| jmacdonald |
| Can anyone tell me what screen I should see the time on the NAV unit other than the setup/adjust time screen? I checked the manual and I am not seeing where this is displayed. |
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| renov8r |
...except in the setup.
Of course, it is always a "What time is it?" away thanks to voice recognition... |
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| Dick at Incline |
| Since the GPS Time is accurate to better than 1-second it would be nice if the Nav System used GPS time, rather than me having to set it. Also, since GPS speed is so much more accurate than the Speedometer, it would be nice if that were available for display as well. :4: |
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| jimfr |
GPS time is quite accurate, and I agree with you, that it would be nice if the MDX used GPS time for the clock.
As for speed, I disagree. GPS speed calculations can drift substantially, depending on signal strengths, losing satellites, orbit drift, clock drift, atmospheric interference, processing lag, etc. etc. I've seen my WAAS-equipped GPS vary by as much as 8 MPH (travelling at a speed of 40 MPH) at a constant speed (level ground, no RPM change).
And GPS altitude, IME, is not very accurate at all. |
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| Dick at Incline |
quote:
Originally posted by jimfr
As for speed, I disagree. GPS speed calculations can drift substantially, depending on signal strengths, losing satellites, orbit drift, clock drift, atmospheric interference, processing lag, etc. etc. I've seen my WAAS-equipped GPS vary by as much as 8 MPH (travelling at a speed of 40 MPH) at a constant speed (level ground, no RPM change).
And GPS altitude, IME, is not very accurate at all.
Now that DoD has stopped "Selective Availability" altitude is very much better than it was a couple of years ago. Now days, typically better than 100-feet---not too bad.
AS far as speed-error goes, I think you are invoking too much science and not enough practical. Orbit Drift is a long term effect that will not bother speed measurement, and clock drift?--it's an atomic clock on the Satellite, that locks your GPS receiver clock, virtually zero drift. In addition, any change in orbit is corrected by Ground Stations communicating with the Satellite. Atmospherics also represent a long term drift that will slowly affect position accuracy, but negligible effect on speed measurement.
I have used Garmin GPS for more than 10 years and find they are much more accurate and consistant than speedometers on my various cars and motorcycles. I never turn-on WAAS and that may be a source of your "processing lag". signal strength and losing satellites certainly can cause errors, but the Garmin units track up to 12 satellites, and I think they use only the best 4 for all the measurements--so losing 2 or 3 out of 8-12 will have no significant effect. If you really want to see how much drift you have, stop dead still--If all the effect you site above are causing the errors you claim, then you would see speeds up to 8mph, when your standing still. I don't think that will happen. Also to see "Drift" look at the change in Lat/Long position--see how slowly it changes.
Bottom line: GPS speed is very accurate, typically better than 1-mph. Speedometer errors on my Motorcycles read typically 4-6 mph high. |
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