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limited lighting, or intentional interference. This raises
the need for robust, low-resource navigation
alternatives. Echolocation-based positioning, inspired
by biological systems, offers such potential, especially
for swarming UAVs operating without central
coordination or communication.
This study introduces a lightweight echolocation-
based approach for relative positioning by
implementing and evaluating a passive acoustic
method using Time Difference of Arrival and signal
strength, demonstrating the potential of echolocation
as a complementary technique in hybrid navigation
systems for GPS-denied environments.
The remainder of the article is structured as follows.
Section 2 discusses the literature on acoustic
navigation. Section 3 presents the physical phenomena
that can be used to develop a system for the acoustic
localisation of mobile objects. Section 4 presents the
concept of the laboratory setup and the results of
studies on the suitability of acoustic signals for
determining object positions. Section 5 provides
concluding remarks.
2 RELATED WORKS
An example of sound localisation is the Simultaneous
Localisation and Mapping (SLAM) algorithm [1]. This
solution is designed to localise a drone indoors and
calculate its position based on direct and reflected
signals. The drone is equipped with four microphones
and a siren. The siren emits sound at a specific
frequency received by the microphones at two
moments: right after emission and after reflecting off
the walls. By analysing the variability of the sound's
amplitude and phase and filtering the results, the
positions of obstacles and the objects relative to them
are calculated. This approach enabled the drone to
move within a room without colliding with obstacles.
A similar solution to the audio localisation problem
is using a phone to map a room [2]. Using the built-in
devices in the phone, a microphone and speakers emit
inaudible sounds. The microphone registers the echoes
reflected from objects. The difference between these
solutions lies in mapping: in the first example, it was
performed in real-time from the drone's perspective,
while in the second case, locations were assigned
features during mapping. Such an approach allowed
for error correction during localisation by comparing
features. Research into localisation methods used by
bats is a base for both solutions.
The described solutions were applied indoors,
where the objects in the environment were fixed. The
situation changes when multiple objects, such as
drones, move in open spaces. Besides accounting for
static obstacles, the relative positioning of mobile
objects in open space must be introduced. In both cases,
mobile objects must be equipped with microphones
and speakers that transmit and receive sound signals.
These signals allow objects to locate each other using
differences in signal delay times and signal strength.
Solution presented in this article replaces echo as
the main localisation method with direct signal
transmission and reception between different
components of the navigation system. As a result, it
enables smooth transitions from indoor to outdoor
environments and significantly shortens localisation
time by avoiding the need to wait for reflected sound
signals. The developers focused on relative
positioning, intentionally leaving out absolute
localisation due to the system's specific application.
Although echolocation is not most commonly used
as method for swarm localisation, recent surveys[3]
highlight need for diverse navigation solutions.
Making it more reliable in different environments and
resistant for various external factors. Current methods
rely on vision, LiDAR and wireless communication,
each with own specific limitations. In this context
acoustic localisation could emerge as complementary
or alternative approach. This study explores potential
of echolocation to fill gaps in scenarios where
traditional sensors may fail.
3 SOUND AND ECHOLOCATION
Animals use sound far better than humans to
determine their position relative to other elements in
their surroundings. Echolocation, also known as
biosonar, allows them to navigate, search for food and
hunt. To locate objects in space, they use sound
reflected off the surfaces of objects or other animals.
Among the animals that utilise this navigation method
are well-known bats, dolphins, whales, some bird
species and hedgehogs. Interestingly, some blind
individuals have developed the ability to echo-locate.
The reasons why this skill has evolved include:
− navigating in environments with limited visibility,
such as darkness, murky water or fog;
− detecting prey;
− identifying obstacles along a path.
All these reasons connect to localisation and
navigation. Therefore, it can be concluded that
echolocation is a practical navigation method. Despite
many years of evolution and change, many organisms
still use it as a fundamental method.
Marine animals such as dolphins and whales [4] use
high-frequency sounds produced by forcing air
through their nasal passages. These sound waves
travel to the forehead and are focused into a beam.
When the signal reflects off an object, it is received by
the animal's jaw and transmitted to its ears. This
process allows the animal to determine the object's size,
direction, speed and distance.
Echolocation in the air works somewhat differently.
Bats produce sound in their larynx and emit it through
their mouths. These sounds can reach up to 140
decibels, but their frequency is too high (ranging from
20–80 kHz [5]) for humans to hear. Bats use reflected
sound to identify the size and hardness of objects, and
they can detect small insects at distances of up to about
5 meters. To avoid deafening themselves by producing
such loud sounds, they deactivate their middle ear just
before emitting the sound and restore their hearing just
before the echo returns.
Sound is a physical phenomenon involving the
creation of a longitudinal acoustic wave that represents
a disturbance in the density of a medium. These waves
propagate through gases, liquids and solids. Based on