Joined
·
15 Posts
Is there an altitude readout on the 2005 NAV system?
Altitude readout
1 - 20 of 53 Posts
Joined
·
15 Posts
It is interesting that Toyota does not supply altitude readout when it is a readily available function of GPS. In other words, to get the information they use to give us distance and time enroute, they also get altitude. Iwonder why they don't give it to us?
For those of you who are joking about barometers, I'm certain you realize a barometer give you height above sea level, not pure alititude.
For those of you who are joking about barometers, I'm certain you realize a barometer give you height above sea level, not pure alititude.
GPS altitude accuracy is very poor. Military systems do not rely on it, and use baramotric pressure altimiters. The geometry is just too poor for useful readouts. However, the point that a map database that includes altitude could serve as a proxy for an actual measurement. With a DVD, there should be enough storage for at least POI altitudes. I guess they figured it wasn't useful for navigation purposes.
My little garmin toy gives me the vertical velocity, to a precision of 0.1m/s.DanMan32 said:
The altitude is off by 5-10 meters, okay for curiosity but noway good for landing purpoises, that's why aircrafts rely on barometric altitude, but that still need adjustment with the barometric pressure red out from tower control.
Thats because barometric pressure fluctuates continuously and instant readings are transmitted continuously by air traffic control. (towers and control centers) On flights above 18,000 feet all aircraft altimeters are reset at 29.92 inches (the standard used) so all aircraft separation at altitude is the same. On all autoland approaches now the radio altimeter is the primary source of info below 200 feet.
"My little garmin toy gives me the vertical velocity, to a precision of 0.1m/s. "
Just because the display includes a tenth's digit, it doesn't mean it is accurate to that degree. It probably does some averaging to smooth it out so it appears so, but I can guarantee you it is not that accurate, since even the military units aren't close to that.
There is a myth about GPS accuracy based on the stated "error" many hand-held units display. This is not the actual accuracy, but the subset of this, specifically the degradation caused by less-than-ideal satellite geometry. Other error sources, including the psuedorandom noise added by DoD, are typically many times this value. That display sells a lot of units but it is a lie to call it a measurement of "accuracy."
Horizontal accuracy of the Prius GPS is on the order of a little less than 100 feet (85 by my comparison to a PLGR unit after several minutes of averaging), and vertical accuracy would be much miuch worse than this, regardless of what the display implies.
Just because the display includes a tenth's digit, it doesn't mean it is accurate to that degree. It probably does some averaging to smooth it out so it appears so, but I can guarantee you it is not that accurate, since even the military units aren't close to that.
There is a myth about GPS accuracy based on the stated "error" many hand-held units display. This is not the actual accuracy, but the subset of this, specifically the degradation caused by less-than-ideal satellite geometry. Other error sources, including the psuedorandom noise added by DoD, are typically many times this value. That display sells a lot of units but it is a lie to call it a measurement of "accuracy."
Horizontal accuracy of the Prius GPS is on the order of a little less than 100 feet (85 by my comparison to a PLGR unit after several minutes of averaging), and vertical accuracy would be much miuch worse than this, regardless of what the display implies.
The DoD turned of psuedorandom noise May 2000.KTPhil said:
See the Garmin site for a good explanation on GPS and sources of GPS signal error.
http://www.garmin.com/aboutGPS/
"The DoD turned of psuedorandom noise May 2000." Yes, you are correct about the pseudorandom noise: Selective Availability was turned off about 15 years ago.
The noise that contributes to the errors is primarily from atmospheric effects, uncorrected in the normal C/A code transmissions received by civilian receivers (SPS). DoD still severely limits the availability of receivers that can decrypt the Y-code transmissions, required for high accuracy (PPS), which correct for this effect.
Typical accuracies obtainable with the normal C/A-code receivers like the civilian handhelds (and the prius Nav) are in the range of 60-70 feet under ideal conditions. I measured the Prius at 85 feet last month with a clear sky view. Averaging can improve this somewhat for a stationary device, but of course this is seldom possible in a car. Garmins' claim of 15 meter accuracy is, well, let's call it a best/best case: best satellite geometry, long-duration averaging, and a generous bell curve spread from which they choose a given percentile of readings. In real-world conditions, the Prius' 85 foot reading is typical.
The noise that contributes to the errors is primarily from atmospheric effects, uncorrected in the normal C/A code transmissions received by civilian receivers (SPS). DoD still severely limits the availability of receivers that can decrypt the Y-code transmissions, required for high accuracy (PPS), which correct for this effect.
Typical accuracies obtainable with the normal C/A-code receivers like the civilian handhelds (and the prius Nav) are in the range of 60-70 feet under ideal conditions. I measured the Prius at 85 feet last month with a clear sky view. Averaging can improve this somewhat for a stationary device, but of course this is seldom possible in a car. Garmins' claim of 15 meter accuracy is, well, let's call it a best/best case: best satellite geometry, long-duration averaging, and a generous bell curve spread from which they choose a given percentile of readings. In real-world conditions, the Prius' 85 foot reading is typical.
1 - 20 of 53 Posts
