2.7 EXISTING WORK IN SMALL AERIAL VEHICLES
The increased availability of small, low-cost autonomous and
remote aerial vehicles has seen a surge in their use for various research and
remote sensing applications. More advanced control methods for careful
trajectory following and maneuvering have been explored under controlled
environments where precision position sensing is available via a video tracking
system or similar (Mellinger, 2012). Simultaneous localization and
mapping
(SLAM) has been very successfully implemented in indoor environments using
laser scanners and cameras (Achtelik, 2009). Outdoor environments provide
additional challenges including wind disturbances and larger scale environments
that may challenge SLAM algorithms, though recent work has focused on improving
the autonomous operation of quadrotors in feature-rich outdoor environments
(Wendel, 2011). Remotely-operated aerial vehicles with little or no built-in
autonomy are commercially available through several manufacturers and used not
only for research, but also aerial reconnaissance by law enforcement agencies,
rescue services, and others.
The
application of an autonomous aerial vehicle to marine environmental sensing
presents challenges most similar to those tackled by remotely operated
commercially operated vehicles. The environment is very feature-poor, and hence
not amenable to the large body of research available for navigation and control
of quadrotors in a more controlled environment (Achtelik, 2009). The desired operation ranges also greatly
exceed those typically supported by commercial systems, which typically rely on
the vehicle remaining within eyesight of an operator. The unique marine
environment exposes the vehicle to harsher external conditions, primarily wind,
and imposes a very high penalty, complete loss, for any system failure, whether
it be in software or hardware (Achtelik, 2009).
2.8 SENSORS
The sensors described here are just a few of the
most common sensors typically found on autonomous marine platforms.
2.8.1 Conductivity, Temperature, and Depth (CTD)
Sensors:
Possibly the most common instrument used in oceanographic experiments, a CTD
can measure water temperature, salinity, and density. Modern CTDs are small
enough to be mounted on almost any vehicle while still providing excellent
accuracy. A CTD is usually the first step in studying any environmental
phenomenon. Additional sensing capabilities for parameters such as pH and
dissolved oxygen can also be built into CTD instruments (Achtelik, 2009).
2.8.2 Mass spectrometers and flourometers: At their most
fundamental level, mass spectrometry is used to measure the mass-to-charge
ratio of charged particles and determine elemental composition or the chemical
structure of molecules. Underwater mass spectrometers provide this
functionality in a small package that operates on samples of water, usually
provided by an internal pump and filter apparatus (Boykov, 2001). They are useful for identifying low molecular
weight hydrocarbons and volatile organic compounds associated with pollutants.
Flourometers perform similar measurements by examining the intensity and
wavelength of electromagnetic emissions from a sample after excitation. They
identify higher molecular weight compounds including organic entities such as
chlorophyll. Together the two techniques can give us a more precise picture of
the various chemicals present in a water sample (Curcio, 2005).
2.8.3 Acoustic Doppler Current Profiler (ADCP): An ADCP can perform
remote current measurements by sending acoustic pulses and measuring the
Doppler shift in the return reflected by particles in the water. Through the
use of multiple beams, an ADCP can determine the three-dimensional current flow
at multiple distances from the sensor. Range can vary from just a few meters to
hundreds of meters, with accuracy typically decreasing with range as lower
frequencies must be used. ADCPs are frequently mounted facing downwards from an
AUV or surface vehicle to measure current throughout the water column
(Curcio, 2005). Some units can also
function as a Doppler Velocimetry Logger (DVL), locking onto the seafloor and
measuring the velocity of the vehicle rather than water currents (Curcio, 2005).
2.8.4 Imaging sonar: While less applicable to
environmental sensing, imaging sonars still bear mentioning here because of
their widespread use on underwater vehicles. Side-scan sonars are commonly used
on AUVs or towed behind surface ships to image a wide area of seafloor to
either side of the vehicle. Varying frequency, usually in the several hundred
kilohertz range, provides a tradeoff between swath width and resolution. Other
types of sonar are designed to more accurately image smaller areas or even
characterize the sediment or rock under the seafloor (Curcio, 2005).
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