CommunicationPower is a limitation
The
main constraint of developing a successful means of data transmission
over such large expanses of space is power. Because the antenna that
will transmit the data from Epsilon Eridani to Earth would be operating
anywhere from 10W to 100W, realistic power sources would only allow such
a transmission to occur for limited periods of time. Thus, the design
of the communications link will be tailored to allow for at least one
image to be transmitted in a realistic amount of time.
Design choice for the dish
One
way to maximize the distance over which a signal may be transmitted is
to ensure that a perfectly parabolic, high gain, very large antenna is
included in the design. Typical parabolic antennas that are currently
being launched into space are on the order of 22-meters in diameter;
comparing this to the Voyager satellites, these are quite large. It is
reasonable to predict the successful launching of a dish that is
25-meters in diameter, and these are what we will use.
The spacecraft will utilize a hybrid inflatable parabolic dish antenna because this design will minimize the weight, volume, and cost of the communication system. This design makes it feasible to send the needed 25m diameter parabolic dish antenna as the antenna uses a small traditional rigid parabolic dish with an inflation deployed in-situ rigidized reflector.The inflatable system stows compactly underneath the rigid parabolic dish and is deployed using an inert inflation gas. The deployed inflatable reflector is made of a composite material that is flexible and hardened after being deployed using a chemical trigger. The inflated portion of the system can be made as light as 2 kg/m2.
Stowed Configuration of the Hybrid Dish Antenna
Deployment Concept for the Inflatable Reflector
Deployed Hybrid Inflatable Antenna
Design choice for the carrier frequency
Another
way to maximize the feasible distance of transmission is to reduce path
loss. Path loss occurs in proportion to wavelength, and for small
wavelengths, or high frequencies, the less path loss will occur. Due to
inevitable irregularities of the shapes of the parabolic antennas of
transmission and reception, very high frequencies are not feasible for
use; the wavelength of the received/transmitted frequency must not be on
the order of the expected aberrations on the dish. A good estimate of
the upper bound of a transmission is roughly 100GHz, whereas, the signal
will corrode very quickly due to path loss if it isn't above 1GHz. To
hope for the best, the carrier frequency will be 60GHz.
The effect of noise power on the design
In
order to receive a reliable signal, the received signal should be above
the noise floor by a reasonable degree. This noise floor has
contributions from the background radiation of space, the noise
contributed by the low noise amplifier (LNA), and the total bandwidth of
the system. Space has been measured to have a noise of 3K. Modern LNAs
provide a noise figure of about 2dB at freezing temperatures, which
translates to a black body radiator of about 28K in space (not very much).
Finally, for the purpose of maximizing the feasible distance of
propagation, we want to use the lowest bandwidth possible, and this is
possible if we plan to send bits very slowly with a realistic
excitation, the raised cosine pulse. |
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