[aprssig] Mic-E Issues

[Curt, WE7U]

On Tue, 15 Jul 2008, Steve Noskowicz wrote:

> I just dug through the Mic-E section of the spec to satisfy a
> long-time curiosity.  Mic-E is basically a table based lookup and
> base 91 is, well, base 91.  Just as base 10 (decimal) base 2
> (binary), base 16 (hexidecimal) are number systems.  From memory,
> since ASCII has many characters (instead of just 10 (like decimal)
> you can use many of them (91 in this case) to send data as ASCII
> text.
> Did I get that right fellas?

Yep.

Another little tidbit that might not be easily evident from reading
the spec is that Mic-E in my section of the U.S. gives a precision
of about 40' by 60'.  Base-91 gives something like 2' x 3'.  I added
precision rectangles to Xastir a while back that visually show you
where the station _might_ be based on what type of packet they are
transmitting.  You can easily see the difference between Mic-E and
Base-91 stations.  Also between various types of NMEA sentences
based on how many digits of precision they are transmitting.

Of course this begs the questions:  What datum are these people
using in their GPS and what is the accuracy of their current GPS
position?  Without transmitting HDOP and datum and decoding that in
the display software, this improved precision capability of Base-91
is partially unused.

-- 
Curt, WE7U.				archer at eskimo dot com
http://www.eskimo.com/~archer
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