Aerospace and Electronic Systems Magazine June 2017 - 8

State-of-the-Art Space Mission Telecommand Receivers

ACQUISITION AND TRACKING
Signal acquisition and tracking will potentially operate at SNRs
lower than those currently expected, and this, depending on the
configuration, may prove extremely challenging. Identification
of receiver processing bottlenecks is done for DS and NE missions. Whereas the former is representative of very low SNR scenarios, the latter is representative of high data-rates scenarios.
In both cases, residual carrier modulation is typically used for
the TC link. For NE missions, the transponder is configured to
receive a Manchester encoded data phase modulated onto the
carrier modulation, a scheme referred to as PCM SP-L [15]. On
the other hand, in DS missions the transponder is configured to
receive nonreturn-to-zero (NRZ) data modulated on a sinusoidal-wave subcarrier that is then phase modulated onto the carrier (PCM NRZ-L modulation [15]). Representative modulation
parameters for the two scenarios are reported in Table 3. For the
DS case, the symbol rate used is the minimum one allowed by the
standard [15], providing the most demanding scenario for acquisition and tracking, i.e. the lowest SNR.
From a receiver perspective, the architecture is similar for
both scenarios in Table 3, with the main difference that PCM
SP-L does not require subcarrier tracking. Therefore, in tracking
mode, the main modules in the receiver are: a carrier tracking
block, typically implemented by a phase-locked loop (PLL); a
subcarrier tracking block when necessary, typically implemented as a Costas loop; and a symbol timing tracking, implemented
as a data transition tracking loop (DTTL). The data transition
detector in the DTTL has to be slightly adapted depending on
the encoding type, but its operation is basically the same. These
three tracking loops initiate their operation after the signal has
been detected and acquired, process referred to as "acquisition
mode".
In acquisition mode, the TC standard [15] specifies the use of a
symmetric triangular carrier sweeping procedure with the purpose
of locking to the carrier frequency. In this stage, the ground station
(G/S) transmits an unmodulated carrier whose frequency is swept
around the nominal carrier frequency. At the spacecraft end, a 2nd
order PLL is used to lock onto the carrier as soon as it is in the
pull-in range of the loop. One of the main challenges is the low

SNR, particularly with low data-rates. This is clear from the relation between C/N0 and Es/N0, where C is the carrier power, N0 is
the one-side noise power spectral density, and Es is the energy per
channel symbol.
As we will show in the following, the new LDPC codes can operate at SNRs as low as Es/N0 ≈ 0 dB (at CER = 10−5). Correspondingly, a C/N0 ≈ 8.9 dB-Hz results for the lowest symbol rate of
7.8125 symbols per second (sps) for DS missions, which is below
the typical threshold (C/N0 ≥ 10 dB-Hz) for correct carrier acquisition and tracking based on PLL closed-loop architectures, even for
very small loop bandwidth. For NE missions with data-rates above
8 ksps, such limitation does not occur.
Hence, in order to reduce the noise contribution to the tracking loops, it is required to reduce the loop bandwidth as much
as possible. This is in contrast to having a sufficiently large loop
bandwidth to increase the pull-in range and allow signal lock.
An enhancement aimed at reducing the loop bandwidth at the
tracking loops is to improve the signal acquisition via a fast Fourier transform (FFT) processing of the data, which provides an
enhanced estimate of the resting frequency of the tracking loop.
No sweeping from ground is required if FFT-based acquisition
is used.
In tracking mode, the carrier tracking loop could eventually reduce its loop bandwidth since the carrier sweeping would
no longer be applied at the G/S during the transmission of the
modulated carrier, and thus the dynamics of the signal are lower,
i.e. dictated only by the Doppler. For the three different tracking loops, Table 4 summarizes the different methods that can
be considered, including enhanced architectures. The first row
per tracking loop describes the legacy technique implemented
in today's receivers.
Carrier tracking in the baseline receiver is performed with a
2nd order PLL, which is compromised in the DS scenario by the
noise reduction versus dynamic range trade-off. That is, the PLL
has to operate at very low SNR, which implies to use a very low
bandwidth, but the incoming signal is affected by a moderate carrier Doppler rate, being more suitable to cope with such dynamics to increase the loop bandwidth. In the DS case, the filter is
not able to meet both requirements at the target SNR and the loop
is not likely to lock to the incoming signal's carrier phase. The

Table 3.

Parameter Specification of Reference Scenarios
Parameter

Near-Earth (e.g. Lagrange mission)

Modulation type
Modulation waveform

Residual carrier
PCM SP-L

PCM NRZ-L

Direct on carrier

Sine-waveform subcarrier

Typical modulation index

1.0 radians

1.2 radians

Nominal symbol rate

64 ksps

4000/29 = 7.8125 sps

Carrier frequency
Subcarrier frequency

Deep-Space (e.g. ExoMars mission)

X-band
Does not apply

IEEE A&E SYSTEMS MAGAZINE

16 kHz

JUNE 2017

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine June 2017