There are many considerations for the communications hardware on the
WISP. The WISP has a streamlined shape for ease in movement through
melted ice. Therefore, a traditional antenna cannot be used here.
Furthermore, an extremely low frequency (
200 Hz) is needed to
penetrate the ice, and so an acoustic transducer is used for
communications from the WISP to the lander.
A fairly powerful acoustic sounder is required to generate signals
strong enough to travel 10 km. Assuming that the sounder can produce
100 W of sound, the sound intensity 1!km away would be
69 dB re 10-12 W/m
.
(This assumes that the sound is evenly
distributed over a perfect sphere a kilometer away.) At 10 km from
the sounder, the intensity is 49 dB: still an easily detectable
signal. Sound absorption by the ice is estimated at less than
0.1 dB/km at 200 Hz, and is ignored here. (Sound attenuation in still
air is less than 1 dB/km [8]. It should be much less for
solid ice.)
The link budget for the communication between lander and WISP is not quite complete because the acoustic requirements are not the same if there was just an antenna on the WISP. The data rates of the WISP consist of telemetry, standard measurements such as temperature, spectrometer, and black and white photographs. The total data rate was calculated to be 75 bps. The power required for the WISP to communicate acoustically will be kept below 150 W.