Doppler Rate Aid Principle And Architecture - Safran CORTEX CRT Quantum User Manual

3.6.3.5.4. Doppler Rate Aid Principle and Architecture

Given the current Ranging receiver performances, it appears that, to obtain accurate measurements at low signal
to noise ratio, the PLL bandwidth should be narrowed. On the other hand, if the Doppler rate is high, the
selection of a wider PLL bandwidth is required to reduce the error bias. To be able to cope with Doppler Rate at
very low ranging SNR, the classical PLL is improved by adjoining it a Doppler Rate Aid software module.
Carrier Doppler shift measurement is performed by the IF Receiver. Having a priori knowledge of the complete
system frequency plan (ground-station up-link and down-link Radio Frequency, spacecraft operating mode,
spacecraft turn-around ratio, ...), the Doppler shift affecting the major range tone can be deduced from the carrier
Doppler shift (see the previous equations for definition of the ratio between the carrier Doppler shift and the tone
Doppler shift").
The Doppler Rate Aid principle is that the ranging receiver uses (as expected tone frequency) the emitted tone
frequency corrected by the estimate of the Tone Doppler shift. Hence, because the PLL has only to handle the
residual phase error, the bias should disappear and, even in presence of high Doppler Rate, low bandwidth PLL
could be used.
To configure the Doppler Rate Aid module, two additional parameters are introduced:


At the ranging session end, the RAU fills the new status PLL Frequency Offset that gives the remaining offset
frequency handled by the Ranging PLL.
IF
S (t)
Receiver
rx
f
dop_carrier
© Safran Data Systems
COMMAND RANGING & TELEMETRY UNIT CORTEX
CRT QUANTUM USER'S MANUAL
Doppler Rate Aid / Doppler Aid (Off / Doppler Rate)
Carrier Doppler shift/Tone Doppler shift ratio (expressed in floating format).
Ambiguity resolution module :
Minor tones processing
Rough delay estimation :
N = number of 2

X (t)
RG_rx
f
tx_tone
Major tone
synthesizer
(t)
Figure 77: Doppler Rate Aid Architecture
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Ranging

-rotations on the Major tone
(t)
Low pass &
Rone
decim. filter
f (t)
dop_tone
Ratio factor
Safran Data Systems
.
Ref. DTU 100042
Is.Rev. 5.17
Date: Dec.
03, 2021Sept. 30, 2021
  
(t) = (t) + 2 N

Rone Rone mod2
Phase
accumulator
PLL
filter

(t)
residual
VCO

(t)
Rone
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