Do civilian GPS unts do accurate altitude?

[Brian Elfert]

Do civilian GPS units give accurate altitude readings? A friend of mine said that the government does not allow civilian GPS units to receive accurate altitude data. Supposedly the feds don't want GPS used for missile guidance.

I was trying to use my GPS to take altitude readings in my yard and the altitude was varying all over by up to 100 feet when the GPS was sitting on the ground. It would settle on a number for maybe 15 seconds and then it would be all over the place again and finally settle on a totally different number.

[Eric DeSilva]

Civvie GPS, as far as I know, isn't restricted in that manner. What he was probably referring to is that GPS had something called "selective availability" whereby the last bits of the GPS signal were scrambled and only available to military units with SA decryption. However, given a shortage of mil-spec GPS units in the first Gulf War, SA was turned off and GIs got run of the mill commercial units. When the average Joe's accuracy in the US went from 50m to 5m overnight, it was hard to turn SA back on, and its been off ever since.

That said, there are more or less accurate GPS units, and more accuracy costs you more... The purely commercial ones are OK, but you can get much better accuracy from industrial GPS systems. Then again, you can buy 30 TomToms for the price of a good GPS.

[Brian Elfert]

The GPS unit I am using is a $99 Garmin Etrex I bought two years ago. The speed and such is very accurate, but the altitude is terrible. A contractor told me that the better GPS units don't actually use GPS for altitude.

My friend was specifically referring to altitude only, not the selective availability.

[Stephen Beckham]

Brian - have you checked the number of birds your system can see? If you can't see more than four, your altitude will be terrible. Does your system allow you to see which satellites you are reading and their quality?

If the quality is dropping or you've got some kind of ground interferance your system will jump all over...

Try the system on top of a shed or car - away from power lines or transformers. Science wouldn't dictate interferance from them - but who knows... Research online and see what kind of interferances or items close to your system can 'jam' it.

[Eric DeSilva]

The altitude error is much greater because it is a satellite based system. If you think about it, the best satellite positions for a perfect read are going to be evenly distributed in an imaginary sphere surrounding you. Unfortunately, since you are standing on the earth, that rules out half the sphere because you need line-of-sight to the satellite. As a practical matter, it even rules out a constellation with satellites close to the horizon. So, generally speaking, your fixes will be overhead--which means that the cumulative error is mainly in the vertical plane. So, I think the offhand estimate is vertical error = 1.5x horizontal error. But, most commercial units advertise achieving approx. 5-15m accuracy, so the vertical error can be 22+m. For something like the Trimble GeoXH, the accuracy can be as good as 20cm, so vertical accuracy is probably like 1/3rd of a meter. All depends upon your application and how much you wanna pay for redundant receivers, post processing power, etc. Course that little puppy will cost you $6K.

[Curt Harms]

Here's a wikipedia blurb that might shed some light-
http://en.wikipedia.org/wiki/WAAS#Comparison_of_Accuracy

Of course the receivers are 12 channel units.

[Lee Schierer]

At our construction site, they use a differential GPS to locate pipes and underground wiring runs and connection boxes. It is highly accurate for location and altitude to within a few inches. Most consumer GPS units don't work well for altitude. My Garmin for my boat seems to be more accurate than my Friend's Magellan for his boat with regard to altitude. The Navigation units for road maps have little need for altitude so I don't think they even display it.

[Mike Henderson]

Here's a wikipedia blurb that might shed some light-
http://en.wikipedia.org/wiki/WAAS#Comparison_of_Accuracy

Of course the receivers are 12 channel units.
That is differential GPS (http://en.wikipedia.org/wiki/Differential_GPS), not standard GPS. The way differential GPS works is that you have a GPS receiver on the ground at a known location. The signal you receive at that location will give you what the location would be based on the GPS signal. The difference between the actual location and the location given by the GPS receiver is the error differential. Conceptually, that error is then broadcast to the people using the differential GPS system and it is used to correct their GPS reading.

The advantage of this approach is that even when selective availability (signal dithering) is applied, the differential GPS system is accurate.

Mike

[Standard GPS is not very accurate in altitude - not near the ground location accuracy.]

[mike holden]

Brian, the GPS unit uses three dimensional triangulation from satellites to determine position. The operative word is "three". The unit will give your height just as accurately as your polar coordinates. Remember though, it is averaging the results from all the sattelites it can "see" and some sattellites will go in and out of range due to who knows what all - sunspots, clouds, tree branches, radio interference etc.
This is why Geocaching is a challenge - if the GPS was dead accurate, then finding the cache would be too easy. In caching, we figure at best a 20 foot radius circle for both the placement and the search, so when we "arrive at destination" we feel we are within 80 feet.
It is this reason that there is a "track on road" setting on your GPS so that it will show you on the road as you drive, it would be quite disconcerting if it were to show you in the ditch alongside the road, wouldn't it?
If you think of yourself in the center of a sphere, 20 feet in radius, then the readings start to make a lot more sense.
Mike
Ti Si Muovo - Still it moves!

[David Freed]

I have been using Delorme Street Atlas USA for several years. this package installs on your laptop, and the antennae plugs into and gets its power from a usb port. While this program is not perfect ( I don't think any of them are), it does a great job for me. They bring out (and I buy) an updated version every year. As for how accurate the altitude is, I'm not sure, but it doesn't bounce around. If you go up or down a hill, it will change as you move, but when you stop, it stops changing. You can choose to see right on the screen how many satellites you are receiving from and their location, or the space saving little stoplight; red- 1 or no sat, yellow - 2 sats, green - 3 or more sats. I have seen as many as 7 or 8 talking to my laptop, but it is usually 3 to 5. I don't know what the package with the antennae costs now since I have only bought the software for a few years, but it was around $140 when I bought it.

[Randal Stevenson]

There is a DOD restriction, that altitude in these units don't work above a certain level (something like 10 or 20,000 feet). More then that, and you need to obtain clearance from them.

My Uncle was a surveyor, that used GPS all the time, when he got injured and retired from it, he was hired by (then Thales) Magellan. We had this discussion over something (I believe one of the Balloonist world attempts).

Otherwise, the others have stated correctly (variables such as satellite count, line of site, etc)