Phase II Final Report - NASA's Institute for Advanced Concepts

Chapter 4.0 Entomopter Flight Operations
4.3 Entomopter-borne Active Emitters for Navigation and Communication
and hazes with spatial dimensions greater than 2,000 km, and local dust storms include clouds
and hazes with spatial dimensions less than 2,000 km. Reference [123] describes Mars dust as
consisting of primarily basalt and montmorillonitic clay. Using a corresponding dielectric constant,
it was shown in [123] that at Ka-band, large dust storms can cause as much as 0.3 dB/km
or more of loss, with normal dust storms causing about 0.1 dB/km. Most large dust storms occur
in the southern hemisphere during later spring and early summer when the southern hemisphere
suddenly becomes hot [123]. It is also important to keep in mind that dust attenuation is proportional
to operating frequency. Thus, more attenuation will occur at higher operating frequencies.
Although it is important to be aware of the potential presence of these dust storms, for the purposes
of this analysis Entomopter operations will be curtailed during the presence of dust
storms. This is due not only to degraded RF performance, but due to the uncertainties of flight
during these storms.
Attenuation will also be caused by scattering of the signal from sharp discontinuities in objects
in the communication path. As discussed in [106], a scattering cross-section can be defined for
any object as the ratio of the total power scattered by the object and the power-flux density incident
on the object. For sharp, long edges (several wavelengths long), the effective scattering
cross-section A s can be described as
A s
= KλL
,
where L is the object edge length, λ is the wavelength, and K is a dimensionless constant less
than one, having a magnitude that depends on the scattering object. From this relationship, it is
evident that scattering is reduced with higher frequency. Golshan and Ho indicate that signal
blockage caused by objects or terrain features on Mars can be a significant problem unless the
communications are limited to a small area in a fairly flat terrain with relatively few large rocks.
They recommend robust communication protocols or operational procedures to avoid loss of
data from such scattering interference, or suggest using Mars orbiting relays to reduce blockage
problems. They also suggest positioning the base station on a commanding location to reduce
blockage probability and extend line of sight communications.
The baseline scenario used for this study assumes that the ground is flat and there are no obstacles,
thus scattering losses will be neglected. However, when considering an operating frequency,
the reduced scattering cross-section (and thus reduced scattering losses) with increasing
frequency should be considered.
4.3.3 Frequency of Operation
The frequency of operation must be coordinated with those already planned for future Mars missions.
This would include the link from the Mars surface to an orbiting relay satellite (UHF) or
direct links from the Mars surface to the deep space network (Ka-band and X-band) [70]. When
choosing an operating frequency, propagation losses due to oxygen and water vapor are a concern
on Earth but are orders of magnitude less on Mars, as shown in Figure 4-7. Because size
and weight are important criteria for the Entomopter, it is desirable to operate at higher frequencies
so that antenna size is indirectly proportional to frequency. This also has the advantage of
reducing scattering losses, as mentioned in the preceding section. One must also consider available
hardware, which becomes difficult to realize at higher frequencies. In addition, link budgets
241