FIS-B Weather in the Cockpit, the Free NEXRAD Picture Coming Up From ADS-B, and the Latency Trap That Makes a Storm Look Where It Was, Not Where It Is

FIS-B is a free weather broadcast that sends NEXRAD radar, METARs, TAFs, winds aloft, PIREPs, and advisories to your cockpit over the 978 MHz ADS-B link. It is one of the most valuable tools a light-airplane pilot has gained in a generation, but the radar picture it shows is strategic, not tactical: by the time it reaches your screen, the image can easily be five minutes old, and its effective age can push toward 15 to 20 minutes. Use it to stay tens of miles away from weather, never to thread a gap between cells.

What Is FIS-B and How Do You Get It?

When the FAA built the Automatic Dependent Surveillance-Broadcast (ADS-B) system, it used two separate radio links. Most of the world and most larger aircraft use 1090 MHz. In the U.S. light general aviation world, there is a second link at 978 MHz, called the Universal Access Transceiver (UAT).

The FAA used spare capacity on that 978 MHz link to broadcast a free package of weather and flight information to any aircraft listening. This is Flight Information Services-Broadcast (FIS-B). If you have ADS-B In on the 978 MHz link, you get it automatically - no subscription, no monthly bill.

The package is genuinely impressive: regional and national radar mosaics, METARs and TAFs, winds aloft, pilot reports (PIREPs), TFRs, NOTAMs, special use airspace status, and AIRMETs and SIGMETs. Twenty years ago you would have needed a flight service briefer on the phone to get half of it.

Where Does the Cockpit Radar Picture Come From?

The star of the package is the NEXRAD mosaic. NEXRAD stands for Next Generation Radar, the network of large ground weather radars the National Weather Service operates across the country - the spinning dishes housed inside the giant white “golf ball” domes on hilltops.

As a strategic tool, it is superb. Seeing a line of weather 200 miles ahead and deciding early whether to deviate east, west, or land and wait is a capability light-airplane pilots simply did not have a generation ago. It has saved lives.

Why Is Datalink NEXRAD Older Than It Looks?

The radar picture is not live, and it was never designed to be. Delay accumulates at every link in the chain:

1. The radar scan. A NEXRAD dish sweeps in a circle, tilts up, and sweeps again to build a 3D volume scan. One full volume scan takes about four to six minutes, so the data is already minutes old before it even leaves the site.
2. The mosaic. Individual radar sites are stitched into the seamless national composite, which takes additional time.
3. The broadcast schedule. Regional high-resolution radar goes out roughly every couple of minutes; the national picture updates less often.
4. Reception. Your aircraft has to fly within range of the ground stations to actually receive the broadcast.

Add it all up and the radar image on your screen can easily be five minutes old, with an effective age pushing toward 15 to 20 minutes on a bad day.

The Latency Trap: Why the Timestamp Lies

Here is the part that has killed people. Your display shows an age - a little timestamp that might read “2 minutes.” You glance at it and think two minutes is basically now.

But that number tells you how long ago your aircraft received the broadcast - the age of the transmission, not the age of the weather in the picture. The moment the radar physically swept that thunderstorm could be 10 to 12 minutes before the timestamp your box displays. The clock starts when the data is packaged and sent, not when the storm was seen.

The FAA studied this after fatal accidents in which pilots used cockpit radar for tactical maneuvering - threading gaps between cells. The NTSB found that the gap the pilot flew for had already filled in by the time the aircraft arrived. The picture said clear; the sky was full of hail.

Why Thunderstorms Make This So Dangerous

Thunderstorms are exactly the weather where latency bites hardest. A mature cell can build at thousands of feet per minute. A gap between two cells can close in the time it takes to fly to it. And a storm system tracking across the ground at 30 to 40 knots means the red blob on your screen is already downrange of where it appears. You are looking at a photograph of the past and flying into the present.

What Are the Real Pros and Cons of FIS-B Weather?

The pros are real. It is free, nationwide, subscription-free, and delivers strategic weather awareness at a range light aircraft never had. For the big-picture decision - go or no-go, deviate now or later, land and wait - FIS-B NEXRAD is excellent. Many pilots bought ADS-B In for the traffic and got the weather as a bonus.

The con is equally real. This is a strategic tool wearing a tactical tool’s clothing. It looks precise and live, with colors, sharp edges, and a stopwatch-style timestamp - and everything about that presentation invites you to trust it up close, which is exactly where it fails.

The rule every safety organization landed on is worth memorizing: Do not use datalink NEXRAD to penetrate a line of weather or thread between cells. Use it to stay far away from the whole area - give convective cells a wide berth, commonly quoted as 20 miles - and never assume a gap on the screen is really there. For true tactical, close-in avoidance of a specific cell, you need onboard radar or a lightning detector like a Stormscope - something sensing the weather in real time from your aircraft, with no delivery chain behind it.

Is Paid Satellite Weather (SiriusXM) Any Better?

The satellite subscription services solve a different piece of the problem. Satellite weather often refreshes a little faster and reaches you at low altitude and on the ground, where FIS-B ground stations can’t always reach - because it comes down from a satellite instead of up from a tower.

But it is the same underlying NEXRAD data with the same fundamental latency baked in. Paying for the subscription buys you a fresher delivery of an old photograph; it does not make the photograph live. Neither product is tactical. Both are strategic. Anyone who claims their weather subscription lets them pick through a squall line will eventually have a very bad afternoon.

Is FIS-B Getting Better?

The technology keeps improving at the source. The NEXRAD network was upgraded to dual polarization, sending its pulse both horizontally and vertically. That lets the radar distinguish rain from hail, snow, and even flocks of birds far better than the old single-polarization radars.

The radars can also run a mode called SAILS, which sneaks in an extra low-level sweep partway through the volume scan, updating the near-ground picture more often when severe weather is around.

But none of that fixes the delivery chain. As long as the data must be scanned, stitched, packaged, scheduled, and broadcast, there will be latency between what the radar saw and what you see. The pipe is the problem, not the radar - and there is no version of FIS-B on the horizon that turns that strategic picture into a live tactical one. The physics of the delivery won’t allow it.

Who Builds and Delivers This Data?

Credit is shared across several organizations. The radar network belongs to the National Weather Service and the FAA. The FIS-B broadcast infrastructure and ground stations were built and operated under the FAA’s ADS-B program. The avionics that turn those radio bits into a picture come from the industry - Garmin, Avidyne, and portable options like ForeFlight paired with a receiver on the glareshield. Every handoff between the storm and your eyeball adds a few more seconds of age.

Key Takeaways

• FIS-B weather is free over the 978 MHz UAT link with ADS-B In - no subscription required.
• The cockpit NEXRAD picture can be 5 minutes old, with effective age reaching 15–20 minutes.
• The on-screen timestamp shows when you received the data, not when the radar saw the weather - the storm can be 10–12 minutes older than the number suggests.
• Treat datalink radar as strategic: keep your distance in tens of miles and never bet the airplane on a gap between cells.
• For tactical cell avoidance, use onboard radar or a lightning detector - paid satellite weather carries the same NEXRAD latency.