COMMAND RANGING & TELEMETRY UNIT CORTEX
CRT QUANTUM USER'S MANUAL
Ref. DTU 100042
Is.Rev. 5.17
Date: Dec.
03, 2021Sept. 30, 2021
This document is the property of Safran Data Systems.
It cannot be duplicated or distributed without expressed written consent.
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:
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.
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
are the correlation values normalized on 1.
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:
92
(Integration time = 5s)
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.
To Next Page
Loading...