IEEE Aerospace and Electronic Systems Magazine - July 2020 - 28

Improving Small Satellite Communications and Tracking in Deep Space

Figure 1.
In this artist's illustration, a spacecraft equipped with a DPS is using images of the moon's features and the stars to navigate autonomously
(Credit: Joe Guinn, JPL).

problem of rewriting the software for each project. Conceptually, the DPS is a standalone piece of hardware that performs navigation needs of the mission by carrying sensors
and cameras, a processor, navigation algorithms, and a standard interface to the flight computer [4], [5]. In certain cases,
the DPS may use landmarks on the surface of, for example,
Mars or the moon, and guide the vehicle relative to them. In
such situations, the DPS has to have advance knowledge of
landmarks on the surface of interest. Figure 1 illustrates this
concept.
Many missions can use the DPS to meet their navigation
needs. This will free projects from developing their own
navigation system because they would be able to obtain a
DPS unit to perform autonomous navigation, a feature that
could be of much interest to the SmallSat community.
Hence, DPS can potentially save the cost and time for missions that may choose to use it in the future. Autonomous
navigation also simplifies operations. For example, if the
Mars Cube One (MarCO) CubeSats [6] were equipped with
DPSs, they would have been able to navigate to Mars with
little or no ground navigation team involvement, albeit with
lesser accuracy at least during cruise, for more on MarCO,
see Part I of this article. For SmallSats like MarCO, cost savings resulting from not having to develop a navigation system and a reduction in operational cost would be welcome.
The DPS concept as a piece of the separate hardware that
can be added to the spacecraft to perform navigation needs
of the mission may or may not be suitable for small CubeSats
because of the mass and size of such a device. In its most
stripped-down form, DPS could simply be the core flight
software residing on the spacecraft computer and utilizing
an existing onboard imager (science or engineering) and performing the autonomous navigation function. So, in this
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extremum, the mass and volume of the DPS-unique hardware would be close to zero. This instantiation, i.e., virtual
linkage of software and existing spacecraft hardware, may
be desirable for CubeSats because it does not add mass to
the satellite. For extended and advanced capability, dedicated imaging and computing hardware would likely be
required, but this would apply to missions unlikely to be
undertaken by a very small craft such as a small CubeSat;
however, it would be suitable for larger CubeSats (for example of size 12 U or greater), MicroSats, and larger spacecraft.
At the other extreme, the hardware could range upwards
to a system, which includes a two-dimensional (2-D) gimbal,
multiaperture imager, antenna, deep space atomic clock (see
the "One-Way Radio Navigation" section), a radio similar
to the Iris radio (see the "Proximity Radios" section), and a
processor; hence, capable of performing both optical and
radio navigation without relying on the spacecraft processor.
Such a system could service even the most demanding need
to provide full navigation services, as well as landing backup
for human flights. One could envision suites of the hardware
anywhere in between this "ideal" configuration, and the virtual CubeSat system.

NAVIGATION QUALITY
Earth-based radio navigation uses the DSN and can be very
accurate. Doppler and range observables are distance insensitive, except for extreme cases when channel signal-tonoise ratio may become an issue. Cross-range information
obtained from Delta-DOR measurements, on the other hand,
degrades with distance because cross-range uncertainty
increases with distance for a fixed angular accuracy; nevertheless, this only becomes a concern for extremely large

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JULY 2020

IEEE Aerospace and Electronic Systems Magazine - July 2020

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