Aerospace and Electronic Systems - April 2019 - 51

Berner
spacecraft could receive the uplink at the same time, and
be coherent at the same time.
As simple as this sounds, there are some issues that
would need to be resolved to implement it. First of all, the
CubeSat transponder would need to be modified to use the
nonstandard ratio; that will be discussed in "Generating
carriers using modulation" section. Second, the DSN
scheduling system would need to be modified to indicate
to the DSN operators and to the operations automation
that the uplink is being shared by multiple spacecraft (currently, the system only allows for the uplink to be assigned
to one spacecraft). Next, the tracking data processing
would need to be modified to associate the shared uplink
with each different downlink (for providing the Doppler
and ranging data to the mission). Finally, how the mission
could provide the command data to be uplinked to the
spacecraft would need to be resolved-the standard space
link extension (SLE) protocol (which is used to transfer
data between the mission and the DSN equipment) only
allows a single connection to the uplinking antenna at a
time; either different time periods for commanding will
have to be allocated to each CubeSat (another scheduling
issue) or a nonstandard implementation of the SLE protocol will have to be developed to allow multiple connections and data multiplexing.
None of these are show stoppers, but they will take
investment in development to resolve. The shared uplink
concept is a viable solution for the initial acquisition case
when all the spacecraft have relatively the same dynamics.
It would not be as useful in a Mars case, where the dynamics of the different spacecraft vary, due to their relative
motion to Earth (spacecraft on the surface versus an
orbiter moving towards Earth versus an orbiter moving
away from Earth). In that case, there is a need to Doppler
compensate the uplink (to reduce spacecraft receiver
tracking losses) and the compensation for one spacecraft
would not be the same as what would be needed for the
second spacecraft.

GENERATING CARRIERS USING MODULATION
Another way to increase uplink capability would be to use
modulation to split the transmitted carrier into two or more
carriers [19]. Assume that there is a carrier, at frequency fc ,
modulated with command data. Now, instead of ranging
modulation a squarewave or a sinewave with frequency fdelta
is added to the modulation. The resulting output will be the
carrier and the harmonics of the subcarrier modulation, at
frequencies þnfdelta. The frequencies and relative power levels depend on the subcarrier type selected (square or sinewave) and the modulation index used. If the two frequencies
fc and fdelta are selected correctly, they could provide
uplinks to several different spacecraft.
This has one advantage over the shared uplink, namely,
it does not require any modifications to the CubeSat
APRIL 2019

transponder; but the frequency separation is limited to the
frequency of the subcarrier, which may not be enough to
protect the coherent downlinks from interfering with each
other. Also, it has the same issues as the uplink sharing,
namely, needing modifications to the scheduling, tracking,
and command processing, and not being useful in a scenario
with different spacecraft dynamics. Plus, it may require
additional processing on a pass-by-pass to select the two frequencies, which would add to the operational complexity.
However, if modifying the transponder is to be avoided, this
does provide an improvement in the initial acquisition case.

DUAL EXCITERS
A third method to increase the number of uplinks is to
install a second signal generating assembly (called the
exciter) and sum it with the original signal prior to the
input to the transmitter. This provides two independent
carrier signals, each with its own modulation, which alleviates the issue of different spacecraft trajectories.
There are several challenges with this option. The biggest issue is due to the fact that the transmitter uses a klystron for the signal amplification (up to a 20-kW output) and
klystrons are nonlinear devices. When the two independent
signals are input to the klystron, multiple intermodulation
products are created. These products are potentially strong
enough to interfere with other spacecraft or can be outside
the allowed spectrum allocation for the DSN. To combat
this, filtering must be designed to limit the radiation of the
intermodulation products; this filter may not be a simple
design. Also, the desired output signals will be at least half
the power of a single signal (the DSN has plans in the future
to install an 80-kW transmitter in one antenna at each complex, which will minimize this issue). Finally, the cost of
implementing a second exciter and the filtering scheme will
need to be traded off against the potential benefit.

SPACECRAFT MODIFICATIONS
To take advantage of some of the options previously mentioned, or to increase the range of those options, some modifications to the spacecraft transponder need to be made.
Given the current generation of digital transponders, which
are software defined radios, these modifications should be
possible without needing a major redesign.

NONSTANDARD TURNAROUND RATIOS
As discussed previously, having a nonstandard turnaround
ratio would allow for the sharing of the uplink signal by
multiple CubeSats during the initial acquisition period
(and other times, when there are multiple spacecraft in the
antenna beamwidth). But, once the CubeSat is no longer
in a large group of spacecraft sharing the uplink, it is
desired that the standard turnaround ratio be used. This

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Aerospace and Electronic Systems - April 2019

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