Mode S Transponders

pscraig

Well-Known Member
I have heard that Mode S transponders encode much more information than a standard Mode C, but while searching the FAA registration database, I noticed *all* aircraft have an assigned, presumably predetermined, Mode S code. When ATC pings the transponder, does it reply with all of the information, or does the ATC computer have a database of aircraft information and look it up as needed?
 

highspeed

Well-Known Member
I think mode S is more for a TCAS use.
If you have "TCAS I" you will be given alerts of other aircraft in your path. If you have "TCAS II" you will be given an alert, as well as a resolution, ie "climb, climb." The TCAS system is transponder based system, so if a plane is out there flying with out a transponder, or they have it turned off, TCAS will show nothing, and make no advisory.

The mode S/C differences are in the resolution TCAS gives.
If you are flying a plane with Mode C and no TCAS, you look for traffic on your own. ATC will be receiving secondary info from your mode C. Also, other aircraft with TCAS will know you are there.
If you have Mode S and TCAS II, you will see other aircraft on the TCAS, and will be given a conflict resolution. The part that makes Mode S nice, is if two planes, both with Mode S and TCAS II are flying at each other, as in like two B757's, Both planes will get a conflict resolution, but both plane's transponders will communicate with each other, and give a different resolution. ie, one will be told to climb and the other will tell you to decend.

So if that hasn't confused you more then before, TCAS I gives alerts, TCAS II gives alerts and resolutions.
Mode S planes w/ TCAS II will coordinate the best resolution.

hope this helps, if not i'd bet someone else can explain it a bit better.
 

zuckerwar

New Member
I think that mode S refers to transponders with four selectable digits, i.e. the ability to input a discrete code out of 4096 possibilities (4 x 4 x 4 x 4).

Mode C refers to the abliity to report your pressure altitude.

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highspeed

Well-Known Member
</font><blockquote><font class="small">In reply to:</font><hr />
I think that mode S refers to transponders with four selectable digits, i.e. the ability to input a discrete code out of 4096 possibilities (4 x 4 x 4 x 4).

Mode C refers to the abliity to report your pressure altitude.

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I don't think thats correct. I believe they both report pressure altitudes. That is how both ATC and each aircrafts TCAS II can coordinate different resolutions. If they both didn't send out altitudes, TCAS (or ATC) wouldn't know whether to climb or decend you.
 

Eagle

New Member
The Mode S is a secondary surveillance and communication system which supports Air Traffic Control (ATC) and could support other data link services, such as weather information, directly to the cockpit. Each Mode S transponder equipped aircraft is assigned a unique address code. Using this unique code, interrogations can be directed to a particular aircraft and replies can be unambiguously identified. Mode S limits its interrogations to specific targets, and proper timing of interrogations permits replies from closely spaced aircraft to be received without mutual interference. Mode S also provides monopulse detection for improved azimuth accuracy and includes RMM capabilities. The Mode S also interrogates and receives aircraft position and altitude information from Air Traffic Control Radar Beacon System (ATCRBS) transponder equipped aircraft. As a back-up, the Mode S has the capability to operate as an Air Traffic Control Beacon Interrogator (ATCBI). As of the end of 1998, there are approximately 140 Mode-S sensors commisioned in the backup mode (IBI) with around 90 of those fully commissioned in Mode-S.
 
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