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[aprssig] Re: The best resolution of position from APRS

Wes Johnston aprs at kd4rdb.com
Wed Jan 4 12:24:54 UTC 2006


One degree is 60 nautical miles.... 69 statute miles... 111km - but only
in the North-South direction.  East-West distances must take into
consideration the convergance of longitude at the poles.  Cosine takes
care of this.  So, since I live at approx 30degrees north, the COS of 30
is .8666.  So one degree of LONGITUDE for me is worth 96km.  So if I
were to draw a 1 degree "square" box around me, it would be 111km tall
and 96km wide.

>From there, you just whittle it down to minutes by dividing by 60...

1 degree = 111 km
1 minute = 1.85 km
.1 minute = 185meters
.01 minute = 18.5meters
.001 minute = 1.8 meters

Of course each of these numbers has to be "shrunk" by the COS of your
lattitude.

The mic-e spec goes down to 18 meters north south granularity.

Wes


Andrew Rich wrote:
> So how do you equate a lat and lon change to distance ?
> 
> For example
> 
> 07201.75W moves to
> 07201.76W how far phyiscally have I moved ?
> 
> 07201.7502W moves to
> 07201.7503W how far phyiscally have I moved ?
> 
> 
> -----------------------------------------
> Andrew Rich - VK4TEC
> vk4tec at tech-software.net <mailto:vk4tec at tech-software.net>
> www.tech-software.net
> Brisbane AUSTRALIA
> 
> 
> 
> -----Original Message-----
> From: aprssig-bounces at lists.tapr.org
> [mailto:aprssig-bounces at lists.tapr.org]On Behalf Of Stephen H. Smith
> Sent: Wednesday, 4 January 2006 6:20 PM
> To: TAPR APRS Mailing List
> Subject: Re: [aprssig] Re: The best resolution of position from APRS
> 
> 
> vk4tec at tech-software.net wrote:
> 
>>So try this
>>
>>Put your TNC into converse mode, you know when you hit enter it transmits.
>>
>>But instead hook up your GPS receiver.
>>
>>In theory every line of NMEA data has a return, so the TNC should just
>>UNPROTO it out
>>
>>Quick and nasty APRS
>>
>>
> 
> This is exactly how the original APRS trackers worked 10 years or so
> ago.  GPS receivers plugged into the serial ports of pre-APRS-aware
> TNCs.   You had to slow down the NMEA reporting rate from the GPS and
> turn off all but one NMEA sentence; else the TNC would try to beacon
> multiple GPS strings every second.
> 
> The first generation of APRS-aware TNC firmware (TAPR Ver 1.1.9 for
> TNC2s like MFJs and early Kantronics) provided for constantly updating
> several beacon comment fields in the TNC by capturing specified GPS
> strings at whatever repetition rate the GPS spewed out.   The TNC could
> then be set to beacon these strings at reasonable rates with the usual
> BEACON EVERY n commands.    Consequently, you could now use most GPS
> receivers as-is in the default mode of multiple sentences output every
> second.
> 
> There is an APRS standard for these dumb NMEA trackers.  Set the
> destination address to GPSxyz (instead of the customary APnnnn). The
> three characters in positions x,y, and z determine the APRS symbol to be
> displayed.
> 
> The main disadvantage of the raw NMEA approach is wasted air time on
> crowded channels.  The raw NMEA string is about 3 times as long as a
> standard APRS format posit, and about 10 times as long as a Mic-E format
> posit.    Besides wasting air time, the posit is less likely to get
> received since the longer string is much more likely to be hit by a
> mobile fade, noise, or get stepped on by another station.
> 
> 
> 
> 
> 
> 
> 
> 
> Stephen H. Smith    wa8lmf (at) aol.com
> EchoLink Node:      14400    [Think bottom of the 2M band]
> Home Page:          http://wa8lmf.com
> 
> "APRS 101"  Explanation of APRS Path Selection & Digipeating
>   http://webs.lanset.com/wa8lmf/DigiPaths
> 
> Updated APRS Symbol Chart
>   http://webs.lanset.com/wa8lmf/miscinfo/APRS_Symbol_Chart.pdf
> 
> 
> New/Updated "Rev G" APRS     http://webs.lanset.com/wa8lmf/aprs
> Symbols Set for UI-View,
> UIpoint and APRSplus:
> 
> 
> 
> 
> 
> 
> 
> 
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> 
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