Holger Wirtz wrote: > 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. >>> > Hi, I've had a chance to review the path loss formulae
Very simply, the height factors of the TX and Rx aerials is only used to translate what might be an oblique angle into a horizontal distance, in all reality when ATC becomes important the path distance between point to point and terrestial distance can be ignored. The frequency factor can be formed into a constant mid-band freq of say 118Mhz As has been stated, the rain attenuation etc can be ignored as being insignificant at VHF Assume standard TX power of 25W ( I stand corrected 25W= 14dBW or 34dBm ...not 17/37) Assume standard RX sensitivity of -114dBm for 12dB SINAD (roughly translated signal to noise) Assume all cable losses are balanced by aerial gains (eg no extra losses or gains) with all these assumptions a distance of 461km pops out of the formulae. what is perhaps more significant and can't be assumed or ignored is co-channel interference (previously referred to as "clutter" I think) as a starting point I would use 461 km and concentrate on simulating clutter or doubling and maybe making distant signals more noisy ( double the noise for each extra 100 km) Cheers Dene ------------------------------------------------------------------------- Take Surveys. Earn Cash. Influence the Future of IT Join SourceForge.net's Techsay panel and you'll get the chance to share your opinions on IT & business topics through brief surveys-and earn cash http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
