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[aprssig] Re: DAO quantified.

Wes Johnston, AI4PX wes at ai4px.com
Sun May 11 17:58:36 UTC 2008


Someone just asked me off list what DAO is...

 DAO in a nushell is one extra digit of precision on a normal aprs packet.

Here's a snippet from Bob's page..

FORMAT:  The format of this APRS Precision-and-Datum option is the presence
of a 5 byte field !DAO! appearing anywhere in the position comment field.
This option is backwards compatible beacause the basic DDMM.HH/DDDMM.HH
format is retained and will still be decoded by all existing applications.
The !DAO! simply provides the additional precision down to a foot or so
and also identifies the datum:

   !DAO! - is fixed length anywhere in the position commment
    D    - is the datum identifier (base-91)
     A   - is the added LAtitude precision (base-91)
      O  - is the added LOngitude precision (base-91)

RECOMMENDATION:  Recommend placing this option on the END of all other
position comment text.  This way it does not displace any human readable
comment text that is otherwise desired to display on older systems.  This
added option may extend beyond the existing "viewable" limit of 57 bytes
currently in the spec since these added bytes are not used by legacy
systems anyway.

PRECISION:  There are three degrees of precision offered by this option.

    1) HUMAN READABLE, Thousandths of a minute.  This is good to the
       nearest 6 feet or so and being human readable gives even users
       of the D7 and D700 or any other existing application the ability
       to human read position to 3 decimal digits of Minutes.  This
       format is identified if the Datum byte is in uppercase.

    2) BASE-91.  This adds additional precision to the nearest 91/ten
       thousandth of a minute, or about 4 decimal fractional digits of
       a minute or about one foot.  This format is identified if the
       datum byte is in lowercase.

    3) NULL.  If the A and O bytes are SPACE characers, then they
       are only there to fulfill the !DAO! format and imply NO ADDED
       precision.  This is used when one wants to send DATUM info
       but without claming added precision.  This use of space characters
       to imply lacking digits of precision is consistent with the
       existing APRS ambiguity system.

By the way, for those of you wondering about my tag line... My 8 year old
daughter's name is Hope.  Must be something about that missing Y
chromosome.  Talking excessively does seem to run on that side of the aisle.
-- 
Wes
---
Where there's silence, there is no Hope.
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