Gps Question

[ChairmanoftheBORED]

This pops into my brain every once a while and for some reason its remained here today. Perhaps its because I am onto my second G&T..

 

So! I have noticed on the old Garmin 296 (or any GPS) that when you overfly your selected waypoint it begins to give you your ETA back to the fix immediately and progressively expands the amount of time it will take to get there. Correct? sure...

Cool...

 

MY question is: WHY considering that you have just proceeded directly above and beyond your waypoint that it does not immediately default to infinity? After all - you are heading completely in the opposite direction and as far as that GPS knows you might never turn around but return to that point. Go directly 10 miles down the track beyond that point and that waypoint might still only be an hour away according to the GPS, which pretty much eliminates my thoughts pertaining to some percentage of the horizontal track contributing to the Time to Destination.

 

...after all - it is the complete opposite direction.

 

I guess if you stopped over your target and ascended vertically it would read zero for sure...?

 

What am I missing here and should I stay off the Gin?

 

Steve

[Freefall]

I think it calculates the ETA or ETE based on ground speed and distance alone. I dont think it cares what your vector is.

[chopperman]

You told it where you want to go and until you tell it different it calculates the time it will take to get there. It can't tell you have changed your mind!!!! It gives the time at your current speed it will take to return to the location (of which you told it where to go to) if you overfly and want to return.

It's like a wife, it cannot read your mind as much as you wish they could.

[sk76driver]

It's like a wife, it cannot read your mind as much as you wish they could.

 

 

And vice versa!

[212wrench]

Can anyone answer me this question, does a GPS give you distance point to point, for example from Calgary to New York, through the earth or would it give distances over the curved surface?

[407 too]

I'll take a stab at it wrench, I believe the better aviation GPS units actually calculate distances using Great Circle.

 

the portable gps units you buy for hunting etc. are probably using straight line.

 

(CAVIAT - i'm usually wrong more than i am right :shock: )

[Blackmac]

Straight Line, so watch out for the big hills.

[Gary Watson]

Aircraft and marine GPS units both calculate in Great Circle Route AFAIK. I will check this out tomorrow on one of our portables (296) Most data-based navigation systems use GCR even for short distances but you don't notice it.

[Gary Watson]

Of course you could go to the Garmin site and read the manual

[justfly]

Can anyone answer me this question, does a GPS give you distance point to point, for example from Calgary to New York, through the earth or would it give distances over the curved surface?

From Garmin's Glossary:

Distance

The length (in feet, meters, miles, etc.) between two waypoints or from your current position to a destination waypoint. This length can be measured in straight-line (rhumb line) or great-circle (over the earth) terms. GPS normally uses great circle calculations for distance and desired track.