[aprssig] APRS Ambiguity and Table Entry Method
bruninga at usna.edu
Thu Jul 3 13:29:12 CDT 2008
>> For the past several months we have been seeing
>> reports like "40 . N/08846.42W".
That is position ambiguity of 4 digits. Indicating that the
sending station is only transmitting his position to the nearest
60 miles in the vicinty of 40N and 89 west. Meaning he is
probably in the vicinity of Decatur Illinois.
Setting ambiguity like this allows one to appear on the global
APRS system with a position but without having to fuss with
having a GPS on all the time. Or transmitting a WRONG position.
If the LONGITIUDE was also 40, then that is a SPECIAL CASE
defined for FIXED events with known positions, such as a
marathon, or all the hotels in the Dayton Hamvention pamphlet,
etc... In that case, TABLE-LOOKUP for that event is used to
determine the position. This way, again, D7 hand held users can
place them selves anywhere throught an event and even inside
buildings just by entering a 3 digit table value... Here is how
it is defined in APRS1.2 addendum.
TABLE METHOD: This second method can be used to place a station
(or object) in the vicinity of any of up to 999 pre-tabled
positions, with only the simple entry of a 3 digit number. We
did this by declaring the area of N40,00 and W40,00 out in the
middle of the Atlantic Ocean as special and then using the
hundredths digits to allow entry of the pre-identified
locations. Typically, we use the first digit as a "table"
selection and then the other two digits are the selection in
that table. Many events have so few points, that a table of 0,
and then just 2 digits can be used. For example, using the POS
key on the D7 radio and entering this location would place the
user in the vicinity of location number 23:
N40 00.00 <== the .00 indicates table 0 and 0 ambiguity
W40 00.23 <== the .23 here indicates location 23 in table 0
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