Military Radar Can Make Your Router Switch Wi-Fi Channels

You might notice that every so often, your dual-band router drops its 5 GHz network without warning. Phones and laptops connected to it drop down to the slower 2.4 GHz band, video calls begin to stutter and freeze, and a few minutes later, everything comes back on a different channel. If you take a look in your router’s event log afterward, you might notice the word “RADAR” appearing throughout.

That’s Dynamic Frequency Selection, or DFS, in action. According to ASUS, it’s how the router gives way to military radar, satellite communication and weather radar that has priority on large parts of the 5 GHz band. In the United States, Federal Communications Commission (FCC) rules require a router to listen for radar on one of those shared channels for 60 seconds before it transmits, to stop within 10 seconds of hearing one and to stay off that channel for at least 30 minutes afterward. DFS is switched on by default, and the FCC requires it to stay that way in the shared bands.

The radar that’s most likely to interfere with a US home network isn’t the military, though, it’s those found at airports. The Federal Aviation Administration (FAA) runs 45 Terminal Doppler Weather Radar systems that protect 46 of the country’s busiest airports, from JFK and O’Hare to Dallas and Denver. Their job is to spot wind shear, the sudden and potentially dangerous change in wind speed and direction that has historically caused aviation accidents during approach and departure; they transmit between 5600 and 5650 MHz right in the middle of the 5 GHz Wi-Fi band. Per the FAA, no wind shear accident has occurred at a TDWR-protected airport since the system was commissioned in 1994.

In early 2009, the FAA began detecting interference to those radars caused by unlicensed 5 GHz equipment, prompting a joint investigation with the National Telecommunications and Information Administration (NTIA). Enforcement followed, and in August 2019, the FCC proposed $25,000 fines against three wireless ISPs — Boom Solutions, Integra Wireless and WinPR — after their equipment interfered with the weather radar at the San Juan International Airport in Puerto Rico. In each case, the FCC said the companies had turned DFS off. WinPR’s penalty was confirmed that November and Boom Solutions’ in early 2020. While those were outdoor point-to-point links and not home routers, they’re a good example of why router manufacturers can’t simply drop a feature because it annoys their customers.

That government-mandated 60-second listen happens each time a router spins up on a DFS channel, which is why 5 GHz can take longer than 2.4 GHz to reappear after you restart your router. ASUS says the wait can take anywhere from one to 10 minutes depending on the country. The upper end of the scale is in Europe, where the ETSI standard governing 5 GHz Wi-Fi demands a 10-minute check on any channel that overlaps the 5600 to 5650 MHz weather radar slot.

Radar detected mid-session triggers the 10-second exit and 30-minute lockout. FRITZ! says its routers steer connected devices to 2.4 GHz while they wait, “seamlessly” for devices that support the 802.11k and 802.11v roaming standards and by briefly dropping the connection for older ones. ASUS, meanwhile, notes that a radar hit on a 160 MHz channel can push the router down to 80 MHz, and that a router set to a fixed channel won’t move back once the radar goes away.

So, it’s fine to use DFS channels. They’re legal, enabled out of the box and most of the 5 GHz band is made up of them; only channels 36 to 48 and 149 to 165 are radar-free in the United States. The trade-off is occasional interruptions if you live near somewhere like an airport or military base. FRITZ!’s threshold is radar detected continuously or every hour, at which point it recommends pinning the router to channels 36 to 48.

ASUS provides instructions for disabling automatic DFS channels that also tell users to untick 160 MHz channel bandwidth, because the widest 5 GHz channels can’t be built without the radar-shared spectrum.

So, by turning DFS off, you’ll get some extra stability at the cost of losing out on the fastest 5 GHz connection your router can offer. That’s worth thinking about before you go hunting for tweaks that could speed up your router. You’ll find these settings alongside the options you’d change to improve your router’s security, usually under the 5 GHz wireless menu.