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

Chapter 5.0 Potential Payload Functions Using a Communication/
Control Subsystem
Surface topography over a large area can be performed through multiple flight tracks and data
combining from each track to create a map of the desired area coverage. Alternatively, a scanning
antenna mounted underneath the Entomopter can be used to cover a larger surface, avoiding
extraneous (and fuel-intensive) flight paths. Referring to the sunflower antenna
configuration mentioned above, instead of a single sunflower petal mounted beneath the Entomopter
to perform altimetry, half of the full circular antenna can be mounted and elements
switched on and off to scan a larger swath width, S, compared to a single element. The shape and
number of elements can be varied to provide adequate gain and along-track and cross-track resolutions,
Xa and Xr, respectively. A representative illustration is shown in Figure 5-1. One of the
main constraints is that the scan should be completed before the Entomopter moves a distance
equal to the along-track resolution, Xa. This imposes a minimum dwell time, t d , constraint at
each position [72] of:
where n is the Entomopter velocity.
t d
<
Xa Xr
S v
h
Xr
S
Xa
Figure 5-1: Geometry for a Scanning Imaging Radar Altimeter
Three-dimensional mapping can be performed by taking two images of the same area with two
different angles of incidence. One method to perform this mapping is to use interferometric synthetic
aperture radar (ISAR). Interferometry is based on cross-correlating two SAR images of
the same scene with slightly different incidence angles. The 3D position is estimated by measuring
the slant range and the phase difference. This could be achieved using a single Entomopter
with two antennas offset from each other, or using two Entomopters, each equipped with an
antenna, and flying in formation so as to acquire two images of the same surface at different
angles, as shown in Figure 5-2. When the SAR images are synthesized, each resolution cell has
an amplitude and phase angle between 0 and 2π radians. The phase is an ambiguous measure of
the two-way distance, d, between the antenna and a point on the ground.
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