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GPS altitude reporting question

andrewsw16andrewsw16 Member Posts: 10,728 ✭✭✭
edited May 2018 in General Discussion
Got a question for you guys familiar with how our automobile GPS works to report altitude. I was recently driving through the Rockies with altitude reporting in a small window in the corner of my GPS and I started wondering just what was the source of the reported altitude. Does the GPS calculate the altitude by: 1. triangulating the signals from 3 or more satellites, 2. by an internal air pressure sensor like an aircraft altimeter, or 3. does it just reference my geographic location to a stored database of altitude data such as DTED or DEM?
Just curious.[:)]

Comments

  • grdad45grdad45 Member Posts: 5,304 ✭✭✭✭
    edited November -1
    Probably. But that is just a guess. Never wanted to know how high I was, just enjoyed the trip. [}:)]
    Actually, I have no idea.
  • oldWinchesterfanoldWinchesterfan Member Posts: 1,051 ✭✭✭
    edited November -1
    I'm pretty sure it's #1. In addition to a GPS like yours, I also have a GPS based altimeter app on my phone that is not based on roads and works anywhere the phone has line of sight access to the south.
  • allen griggsallen griggs Member Posts: 35,183 ✭✭✭✭
    edited November -1
    Great question I have been wondering the same thing. Love to see the altitude on my GPS in the big rig.
    Driving from Shreveport to El Paso, you are at, I am not sure but maybe 100 feet at the Big River.
    As you go west, you gradually, imperceptibly climb, and you go through passes in Texas that are at 5,000 feet! Fascinating.
    El Paso is at about 3700 feet.

    By the way, HeDog will straighten you out, it is elevation not altitude.
  • He DogHe Dog Member Posts: 50,947 ✭✭✭✭
    edited November -1
    If it is on the ground it is elevation, not altitude. Your GPS references your geographic location.

    I have an altimeter which uses air pressure. It is accurate to within about 50 feet of elevation.
  • kidthatsirishkidthatsirish Member Posts: 6,985 ✭✭✭
    edited November -1
    Typically your GPS will do one of two things.

    If it has a decent memory, it will use the position calculated by a minimum of three satellites to pull a stored premeasured elevation above MSL via its database.

    If it does not have a data base, it will use the calculation from a 4th satellite or more to determine what your elevation is. (3 satellites required for a precision location on a 2d map, the 4th satellite will give "altitude" or in your specific case "elevation") For this illustration picture a minimum of 4 spheres (satellites with time stamps) orbiting a much larger sphere (earth) that intersect in three dimensions, the receiver "knows" which satellite is on what side of the earth and the satellite ID information is transmitted with a timestamp.
  • gunnut505gunnut505 Member Posts: 10,290
    edited November -1
    GPS works on math.
    The World Geniuses created a mathematical surface to (almost) model the undulating surface of our planet; it's called the Geoid, and has gone through innumerable iterations to more closely match the surface.
    The GPS satellites know fairly closely how far they are from the center of the planet, since they are in intersecting planes in their orbits.
    The magic starts with the satellite pulses, collected by the antenna on your device, then run through the GPS engine, and compared to ephemeris information (satellite orbits and times); it determines the shift in time between the satellite and the device via sophisticated timers/clocks, and computes a 2D position.
    The GPS device adds data from more satellites to confirm 3D position, and uses the center of the Geoid as a corner of that triangle.
    It then has to compute a relationship between the Geoid, the center of the planet, and the local terrain.

    All that means is that someone's $1500 iPhone won't accurately reflect the true elevation, because the gear that will do that costs many thousands more.
  • wiplashwiplash Member Posts: 7,146 ✭✭
    edited November -1
    Ambient Pressure on mine.
    There is no such thing as Liberal Men, only Liberal Women with Penises.'
  • MobuckMobuck Member Posts: 13,733 ✭✭✭✭
    edited November -1
    The ones we use for work(Federal agency) utilize the same technology the military uses so I'm not allowed to tell you how it works.
  • Rocky RaabRocky Raab Member Posts: 14,130 ✭✭✭✭
    edited November -1
    If it's GPS, it works by triangulation. Your unit measures the exact distance to as many satellites it can "see" and from that, determines a position in three dimensions. (changing position allows it to calculate speed, also) It doesn't care if you are on the surface or not.

    The accuracy that can be achieved depends on your unit, and the level of information your unit is allowed to access, as well as how many satellites you can "see" at the moment, and the quality of the signal. Military, aviation, and survey equipment are all much more accurate than a civilian hand or vehicle unit.
    I may be a bit crazy - but I didn't drive myself.
  • AlpineAlpine Member Posts: 15,041 ✭✭✭✭
    edited November -1
    I have taken my road Garmin GPS on an airplane (with permission) and it reported the same speed and altitude as the display showing on the screen monitors on the seats.

    As stated previous the good GPSs work by math and triangulation. More powerful GPS satellites that are being launched will bring position resolution down to 1 meter. That is a lot of math and numbers to the right of the decimal.
    ?The problem with socialism is that you eventually run out of other people's money.?
    Margaret Thatcher

    "There are three kinds of lies: lies, damned lies and statistics."
    Mark Twain
  • nmyersnmyers Member Posts: 16,875 ✭✭✭✭
    edited November -1
    I had a 1st gen Garmin that showed you which satellites it was receiving. It would show my position with 2 satellites, but elevation required a 3rd. So, gunnut's explanation makes sense.

    I checked my altitude with my iphone while sitting on a beach, & it showed I was at 85'. I wasn't in the surf, but I wasn't > 10' above the waves.

    Neal
  • Rocky RaabRocky Raab Member Posts: 14,130 ✭✭✭✭
    edited November -1
    The tells you that your iPhone GPS is only accurate to about 75 feet. Fine for most mundane uses, of course.
    I may be a bit crazy - but I didn't drive myself.
  • Smitty500magSmitty500mag Member Posts: 13,603 ✭✭✭✭
    edited November -1
    Anybody here ever driven though Death Valley with their GPS? I wonder what the read out would show when you're below sea level?
  • gunnut505gunnut505 Member Posts: 10,290
    edited November -1
    Try your GPS device on a jet.[:)]

    As you approach 600mph, you may see something cool.

    Almost all GPS devices have a DOD/DHS cutoff installed that ostensibly prevents their use as guidance packages in airborne threats. The only one I've seen that didn't have "vector handling" was attached to the aircraft used for remote sensing/LIDAR/Photogrammetry, and it would display absolutely everything, including cross-wind effect, yaw, altitude as ASL & AGL, bearing, heading, altitude density, and time of flight.

    Wouldn't fit in your car, anyway[:)]
  • Rocky RaabRocky Raab Member Posts: 14,130 ✭✭✭✭
    edited November -1
    If you are below sea level, you'd see negative numbers, of course.
    I may be a bit crazy - but I didn't drive myself.
  • gunnut505gunnut505 Member Posts: 10,290
    edited November -1
    quote:Originally posted by Alpine
    I have taken my road Garmin GPS on an airplane (with permission) and it reported the same speed and altitude as the display showing on the screen monitors on the seats.

    As stated previous the good GPSs work by math and triangulation. More powerful GPS satellites that are being launched will bring position resolution down to 1 meter. That is a lot of math and numbers to the right of the decimal.






    There's a lot of numbers after the decimal because the math calculates the latitude, longitude, elevation, and Distance.
    If your calculated position has an accuracy ratio of 1 part in 10 Million, that's accurate to one foot in ten million feet.

    When I stopped Surveying, the Block lll satellites were almost all airborne; they had the improved Cesium/Rubidium "clocks", and had improved the accuracy/repeatability to within a spinning dime for XYZE positions.
    They also broadened the horizon by adding the civilian band & adopting the circular polarized antennas to increase gain at the SV.
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