IEEE Aerospace and Electronic Systems Magazine - July 2020 - 38

Improving Small Satellite Communications and Tracking in Deep Space

Figure 8.
Artist's impression of two SmallSats conducting radio occultations at Mars in order to monitor the Martian atmospheric pressure-temperature profile for future entry-descent-landing activities.

the Mars system and small bodies [32]. More generally, Mars
Express conducted only a limited number of relatively distant
fly-bys of Phobos. Because of its weak gravitational field, a
larger number of closer fly-bys would significantly improve
the knowledge of the gravity field, and thereby its interior
structure. Phobos has long been recognized as having an
anomalously low bulk density, and an improved gravity field
would likely yield benefits in understanding its formation
and subsequent evolution.
Looking to other target bodies, a SmallSat communications link could be used to probe atmospheric structure and dynamics in a manner similar to what has been
described for Mars. Mission architectures could include
small spacecraft networks, atmospheric vehicles (balloons), or probes. Dual-link Doppler measurements
(spacecraft-spacecraft, spacecraft-probe, spacecraft-
balloon, balloon-balloon, etc.) allow derivation of 2-D
wind vectors and absorption on the radio link can be
used to infer concentrations of atmospheric constituents.
A global network of small spacecraft enables surveying
new planetary targets and yields information on surface
properties at spatial scales important to the safety of
landers and rovers.

RADIO SCIENCE REQUIREMENTS
All spacecrafts have telecommunication links that can be
utilized for addressing Strategic Knowledge Gaps or
obtaining high-value science. Key requirements for the
SmallSat radio to be used for radio science investigations
are the following.
38

1. Simultaneous dual-frequency transmit-receive: Usually, radio-wave propagation effects through a medium
are frequency-dependent, and dual frequency operations are required in order to characterize or mitigate
effects of dispersive media. Depending on the
radio science investigation, a medium's properties
must be measured to a high precision in order to distinguish its effects from those of other forces on the
spacecraft. Many radio science investigations use dual
X-Ka-band observations in order to mitigate the effects
of propagation through an ionized medium (ionosphere
or solar wind). However, some atmospheric investigations may require dual S-X-band observations.
2. Precision oscillator input: Radio science investigations typically require a measurement of the phase
of the signal. In order to measure this phase to sufficient precision, a frequency reference at the level of
an ultrastable oscillation (USO) is required (equivalent to 10À13 over 1000 s). Whereas the radio is not
required to provide USO-level capability intrinsically, it should be able to accept an external frequency reference input.

SUMMARY: STUDY FINDINGS ON SMALL SATELLITE
NAVIGATION, PROXIMITY LINKS, AND
COMMUNICATIONS LINK SCIENCE
SmallSats have a great potential to be used for planetary
and deep space applications. Their communications and

IEEE A&E SYSTEMS MAGAZINE

JULY 2020

IEEE Aerospace and Electronic Systems Magazine - July 2020

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