IEEE Aerospace and Electronic Systems Magazine - July 2020 - 37

Davarian et al.
Tolerant Networking (DTN)") and add S- and Ka-band slices to the radio. For the moon scenario, S-band for proximity is preferred because regulations do not permit UHF
emissions on the far side of the moon. It should be emphasized that the choice of frequency and antenna type is mission dependent. Therefore, as more diverse missions are
considered for a return to the moon, proximity link frequency and the antenna type must be judiciously selected
to achieve the best results.

PROXIMITY NAVIGATION
Proximity navigation refers to scenarios in which no direct
connection to the DSN is used for navigation. In this case,
the SmallSat of interest communicates with its ground operations via a relay spacecraft (usually a mothership), Figure 2.
There are a few examples of optical proximity navigation in
deep space [28], but there is only one example of radiometric-based proximity navigation. The Phoenix lander [33],
lacking a DTE connection, had to rely on its proximity link
to determine its position on the Martian surface. The UHF
link with Odyssey, a Mars orbiter, was used for this purpose.
The collected 2-way Doppler data were downlinked in the
orbiter telemetry, extracted from it, and processed on the
ground. Phoenix position on Mars was determined to better
than 30-m (3-sigma) using several long diverse passes. This
was possible because the orbit of Odyssey was known accurately. Since, in the case of Phoenix, data processing was
performed on the ground, there are no examples of onboard
processing of radio-based proximity navigation.
An unpublished JPL study considered intersatellite navigation where the performance of one-way Doppler measurements for spacecraft relative position determination was
investigated in the Martian environment. In an alternative
approach to proximity navigation, CCSDS Proximity-1
Space Link protocol defines and provides services for time
sample collection and exchange between spacecraft. Accurate time exchange between a SmallSat and a mothership
allows for SmallSat position determination. An efficient and
distributed time exchange and processing algorithm that can
be directly infused into the CCSDS Proximity-1 Space Data
Link is described in [30]. Implicit in the above-mentioned
approaches is the assumption of an accurate clock onboard
the spacecraft. High precision clocks may be too costly for
many SmallSat missions. However, for missions that require
moderate navigation performance, a cost-effective approach
is to use a chip-scale atomic clock as discussed in the "OneWay Radio Navigation" section.
Another option for proximity navigation is to equip at
least one end of the link with a transponder (a radio with
coherent two-way communications) as was the case for the
Phoenix Mars lander discussed earlier. However, this
approach is limited to communications between two radios
at a time-whether that be intersatellite or between the
SmallSat and the ground station. The advantage to using a
JULY 2020

one-way tracking architecture is flexibility and extensibility. The one-way method allows for communications
between multiple satellites. Therefore, this method should
be further investigated, and the merits of the competing
approaches should be examined via analysis, laboratory
tests, flight implementation, etc.

RADIO SCIENCE INVESTIGATIONS WITH SMALLSATS
Radio science observations utilize precise monitoring of the
communication links between a transmitter and receiver in
order to detect forces acting on the spacecraft or to infer the
properties of an intervening medium that affect the links.
Historically, radio science observations have been conducted between a spacecraft in deep space and antennas of
NASA's DSN. However, advances in small spacecraft technologies, including possibilities opened up by the appearance of capable radios, such as Iris, expand the range of
potential mission concepts. For example, the high-precision
gravitational field measurements of the moon by the twin
spacecraft Gravity Recovery and Interior Laboratory
(GRAIL) [31] can be extended to even higher order, local
gravity fields by flying the small probes at very low altitudes,
lower than acceptable for larger, costlier spacecraft. The sections below provide examples of potential radio science
investigations and requirements that must be levied on
SmallSats to support this capability.

SMALLSAT RADIO SCIENCE OPPORTUNITIES
At Mars, small spacecraft equipped with a suitable radio
could be used for radio science investigations with applications to both future human exploration and planetary
science of characterizing the planet. Analogous to GRAIL,
crosslink architecture could be used to improve the knowledge of the Martian gravity field. As at the moon, an
improved gravity field of Mars could be used for more
detailed descent and landing planning, comparison with
topographic imagery provided by orbiters, such as MRO,
and identification of subsurface structures.
Crosslinks between small spacecraft could also be
used to characterize and monitor the atmospheric pressure-temperature profile, via the technique of radio occultations. Notably, using small spacecraft crosslinks can
provide substantial increases in spatial coverage and temporal resolution as compared to the traditional spacecraftDSN links. A JPL study has found that there are no obvious technical barriers for SmallSats to conduct radio
occultations at Mars, graphically illustrated in Figure 8.
Within the Mars system, the communication link of a
SmallSat could be used to probe the interior structure of the
Martian moons Phobos and Deimos. Early work with a Mars
Express-DSN link suggests that Phobos has a high interior
porosity, a topic of strategic knowledge gaps identified for

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