Question about Transponders

[Typhoon1244]

I thought I was a pretty bright guy, but my F/O asked me a question last week that has me stumped: why is it that the last digit of any squawk can't be an "8" or a "9?"

Yeah, yeah, I know: those numbers aren't even in the transponder. But why not? Is it a technological, historical, or regulatory issue?

 

[chawbein]

Don't question the voodoo that controls the transponster. The black helicopters will come and take you away.

 

[Typhoon1244]

The black helicopters will come and take you away.

And they're all squawking "8989!"

C'mon, seriously: anybody know the reason for this?

 

[ShawnC]

Don't quote me, it had to do with the computer system that was in place at the time when the Mode C transponder was implemented. It couldn't do more than 8 digits (0-7) per a place.

 

[dmspilot00]

I think it has to do with computers thinking in terms of 8. The transponders we use are often called 4096-code transponders (that's 8^4).

I just did a google search, this page http://www.airsport-corp.com/modec.htm might be some help.

 

[chawbein]

I think it has to do with computers thinking in terms of 8. The transponders we use are often called 4096-code transponders (that's 8^4).

I just did a google search, this page http://www.airsport-corp.com/modec.htm might be some help.

I can't believe you posted that on a public message board! Where's my foil hat.

 

[dmspilot00]

I can't believe you posted that on a public message board! Where's my foil hat.

I don't get it.

Seriously though, the page I linked to will tell you more than you ever wanted to know.

 

[chawbein]

It's not that funny. I'm just being stupid for the sake of stupidity. Making fun of conspiracy theorists.

 

[Flyingtoohigh]

It's not that funny. I'm just being stupid for the sake of stupidity. Making fun of conspiracy theorists.

So I can take off my foil hat, too?

 

[chawbein]

At your own risk. The FCC will be able to beam the mind control ray directly into your brain. You can combat this by digging an 8' deep hole and covering it with a galvanized wash tub.

 

[TonyC]

I thought I was a pretty bright guy, but my F/O asked me a question last week that has me stumped: why is it that the last digit of any squawk can't be an "8" or a "9?"

Yeah, yeah, I know: those numbers aren't even in the transponder. But why not? Is it a technological, historical, or regulatory issue?

The transponder code is a Base 8 number, and it's based on the development of the primative system of automatically identifying friends (from foes) using radar.

Typically we deal with a Base 10 (decimal) numbering system, where there are 10 possible values for each digit in a number (0-9). Most of us are also more or less familiar with the Base 2 (binary) system where there are 2 possible values for each digit (0 & 1). You may not be aware that you are also quite versed in Base 60 and Base 24 systems - - 60 seconds make up a single minute, 60 minutes in an hour, 24 hours in a day.

What you might not realize right away is that Base 8 is a convenient shorthand for long numbers expressed in Base 2. It capitalizes on the computer-friendly, or prior to modern day computers, electronics-friendly binary numbering while using a more decimal-like naming convention.

If you'll remember back to your kindergaten days, you might recall the PLACES of digits have significance. Having a 3 in the first place means you have 3 units. The same number in the next position to the left means you have 3 TENS. The next place over is for TEN-SQUARED's, or Hundreds. And so on.

In the Base 8 system, the places have different meanings. The first place is units, the next is EIGHTS, the next is EIGHT-SQUARED's (64 in the decimal convention), and the next is EIGHT-CUBED's (decimal equivalent: 512). A significant advantage of the Base 8 system in electronics is it lends itself to convenient translation to Binary.

Here is a table that expresses the same values in Decimal, Binary, Base 8 and Base 16:

BASE        BASE       BASE      BASE
10           2           8        16
======================================
 0        0 0 0 0        0         0
 1        0 0 0 1        1         1
 2        0 0 1 0        2         2
 3        0 0 1 1        3         3
 4        0 1 0 0        4         4
 5        0 1 0 1        5         5
 6        0 1 1 0        6         6
 7        0 1 1 1        7         7
 8        1 0 0 0       10         8
 9        1 0 0 1       11         9
10        1 0 1 0       12         A
11        1 0 1 1       13         B
12        1 1 0 0       14         C
13        1 1 0 1       15         D
14        1 1 1 0       16         E
15        1 1 1 1       17         F

You may notice a simple pattern, a direct comparison of Base 10 and Base 8 numbers through the number 7. Considering the simplicity of the electronic circuitry required to connect 4 ON/OFF switches (simplistic but applicable comparison), Base 8 is a logical choice for a rudimentary electronic device.

So, when you're squawking 1200, you're really squawking 1 x 512, plus 2 x 64, plus 0 X 8, plus 0 x 1...
In decimal that would be 640...
In binary that would be 0001001000000000

Decimal would be easiest for you and me to enter, Binary would be easiest for a technician to build, and Base 8 is a pleasant compromise.

Be grateful they didn't choose to use Hexidecimal (Base 16, where the values are 0-9 and A-F)!

 

[DGdaPilot]

Wow...don't know whether I feel smarter or dumber now.

 

[pilotman2105]

Re: Re: Question about Transponders

The transponder code is a Base 8 number, and it's based on the development of the primative system of automatically identifying friends (from foes) using radar.

Typically we deal with a Base 10 (decimal) numbering system, where there are 10 possible values for each digit in a number (0-9). Most of us are also more or less familiar with the Base 2 (binary) system where there are 2 possible values for each digit (0 & 1). You may not be aware that you are also quite versed in Base 60 and Base 24 systems - - 60 seconds make up a single minute, 60 minutes in an hour, 24 hours in a day.

What you might not realize right away is that Base 8 is a convenient shorthand for long numbers expressed in Base 2. It capitalizes on the computer-friendly, or prior to modern day computers, electronics-friendly binary numbering while using a more decimal-like naming convention.

If you'll remember back to your kindergaten days, you might recall the PLACES of digits have significance. Having a 3 in the first place means you have 3 units. The same number in the next position to the left means you have 3 TENS. The next place over is for TEN-SQUARED's, or Hundreds. And so on.

In the Base 8 system, the places have different meanings. The first place is units, the next is EIGHTS, the next is EIGHT-SQUARED's (64 in the decimal convention), and the next is EIGHT-CUBED's (decimal equivalent: 512). A significant advantage of the Base 8 system in electronics is it lends itself to convenient translation to Binary.

Here is a table that expresses the same values in Decimal, Binary, Base 8 and Base 16:

BASE        BASE       BASE      BASE
10           2           8        16
======================================
 0        0 0 0 0        0         0
 1        0 0 0 1        1         1
 2        0 0 1 0        2         2
 3        0 0 1 1        3         3
 4        0 1 0 0        4         4
 5        0 1 0 1        5         5
 6        0 1 1 0        6         6
 7        0 1 1 1        7         7
 8        1 0 0 0       10         8
 9        1 0 0 1       11         9
10        1 0 1 0       12         A
11        1 0 1 1       13         B
12        1 1 0 0       14         C
13        1 1 0 1       15         D
14        1 1 1 0       16         E
15        1 1 1 1       17         F

You may notice a simple pattern, a direct comparison of Base 10 and Base 8 numbers through the number 7. Considering the simplicity of the electronic circuitry required to connect 4 ON/OFF switches (simplistic but applicable comparison), Base 8 is a logical choice for a rudimentary electronic device.

So, when you're squawking 1200, you're really squawking 1 x 512, plus 2 x 64, plus 0 X 8, plus 0 x 1...
In decimal that would be 640...
In binary that would be 0001001000000000

Decimal would be easiest for you and me to enter, Binary would be easiest for a technician to build, and Base 8 is a pleasant compromise.

Be grateful they didn't choose to use Hexidecimal (Base 16, where the values are 0-9 and A-F)!

Makes sense. To make a gross generalization, it is a computer thing. It goes back to the fact that computers only understand 0s and 1s ("on" or "off"). The base 8 system is simply a way to put the binary system into something more meaningful and understandable in the cockpit. Just as TonyC mentioned, base 16 would be a PITA to understand. And so would binary.

"Cessna 12345, squawk 1010000000." (A binary number)

Instead, we now have:

"Cessna 12345, squawk 1200." (A base 8 number)

 

[sky37d]

COOL
It makes sense. I learned stuff here. I remember base 8 numbers, long time ago.

 

[JungleJetFO]

Stupid me, I thought they didn't use 8 and 9 since they can look like a 3 from a distance.

 

[Typhoon1244]

Wow...don't know whether I feel smarter or dumber now.

Dumber. Definitely.

Thanks, Tony. I will add that to my repository of fascinating and useless knowledge!

 

[JCJ]

occasionally if you find a really old book about instrument flying it may reference certain codes on the the two digit transponders

00 - ADC (Air Defense Command of cold war era) interceptors

12 - VFR

76 - Radio Failure

77 - Emergency

which of course evolved into the current common transponder codes

 

[Timebuilder]

Cool. Base 8 notation. It never occurred to me.

Hexadecimal notation was bad enough. It made smoke come out of my ears.

Nice call!

 

[hydroflyer]

I still find it amusing to hear an airplane call up for flight following or IFR and repeat a transponder code to the controller which has a 8 or 9 in the code. Apparently not everyone has heard of the base 8 system in the transponder.

 

[Dan CFI/CFII]

Tony's got a real good explanation there... Real simple.

But there's a reason they limit transponder codes tot he Octal (base 8) system. If you look at his table, you'll see that for decimal 0-7 that the first digit is always zero. If we simply never allow any transponder a digit higher than seven, there will never, ever be a code that requires a full four bits of data. Basically, we only need the last three digits off of Tony's table to transmit transponder codes if they are limited to the 0-7 range.

If we were to have transponders with the numbers 8 and 9, it might make more sense to most pilots, but you will notice that you will only gain two more codes for having to transmit an extra bit of data for the other 8 codes.

So when you transmit 1200 on your transponder, it is really sending out something that looks like this: <reference/start pulse> 001 010 000 000 <end/terminaton pulse> That's four bits of data less that we had to send and that the transponder and the reciever did not have to even think about.

It doesn't sound like much now, but remember that these things were designed a long time ago. Having to deal with that extra four bits of information for each transmission was a LOT more work, for what was considered only a marginal gain.

Great explanation Tony! I spent a LOT more time in school learning about avionics, and would have killed for such a simple explanation of such things.

Dan

PS-if you REALLY want to confuse yourself, read up on the differences between Octal (or Hexadecimal) and Octal Notation. That stuff can mess you up! If you do, instead of making drunk posts, you'll be making confused posts Now you know why everything in computers is done in multiples of 8 (you might remember that you used to buy RAM and it would come in, say, 256K, 512K, 1024K sticks...) or 16. It's that Octal or Hex Notation of decimal numbers leaves no unused possible combinations.