This follows a warning from the US Navy that 5G mobile networks could interfere with weather satellites. The FCC began selling spectrum in the 24 GHz band in March.
Democrat Senators Ron Wyden and Maria Cantwell asked Pai "not to award any final licences to winning bidders for future commercial broadband use in the 24 GHz spectrum until the FCC approves the passive band protection limits that NASA and the National Oceanic and Atmospheric Administration determine are necessary to protect critical satellite-based measurement of atmospheric water vapour needed to forecast the weather".
In its letter, the US Navy said: "Remotely sensed observations (water vapour) may be degraded or lost due to growing interference from the broader adoption of 5G; specifically, in the 24 GHz bands.
The two Democrats pointed out that the FCC had begun the auction of spectrum between 24.25 and 25.25 GHz for future commercial broadband use despite objections from NASA, NOAA and the American Meteorological Society.
They said these entities had all argued that out-of-band emissions from commercial broadband transmissions in the 24 GHZ band would disrupt the collection of water vapour data measured in a neighbouring frequency band (23.6 to 24 GHz) that meteorologists relied on for weather forecasts.
Listing the problems with operating in the 24 GHz band, the Navy letter further said NOAA and NASA had conducted studies that showed interference in passive collection at the 23.6-24 GHz band from the adjacent 5G band (24.25 GHz).
"As such it is expected that interference will result in a partial-to-complete loss of remotely sensed water-vapour measurements. It is also expected that impacts will be concentrated in urban areas of the United States first."
The letter said an additional assumption was that if the US expanded into the 24 GHz band, other countries would follow suit and thus impacts would eventually be worldwide, concentrated near densely-populated areas.
Thanks to Ars Technica for links to the two letters.