Datalink weather, either from ADS-B or SiriusXM, is an essential tool for almost all pilots. Once you’ve flown a cross country with in-flight radar, up-to-date METARs, and visual AIRMETs, it’s awfully hard to go back to flying without it. It makes flying safer, easier, and more comfortable – a rare combination.
If you’re considering a datalink weather receiver for your iPad (like a Sentry, Stratus, or GDL), one of the first decisions you’ll face is the source of your weather data: ADS-B or SiriusXM? Both are reliable systems that deliver the same key information, so neither one is a bad choice. But there are important differences to consider. Let’s review each option.
There are enough acronyms to confuse even the most experienced pilot, so let’s begin with the essential information. If you’re shopping for a weather receiver, you probably know the quick features of ADS-B and SiriusXM.
- Information is broadcast up from ground stations
- No monthly subscription is required
- Includes the essential weather information (radar, METARs, TFRs, etc.)
- Information is broadcast down from geostationary satellites
- A subscription is required ($30-100/month)
- Includes higher end weather products (satellite, storm cell information, etc.)
All of those bullets are correct, and they summarize the main differences. A deeper dive into the details, though, may help you make a smarter decision.
The first thing to consider is coverage area. After all, the best weather receiver is useless if you won’t be able to access the information you want. And here’s the first major difference between the two systems.
ADS-B uses a network of over 700 ground stations to broadcast weather on the 978 MHz frequency. Like a VOR, if you have a radio tuned to the right frequency (and with an ADS-B receiver, you do) then you’ll get weather. Also like a VOR, reception is based on line of sight, so higher altitude improves reception and mountains prevent reception. East of the Mississippi, ADS-B coverage is quite good but you likely won’t receive weather on the ground (at Sporty’s airport we typically get reception at 200-300 feet and the closest tower is about 25 miles away). Almost the entire country has coverage at 3-5,000 feet AGL, but if you’re flying over the Rockies at low altitude, coverage can be spotty.
Here’s a map from the FAA, estimating ADS-B coverage at 5,000 feet AGL:
It’s worth noting that ADS-B uses different types of ground stations that transmit different weather products. In everyday flying, this doesn’t matter – one ADS-B ground station is enough to get weather, but you’ll typically receive somewhere between 3 and 12 towers. The only time this difference between towers comes into play is if you’re only receiving one tower and it’s a surface station. In that case, you might not see national (CONUS) radar.
SiriusXM, since it uses satellites, has no altitude limitations – you’ll receive all weather products even on the ground. This makes it ideal for pilots flying at low altitude in remote areas. It also offers some coverage in southern Canada and the Caribbean, but it’s important to note that satellite reception does not guarantee there is weather data for your location. That is, you may get good SiriusXM reception in the southern Bahamas, but there is no radar data to display.
Here’s the coverage map for SiriusXM:
After reception, the next difference is in the weather products that ADS-B and SiriusXM broadcast. Both transmit the most important ones, including: NEXRAD radar, METARs, TAFs, PIREPs, AIRMETs, SIGMETs, lightning, and TFRs. Those are the tools most pilots need to avoid thunderstorms, IFR conditions, and restricted airspace, but there are some details to consider.
SiriusXM offers four main subscription packages, and depending on the subscription level you can access additional weather products. This includes freezing level graphics, surface wind forecasts, and both cloud-to-cloud and cloud-to-ground lightning (ADS-B only shows the latter). Note that you can suspend SiriusXM service for up to six months per year, so with a Garmin GDL 52 a seasonal flyer could use SiriusXM during the peak flying months and ADS-B during slower months.
Perhaps the most useful additional product on SiriusXM is Storm Cell Attributes. This adds echo tops, direction of movement, and speed of movement to the typical radar image. These extra data points can help you determine whether that yellow cell is convective or just rain. In the example below, that line of weather has tops between 25,000 feet and 45,000 feet, and is moving east at a fairly good pace. That suggests real convection and a nasty ride:
While SiriusXM has the higher end weather products, ADS-B has closed the gap recently by adding lightning, cloud tops, Center Weather Advisories, and icing forecasts. The biggest thing missing from that list is satellite, although we would put that in the nice-to-have category, not the must-have category.
Also note that while SiriusXM transmits all weather data simultaneously, ADS-B will often only show METARs and TAFs within about 500 miles of your airplane. Here’s a side-by-side comparison of ADS-B and SiriusXM weather products:
One final topic that gets a lot of attention when it comes to weather products is radar. You’ll often hear something like, “ADS-B radar is blocky; SiriusXM is high resolution.” That’s sort of true, but it doesn’t tell the whole story; it depends on two questions. First, what receiver are you using and where are you displaying the radar (ForeFlight on your iPad, G1000 screen in the panel, etc.)? Some apps and avionics do a lot of radar smoothing to make it look higher resolution than it really is. That’s not a bad thing necessarily, but it doesn’t have much to do with the raw radar data. Make sure you understand your system and what it’s showing.
Secondly, while SiriusXM has a single resolution nationwide, ADS-B uses a higher resolution regional NEXRAD and a lower resolution national NEXRAD image. Here’s a side-by-side comparison of ADS-B regional radar and SiriusXM, as displayed in ForeFlight. As you can see, the resolution is basically the same:
The difference is that SiriusXM radar is full resolution nationwide. In the example below, we can see full resolution radar in northern Wisconsin, even though our airplane is 800 miles away:
With ADS-B radar, you can see higher resolution radar close to the airplane (near Tulsa, Oklahoma), but further away the radar gets blockier (into Mississippi). That blockier image is the national radar picture, and as you get closer to it, the image will change to the regional radar image:
In our experience, the two resolutions of ADS-B radar are not a major limitation unless you’re flying a high performance aircraft. Red is red, no matter how good the resolution. In a Cessna 172 or a Cirrus SR22, for example, 250 miles (the range of regional radar on ADS-B) is a long time – probably over two hours in the Cessna. The fact that longer range radar is blocky does not change the way we fly because there’s plenty of time to look at the higher resolution radar when we get closer.
On the other hand, if you’re flying a jet at 400 knots, that blockier ADS-B radar beyond 250 miles may be a more important limitation. It does make long range planning a little more difficult at high speeds.
In our experience, SiriusXM tends to overestimate the weather compared to ADS-B. This depends on the app to some extent, and conservative is a good way to fly, but there are many days where a green return on SiriusXM does not show up on ADS-B. Here’s an example:
One reason for this difference is that ADS-B and SiriusXM show different colors at different levels of reflectivity (dBZ). Note the chart below, where 15dBZ is green on SiriusXM but blank on ADS-B:
One final detail to consider is base vs. composite reflectivity. This is a complicated subject, but in essence, base radar shows the lowest scan angle from a radar site – what’s coming out the bottom of the cloud. Composite radar shows the highest reflectivity from all scan angles – the worst case, even if the only red in that cell is at 35,000 feet. Composite radar is the most conservative option, since as pilots we fly in three dimensions and we want to know what the weather is doing aloft. For this reason, most pilots use composite reflectivity, and both ADS-B and SiriusXM transmit this product.
However, SiriusXM offers the option to view base reflectivity as well. This is mostly useful as a spot check or for real weather geeks who want to get a detailed view of the weather. If the composite picture shows solid green but the base picture is clear, you can probably assume the rain is at high altitude and none of it is reaching the ground. This could be useful for a VFR pilot who will be flying under an overcast – will there be rain showers to reduce visibility?
Here’s an example of base and composite reflectivity in ForeFlight. You can see that composite shows a lot more yellow, suggesting that most of the rain is aloft and only light rain is falling at the ground:
As you can see, a detailed discussion of ADS-B vs. SiriusXM quickly gets confusing. But strip away all the talk of dBZ and ground stations, and the message is clear: fly with some type of datalink weather. The similarities are much more important than the differences, and when used properly we believe either service can improve your safety.
If you fly a piston airplane 50 or 75 hours per year, we think ADS-B is a perfectly good solution. You won’t miss the higher end weather products and the lower resolution national radar is not a practical limitation. Since there’s no subscription, it’s a great value – just buy a receiver and start looking weather.
If you fly a high performance airplane, fly at low altitude or fly in Canada, SiriusXM is probably worth the extra money. You won’t have to worry about different radar resolutions and some of the additional weather layers can be very helpful. Especially if you fly a lot of IFR, having a satellite image or storm cell information can make the difference between an easy flight and an uncomfortable one. You can even add SiriusXM audio so passengers can listen to music or sports in flight. Just remember you’ll be paying at least $180/year extra for SiriusXM over ADS-B – only you can decide whether that’s a good investment for your flying.
Whichever system you fly with, make sure to use it properly. This Flying magazine article has some good advice for using datalink weather as a safety tool and not as a crutch. Remember, it’s for big picture awareness and large deviations – not for picking your way through a tightly packed line of storms.