worse.

NOAA, NASA, and U.S. Navy are quite concerned about this issue, with the Navy writing a strong statement of the potential harm.  On Thursday, NOAA Administrator Neil Jacobs warned of a potential loss of  1/3rd of current forecast skill.  These warnings need to be taken seriously.

The key now is to have close coordination between the FCC and NOAA/NASA/DOD, as well as other international players, to ensure that spectra close to the weather observing frequencies are not used and, if there are, investments in high-quality transmitters, with effective filters, are required by law.  

Improved forecast skill derived from weather satellites has had huge positive impact on saving lives and property, and in fostering economic growth.  Reasonable actions must be taken to protect the value of weather observations from space.