Aerospace and Electronic Systems Magazine June 2017 - 4

Feature Article:

DOI. No. 10.1109/MAES.2017.160079

State-of-the-Art Space Mission Telecommand
Receivers
M. Baldi, M. Bertinelli, F. Chiaraluce, P. Closas, P. Dhakal, R. Garello, N. Maturo, M.
Navarro, J. M. Palomo, E. Paolini, S. Pfletschinger, P. F. Silva, L. Simone, J. Vilà-Valls1

INTRODUCTION

Since their dawning, space communications have been among the
strongest driving applications for the development of error correcting codes [1]-[3]. Indeed, space-to-Earth telemetry (TM) links
have extensively exploited advanced coding schemes, from convolutional codes to Reed-Solomon codes (also in concatenated form)
and, more recently, from turbo codes to low-density parity-check
(LDPC) codes. The efficiency of these schemes has been extensively proved in several papers and reports.
The situation is a bit different for Earth-to-space telecommand
(TC) links. Space TCs must reliably convey control information
as well as software patches from Earth control centers to scientific
payload instruments and engineering equipment onboard (O/B)
spacecraft. The success of a mission may be compromised because
of an error corrupting a TC message: a detected error causing no
execution or, even worse, an undetected error causing a wrong execution. This imposes strict constraints on the maximum acceptable detected and undetected error rates.
Space TC links have peculiar characteristics, which are challenging for designers: 2
C

1
2

The complexity is highly asymmetrical: TC messages are
originated on ground (with mild complexity constraints) and
received O/B, where resources are very limited.
Links are asynchronous and bursty: messages are transmitted sporadically and a proper procedure must be adopted to
detect them prior to decoding.
Reliability is a key issue, even more for emergency commands,
that may require a codeword error rate (CER) as low as 10−5
and an undetected codeword error rate (UCER) as low as 10−9.

Affiliations and addresses for all the authors appear on page 15.
It is interesting to note that some of these features are in common with
new, completely different applications, like ultra-low latency high-reliability 5G scenarios.

Corresponding author's address: Marco Baldi, Dipartimento
di Ingegneria dell'Informazione, Università Politecnica delle
Marche, Via Brecce Bianche, 12, Ancona, 60131 Italy, E-mail:
(m.baldi@univpm.it).
Manuscript received March 29, 2016, revised July 27, 2016, and
ready for publication September 9, 2016.
Review handled by H. Liu.
0885/8985/17/$26.00 © 2017 IEEE
4

Short codes are needed since some commands, e.g., emergency ones, are very short (2 bytes, typically) and must be
received with limited latency. This problem is exemplified
by a tumbling spacecraft in deep-space (DS), operating in
very low power regime and providing limited availability
of the link: in such a case, the command must be received
within a short time window.
DS links are typically characterized by low data-rates, from
few kilobits per second down to tens of bits per second (bps),
due to link budget constraints: bandwidth and transmitted
power from ground are not a key issue, but the distance to
be covered is very long and the received power is limited.
The challenging channel conditions, due to high Doppler
dynamics and low signal-to-noise ratio (SNR), impose serious constraints on the receiver implementation, pushing the
synchronization sub-systems to their limits.

So far, the TC link requirements have been satisfied by combining sufficient power transmitted from ground with traditional
coding techniques, offering overall marginal coding gains but good
error detection capabilities. To do this, typically, the error correction capability of the coding schemes is exploited first, then error
detection is applied to check if some residual error still remains. In
this case the message is rejected, to heavily reduce the possibility
of accepting a wrong command.

NEW REQUIREMENTS AND THEIR IMPLICATIONS
In very recent years, TC links for new missions have evolved
toward more and more demanding requirements, whose fulfillment necessarily imposes the adoption of error correcting codes
approaching the limits of communication channels. The TC link
range is becoming huge, reaching hundreds of millions of kilometers for DS missions. This becomes a problem due to the limited O/B resources, potentially resulting in operation at extremely
low SNR. Missions based on small landers (e.g., rovers) on distant
planets are typical examples: given their constraints (e.g., small
antennas), signals are received with low SNR values. In general, it
is very difficult to command such units directly from Earth without
relying instead on an orbiter around the planet, which might share
similar SNR issues. Furthermore, next generation mission uplinks,
especially near-Earth (NE) ones, will require very high data-rates,

IEEE A&E SYSTEMS MAGAZINE

JUNE 2017

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