Transponder

[NuevaLuna]

It seems more and more commom these days for ATC to assign us a new code half-way thru the flight. With computer technology, shouldn't the original code be okay the whole flight?

Thanks

[TFaudree_ERAU]

There are only 3076 transponder codes for use. There are more than that many airplanes in the air with an ATC assigned code at the same time. The law of averages means somewhere along the line, 2 airplanes with the same code will be within range of the same radar and be in conflict.

[DE727UPS]

"With computer technology, shouldn't the original code be okay the whole flight?"

The ATC computer system isn't as high tech as one would assume.

[Fox Xray]

Throughout the nation each ARTCC is assigned X amount of codes to their host computer systems. The transponder has 4096 codes that can be used, minus the 64 or so non discrete codes and minus a few others we're left with say about 4000.

You depart MIA landing JFK, you fly through several centers, ZMA, ZJX, ZTL, ZDC, and ZNY. Volume in these ARTCC's is very high, sooner or later you'll most likely enter airspace where your code is already being used, in that instance the host computer assigns your flight another code.

The fact that you are seeing this more and more shows you that there are high volumes of air traffic in the NAS.

[Fox Xray]

Originally Posted by DE727UPS

"With computer technology, shouldn't the original code be okay the whole flight?"

The ATC computer system isn't as high tech as one would assume.

Our system is a lot like the old MS DOS. The inputs are all coded, it's very old technology.

[NuevaLuna]

Thanks for the replies.

[aviatorrbt]

it happened to me this week twice! i was in really busy airspace though.

[TFaudree_ERAU]

I was flying a charter from the Richmond, VA area up to Syracuse, NY and back. On the return trip, as SOON as we crossed from ZNY to ZDC, we were issued a new code. I couldn't help but to think it was something to do with crossing the Mason-Dixon line.

[PanJet]

Adding to the original question, is there some sort of system to the codes? I've noticed that I get different codes in different areas. For example, in our area, 95% of the time our codes begin with the decimal 4. If I go to a different area, they begin with something else.

[PanJet]

Originally Posted by TFaudree_ERAU

There are only 3076 transponder codes for use. There are more than that many airplanes in the air with an ATC assigned code at the same time. The law of averages means somewhere along the line, 2 airplanes with the same code will be within range of the same radar and be in conflict.

Another question. If transponders go up through 7799 I believe, why are there only 3076 available codes? (Also, I thought there was 4076, but we'll go with this for now). Are the rest reserved (other than 75, 76, 77, and 12 hundred).

[TFaudree_ERAU]

To tell you the truth, after I read the other reply stating 4096, I'm not quite sure where I got 3076. 4096 sounds more reasonable to me.

In regards to the 4 code. Each facility is assigned a range of codes; internal primary, internal secondary, internal tertiary, external primary, external primary, and external tertiary. Have a look at this PDF and tell me if the codes you usually are assigned are in the respective range of codes of your ARTCC. These are the ones allocated to each center on VATSIM, and I've often wondered if they have any real world similarity.

[averyrm]

7777 is the highest it goes, no 8 or 9, so 4096 possible codes.

[averyrm]

To expand you have 0-7, or 8 digits possible per number, 8x8x8x8 = 4096

[PanJet]

Originally Posted by averyrm

To expand you have 0-7, or 8 digits possible per number, 8x8x8x8 = 4096

Hmm, okay, that makes sense. Okay, I was never great at math, so dumb question. Doesn't the 8x8x8x8 rule mean that you will have no two combinations with the same digits no matter how they are ordered? Meaning 0001 is not distinguished from 0010 or 0100 or 1000?

I guess I was thinking if you went through starting with 0001, then 0002, 0003, 0004..... all the way through 7777 (don't know why I was thinking 7799 earlier), then wouldn't you have 7777 codes minus the reserved ones?

Pardon my ignorance. Like I said, I was never that great at math.

Originally Posted by TFaudree_ERAU

In regards to the 4 code. Each facility is assigned a range of codes; internal primary, internal secondary, internal tertiary, external primary, external primary, and external tertiary. Have a look at this PDF and tell me if the codes you usually are assigned are in the respective range of codes of your ARTCC. These are the ones allocated to each center on VATSIM, and I've often wondered if they have any real world similarity.

Actually, yes, that does correspond fairly close to the codes we get. So then what's the difference between internal and external and primary, secondary, and tertiary?

[SteveC]

Hmm, okay, that makes sense. Okay, I was never great at math, so dumb question. Doesn't the 8x8x8x8 rule mean that you will have no two combinations with the same digits no matter how they are ordered? Meaning 0001 is not distinguished from 0010 or 0100 or 1000?

No, they are different codes. 0010 is different and distinguishable from 0100.

I guess I was thinking if you went through starting with 0001, then 0002, 0003, 0004..... all the way through 7777 (don't know why I was thinking 7799 earlier), then wouldn't you have 7777 codes minus the reserved ones?

Remember, no 8's or 9's:

1st code = 0000
2nd code = 0001
3rd code = 0002
4th code = 0003
5th code = 0004
6th code = 0005
7th code = 0006
8th code = 0007
*************
9th code = 0010
10th code = 0011
11th code = 0012
12th code = 0013
13th code = 0014
14th code = 0015
15th code = 0016
16th code = 0017
*************
17th code = 0020
18th code = 0021
19th code = 0022
20th code = 0023
21st code = 0024
22nd code = 0025
23rd code = 0026
24th code = 0027
*************
25th code = 0030
>
>
>
>
4095th code = 7776
4096th code = 7777

Starting to see the pattern? There are only 8 digits used in each column, not the normal 10. If all ten were used there would be 10 x 10 x 10 x 10 = 10,000 possible codes (remember that 0000 counts as one code as well, therefore 10,000 possible instead of 9,999). The same math applies to base eight numbering system as base 10. Thus 8 x 8 x 8 x 8 = 4,096 possible codes.

Here is another example, only using base 2 (binary, like computers, use only 1's and 0's) instead of base 8 (0-7) or base 10 (0-9). Might be a little clearer to see.

1st code = 0000
2nd code = 0001
3rd code = 0010
4th code = 0011
5th code = 0100
6th code = 0101
7th code = 0110
8th code = 0111
9th code = 1000
10th code = 1001
11th code = 1010
12th code = 1011
13th code = 1100
14th code = 1101
15th code = 1110
16th code = 1111

Only 2 possible digits in each column, therefore 2 x 2 x 2 x 2 = 16 possible codes.

Cool, eh?

[PanJet]

Okay, well, like I said, not good at math. I guess if I had counted a few more digits myself I might have realized that fact. Hmm, I shall now retire to the corner to wear my dunce hat.

Thank you for clarifying. It all makes sense now.