[aprssig] Ambiguity due to GPS

Bill Diaz william.diaz at comcast.net
Sat Jan 7 10:53:08 EST 2006

  See below:

>-----Original Message-----
>From: aprssig-bounces at lists.tapr.org 
>[mailto:aprssig-bounces at lists.tapr.org] On Behalf Of Scott Miller
>Sent: Thursday, January 05, 2006 17:58
>To: 'TAPR APRS Mailing List'
>Subject: RE: [aprssig] Ambiguity due to GPS

>> His transmissions amount to about 16 packets per minute, not counting
>> digipeats.  If we had multiple stations sending this kind of 
>> data at this
>> rate, the local network will quickly become unusable. 

>> We really need to encourage developers to prohibit these 
>> types of abuses on
>> 144.39.  IMO, permitting a user to send such a volume of 
>traffic at 10
>> second intervals should be reconsidered.

>I'm not opposed to adding more informative text to the manual, 
>but I will
>not *prohibit* the user from doing anything they feel is appropriate.

Since few people bother to read the manual or help files, I doubt this will
have much impact on our problem.

>Sending 16 packets per minute on 144.39 is obviously 
>excessive, and the user
>probably needs to be politely corrected on that.  But I've personally
>configured a tracker to send something like 140 
>packets/minute, which is
>perfectly acceptable for telemetry applications on a dedicated channel.
>Won't work too well with a digipeater, though!

I am talking about excessive packets generated by OpenTrac units on 144.39
in a heavily congested area.

Take a look at the following samples captured by ALogger from APRS-IS over a
24 hour period:

!T 23:37:15 !xxxxx-8>APOT02,NS9RC,WIDE1*,WIDE2-1,qAo,N9UOF-1:>MAK-P2 <3
!T 23:37:16
270/008/A=000267 HDOP01.4 SATS06
!T 23:37:17
!T 23:37:43
224/013/A=000279 HDOP01.4 SATS06
!T 23:37:43
!T 23:37:50 !xxxxx-8>APOT02,NS9RC,WIDE1*,WIDE2-1,qAo,N9UOF-1:>MAK-P2 <3
!T 23:37:51
180/024/A=000286 HDOP01.4 SATS06
!T 23:37:51

8 unique packets from an OpenTracker in 46 seconds, and he has a 2 hop path
in a heavily congested area.  Nearby users could concievably hear a total of
24 of his packets within a 1 minute period.  I generally hear 2 copies of
each packet he sends since I am within range of 2 of digipeaters he is
normally heard through.  

And he is hardly unique.

!T 18:44:00
/A=000876 14.5V 19C HDOP01.0 SATS09
!T 18:44:26
W>041/010/A=000873 14.5V 17C HDOP01.1 SATS09
!T 18:44:35 !xxxxx-9>APOT02,WIDE1-1,WIDE2-2,qAo,W9TMW:>OpenTracker on Icom
!T 18:44:46
W>312/014/A=000879 14.5V 19C HDOP01.0 SATS09
!T 18:44:59
W>269/031/A=000880 14.5V 19C HDOP01.0 SATS09

I often hear 2 copies of each packets this station sends as well.

Yet another:

!T 21:31:23 !xxxx-9>APOT02,WIDE1-1,WIDE2-2,qAo,K0GND:!/9mF76&AE>nAG Mostly
!T 21:31:30 !xxxx-9>APOT02,WIDE1-1,WIDE2-2,qAo,K0GND:!/9mDk6&Ac>zEG Mostly
!T 21:31:41 !xxxx-9>APOT02,WIDE1-1,WIDE2-2,qAo,K0GND:!/9mB%6&Ac>qAG Mostly
!T 21:31:47 !xxxx-9>APOT02,WIDE1-1,WIDE2-2,qAo,K0GND:!/9mAq6&@w>dBG Mostly
!T 21:31:57 !xxxx-9>APOT02,WIDE1-1,WIDE2-2,qAo,K0GND:!/9m at w6&?i>x<G Mostly
!T 21:32:03 !xxxx-9>APOT02,WIDE1-1,WIDE2-2,qAo,K0GND:!/9m at 86&?i>p?G Mostly
!T 21:32:13 !xxxx-9>APOT02,WIDE1-1,WIDE2-2,qAo,K0GND:!/9m??6&>[>d<G Mostly

7 unique packets in 50 seconds.  Hardly harmless, since he has a 3 hop path.
Sure glad he is not sending telemetry too or within range of my local

And more:

!T 22:31:06
7 07.7V CNT00021-01C
!T 22:31:16
4 07.7V CNT00022-01C
!T 22:31:24
6 07.7V CNT00023-01C
!T 22:31:32
6 07.7V CNT00024-01C
!T 22:31:40
6 07.7V CNT00025-01C
!T 22:31:50
5 07.7V CNT00026-01C

Imagine what would happen if you have 3 or 4 of these trackers in close
proximity to each other in a heavily congested area.  They could certainly
monopolize the frequency, especially if each is using a 3 hop path.  Not the
kind of product I would want used during a special event involving multiple

Hopefully, we will not see these transmission rates from any balloons or
aircraft operating on 144.39.  It wouldn't be difficult to imagine the kind
of havoc this could cause over a very large geographic area.  I recall an
incident a few years ago.......

As Bob pointed out earlier, one transmission per minute is an acceptable
packet rate.  I feel confident that many of your users are not aware their
transmission rate is far beyond what is considered to be acceptable on a
shared channel.

You really should reconsider the transmission rate your product permits
operators to use.  UiView has an option to limit the number of packets gated
to RF in a 1, 2, or 3 minute period.  Shouldn't be difficult to hardcode
some reasonable limits in the firmware of your product.  This would limit
the harm caused by excessive or abusive transmission rates.

Bill KC9XG


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