Esa Code Standard - Safran CORTEX CRT Quantum User Manual

3.6.3.2.2. ESA Code Standard

For each code, ambiguity solving consists in synchronizing the received code with a local replica of the emitted
code. Given the ESA code properties, only 4 codes positions are possible for the received signal. Compared with
the emitted code, it can be in-phase, in-quadrature, in-opposition or in-inverted quadrature.
By the way, to determine the position of the received code, only two correlations need to be computed:


In ideal condition (no-noise, no non-linearity), we systematically obtain that the normalized correlation of one
correlator output values ±1 whereas the other correlator output is null. As quality indication for ambiguity solving
on each code, we define for each (with the index I) the Quality Indicator QI:
where �� and ��
�������� ��������
This indicator values 1 in ideal condition and decreases to value 0 as far as the noise conditions are degraded
(worst case when ��
��������
To have a confidence indicator, we define the Quality Factor Q as:
This Quality Factor will mainly depend on the noise level (S/No at RAU input) and on the un-compensated
ground or on-board non-linearity (if any). It depends also on the Integration time on each minor tone.
The following Quality Factor values should be observed in various noise conditions, with a Code length of 18, a
Major Tone frequency of 114,754 Hz, a 1 Hz Loop bandwidth, and the Integration time = 1 s:
One should note that the probability of false ambiguity solving is very high when Q ≤ 85.
In fact, if Q < 89, it is recommended to increase the Integration Time. Indeed, as indicated in the previous table,
for a signal-to-noise ratio of 20 dB.Hz, by setting the Integration Time to 5 seconds instead of 1 second, the
Quality Factor increases from 89 to 92.
© Safran Data Systems
COMMAND RANGING & TELEMETRY UNIT CORTEX
CRT QUANTUM USER'S MANUAL
the correlation ��
of the received code against an in-phase replica of the emitted code,
��������
the correlation ��
of the received code against an in-quadrature replica of the emitted
��������
code.
���� ( �� ) = | | ��
are the correlation values normalized on 1.
≈ ��
).
��������
�� = 100 + 10 log [
S/N (dB.Hz)
0
> 45
45
35
25
20
20
This document is the property of
It cannot be duplicated or distributed without expressed written consent.
( �� )| − | �� (��) | |
�������� ��������
min ����(��)]
��=1..��
����������
Quality Factor
97
96
96
93
89
92
(Integration time = 5s)
Safran Data Systems
.
Ref. DTU 100042
Is.Rev. 5.17
Date: Dec.
03, 2021Sept. 30, 2021
Page 156