Understanding pressure altitude and GPS altitude in aviation apps
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Ten years ago, ForeFlight introduced synthetic vision and gave pilots a new way to view the world on their iPads. Nowadays, most other aviation apps offer a similar view. In addition to a 3D view of terrain and obstacles this shows glass cockpit-style tapes for speed and altitude, making it an excellent backup tool in case of panel avionics failure. But if you’ve flown with synthetic vision for long, you’ve probably noticed that the speed and altitude don’t match the panel. What’s going on?
The answer is pretty simple—the panel and the iPad are showing different data from different sources—but the details are important. Here’s a somewhat geeky dive into the details.
Airspeed and altitude differences
Consider speed first. The panel in your airplane shows indicated airspeed (IAS, derived from the pitot tube on the wing), while the iPad shows groundspeed (derived from the GPS in your ADS-B receiver or iPad). These are significantly different things, since one shows the speed at which your airplane is moving through the air mass and the other shows the speed at which your airplane is moving across the ground. If you’re worried about stall speed or best glide speed, indicated airspeed is the number you care about—this is the speed the wing “feels.” Groundspeed is helpful for estimating your time en route, but it is affected by wind and other factors. It’s helpful as a rough guess in an emergency, but it’s hardly a replacement for IAS.
Altitude seems like it should be easier to compute, but similar problems quickly pop up. Take the example below, where we’re flying at 11,500 feet in a Cirrus and connected to an ADS-B receiver. Even though we are level at 11,500 with the autopilot on, ForeFlight shows 11,910 for altitude—a difference of some 410 feet. Your first clue is just below the altitude tape, where the app tells you it’s using GPS for the altitude source.
One source of error here could be GPS accuracy. A GPS receiver needs at least four satellites for a 3D position fix (one that includes altitude in addition to lat/lon), but even with 10 satellites and WAAS correction signals the vertical error can easily be 10 to 20 meters. That’s over 60 feet, which is enough to make a noticeable error, but not enough to explain the 400 foot difference we see above.
Just like we saw with airspeed, the real issue is that you’re drawing data from two different sources: the panel is showing indicated altitude (that is, pressure altitude from the pitot-static system, corrected for the local altimeter setting), while the iPad is showing geometric altitude (from the ADS-B receiver’s GPS).
Pressure and temperature
Geometric altitude does not correct for non-standard temperature or pressure. Most pilots think about pressure, since we’re constantly adjusting the altimeter setting, but even that isn’t enough to explain the difference. Look at the example below, from a completely different flight where we were at 32,000 feet in a Citation. At 18,000 feet and above all airplanes set the altimeter to 29.92 (standard pressure), so that should be pretty close to the GPS altitude, and yet ForeFlight still shows us 300 feet high.
Confused yet?
The answer (or at least most of the answer—we’ve already geeked out enough) is temperature. This plays a big role, especially in summer. Back to the Cirrus flight we’ve been discussing. On this warm day, the temperature at altitude was -5 degrees C, which was 13 degrees warmer than standard (ISA+13 in the jargon). While the altimeter in the airplane can be adjusted to correct for the local altimeter setting, there is no temperature knob to twist. Thus pressure altitude (what the altimeter is sensing) will almost never match geometric altitude (what the GPS is calculating).
You can see the difference from the ADS-B Out Ownship menu in ForeFlight. When connected to an ADS-B receiver, tap the gear symbol at the top left of the Maps page, then tap your ADS-B receiver (in this case Sentry), then tap Ownship. You’ll see under “Location” the two different altitudes.
The science can get a little confusing, but the takeaway is simple: GPS altitude will almost always differ from your indicated altitude and when the temperature is above standard GPS altitude will display higher than pressure altitude. Use it as a ballpark number, not a precise number to fly.
Pressurization check
The difference between pressure altitude and GPS altitude isn’t all bad. In a pressurized airplane you can use the difference to check the status of your pressurization system. Here’s an example where we were flying in a Pilatus PC-12 at 15,000 feet and using a Sentry Plus ADS-B receiver, which has a built-in pressure sensor. This allows you to set one of the data blocks at the bottom of the Maps page in ForeFlight to show cabin pressure—a direct reading of the pressure altitude where the Sentry Plus is mounted. Compare this to the GPS altitude, which should be higher if the pressurization is working properly. In a climb the GPS altitude should be going up much faster than the cabin altitude, and it’s also a good idea to check the cabin pressure number as you climb through 10,000 feet.
Can you display IAS and IALT?
All hope is not lost. You can display actual airspeed and indicated altitude on your iPad, but only if you have the right equipment.
One option is to use panel avionics. The screenshot below shows the Garmin Pilot synthetic vision screen, while flying at 10,500 feet. The altitude tape shows 10,500 on the dot.
Notice the data label above the altitude tape that says “PRESS ALT.” This is your clue that Garmin Pilot is actually displaying pressure altitude, not GPS altitude. That’s only possible because the app is reading pressure altitude directly from the airplane’s air data computer (this particular airplane has a Garmin G500 TXi glass panel) over a wireless connection to the Flight Stream 510. This essentially pulls actual panel-mount sensor data and sends it to the iPad. So in this case, the app shows a perfect reproduction of the panel instruments—you’ll even see IAS above the speed tape, indicating it is showing indicated airspeed.
ForeFlight has also added this option in a recent app update, but once again only if you have the right equipment on board (in this case, a portable ADS-B receiver with a built-in pressure sensor). The app uses the nearest airport’s altimeter setting from a METAR, via the ADS-B receiver, and applies the baro correction. You’ll see the label “BARO ALT” and the current altimeter setting value displayed under the altitude tape when ForeFlight is using this source.
The end result is that you’ll now see the exact same altitude displayed in ForeFlight as on your instrument panel, provided your airplane’s altimeter is set to the same local altimeter setting. This significantly increases the utility of ForeFlight’s backup instruments and synthetic vision display, making it even more useful for instrument pilots who need to get down on the ground after a primary instrument failure in IFR conditions.
Super and informative article. Very helpful. Well done!
I have a question. Lets say your performing a night flight over a mountainous terrain. Your out in the middle of nowhere so your not sure if the pressure setting from the last airport is correct still to accurately allow you to determine if you can clear the mountains ahead at 9500 feet. Should you trust the GPS altitude in order to clear those mountains since the VNC map height is showing a known set height (non pressure affected height on the map) for those mountains?
The only safe answer here is “margins.” I would trust the GPS altitude, but with at least a 500 foot margin and probably even more. It would be rare to see a dated altimeter setting or a GPS-to-pressure altitude error produce a difference of 700 feet, for example.
Like anything in aviation, use all the tools you have, but always give yourself some margins and have a plan B.
I would never trust the GPS derived altitude as I see that constantly incorrect (and I use 3 different GPS sources). I would look for altimeter settings for 3-4 airports even if they are 30-50 miles away to gain an understanding of what my altimeter setting might be “in the middle of nowhere”.
Great article. Thank you. I do have a practical question: Let’s say for sake of example that I am flying low, and I want to make sure that I am at least 1000 feet AGL over populated areas to make sure the FAA doesn’t get upset. Which altitude is more accurate – pressure altitude, or GPS altitude? Put differently, which altitude would the FAA look at to determine whether I am at a legal altitude? Another example would be if I had an engine out and wanted to know how many feet I had available to glide.
It’s hard to say for sure, but in general I would use pressure altitude. That’s what we all fly with on our altimeters.
What about altitude shown for other traffic, on ADS-B? Say I’m at 9500′, and I see nearby traffic displayed at or near my altitude. Is he displaying pressure altitude (altimeter set at 29.92), altitude corrected for local pressure, GPS altitude, or …?
ADS-B altitudes are all pressure altitudes, referenced to 29.92. If the local altimeter setting is not 29.92, your reported altitude will not be your indicated altitude. Go look at FlightAware for your last flight and refer to your reported altitude. It’s likely quite different than the indicated altitude you flew.
But since all ADS-B altitudes are reported as pressure altitudes, your traffic display will accurately show if traffic that is 200´above you, or 500´below you. Your traffic display is calculating YOUR pressure altitude compare to the pressure altitude of detected traffic.
One thing to keep in mind is that many times GPS altitude is calculated as Height Above Ellipsoid whereas what we normally think of as MSL is Height Above Geoid. The nice thing about altitude reference datums is that there are so many to choose from. 🙂 Seriously, though, whenever you talk about altitude you *must* take into account which reference plane you are measuring from. They frequently differ.
+1
In reference to the altitude question, which altitude is ATC seeing on their radar scopes? I’ve often been told the altitude ATC sees is off from my assigned altitude although my altimeter is showing the assigned altitude and I’m using the assigned pressure. When i ask what altitude the are showing me at, it matches my ADS-B altitude.
Could it be time to switch to GPS altitude for all cases and systems? I realize not all GA aircraft have GPS capability perhaps, but with ADS-B isn’t the number of aircraft without GPS reference greatly reduced?
Thanks,
Steve
The article states “ A GPS receiver needs at least four satellites for a 3D position fix (one that includes altitude in addition to lat/lon),”. Please correct this misguiding statement. I’m a DPE that hears this over and over from new instrument students who get the impression that there is such a thing as a GPS 2D fix and that another satellite provides altitude. FALSE.
The GPS works on the concept of intersections that exist at known distances from multiple reference points (satellites). Let’s start with one satellite in view which the GPS receiver calculates is 10000 miles away. What is the solution set of ALL possible points 10000 miles away from one point? It is a sphere 10000 miles in radius from the satellite. With 1 satellite in view, we have an infinite number of solution locations. Let’s add another satellite which is found to be 12000 miles away. With two sats in view, we know that we are simultaneously 10000 miles away from the first sat, and 12000 miles away from the seconds. Imagine two spheres, maybe a beach ball and a volleyball, pressed together. Imagine that they can pass thru each other, but think about the shape of their intersection in space, the points that are simultaneously 10000 miles from the first and 12000 miles away from the second. The intersection of two spheres is usually a circle, with still an infinite number of points. Now bring in satellite 3, which defines another sphere. We now consider what the possible solution points are when a circle intercepts a sphere. Image a hula hoop that can pass thru a beach ball. There are usually 2 points of intersection, where the hula hoop enters the beach ball and where it exits. With 3 satellites where have narrowed our possible location down to TWO points, but we still don’t know which one. We could make some reasonable guesses if one point was 10000 above KLAX, and the other was deep inside the earth. But what if one point were 10000’ over KLAX and the other is 5000’ over KDFW? Only by having a 4th reference can we determine which is the true position. An local altitude reference “might” work if the two possible locations differ greatly in altitude. OR, we can bring in a 4th satellite which will pin down which of the two possible location meets the simultaneous distance requirements from all four satellites. Bottom line, the GPS system solves a set of simultaneous equations from 4 (or more) satellites to provide a fix, a 3D location. There is NO SUCH THING as a 2D fix using GPS, It’s a 3D fix, or nothing.
So often I hear that it takes 3 sats for a 2D fix, and a fourth to add altitude. Nope. 3 sats don’t provide any single fix. 4 sats are the minimum to provide a solution, and it’s a 3D solution. Sorry this is such a long post!
I think I understand what you’re trying to say, but the whole world (including NOAA) seems to be against you: https://oceanservice.noaa.gov/education/tutorial_geodesy/geo09_gps.html
I just read the above referenced article. It says exactly what I described above, but in a much more concise way. I’ll bookmark it for future reference! Thank you.
Three satellites will define two points in space (both of which are 3D). The fourth one is needed to determine one location in space. Until the receiver has 4 references (3 sats and an altitude, OR 4 sats with no altitude) it can’t provide a solution. But the 4th satellite does NOT provide an altitude, it’s used to eliminate one of the two possible positions defined by the 3 satellite calculation.
Whereas 3 sats and an altitude *might* select the correct fix, it will run into a problems if of both of the candidate fixes are at the same altitude, the quandary persists. That’s why altitude assist GPS units are pretty rare.