Vision and Voyages for Planetary Science in the - Solar System ...

e conducted within the Discovery program, and there are intriguing possibilities for human missions
to NEOs supported by NASA’s Exploration Systems Mission Directorate (ESMD). Instruments
(mapping cameras and spectrometers) for orbital characterization are already developed, but sampling
instruments, especially for accessing the subsurface, require development.
The Deep Space Network
The Deep Space Network (DSN) is a critical element of NASA’s solar system exploration
program. It is the only asset available for communications with missions to the outer solar system,
and is heavily subscribed by inner solar system missions as well. As instruments advance and larger
data streams are expected over the coming decade, this capability must keep pace with the needs of
the mission portfolio. In addition, future capabilities afforded by optical communication, transponder
advances, advanced software, and other means may provide future increases in returned data volumes
and will be important to meeting mission demands.
The DSN is composed of three stations located in Goldstone, California, Madrid, Spain and
Canberra, Australia, along with operations control and other services in the United States. Each
station has one 70-meter antenna, one 34-meter High Efficiency (HEF) antenna, and at least one 34meter
Beam Wave Guide (BWG) antenna. There is an additional BWG at Madrid and two more at
Goldstone. These antennas support over three dozen missions with downlink and uplink capabilities
in S-band, X-band and Ka-bands (limited). Collectively, these stations can provide nearly continuous
full-sky coverage.
The 70-meter dishes are in high demand, particularly during critical events, because of their
downlink capability, sensitivity, and ability to satisfy other mission requirements. As such, they are
heavily oversubscribed, and present deep space missions are limited mostly by downlink rather than
onboard storage. For example, the Cassini mission routinely must choose which data to play back, as
the capacity of its solid state recorder exceeds what can be played back to Earth within allocated
passes (Table 10.2). The DSN must also contend with aging infrastructure, particularly the 70-meter
antennas which were constructed in the 1960s. Nonetheless, the DSN continues to perform
extraordinarily well, returning data with a very low drop-out rate and achieving command and
telemetry availabilities of better than 95 percent to most operating missions.
The DSN’s current budget supports expansion of Ka-band downlink capability, and addition
of two 34-meter BWGs at Canberra and one at Madrid by 2018. The longer-term configuration goal
through the end of the decade includes plans for one more 34-meter BWG at each station by 2023 to
nearly mimic the capability of a 70-meter antenna, while keeping the 70-meter antennas operational
for as long as possible. In addition, there are plans for higher power spacecraft transmitters,
development of a Universal Space Transponder, and increases in on-board data compression and
selection coding techniques.
Future demands on the DSN will be substantial. There is an ever-growing need for downlink
capacity. Sophisticated next-generation instruments can generate terabits of data, or more, in short
time periods. With advances in, for example, LIDAR, synthetic aperture radar, and hyperspectral
imaging, missions will require Ka-band, and higher, transmission rates to handle these data, even with
improvements in on-board data processing.
Solar system exploration also requires either 70-meter antennas or equivalent arrays to
achieve the sensitivity needed for distant outer solar system missions, such as to the inner Kuiper
Belt. In addition, critical event monitoring and other operations of missions to closer bodies also
require the high sensitivity and downlink margin of larger apertures.
The DSN must be able to receive data from more than one mission at one station
simultaneously. If new arrays can only mimic the ability of one 70-meter station and nothing more,
missions would still be downlink constrained and would have to compete against one another for
limited downlink resources.
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
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