GPS Geeks: How accurate is GPS altitude.

Or at least GPS geeks geekier than I…

Let’s say I have a “3-d fix” (at least 4 satellites).

How much error do I have in that dimension? Is it the same as it would be if it were on the ground?

How accurate would it be in comparison to a properly calibrated sensitive pressure altimeter?

The reason I ask is because starting Monday, I’m starting to learn to fly hot air balloons. The guys I’m learning from pretty much don’t use GPS, but I’d personally prefer to use it. I want to know if GPS would be a suitable alternative to a barometric pressure-based system (the regulations don’t state that the altimeter must be pressure-based. The minimum equpment is an altimeter and VSI.)

If the error is similar to the ground-based error (~5 meters with WAAS), then it’s not big deal (Considering the balloon envelope is several times the size of the error). But if it’s any bigger, then I need to look in to picking up some personal instruments. Personally, I’d prefer to get by with only one piece of equipment.

An interesting article here.

Personally I’ve seen swings of 140ft in a 5 minute span sitting in a parking lot.

This is why some of the handheld units aimed at outdoor recreation include a barometric altimeter and an electronic compass. Such as Garmin’s eTrex Vista:

(Before you buy one, you might wish to wait for the rumored Cx model with the SiRF chip and transflash memory)

The figures I’ve heard suggest that GPS based altitude readings on consumer units have 3 times the error of the position readings.

This is correct - the Federal Aviation Regulations simply state that an altimeter is required in a balloon. But in other places they get into such things as procedures for testing altimeters, which seem to deal exclusively with pressure-based sensors. So this may need further checking.

While GPS altitude is rarely as stable as a pressure-based indication, I’m not sure this is important in a balloon - when you really care about altitude (as when you’re landing), you tend to rely on visual judgment. An exception to this would come if you plan to fly where airspace avoidance might be an issue (e.g. you need to stay below some overlying controlled airspace).

It may be worth noting that the errors in GPS altitude are more or less independent of height and temperature, whereas barometric indications aren’t. If you plan to fly much over 20,000’ (most unlikely in a balloon, but not impossible) then GPS altitude may be more accurate.

As far as the regs are concerned, I’m in Canada, and they’re somewhat different. I haven’t found any parts on testing yet. (I’m pretty up-to-date on the regs right now, as I’m just going through my commercial licence/multi/IFR ratings for fixed wing.)

And you’re right, I really don’t care that much about altitude, besides the rare airspace-evasion, and reporting my altitude should I wander in to a control zone. In that situation, accuracy really only needs to be within ~200 feet, considering the balloon itself is several stories high. The only other real use for an altimeter is find out at what altitude you are when you start moving a certain direction, but this doesn’t need to be accurate, just consistent (when the gps says 450 feet, the wind seems to be going about 275 degrees, and at 575, it’s 267 degrees… Whether I’m really at 450 feet or 575 feet doesn’t matter, provided it’s the same altitude as before)

Obviously, if I were in anything except a balloon, I wouldn’t even consider relying on GPS for altitude (hell, I don’t even even use it as a primary navigation aid, only as a second or third backup… I’m not always going to be in an aircraft with GPS equipment, so I try to keep my other skills sharp.)

That’s exactly the kind of article I’m looking for, thank you.

And I’ve seen those swings too, but only when the gps was having trouble locking on to sats, and switching between them. Once a positive lock is established, I find those errors disappear completely. Being in the air, there’s nothing to block the readings, so the GPS usually stays locked to the same sats for the whole time.
See, my problem is ballooning is so uncommon, (there’s less than 250 balloon pilots across Canada) that the regulations are rather vague (which appears a good thing, as ballooning seems to be more of an ‘art’ than a precise discipline, such as fixed-wing IFR, and therefore tends to need more lattitude) and any information on ballooning is extremely scarce.

Right now I’m using a basic yellow eTrex (the newer one, with WAAS), which is good as far as navigation is concerned (fixed wing, hooked in to a Palm Pilot with an airpace/aerodrome database, and obviously only VFR and as a secondary reference to my normal VNCs and WACs)

I’ve looked in to the GPS units with altimeters, as yabob suggests, but some seem to have a warning that fast ascents and decents can damage the altimeter (or at least throw off its calibration). Though looking through various GPSrs, the Geko 301 has :

Which in my mind, would only be of use to pilots. Perhaps I’ll look at investing in one of those, and leave my eTrex to my fixed-wing flight bag.

I’m having hard time understanding how this works mathematically/geometrically. GPS works by determining a position in space. It doesn’t use one method to determine where you are in an X-Y plane then a different method to determine your Z. Why would error in one direction be any different than error in any other direction?

(BTW once I took my GPS on a commercial flight and it was kind of cool to see that speed go over 500 MPH. I didn’t think to check the altitude, though.)

GPS works by measuring distances to multiple satellites at known locations. (Or to be strict, the difference in distances to multiple satellites.) To determine longitude you’d compare distance to one satellite towards the east and another one towards the west. To determine altitude, you compare distances to one satellite high up in the sky and another one somewhat lower in the sky. The difference is much smaller.

To get a grid cordinate on the surface the GPS uses Triangulation with mulitple sats, to get altitude it uses the measurement for the bird closet to being overhead, if it is not directly overhead you can throw the measurement out.

I deal with alot of boat equipment and when GPS first came out with Mulitplexing recievers boaters would complain about the inaccuracy of the Altitude reading, although on the water Altitude doesn’t matter cause you are at Sea Level or a fixed height above sea level on a lake or inland body of water. Where the barmetric altimiters came in to play is in the outdoor market for hikers and the like.

Another thought for you is to buy a Suunto watch or something similar that has a barametic altimiter built in.

GPS receivers don’t do triangulation; they do time-difference measurements (pretty much as scr4 has said).

It is true that the accuracy of altitude measurements will improve when satellites are closer to directly overhead. One reason that altitude accuracy is not as good as lat/lon accuracy is simply that the area of the sky that’s close to directly overhead is rather small compared to the total area, so satellites don’t spend much time there.

It’s fairly important to use pressure based altitude measurement in all forms of aviation…because that is what all the other aviators are using.

In a balloon you probably can’t fly at odd thousand’s +500’ when headed generally east, but if you happen to be at 7,500’ pressure altitude, you’ll know to keep an eye out for traffic coming from the west. You’ll also be able to maintain 7000 feet if you choose, so most of the power traffic would tend to be either 500’ above you are 500’ below you. The pressure altitude is also what allows you to stay out of restricted airspace around airports and such.

You’ll also see phantom jumps up and down in altitude as various satilites pass in and out of view.

Finally, Kollsman altimiters are about as reliable as things get. No reliance on batteries, satillites, etc. And they present an analog display. Should you lose your rate of climb indicator, the motion of the skinny hand on the altimeter will serve you much better than a digital display scrolling along…and it provides a cross check for the vario.

Since navigation in a ballon is dependent on the wind, and the pilots observations and intuitions, I suspect the distraction of GPS would lead to worse, rather than better navigation.

The ONLY good use I see for a GPS in a balloon is to help your crew locate you after you land.

Provided the other aviators are using the proper altimeter setting for the area, the difference between gps altitude (barring the GPS errors) and pressure altitude should be negligible, as the altimeters should be at the same height. And it’s not like we takeoff at the airport either, so we don’t necessarily have the right altimeter setting if we do use pressure altitude.

Generally balloons will stay below the altitude that the cruising altitude orders apply (which is 3000’ AGL here in Canada. I assume it’s similar in the US), and it’s pretty difficult to keep an eye open for other traffic when you’re a balloon (they keep an eye open for you… Especially when you’re a 100 foot brightly coloured floating sphere.

I do realize that the analogue altimeters are more reliable than electronic instruments, but I’m looking for a solution that’s all in one package, is waterproof and shockproof, so that rules that one out.

GPS when you’re flying is more of a guide to a balloonist. It’s not so much as navigation aid to the pilot, as opposed to the other pilots flying with you (should they want to get on the same wind as you, you tell them what alt you’re at, they can join you, and you’ll be moving the same speed.) It’s actually quite useless to crew members finding you (I’ve crewed for a few years now) it doesn’t help unless you both have something like this, which is what I’m currently looking at picking up.

Yes, it’s the same. Above 3000’ AGL, the odd-thousand-plus-500-easterly VFR rule applies.

In the US, balloons have right of way over all other non-emergency traffic anyway.

Thanks for the description of an aspect of aviation fairly few pilots experience.