GPS and fog; and altitude

[djlewis]

OK, I reread Alex and Andrew's account, and they didn't say that the fog was the cause of their temporary GPS failure on the Maine Island , just that their GPS did take a vacation in fog and returned later on a sunny day.

So, I'm glad to hear that GPSs work fine in fog, and that I was worrying under a misconception.

On a non-kayaking note -- on a recent airplane flight I hooked my new GPS to a laptop running Delorme Street Atlas. Lo & behold it worked great, showing a lovely track across the countryside, 500-600 mph, and ~two~ altitudes of about 4000-6000 and 32000-34000 feet. Now if there hadn't been solid cloud cover the whole trip, I could have looked down and actually identified all those towns and features that usually whiz anomyously past.

Question -- I assume the two altitudes are as determined by (a) air pressure (inside the passenger cabin, of course) and ( triangulation from the satellites. But I can only get the former, air-pressure version on the GPS unit itself (a Garmin GPSMap 76CS). Anybody know how to get a triangulated altitude? Of course, it doesn't matter much for kayakers at sea level, but it might help calibrate the barometer.

--David

[donperry]

As I understand, when auto-calibration is 'on' the altitude is computed within a hundred feet or so of your actual altitude. When auto-calibration is 'off' the baritron is decoupled from the position computation and the altimeter reports relative pressure & changes. The baritron is highly sensitive and the altimeter should be manually calibrated (at a known elevation) when the unit is turned on.

Altitude computed from the almanac and ephemeris data is a coarse number compared with the close latitude and longitude reported on your WAAS enabled receiver. Garmin GPS receivers record altitude as part of the tracklog even if they don't have an altimeter. However, you need Garmin MapSource software to retrieve it from the receiver's memory. DeLorme, Maptech and Topo don't retrieve those values because they would probably conflict with their own mapped surface elevations.

[christopherG]

david, david, david (ye of little faith!),

did i not tell you it works fine at thirty or forty thousand feet?

i do not see how you could triangulate (i'm no mathematician) from various satellites: who the heck knows where they are? their positions are not published, as far as I know, and neither does it matter...

i think don perry is probably bang-on with his idea about the internal baritron/barometer -- you were perhaps getting real altitude from satellites and the internal device was giving you cabin altitude? 6000' is exactly in the ballpark...

[John Leonard]

Not 100% sure as I dont own a 76CS, but a friend has a vista (my legend has no barometer) that I have played with a bit. You should be able to turn off the barometer and magnetic compass in order to conserve the batteries. With those off the altitude data it gives you will be calculated from the satelites. You may also be able to change the data it provides by going to the box giving you the altitude, "clicking" on it, and getting a list of data that it can give you. All of the Garmins are pretty configurable like that. Even with the barometric altimeter uncalibrated it will probably be more accurate than the satelite data as there are inherent innacuracies in calculating altitude from satelites (especially at sea level).

I have had my gps refuse to lock on numerous times in a variety of weather. I would guess that the fog was not the true cause of the failure. They are handy backups if you are not 100% sure of your exact position, but I wouldn't want to trust my life to it. Mainly I use it to get speed and distance travelled and occasionally navigation information.

John

[Dee Hall]

It is my understanding that the GPS does indeed calculate position using the satellites, however I believe that it is quite slow (requires lots of averaging to get accuracy). My guess is that the pressure based altimeter is used for rapid changes in altitude and that it is periodically or continuously recalibrated using the satellites.

Altimeters need to be recalibrated frequently with local barometric reports. Their accuracy is only as good as the accuracy of these reports. The GPS eliminates the need for these readings.

[djlewis]

Google comes through with...

http://gpsinformation.net/main/altitude.htm

(Searched: GPS altitude)

Gist -- GPS receivers do measure altitude by triangulation from the satellites, but for a number of reasons, it isn't nearly as accurate as horizontal (lat/long) position. Among those reasons are (a) satellite/receiver geometry; ( the shape of the earth -- it's not something that can be modelled easily with mathematics, so GPS manufacturers use different simplifications; these can result in mistakes on the order of +- 40 meters (like walking on the shore at obvious sea level and being told you are 40 meters below sea level).

I still don't know in detail how barometric pressure is used to determine more accurate altitude, except for the obvious. Some thoughts: (a) it obviously requires calibration for altitude; ( barometirc pressure can and does change significantly in as little as minutes, but more typically hours; that would complicate reading altitude from barometric pressure. Also, if you are changing altitude, like hiking up a mountain, it seems to me that would interfere with accurate barometer readings.

Luckily for us sea kayakers, we operate mostly at constant sea level, so the altitude variable doesn't in fact vary. So it seems that would enable us to get a meaningful baromoter reading, which is, of course, what concerns us most (like when dropping rapidly).

Here's another nice article on this stuff, though not with much about altitude.

http://www.cmtinc.com/gpsbook/

--David.

[bob budd]

Satellites are generally held in geosynchronous orbit so antennae can be pointed them for the most efficient communications. Their location must be known by all communicators and is probably a matter of public record. Triangulation uses the time it takes a signal to travel to and from the satellite. If you bounce off one satellite you can draw a circle around that satellite. A second satellite produces a second circle usually with two intersecting points of the two circles. A third satellite should produce a third circle and usually a single intersecting point of the three circles. The use of three points leads to "tri"-angulation, having nothing to do with triangles, but rather the three angles from the source to the points measured. As these signals travel near the speed of light the time involves is quite short and a single measurement is generally considered an estimate. A running average of estimates provides a more accurate measurement.

[donperry]

Many communications satellites are orbiting in geosynchronous orbits at about 26,000 miles. But the GPS constellation is held in fast moving 12,000 mile polar orbits. Almanac data identifies each bird and it's nominal orbit and is updated every few days. Ephemeris data changes frequently and gives deviations from the nominal orbits.

The GPS receiver reports its first fix after it has the current almanac data, ephemeris data and a timing beacon from each satellite in view. Thereafter it only needs the timing beacons to make the computation since the other values are registered in the processor.

[DeCourcy]

"...various satellites: who the heck knows where they are? their positions are not published, as far as I know,..."

If anyone's interested in seeing where the various satellites are go to this website: http://science.nasa.gov/Realtime/JTrack/

Look at the 3D version too.

Jim.