IEEE - Aerospace and Electronic Systems - September 2019 - 52

Regenerative Ranging for JPL Software-Deﬁned Radios
Table 3.

Table 4.

Ranging Code Coefﬁcients used for Integer Delay Recovery
Component
code index n

Galileo Scenario Parameters
Regenerative ranging
þ Telemetry

504,735

721,050

Eb =N0

4.5 dB

642,390

2.1 m

134,596

850,080

175,560

CONCLUSIONS AND FUTURE DEVELOPMENT

These coefficients are valid for the CCSDS T2B and T4B ranging
codes.

consequences of these smaller signal levels was that ranging could not be done to Galileo, whose transponder had a
nonregenerative ranging channel, when the spacecraft was
near Jupiter. (However, ranging was done early in the mission, as the Galileo spacecraft approached the earth during
each of two earth-gravity-assist flybys.)
Using the largest available (70 m) antenna of the DSN,
the downlink PT =N0 was, for the near-Jupiter phase of the
mission, 20:5 dB Á Hz at a range of 7:3 Â 108 km [15]. Even
if telemetry were turned OFF, the largest downlink PR =N0
that could be achieved would have been À13:3 dB Á Hz, too
small for ranging.
In order to test the efficacy of regenerative ranging,
the following what-if scenario is considered. If the Galileo
mission had implemented PN regenerative ranging, would
range measurements have been possible during the nearJupiter phase of the mission?
Most of the parameters of Tables 1 and 2 apply in this
scenario as well. Those parameters that change are listed
in Table 4.
The downlink ranging modulation index fR was 0.45
rad rms and the required Eb =N0 was 4.5 dB for Galileo.
The CCSDS T2B range code [7] is more appropriate (than
the T4B range code used in the calculations of Figures 1
and 2) for this very-low PT =N0 scenario. The T2B range
code is optimized to work at lower PT =N0 , but at the price
of a larger s R (2.1 m in this example, as opposed to the
0.5 m of Table 1).
Calculations show that regenerative ranging with a
T2B range code would have been possible in this scenario.
Moreover, telemetry with a bit rate of 25 b/s could
have been supported simultaneously. With only telemetry
present on the downlink, the telemetry bit rate was 40 b/s.
In summary, the use of regenerative ranging along with
a reduction in telemetry bit rate permit range measurements in this scenario where they would otherwise be
impossible.
52

0.45 rad rms

This paper discussed a brief theoretical overview of the PN
regenerative ranging technique and its advantages against
nonregenerative ranging. The advantages are twofold: 1)
regenerative ranging provides orders-of-magnitude improvement in ranging SNR at low telemetry rates and 2) regenerative ranging is operationally simpler for tracking station
operations with quicker signal acquisition. Thus far, mission
requirements drive the use of PN regenerative ranging, such
as the New Horizons mission to Pluto with extremely large
propagation loss, or the BepiColombo mission with ultrahigh ranging precision requirements. However, the
improved ranging SNR can instead equate to system trades
for reduced transmit power, reduced ranging modulation
index, or smaller antenna aperture. This reduced cost to the
mission is especially favorable for highly power/mass-constrained spacecraft such as CubeSats and SmallSats.
The implementation method presented in this paper is
not only limited to the Iris transponder, but is directly applicable to other JPL SDRs like the UST. The higher sampling
clock rate coupled with ultralow phase noise performance
on the UST can produce finer resolution navigation products for missions to accomplish challenging orbit determination needs with greater science applications. The dualfrequency uplink/downlink capability on the UST also provides radio scientists with high-performance measurement
capabilities at S-, X-, and even Ka-band to explore planetary atmospheric and gravitational effects. The addition of
PN regenerative ranging would be little, to no cost on the
current JPL SDR radio transponders.
The push in recent years is to move from technology
demonstration missions like MarCO to full-featured science missions exploring interplanetary and deep space
using CubeSats. Several mission concepts are discussed in
the literature to explore other planetary bodies and moons
such as Phobos at Mars, the Galilean moons at Jupiter,
and other outer planets. From a telecommunications perspective, mission enabling technology development such
as advanced-capability SDRs with new functionality
including PN regenerative ranging is paving the path to
the future.

IEEE A&E SYSTEMS MAGAZINE

SEPTEMBER 2019

IEEE - Aerospace and Electronic Systems - September 2019

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