IEEE - Aerospace and Electronic Systems - September 2019 - 46

Feature Article:

DOI. No. 10.1109/MAES.2019.2911002

Regenerative Ranging for JPL Software-Deﬁned
Radios
Krisjani S. Angkasa, James S. Border, Courtney B. Duncan, M.
Michael Kobayashi, Zaid J. Towfic, Thaddaeus J. Voss, California Institute of
Technology, Pasadena, USA
Peter W. Kinman, California State University, Fresno, USA

INTRODUCTION
A spacecraft's radio is a key component used both for
communications and navigation. Previous generations of
transponders for deep space missions have been slow to
evolve and provide new functionality. Today, softwaredefined radios (SDRs) offer more flexibility to meet
various spacecraft telecommunication needs.
This paper describes the implementation of pseudonoise (PN) regenerative ranging in a JPL transponder called Iris, which is a Deep Space Network (DSN)
compatible radio particularly sized for CubeSat/SmallSat applications. This radio was first designed for the
Interplanetary NanoSpacecraft Pathfinder In Relevant
Environment mission [1], along with the first generation of the Iris deep-space transponder [2]. It was further developed and recently flown onboard Mars Cube
One (MarCO) to support InSight Mars Entry, Descend,
and Landing [3]. The approach is general enough to be
used in all of the JPL SDRs, including the Universal
Space Transponder (UST) [4] and the concepts presented here would apply to other digital radio architectures as well.
A ranging measurement starts with the phase modulation of the DSN uplink carrier by a ranging signal, the
bandwidth of which is typically about 1 MHz. Onboard
the spacecraft, at the receiving end of the uplink, the transponder phase locks to the uplink carrier. This permits the
demodulation of the uplink carrier and the reproduction
Authors' current addresses: K.S. Angkasa, J.S. Border,
C. B. Duncan, M.M. Kobayashi, Z.J. Towfic and T.J. Voss
are with the California Institute of Technology; Peter
W. Kinman is with the California State University,
Fresno.
Manuscript received February 9, 2019, revised March 26,
2019, and ready for publication April 10, 2019.
Review handled by F. Davarian.
0885-8985/19/$26.00 ß 2019 IEEE
46

of the baseband ranging signal. Noise originating in the
transponder accompanies the baseband ranging signal.
The transponder's ranging channel processes the baseband
ranging signal plus noise. The output of the ranging
channel is applied to the phase modulator for the downlink
carrier, ensuring that an echo of the ranging signal is on
the downlink.
A transponder can have one of the two types of
ranging channel: a wideband nonregenerative channel
or a regenerative channel. In the past, most NASA
spacecraft have employed transponders with nonregenerative ranging channels. At the expense of additional
complexity, a regenerative ranging channel offers
improved performance.

NONREGENERATIVE RANGING
A nonregenerative ranging channel is one that passes the
baseband ranging signal, along with some of the uplink
noise, through a wideband filter and an automatic gaincontrolled amplifier. The filter must have a large enough
bandwidth to pass the ranging signal without significant
distortion. For a typical 1-MHz ranging signal, the bandwidth is about 1.5 MHz.
For most deep-space applications, the noise power
exceeds the ranging-signal power in the ranging channel. The signal-to-noise ratio (SNR) in the ranging
channel depends on uplink power and incurs a space
loss proportional to 1/R2, where R is the tracking station
to spacecraft distance. The SNR of the ranging signal
received on Earth depends on the SNR of the ranging
signal received at the spacecraft and on the spacecraft
transmit power. The received signal at Earth also
incurs a space loss on the downlink that is proportional
to 1/R2. For the weak signal case, with small ranging
SNR at the spacecraft, the terms for ranging signal SNR
at the spacecraft and for downlink space loss are nearly
multiplicative factors for calculating the SNR received

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SEPTEMBER 2019

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