[aprssig] MicE Position Ambituity in APRS (Robert
Alex Carver
kf4lvz at yahoo.com
Tue Jan 8 12:18:19 CST 2008

>
> Message: 11
> Date: Tue, 8 Jan 2008 12:53:41 0500
> From: "Robert Bruninga" <bruninga at usna.edu>
> Subject: [aprssig] MicE Position Ambituity in APRS
> To: "'TAPR APRS Mailing List'"
> <aprssig at lists.tapr.org>
> MessageID:
> <043f01c8521f$678c4950$42577a83 at ewlab.usna.edu>
> ContentType: text/plain; charset="USASCII"
>
> > I have a D700. What does *it* do to
> > the GPS Lat/Lon data when Posambig is
> > turned on?
>
> In the normal APRS format. Unavailable digits of
> precision are
> replaced with a SPACE.
>
> In the D700 (MicE format), unavailable digits of
> precision in
> the latitude are replaced with the letter "Z" (not
> zeros). The
> Longitude then is assumed to have the same level of
> precision.
>
> When a person enters his estimated position into a
> D7 or D700
> manually, he only enters those digits he knows.
> Then he uses
> the position ambiguity menu to INDICATE how many
> extra digits of
> precision he is not using. The result is exactly
> the same.
>
> On decoding of the MicE format, the position used
> by the client
> program should not insert "0's" into those unused
> positions, but
> SPACES just like the full APRS format does. Again,
> this is not
> truncation, but reproducing at the receipent exactly
> what the
> sender intended.
>
Sorry, Bob, but what you describe is precisely the
definition of truncation. If I truncate something, I
lop off a piece of it and leave nothing behind. The
value "34.4567" truncated to two decimal places is
"34.45__". Rounding is a different story since I may
change one of the digits. But pure truncation is
exactly what you've done.
The application is going to insert not just zeros but
a whole range of values to plot the result.
If I've got "34.45__" then the full range of values
between 34.4500 to 34.4599 when truncated result in
the value you just transmitted. So the application
should rightly plot a line from 34.4500 to 34.4599.
This should happen for latitude and longitude. Two
perpendicular lines are now generated, sweeping
through a range of values on the surface of a sphere
results in a spherical trapezoid (a degenerate
spherical trapezoid at the poles which is a spherical triangle).
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