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## 8364Red Dot vs Dead Reckoning + More

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• Apr 1, 2011
Very well explained James. I have isolated the problem, implemented the
solution, and am about to release a new Dev version for those of you
that can (safely) see your screen while mobile can test (and hopefully
verify) my correction. I base the Dead Reckoning of ME on whether or
not you have Configure / Beacon / CourseSpeed checked. If so, ME will
dead reckon based on the (hopefully) same information a recipient of
your last beaconed packet would be using. If you don't beacon
CourseSpeed, then I don't DR ME since you KNOW where you are and where
you're heading and you haven't told anyone else anyway.

As for the difference/similarity of ME's DR vs the Red Dot.... If you
measure the deviation between ME's actual position and the corresponding
DR symbol, scale that according to the Configure / Genius / Forecast
Error setting, you'll have a bearing (from actual to DR) and a
percentage of error (distance / Forecast Error * 100%). Plot that
value on the circle based on the compass points where touching the
circle = 100% and you have the red dot, a Visualization of the range and
direction of the error between a time-based extrapolation of your last
beaconed position, speed, and course vs your actual current position, in
other words, the red dot displays the magnitude and direction of the
error between where an outside observer thinks you are based on the time
since and values within your last beacon and where you know yourself to
actually be.

If the DR is on top of you (constant, straight line motion), the red dot
will be on top of you and invisible. You won't see the DR symbol
either, most likely.

If you speed up (still in a straight line), the DR symbol will start
falling behind, and the red dot will start drifting towards the circle,
lagging directly behind your current direction of motion, but the DR and
red dot will be at different rates because the red dot is a percentage
of the allowed Forecast Error (when it touches the circle), and the DR
will be where that outside observer would think you are. So, the DR
moves according to map scale and the red dot moves according to the
magnitude of the allowed Forecast Error.

If you slow down (still in a straight line, we can do that here in
Florida for long distances), the DR symbol will start pulling out in
front of your direction of motion, and so will the red dot, again at
different rates. If you take the distance between your actual position
and the DR symbol, divide it by the configured Forecast Error, that's
relatively how far the red dot will be between the center and edge of
the circle, still in your direction of motion.

I'll leave it as an exercise for the reader to figure out where the DR
and red dots will go if you vary just the direction and maintain speed
(should drift away from you at 90 degrees) or if you vary both speed and
direction.

For extra credit, you can play the "red dot game" (yes, this is
absolutely worse than just glancing at the APRSISCE/32 screen while
driving). Purposely vary your speed and/or direction to control where
the red dot is on the circle. Hint: it's a lot easier if you have a
very long stretch of very straight road and works fairly well at giving
you something to do to stay alert, even if not truly looking where
you're going). Hint #2: it's highly discouraged to attempt to control
the red dot in any orientation other than directly ahead or behind your
current heading unless you're driving on a salt flat or something.

Now, to quit typing, do a few release notes entries, and get this
development release out there for testing!

Lynn (D) - KJ4ERJ - Author of APRSISCE for Windows Mobile and Win32

On 4/1/2011 1:52 PM, James Ewen wrote:
> On Fri, Apr 1, 2011 at 10:35 AM, apenadragon<kc2ant@...> wrote:
>
>>> So, I just need to fix ME's DR to use the last beaconed speed/heading
>>> rather than the current GPS speed/heading and all will be well.
>> Do you?? personally I don't see the issue... the DR is a projection of where
>> the station "should" be...
> Actually Adam, it sounds like you're missing the issue.
>
> If you take the last reported speed and heading and project that from
> the last reported location, as time progresses, the dead reckoned icon
> will move to where the station "could" be.
>
> The problem with what we are seeing is that Lynn is using the CURRENT
> speed and heading, and projecting a possible location using that
> information from the last reported location.
>
> Lets suggest that a position was reported, with a speed of 60 mph, and
> a heading of 90 degrees. After one minute, the dead
> reckoned position should be 1 mile east of the reported location.
>
> The way it is implemented, if I stop driving east after 30 seconds,
> the DR position would return back to the last reported location. If I
> were to turn south and drive for 1 minute at 60 mph (without sending
> an updated position report), there would be a DR object 1 mile south
> of the originally reported position.
>
> Now, if this instantaneous line were to originate from the current
> location, it would still be wrong. The line leaving my current
> location would not be a dead reckoned position, but rather a projected
> location for a future position report based on the current speed and
> course, projected into the future the amount of time since the last
> reported position.
>
> When Lynn gets it straightened out, you'll see something interesting
> happening. As Lynn said, if you continue in a straight line at a
> constant speed, you won't see the projected icon. If the road turns,
> or you change speed, then you'll see the projected (DR) icon continue
> along the last reported vector. Once the Genius settings determine
> that you've strayed too far, a new position report will be sent, and
> the DR icon will start moving away from the reported position again.
>
> James
> VE6SRV
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