Re: [Flightgear-devel] [OT]: Range of radios

[Holger W. <wi...@df...>]

On Tue, Feb 13, 2007 at 05:49:48PM +0000, Steve Hosgood wrote:
[...]
> >So, Holger, if you arrange that all "radio transmission" packets in FG/MP 
> >carry the transmitter's wattage and the location of the source, you can work 
> >out the straight-line distance from yourself (call it 'd'), and then do 
> >something like:
> >    pkt = get_packet();
> >    d = sqrt(sqr(my_x - pkt->x) + sqr(my_y - pkt->y) + sqr(my_alt - 
> >pkt->alt));
> >    if (sqrt(pkt->transmitter_power)/d < my_receiver_sensitivity)
> >          /* I can't hear this guy */
> >          chuck_packet(pkt);
> >    else
> >          decode_packet(pkt);
> >This sort of thing would maybe allow ATC (who might have more sensitive 
> >radios) to hear people that the local Cessna pilots can't hear. And that might 
> >be quite realistic.

The problem with a more sensitive radio is that real radio communication
is unidirectional and my implementaion using VoIP is bidirectional based
on conference rooms (a star topology). The range decision is made on the
position of the plane towards the position of the tower. If this range
is outside a specified range calculation (see your formula :-) and
perhaps a threshold detection) the "plane"-client hangs up the "line".

For the ATC this means that he can hear _every_ voice traffic on his
frequency (he has an infinite sensitive radio). Because frequencies are
often more than one time occupied (world wide) the "virtual frequency" is
a combination of ICAO and frequency.

> >To improve things, you might like to make sure that the straight-line 
> >distance 'd' between yourself and the transmitter does not get close to 
> >ground. You'd have to factor in curvature of the earth and any mountains if 
> >you wanted to get it right. If the straight line gets within a couple of 
> >wavelengths of ground, you start getting attenuation and multipath distortion 
> >and all sorts of stuff(*). For a first cut, ignore all that and just use 'd'!
> >Notice also how you could arrange to degrade packets if they get received 
> >very close to your receiver's sensitivity (you could add noise, distortion 
> >etc) which again would add to realism. My code suggestion above just models an 
> >unrealistic sharp cutoff when the signal gets too weak, but IIRC aviation 
> >radio is AM deliberately because that's *not* how AM radio behaves as it nears 
> >the sensitivity limit.

Ok, I see... your formula and a threshold detection for eliminating
flapping should be a good practice.

Thanks!

Holger

> >Steve.
> >(*) The BBC (amongst others) have done a load of work in this area to do with 
> >predicting service-areas of radio and TV transmitters. Some of it is on the 
> >net.
> 

> -------------------------------------------------------------------------
> Using Tomcat but need to do more? Need to support web services, security?
> Get stuff done quickly with pre-integrated technology to make your job easier.
> Download IBM WebSphere Application Server v.1.0.1 based on Apache Geronimo
> http://sel.as-us.falkag.net/sel?cmd=lnk&kid=120709&bid=263057&dat=121642

> _______________________________________________
> Flightgear-devel mailing list
> Fli...@li...
> https://lists.sourceforge.net/lists/listinfo/flightgear-devel


-- 
#####  #### ##  ##   Holger Wirtz         Phone : (+49 30) 884299-40
##  ## ##   ### ##   DFN-Verein           Fax   : (+49 30) 884299-70
##  ## #### ######   Stresemannstr. 78    E-Mail: wi...@df...
##  ## ##   ## ###   10963 Berlin
#####  ##   ##  ##   GERMANY              WWW   : http://www.dfn.de
GPG-Fingerprint: ABFA 1F51 DD8D 503C 85DC  0C51 E961 79E2 6685 9BCF