IEEE - Aerospace and Electronic Systems - September 2019 - 47

Image credit: NASA/JPL-Caltech

at Earth. The SNR received at Earth is, therefore,
approximately proportional to 1/R4.

REGENERATIVE RANGING
In a regenerative ranging channel, the baseband ranging
signal is detected rather than just filtered. A clean copy
of the ranging signal can be generated, analogous to
how the transponder uses a phase-locked loop (PLL) to
generate a clean copy of the carrier. When the uplink is
a PN code, correlation techniques can be used to detect
the ranging signal. The SNR of the ranging signal
received on Earth depends on the spacecraft transmit
power. Just as for Doppler, telemetry, and differential
one-way ranging downlinks, the received SNR is proportional to 1/R2. Only PN code uplinks are considered
here for regenerative ranging applications. State-of-theart digital transponders provide an opportunity to implement regenerative ranging.
Compared to nonregenerative ranging, PN regenerative ranging provides much higher SNR for the very weak
signals often seen in missions with limited radio frequency
power capability and/or on very distant (outer solar system) spacecraft. This substantially reduces the integration
time necessary to resolve the ranging observable, in some
cases enabling ranging.
Missions with high-gain antennas have not generally
had the driving need for regenerative ranging. However,
missions with lesser telecommunication capabilities such
as CubeSats and SmallSats with low-gain antennas may
need regenerative-ranging performance to meet their navigation requirements. In addition, some radio science
investigations require higher accuracy ranging to meet science requirements. These applications require regenerative ranging.
Currently, only two deep space missions employ regenerative ranging. The New Horizons Mission (equipped with
a transceiver developed by Applied Physics Laboratory)
uses PN regenerative ranging to improve the ranging navigation data type over the extreme distances involved in its
SEPTEMBER 2019

mission to Pluto and beyond [5]. The ESA BepiColombo
mission launched in 2018 uses PN regenerative ranging,
with a transponder built by Thales Alenia Space. This mission expects to achieve a range accuracy of 10 cm from
Mercury orbit, which is needed to investigate the fundamental parameters of gravitational theory [6]. The use of
the modified SDRs described here should make this capability and resulting benefits more widely available for both
large and small missions.

PERFORMANCE ADVANTAGE
The performance advantage of regenerative over nonregenerative ranging is quantified here for an example set of
performance and link parameters. Before the numerical
results are presented, the parameters are identified and the
performance metric is explained.
For both regenerative and nonregenerative ranging,
the examples considered here employ the T4B range code
that is defined by the Consultative Committee for Space
Data Systems (CCSDS) [7]. This range code was designed
specifically for deep-space missions. (Another range code
might have been selected for this study; however, as long
as the same range code is used for both regenerative and
nonregenerative ranging, the result would have been qualitatively the same: regenerative ranging has the better
performance.)
The total signal power arriving at the DSN receiver on
the downlink is denoted here by PT . The receiver noise,
which for a line-of-sight downlink can accurately be modeled as additive, white, and Gaussian [8], is characterized
by a noise spectral density N0 , in units of watts per hertz.
Both PT and N0 are referenced to the same point in the
receiving chain. The ratio PT =N0 , with units of hertz, represents the strength of the downlink, relative to the noise
level.
Telemetry is normally present on the downlink, even
during ranging, and so, telemetry is taken into account
here. The minimum PT =N0 required to support both
ranging and telemetry is taken here as an indicator of

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IEEE - Aerospace and Electronic Systems - September 2019

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