ATC Transponder Systems

Mode A Reply PulsesATC Transponder Systems

Mode A Reply Pulses1, 2, 3, 4, 5, 6, 7, 10

Transponder codes are based on the octal number system.

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In the decimal number system we count from 1 to 9 and then put a 1 in the tens column12345678910Mode A Reply Pulses

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Mode A Reply PulsesIn the octal number system we count from 1 to 7 and then put a 1 in the eights column.123456710

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Position3210Power of ten103102101100Weight of position1000s100s10s1s

Decimal Number SystemMode A Reply Pulses

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Mode A Reply PulsesPosition3210Power of eight83828180Weight of position512s64s8s1s

Octal Number System

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Mode A Reply PulsesAs soon as you get to 8, you are putting a one in the eights column, and a zero in the ones column.

So the highest digit in the octal system is a 7

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Mode A Reply Pulses

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Mode A Reply PulsesThere is so much more..

The way we input the code to the transponder is based on the octal number system, but the way the codes are transmitted to ATC, is accomplished using the binary number system of ones and zeros.

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Mode A Reply PulsesFor every octal digit there is a bit binary code equivalent.

The transponder converts the code to binary before it transmits the information to ATC.

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00001001201030114100510161107111

Binary EquivalentsMode A Reply Pulses

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Mode A Reply PulsesPosition3210Power of two23222120Weight of position8s4s2s1s

Binary Number System

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Mode A Reply PulsesConsider the binary number 5 - 1011014s2s1s

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Mode A Reply PulsesOn a transponder, each digit in the code is assigned a letter. The leftmost digit is A, then B, then C and D.

ABCD

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Mode A Reply PulsesA4A2A1101

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Mode A Reply PulsesThe transponder transmits a series of pulses.

There are two framing pulses and then a spot for each code digit to have three pulse positions that portray the three binary bits that constitute the code number.

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Mode A Reply Pulses

A4A2A1101

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Mode A Reply PulsesWhen the transponder sends the code, a 1 is equal to the presence of a pulse and a zero is the absence of a pulse.

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Mode A Reply PulsesLets input a code of 3571 for mode A

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Mode A Reply Pulses

A = 3in binary thats 011A4 = 0A2 = 1 A1= 1

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Mode A Reply PulsesB = 5in binary thats 101B4 = 1B2 = 0 B1= 1

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Mode A Reply PulsesC = 7in binary thats 111C4 = 1C2 = 1 C1= 1

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Mode A Reply PulsesD = 1in binary thats 001D4 = 0D2 = 0 D1= 1

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Our code will look like this when it is sent to ATC

Mode A Reply Pulses

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Mode A Reply PulsesLook at the codes transmitted vs the placement scheme of things on the next 4 slides. You should notice that wherever there should be a one there is a pulse and wherever there should be a zero the pulse is missing.

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A4A2A1011

Mode A Reply Pulses

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B4B2B1101

Mode A Reply Pulses

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C4C2C1111

Mode A Reply Pulses

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D4D2D1001

Mode A Reply Pulses

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