Micro-Air or other Tiny Transponder on a battery

[Windknot]

One of my favorite quotes from a movie is from "The Core" - hokey premise, great action.......

"All this brain power over here is starting to smell like burning batteries"

Thank God guys like you build airplanes and guys like me DONT!!!!


Sean

[drseti]

Here's my source,

"Figure 1 shows the original 64 code format, and today's 4096 code format under it. Each reply consisted of a framing pulse, some combination of the six possible data pulses, and another framing pulse. It all took place in 20.75 microseconds."

Notice that "consisted of" and "took" are past tense. The 20.75 microsecond burst, and that data description, applied to the old (now obsolete) military transponder standard, with only 64 possible codes, and no altitude data. The give-away is "six possible data pulses." We're talking binary (base two) math. Two to the sixth power is indeed 64. Today's transponder standard uses 4069 possible discrete transponder codes (0000 through 8888, being all octal words), and that takes two to the twelfth pulses to describe in binary (three bits for each of the four octal words). Even at the same data rate, with 64 times as many bits, we're talking 64 times as long to transmit. Add to that the bits required to transmit altitude data, plus those required for forward error correction, framing, timing, and identification, and you can see that today's transponder standard sends a whole lot more data than the old 64-code IFF standard of the 1940s, which takes a whole lot more time to transmit.

I agree with you, however, that the Garmin 12 watt (average) spec seems reasonable.

[Jim Stewart]

Deleted, out of sync.

[drseti]

he's saying that Mode C packs the bits tighter and does it in the same time frame as the old Mode A,

Well, yes, but the bits are all of fixed width (they have to be, or the IF bandwidth of the receiver would have to change, and rise and fall times would have to be faster, to pass those pulses). So, packing more bits into the same time window simply increases duty cycle, which raises average power for a given peak power.

which I assume you are referring to as the military IFF code.

Ah, that's part of the confusion, Jim. No, the 64-code system referred to in that document is not mode A. Mode A is what you get with modern transponders when you switch off altitude reporting, and it still produces 4096 possible codes, 0000 through 7777 (I accidentally typed 8888 earlier -- my error!) The old military IFF standard, which dates to the APX-6 transponder of WW-II vintage, had only 64 possible codes, 00 through 77, and that's the one that they were referring to as only needing 6 data pulses.

Transmitting the 4 digit squawk code takes 12 pulse locations, not 2^12.

Yes, of course. But first off, we're talking four octal words. We can't really call them digits, because digits is a base 10 term, and they go from 0 to 9. Transponders use base 8 math, and their words go only from 0 to 7 -- just turn your knobs and try to get an 8 or 9!

In any case, of course those four octal words can indeed be transmitted in 12 bits (or pulses, if you use one pulse per bit). What I was trying to say is that those 12 bits can produce two to the 12th (that's 4096) possible squawk codes. Their first example used only six bits (pulses), so it produced only two to the sixth (that is, 64) possible squawk codes. At the very best, even if we ignore altitude reporting and overhead bits, we need to send at least twice as many pulses as they were referring to in their "64 code" example.

Confused yet?

[Jim Stewart]

Sorry we got out of sync. Here's my edited post.

I went back and read the whole link that I posted. What I got was that a Mode C transponder always replies with a 14 bit burst of data and the burst is 20.75 microseconds long. This, BTW, appears to be confirmed on the Wikipedia Secondary Surveillance Radar page. If the ground radar sends a Mode A interrogation, the transponder replies with the squawk code encoded in the 14 bits (you only need 12 bits to encode 4096 values). If the ground radar sends a Mode C interrogation, the transponder replies with the altitude (9 bits as I recall) encoded in the 14 bits. Two interrogations, 20.75 microseconds each.

There seemed to be some ambiguity as to whether or not there is a 15th bit somewhere in the data burst. In any case, it's not real relevant.

EDIT: The 15th bit appears when you press the Ident button.