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Re: [gpsd-users] AIVDM sentence buffering

From: Jonathan Fewtrell
Subject: Re: [gpsd-users] AIVDM sentence buffering
Date: Tue, 28 Feb 2012 16:23:57 +0800

Thanks for this.

The situation I have in mind is one where signals are being received from 
numerous vessels at the same time. In such a case, there could very well be 
more than one signal of a given type (particularly types 1/2/3 and 5) received 
within 2 seconds. My concern is that (say) a type 1 signal from Vessel B could 
shadow a type 1 signal from Vessel A received a fraction of a second earlier. 
Furthermore, if their transponders are reliably sending signals at a steady 
frequency, this shadowing could continue for some time, meaning that Vessel A 
might never be detected.

Is this a valid concern, or have I misunderstood?

For info, I am testing in Hong Kong. I consume reports every 1.5s and I 
regularly see more than 100 simultaneous live targets. Admittedly this is an 
extreme case, HK being perhaps the busiest port in the world, but arguably this 
is when AIS ought to be most valuable.

On 26 Feb 2012, at 15:24, Eric S. Raymond wrote:

> Jonathan Fewtrell <address@hidden>:
>> Reading the documentation (
>> and, I am worried that if a read
>> picks up multiple AIVDM sentences only the most recent one will be
>> reflected in the ais_t struct.
> Yes, this is a potential failure mode.  To avoid it, just consume 
> reports from the demon more often than once per 2 seconds.  That
> way the read buffer will never contain both a message and a later
> message of the same type that steps on it.
> I'll add this advice to the client HOWTO.
> -- 
>               <a href="";>Eric S. Raymond</a>

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