AirNav Systems Forum

AirNav RadarBox and RadarBox24.com => AirNav RadarBox and RadarBox24.com Discussion => Topic started by: cossarashill on March 04, 2009, 08:42:53 AM

Title: What tranmission mode? and IF. Bandwith??
Post by: cossarashill on March 04, 2009, 08:42:53 AM

Hi All,
Just a wee query.........

What is the transmission mode ie. FM/AM.............and what IF. bandwidth are s.mode type signals using ????????????

cheers
Ogilvie

Title: Re: What tranmission mode? and IF. Bandwith??
Post by: OKC-Steve on March 04, 2009, 03:19:00 PM

Pulse Position Modulation (PPM).  Pulses basically.

http://en.wikipedia.org/wiki/Air_traffic_control_radar_beacon_system (http://en.wikipedia.org/wiki/Air_traffic_control_radar_beacon_system)

Title: Re: What tranmission mode? and IF. Bandwith??
Post by: CoastGuardJon on March 04, 2009, 07:56:38 PM

Thanks for that excellent link Steve, so I assume from that the transmissions from all aircraft are "pinged" to ground on exactly the same frequency?    Is that frequency 1090 Mhz exactly, or is there a lower and upper range - AM, FM, SSB modes I can just about get my head around, but this digital stuff is a bit "Beyond Our (My) Ken"!, and who remembers the subsequent "Round the Horne"?

Title: Re: What tranmission mode? and IF. Bandwith??
Post by: OKC-Steve on March 05, 2009, 02:25:03 AM

Hopefully everyone airborne transmits on 1090, but equipment fails, and wears out, so that many transponders are off frequency or not running perfectly.  Thus, the specification is rather loose.  The transmitter can be off +/- 1 MHz and still be in spec.

The ground and airplanes also transmit on 1030 MHz to interrogate other transponders, for example TCAS.  The ground not only interrogates Mode-S, but also the older modes.

The transponder pulses are very narrow, and therefore require a high bandwidth receiver.  The spectrum can be 14 MHz wide at 20 dB down, which would cover as much as 88 to 102 MHz on the FM dial in comparison.  So between the uplink and downlink it would be like 88 to 116 or the whole FM dial plus some...  Did I say the signal was wideband :-)