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3.6.11. ESA Code Standard
3.6.11.1.General
The ranging sequence consists of a sequential transmission of a tone which is phase-modulated by a series of
codes, used for ambiguity solving. Initially, the tone is transmitted to allow the transponded tone to be acquired
by the ground equipment (IF Receiver and RAU).
By means of a digital PLL, the RAU produces a phase-locked replica of this tone. Then the lowest code
modulates the tone. Then the code is replaced by the following codes and so on until the longest code. Each
transmitted code is correlated with the demodulated code to determine the number of tone cycles in the range.
When all codes have been transmitted, the RAU continues transmitting the tone, allowing for accurate range
measurement.
The PLL is a second-order whose bandwidth (2Bn) is programmable from 0.1 to 8 Hz (Cortex CRT Quantum) or
0.001 Hz to 8 Hz (CRT eXplorer).
3.6.11.2.Frequency Programming
The tone frequency is programmable through major tone parameters according to the following rule (in kHz):
with the restrictions described in § 3.6.2.1.
3.6.11.3.Code Programming
Ambiguity solving is performed by code transmission. The code sequence is defined by the parameter Code
length (N = 0 to 24). Each code consists in a PCM/PM (modulation index is 45°) modulated binary messages
where bits and tone cycles transitions are synchronized. The binary message number n (where 0 < n ≤ N) is
pseudo-noise sequence of length 2n bits derived from the following expression:
where ⨁ is XOR binary operator, Q
Only even codes are transmitted. Therefore, the last emitted code number (M) can be different from N:
If N = 0, M = 2.
If N is odd, M = N + 1.
If N is even, M = N.
For example, if N = 4, codes 2 and 4 are transmitted. If N = 7, codes 2, 4, 6 and 8 are transmitted.
For a given M, the maximum ambiguity solving is given by the following formula (in seconds):
© Safran Data Systems
COMMAND RANGING & TELEMETRY UNIT CORTEX
CRT QUANTUM USER'S MANUAL
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Safran Data Systems
.
Ref.
DTU 100042
Is.Rev.
5.17
Date:
Dec.
03, 2021Sept. 30, 2021
Page 182
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