ADS-B vs. SiriusXM datalink weather – what’s the difference?

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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.

Garmin’s GDL 52 receives both ADS-B and SiriusXM.

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.

The basics

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.

ADS-B:

  • Information is broadcast up from ground stations
  • No monthly subscription is required
  • Includes the essential weather information (radar, METARs, TFRs, etc.)

SiriusXM:

  • 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.

Coverage

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:

Weather Products

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:

Radar

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:

Final analysis

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.

Shop all ADS-B and SiriusXM receivers here

11 COMMENTS

  1. The Sirius “big deal” is getting radar on the ground. This is a key component of the PIC’s go/no-go decision making when waiting in the FBO or until at-altitude is simply not an option.

    • If you have a ADS-B you will still get all wx information on the ground through an Internet connection such as LTE or 4G If using Garmin Pilot or ForeFlight. These apps will switch over to ADS-B once a loft.

  2. Ability to get wx on the ground, in the airplane, is important. XM is one solution. Having an iPad with Cell data is my solution. It works most places I fly from and provides better info than XM for preflight. Some pilots opt for a slightly cheaper WiFi only iPad, which really limits connectivity.

  3. Al Benzing got it right. Get an iPad with cellular capability for a quick review of weather products before you depart. Remember that convective weather changes pretty rapidly. What you see now will be different in minutes or hours. While enroute, check ceilings, surface wind and visibilities for the complete picture of areas with convective activity. Nexrad isn’t the complete picture at your particular cruising altitude. The money you save on the annual Sirius subscription will pay for the iPad cellular option up front and part of avgas afterwards.

    Alaska? FIS-B weather via ADS-B is available. Sirius is not.

    Like music when you fly? Plug in your iPod. The $30/month, plus, that you don’t pay for Sirius over the years can buy a lot of other things.

  4. The illustrations of XM Radar on Foreflight are not exactly correct. We use a Garmin GDL52 with Foreflight, and a Foreflight XM subscription. When you display Radar Composite, Foreflight will also display all of the Lightning strikes, and in most cases will actually cover up the cell activity. Foreflight has a separate selection tab labeled “Lightning”, and if you select it, there are even more little lightning bolts. It is not a Sirius or Garmin issue, it is a Foreflight issue, and I have been in constant contact with Foreflight, and forwarded in-flight photos of this issue. If I was able to attach a photo here I would. Foreflight informed me that the Lightning strikes are defaulted ON with either Radar Composite or Radar Base selections. It is very frustrating when trying to decipher a storm system when you can’t even see what is going on from all of the Lightning Bolts covering everything. Foreflight claims their engineers are working to either have Lighting displayed or not, like the Garmin Pilot does. In order to get better interpretations of the convective weather situations, I also subscribe to Garmin Pilot just for the less cluttered Radar displays. If anyone would like validation of this issue, feel free to reach out and I will forward photos and the emails from Foreflight admitting to their downfall with the interfacing of XM in their platform.

  5. I used XM for 10+ years and have been using ADS-B in for 2. Besides contrasts mentioned above, the thing that really ticked me off about XM (now Sirius) was the more than insignificant number of flights I was rushing to get out ahead of fast moving weather and found my XM receiver needing an activation refresh before it would receive weather. For those who fly every day or have a staff, no big deal, for me it meant scrambling at the field to make a call to XM and hoping it would load in a timely manner…the fact that XM just shrugged when called on it made it easy for me to leave XM/Sirius behind.

    • I’ve had that issue occasionally, Rich. The good news is that SiriusXM now offers the ability to do a refresh online so there’s no need to call anyone. You can be sitting on the ground and do it here: siriusxm.com/refresh

      • Thx John. On a normal day that would be better. When pushed by incoming weather, the flow disruption is still unacceptable for me as it isn’t evident until powered up outside the hangar. The better fix would be a subscription notice when they roll over to the next code…at least you’d be able to plan/allow for it. Now moot for me, my flying with ADS-B has had it’s issues, but suffices for my short hop convective awareness.

  6. Al & Manny,
    You make good points about having an iPad with cell capability. However, most modern smart phones can function as a hot spot which allows a WiFi only iPad to connect to cell service through the phone. It may be a bit slower, but it saves the cost of the more expensive iPad plus the monthly cost of cell service for the iPad. Unless you are a real data hog, the extra data used for the flying information probably won’t affect your monthly cell phone charges.

  7. The biggest advantage of ADS B over XM is NOTAMS. I use both GDL51 XM and the Sentry ADS B together to provide the most information. ForeFlight uses the weather picture from XM and METARs and NOTAMS from ADS B. Plus I have traffic with ADS B IN displayed on FF.

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