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1
ENHANCING AUV SURFACE COMMUNICATIONS
Lau Su Jun
1, Low Wei Hao
2, Tan Huang Hong
2
1River Valley High School, 6 Boon Lay Ave, Singapore 649961
2Defence Science and Technology Agency, 1 Depot Road, Singapore 109679
ABSTRACT
Existing radio antennae on Autonomous Underwater Vehicles are low in height, posing a
LOS transmission limitation as the RF communication range is restricted to less than 5km. To
tackle this problem, this paper presents various methods of elevating the AUV’s radio
antenna to enable RF communication with the mission HQ or support vessel at extended
ranges. Raising the AUV’s mast is insufficient to achieve a useful range, whereas using
helium balloons to elevate the antenna requires much space on board to accommodate the
bulky gas canisters. This is unfavourable as space is a premium for any AUV. A feasible
design suggested for implementation is to integrate a waterproof UAV into the AUV. Its
advantages include compactness, organic capability, underwater pressure-resistance, re-
usability and modularity of design, making it suited for AUV missions.
INTRODUCTION
Unmanned vehicles do not require human operators on board; they can either be remote
controlled or autonomous -- capable of navigating on their own to complete the missions that
are assigned through pre-programming. Some examples are unmanned aerial vehicles
(UAVs), more commonly known as drones, unmanned surface vehicles (USVs), which
operate on the water surface, and autonomous underwater vehicles (AUVs), which operate
underwater. Unmanned vehicles can greatly extend the range of influence of a manned
platform, but high fidelity wireless communication between the unmanned and manned
platforms is required for proper execution of a mission. AUVs in particular face great
challenges with the data transfer as they have an extremely low surface profile. This paper
looks into extending the radiofrequency (RF) communication range of AUVs by raising its
antenna height. Improved RF communication range and data transmission of the AUV with
surrounding USVs, support vessels or ground mission control HQs can greatly assist missions.
It allows the AUV to quickly transmit sizeable information it has gathered allowing the
operator to make better and faster decisions with more data on hand. New and more complex
instructions can also be sent quickly to the AUV via RF communication to change its mission
parameters.
BACKGROUND INFORMATION
As an AUV moves through water, it relies on underwater acoustics for communication and
navigation as electromagnetic waves are very heavily attenuated when passing through water.
However, underwater acoustic signals suffer from transmission loss, such as absorption and
physical spreading. In addition, acoustic waves have a significantly lower information
compression rate as compared to radiofrequency waves. As a consequence, acoustic
telemetry is limited and communication links are highly unreliable as compared to RF
wireless communication. Current acoustic technology can only support low data rate and
delay-tolerant applications [1]. Hence, over the course of its mission, an AUV will
2
occasionally surface so as to track its position using the Global Positioning System (GPS) and
transmit data to or receive data from the mission monitoring HQ via RF.
Wireless Communication Using RF
Wireless communication can be broadly categorised into line of sight (LOS) and beyond line
of sight (BLOS) communication methods.
BLOS communication methods are required when the transmitting and receiving antennae
are not in direct line of sight of each other, due to the curvature of the earth or terrain
obstructions along the radio transmission path. BLOS is based on the concept of utilising
relays to provide alternative radio propagation paths so as to send the radio signals around the
obstructions, eventually reaching the receiving antenna. Orbiting satellites serve as
retransmission stations that make long range communication possible [2]. Some examples of
BLOS communication methods are the Iridium satellite constellation, and very small aperture
terminal (VSAT) technology.
However, communication through the Iridium system has very low bandwidth and thus a low
data rate [3], making it unsuitable for AUV missions which require bulk data transmission.
Though VSAT technology has much higher bandwidth limits, it requires active stabilisation
and high power [4]. An AUV platform is too small and unstable to mount a VSAT antenna,
and has limited power. The supporting infrastructure is also expensive to build and maintain.
In addition, commercial satellites from companies like Thuraya and Globalstar provide
telecommunication and broadcasting networks which are public channels that may
compromise on the security of classified information [4]. Therefore, BLOS via satellite
communication is not suitable and practical for the highly interactive real-time military
operations which AUVs have to perform.
Thus, LOS communication becomes the more appropriate means of data transmission when
high rates of data transmission and higher security are required. LOS communication is
established when both receiving and transmitting antennae are in visual line of sight with
each other without any terrain or physical obstruction along the RF propagation path. The
effective terrestrial range is typically less than 30km due to the earth’s curvature and
terrestrial obstructions. Transmitter range is a function of many variables, such as operating
frequency, transmitted signal power level, antenna directivity, and antenna height [2].
However, this paper will only be focusing on increasing the antenna height.
Distance to the Horizon
Figure 1 shows the distance from an elevated position to the horizon.
Figure 1: Geometrical distance to the horizon [5]
√ √ ----------------------------(1)
Where DBL = range between boat and lighthouse(km), hB = perpendicular height of boat’s
antenna above sea level(m) and hL = perpendicular height of lighthouse’s antenna above sea
level(m) (Please see Appendix A for formula derivation).
3
Fresnel Effect
An omnidirectional antenna, also known as a dipole antenna, radiates or intercepts RF waves
equally in one plane. Radio waves form a series of ellipsoidal shapes between the two
antennae that have established a communication link. These ellipsoidal shapes can be
separated into the Fresnel zones as shown below.
Figure 2: Fresnel zones [6]
The signal strength is strongest in zone 1 and weakens in each successive zone as wave paths
get longer with increasing distance from the direct straight line path between transmitter and
receiver. Should the non-direct waves be diffracted or reflected by obstacles along their
propagation paths, they may reach the receiving antenna slightly later than the signal
propagated directly between antennae and end up being out of phase with the LOS waves.
This results in the undesirable Fresnel effect caused by phase cancellation of radio waves
which reduces the power and weakens the LOS signal the antenna receives. As a rule of
thumb, 60% of the first Fresnel zone should be kept clear of obstacles in order to achieve the
optimum range and signal quality at the receiving antenna [6]. Phase cancellation can also be
caused by path difference as radio waves arrives at the receiver by more than one paths,
which can result in destructive interference [7].
In the open seas where AUVs are deployed, the absence of obstacles allows the signal to be
free from interference. However, the effective range is still limited due to the curvature of the
earth which could obstruct the Fresnel zones. Thus, it is important to not only raise the height
of antennae on ground mission HQ or support vessels but on the AUVs as well.
Figure 3: Effect of curvature of Earth on effective range (not drawn to scale)
The 60% radii of the first Fresnel zone were calculated to be 7.94 and 4.76m for 10m and
0.5m AUV antenna heights respectively (For full calculations, please see Appendix B)[8]. RF
signal power will be significantly reduced at the receiving antenna when the AUV antenna
4
remains low. Raising the AUV antenna is therefore critical in preventing any obstacles within
60% of first Fresnel zone radius and ensuring quality signal received at greater ranges.
PROBLEM DEFINITION
The aim of this project is to increase the effective range of RF communication by increasing
the antenna height on AUVs. In general, the mast of the AUVs, where the antenna is housed,
rarely exceeds 0.5m in height. The receiving antennae can typically be 10m or more if they
are mounted on a large support vessel.
Ignoring any attenuating effects, the following distances to the horizon are calculated using
Equation (1).
S/No. AUV Antenna
Height/m
Receiver
Height/m
Distance to the
Horizon/km
Percentage Increase
with respect to S/No.
1 /%
1 0.5 10.0 13.8 0.0
2 0.5 15.0 16.4 18.4
3 3.2 10.0 17.7 27.9
4 5.5 10.0 19.3 39.5
5 10.0 10.0 22.6 63.5
Figure 4: Calculated distances to the horizon
Increasing the AUV antenna height by 5m extends the range by 39.5%, and this is greater as
compared to the same increment on the receiving antenna height, which only extends the
range by 18.4%. This can be seen in Figure 5 which shows that the increase in distance to
horizon decreases with increasing elevation.
Figure 5: Increase in distance to the horizon decreases with increasing elevation [9]
The rate of increase in range with respect to elevation (
) drops below 1km m
-1 beyond h =
3.19m, indicating that any AUV antenna height greater than 3.19m will not be very efficient
in extending the RF communication range. As much as efficiency is important in determining
the AUV antenna height, the effective range should be considered as well. Based on
Δ𝐷
Δℎ 1 8 ℎ;
12
𝐷 ℎ12
5
empirical evidence, the effective range of a 0.5m tall antenna is generally a third of distance
to the horizon. Therefore, a 10m tall antenna is needed in order to optimally achieve a
practical range of 7km as shown on Figure 4. The following section evaluates the different
methods of raising the AUV antenna height.
POSSIBLE SOLUTIONS CONSIDERED
Retractable Mast
Figure 6 below shows an AUV mast which can be raised from and lowered to 0.5m by a
linear motor installed within the AUV module. Composed of glass fibre reinforced polymer
(GFRP) which has high permittivity to RF waves, the mast houses GPS and UHF radio
antennae for positioning and data transmission respectively.
Figure 6: Side view of the Retractable Mast
a. Design Advantages
This solution saves space within the AUV hull since the motor is the only bulky mechanical
component. Other components like antennae and cables are small in size and housed outside
the AUV in the mast. As such, the AUV module is compact and can be easily retrofitted to
other AUVs. The antenna mast can easily be raised and lowered within seconds.
b. Design Limitation
Only a small height increment of 0.5m can be achieved. Should the mast be raised too high, it
may adversely affect the stability of the AUV. An antenna height of 1 to 2m can only achieve
a maximum hypothetical range (distance to the horizon) of 14.9 to 16.3km and the actual
range will be much more limited to less than 5km due to the Fresnel effect. This solution is
thus not feasible.
Helium Balloon Designs
1) Disposable Helium Balloons
Disposable Helium Balloons (Figure 7) launches a tethered balloon from the AUV by
inflating it with helium and snips off the tether once the attached antenna has completed its
6
data transmission. Firstly, a folded balloon envelope is inflated with helium gas supplied by
the regulator via an inflation port. The balloon is packed to push itself out of the AUV,
through the hatch and completes its inflation outside the AUV to achieve a size of 4.45m3
(For full calculations, please see Appendix C)[10]. To dispose a used balloon and its antenna,
the snipping motor positioned below the dynamic seal snips off the tether. It utilises the iris
shutter design similar to that found on cameras to cut the tether.
Figure 7: Cross sectional view of the Disposable Helium Balloons solution
2) Reusable Helium Balloon
Similarly, Reusable Helium Balloon design (Figure 8) works by launching the helium balloon
from the AUV to elevate the antenna. What is different is that the balloon can be returned
into the AUV by a winch and electric motor system, located in the waterproof lower
compartment, which winds up the tether. A valve located on the top of the balloon envelope
releases helium gas gradually as the balloon makes its descent. A linear variable differential
transformer (LVDT) is attached onto the winch system to measure the displacement of the
tether and aid in hatch closing after balloon retrieval.
Figure 8: Cross sectional view of the Reusable Helium Balloon solution
7
a. Design Description
In both designs above, the upper compartment(s) are free-flooded, unlike the waterproof
lower compartment which contains bulky parts like compressed helium gas canisters, gas
regulators and wire spools (in Figure 7) or the wire winch (in Figure 8). Dynamic seals fitted
at the compartment partition serve to prevent water from entering the lower compartment
while allowing the tether smooth movement. Static seals are also placed to prevent water
entry from areas where valves supply helium gas from the regulator to the balloon envelope.
b. Design Advantages
Both balloon solutions save power and are capable of maintaining the antenna height for a
prolonged period. This is because balloons rely on lift force acting on it rather than power for
ascension and they are able to remain afloat for hours or even days [10]. As these are modular
designs, they can be retrofitted to other similar-sized AUVs. Unlike the retractable mast,
balloons can raise the antenna to 10m and do not affect the stability of AUVs.
c. Design Limitations
Both balloon solutions face similar limitations. Firstly, the solution will have a very limited
number of uses due to the limited helium gas supply or number of balloons. One balloon to
be fully inflated, at least two 20L gas canisters (0.91m by 0.21m) are required (For full
calculations, please see Appendix D)[11]. Secondly, the number of gas canisters and bulky
components required make the size of the module too large to be practical on-board a space-
limited AUV. Lastly, the automatic deflating and repacking of a balloon poses a cumbersome
and complicated process out at sea. Therefore, both helium balloon solutions have been
deemed impractical.
THE PROPOSED SOLUTION: UAV-AUV INTEGRATION
After a thorough evaluation of pros and cons of the various antenna elevation methods, it was
decided that integrating a mini-waterproof UAV into an AUV to serve as a relay for RF
commuication between the mission HQ and AUV is the most feasible way as compared to
elevating helium balloons or the AUV mast.
Design Desciption
To elevate an AUV’s UHF radio antenna up to 10m, launching a self-contained UAV is
preferred to one from a support vessel because as it gives the AUV organic capability to
overcome its LOS limitation of an extremely low surface profile. Whereas launching a UAV
from ship is not practical as it requires much more power since the UAV would have to travel
further at a greater altitude to establish a RF communication link. Besides, its prolonged
flight time may compromise on mission covertness.
As shown in Figure 9.1, the hull cover opens when the AUV surfaces. The tethered UAV
takes off, drawing power from its own battery. It can achieve 20min hovering time and a
minimum RF communication range of 7km. It will be recovered by landing on water and
subsequently being pulled into the AUV by a tether. Finally, the hull cover returns back to its
place.
8
Figure 9.1: Overview of UAV-AUV Integration
Design Considerations
a. Hull Cover
The opening and closing motion of the hull cover is achieved by two supporting curved spars,
which move in and out of the two electric motors and gearboxes installed adjacent to the edge
of the hull on the GFRP partition. In order to maintain the watertight integrity of this module,
the hull opening has been lined with gaskets. In addition, the hull cover opens outwards, so
that high underwater pressure keeps the hull cover shut. This prevents any damage to the
internal components and systems due to water pressure (Figure 9.1).
b. Upper and Lower Compartments
The upper compartment, where the UAV sits, is free-flooded when the AUV surfaces and
hull cover opens, whereas the lower compartment is designed to be waterproof so that no
water can enter and damage the winch and electric motor system housed inside. Both motors
for hull cover in the upper compartment are designed to be waterproof (Figure 9.2).
Figure 9.2: Cross sectional view of UAV-AUV Integration
9
c. GFRP Partition and Syntactic Foam
The compartments are separated by a GFRP partition, a strong lightweight material that can
be easily moulded into different shapes. This is to create a cavity to insert a wireless charging
platform for the UAV. Besides, GFRP is a corrosion-resistant and relatively cheap material,
making it a good choice for construction for AUVs which operate at sea [12]. The partition
should also be raised as high as possible to minimise the space that can be occupied by water
and maximise the space taken up by low-density syntactic foam [13]. This greatly reduces the
overall weight of the module and improves its buoyancy. A narrow tube is drilled through the
partition and wireless charging platform to provide a path for the tether connecting the winch
below to the UAV above the partition. A dynamic seal is positioned at the opening of this
tube to prevent water entry and allow tether smooth passage through (Figure 9.1).
d. Winch and Electric Motor for UAV Tether
Tethered to the AUV, the mini-UAV will carry its own battery and re-transmitting antenna.
As such, the tether does not transfer information or power and only serve to anchor the UAV
to the AUV. The tether can then be made thin to minimise the turn radius and subsequently
minimise the size of the winch so as to improve the module’s compactness (Figure 9.1).
e. Wireless Charging Platform
The GFRP partition enables wireless charging to occur between the wireless charging
platform and the UAV’s battery. The charging platform, as well as the motors for the winch
and hull cover, will be connected to the AUV’s battery module (Figure 9.1).
f. Tracking Device
A disk-shaped tracking device, which is tracked by the UAV, is placed adjacent to the
dynamic seal to guide it to return back into AUV during recovery (Figure 9.1). The partition
is designed to be bowl-shaped on the cross-sectional plane (Figure 9.2) so that the UAV will
always slide to the bottom centre and return to its original spot. However, ground effect may
pose difficulties during UAV landing as the downwash of air from the UAV’s rotors reacts
with the partition and generates unwanted lift force due to reduction in induced drag [14]
(Figures 10.1 and 10.2 below). The waterproof UAV, which is already buoyant in water, can
be controlled to land on water beside the hull before being pulled into the module by the
tether and winch system (Figure 9.1).
Figure 10.1: Free-body diagram of
airfoil in ground effect hover [15] Figure 10.2: In ground effect
hover airflow pattern [15]
10
Design Highlights
a. Wireless Charging
The use of the wireless charging platform was adopted as a waterproof method to recharge
the UAV for multiple flights. It does away with a powered tether which is cumbersome as it
increases the weight, turn radius and size of the winch.
b. Pressure Hull Module
To ensure the module is pressure-resistant and watertight so as to protect the internal parts
from damage when the AUV dives underwater, the hull cover is designed to open outwards
while the edges of opening are lined with gaskets to provide a waterproof seal.
c. Waterproof UAV
To overcome the challenge posed by ground effect with landing in the AUV, the upper
compartment of the AUV hull is free-flooded and a waterproof UAV is selected. The UAV is
easily recovered by landing on water next to the AUV and being pulled in by the tether.
Design Advantages
This solution has many advantages over the other solutions.
Firstly, a UAV can easily elevate its in-built radio antenna by a significant 10m without
affecting the stability of the AUV. This is more than five times the height of the retractable
mast design whose maximum height is less than 2m.
Secondly, unlike helium balloons, it depends on its in-built battery to generate lift force, and
does not need bulky components such as helium gas canisters and regulators, making the
module very compact and relatively lightweight. This can help to increase the overall
buoyancy of the AUV. In the long run, this solution is more practical than the helium
balloons designs. It has the potential to be used many times throughout the mission and is not
limited by the number of balloons carried or helium gas supply.
Challenges
Similar to the helium balloons, a small UAV may face hovering problems at higher altitudes
with strong winds and may be prone to damage during extreme weathers. In addition, the
free-flooded upper compartment will be exposed to seawater and will require regular
maintenance.
CONCLUSION
This paper shows the possibility of elevating an AUV’s radio antenna to achieve a practical
RF communication range and overcome its low antenna mast height. At the same time, the
solution presented improves the organic capability of AUVs with the integration of a self-
contained UAV. This design greatly helps to reduce the space needed for the internal
components of an AUV module, and its wireless charging method allows thinner cables to be
used in the tether to minimise the winch size. Retrofit is made easy due to the simplicity and
modularity of the solution.
11
FUTURE WORKS
While this paper has looked into RF range extension through elevating a radio antenna from
an AUV with the use of a UAV, the integration of a UAV into an AUV also presents
additional opportunities besides wireless communications range extension. As ever larger
AUVs become available, more sophisticated UAVs might be able to perform aerial
surveillance and reconnaissance duties, further extending the influence range of the AUV.
Another area for possible future work is to build a prototype to test out the concept and
further improve the design for implementation.
ACNOWLEDGEMENTS
I would like to thank Young Defence Scientists Programme (YDSP) for providing me with
this enriching research opportunity at Defence Science and Technology Agency (DSTA)
during my IP4 year-end school break.
I would also like to express my gratitude towards my mentor, Low Wei Hao, and my co-
mentor, Tan Huang Hong, for taking time off their busy schedules to guide me through the
evaluation of different designs of raising AUV antenna as well as the modelling of my final
design using SolidWorks software.
12
REFERENCES
1. T. Melodia, H. Kulhandjian, L.C. Kuo, E. Demirors. (2013). Advances in Underwater
Acoustic Networking via Mobile Ad Hoc Networking: Cutting Edge Directions, Second
Edition, p805-806.
2. Radio Communications In The Digital Age, Volume 1: HF Technology, Edition 2, p1-
23. Harris Corporation, RF Communications Division, USA. (2005).
3. Ian Poole. (n.d.) Iridium satellite technology, theory and frequency bands. Retrieved
from http://www.radio-electronics.com.
4. L.S Tan, S.P. Lau and C.E. Tan. (2011). Improving Quality-of-Service of Real-Time
Applications over Bandwidth Limited Satellite Communication Networks via Compression,
Advances in Satellite Communications, Dr. Masoumeh Karimi (Ed.), p55-60.
5. Andrew T. Young. (n.d.) Distance to the Horizon. Retrieved from http://www-
rohan.sdsu.edu.
6. [6]R. weaver, D. Weaver, D. Farwood. Guide to Network Defense and
Countermeasures, Third Edition, p208-209. Cengage Learning, Inc., USA. (2006).
7. NovAtel Inc report (2000): Discussions on RF Signal Propagation and Multipath, p1-
13. Retrieved from http://www.novatel.com/assets/Documents/Bulletins/apn008.pdf.
8. ZyTrax, Inc. (2015, October 21). Wireless Calculators. Retrieved from
http://www.zytrax.com.
9. MrReid.org. (2010, November 8). How far away is the horizon. Retrieved from
http://wordpress.mrreid.org.
10. J.I. Miller, M. Nahon. (2005). The Design of Robust Helium Aerostats, p1-13.
11. NEON AUTO LTD BOTTLED GAS SUPPLIES. (n.d.) Helium Balloon Gas.
Retrieved from http://www.neonauto.co.uk/gas.
12. Stromberg. (n.d.) GFRP - Glass Fibre Reinforced Polymer. Retrieved from
http://www.strombergarchitectural.com.
13. [13] BMTI ALCEN. (n.d.) Syntactic Foams: Deepwater Buoyancy for ROV/AUV.
Retrieved from http://www.bmti-alcen.com/en.
14. [14] G. Beare. (n.d.) IGE, OGE and Recirculation. Retrieved from
http://www.helis.com.
15. [15] Paul Cantrell. (n.d.) Ground Effect. Retrieved from http://www.copters.com.
13
APPENDIX
Appendix A: Derivation of Distance to the Horizon Formula
With reference to Figure 1.1, the secant-tangent theorem states that 2 ------(1.1)
Figure 1.1 [5]
Make the following substitutions to equation (1.1)
-
- ℎ
- (km)
2 ℎ ℎ
√ℎ ℎ √ℎ ℎ --------------------------------------------------------------------(1.2)
where r is the radius of the Earth(km).
Given that r = 6378km, the height of observer above sea level is negligible as compared to
the radius of the Earth. Therefore, h can be disregarded and equation (1.2) becomes
√ ℎ ----------------------------------------------------------------------------------------------(1.3)
Substitute r = 6378km into equation (1.3) to derive distance to the horizon formula [5].
√ 8
1 √1 ℎ √ℎ ----------------------------------------------(1.4)
Appendix B: Workings for First Fresnel Zone Radius
The formula to calculate Fresnel zone radius at a point P between the endpoints of the link at
the antennae is given by
√
: ------------------------------------------------------------------------------------------(2.1)
where Fn is the nth
Fresnel zone radius(m), is the wavelength of transmitted signal(m), d1 is
the distance from P from one end of link(m) and d2 is the distance of P from the other end of
link(m) [8].
Wavelength of signal can be found by
299 792 458
------------------------------------------------------------------------------------(2.2)
where c = speed of electromagnetic waves(m s-1
) and f = frequency of signal(Hz) [8].
Given that the signal transmitted by UHF radio antenna ranges from 0.3GHz to 3.0GHz. If
two antennae of 10m in height establish a 7km link, the largest radius in the first Fresnel zone
is in the middle at 3.5km. Using equations (2.1) and (2.2),
1 √1 9993 35 35
7 1
1 .
Since 10m is greater than 7.94m, 60% Fresnel zone will be free of obstructions.
14
If 0.5m AUV antenna and 10m receiving antenna establish a 7km link, the Fresnel zone at a
point one-tenth of link from the 0.5m antenna is
1 √1 9993 7 63
7
Since 0.5m is smaller than 4.76m, there will be obstruction due to the ground within 60%
of the first Fresnel zone.
Appendix C: Workings for Volume of a Spherical Balloon According to Archimedes’ principle, the buoyant force equivalent to the weight of displaced
fluid in the form of a spherical shape is calculated as
4
3 3 ---------------------------------------------------------------------------------------(3.1)
where Fb is the buoyant force(N), is the density of surrounding air (1.23g/m3) , g is the
gravitational acceleration of 9.8m s-2
and r is the balloon radius(m).
The net static lift of balloon is calculated by subtracting the weight of the envelope and
enclosed helium from the buoyant force.
4
3 3 1 1 2 -----------------------------------------------------------(3.2)
where FL is the net static lift(N), is the density of helium (0.179kg/m3) and is the mass
per unit area of envelope material(kg/m2) [10].
The mass that balloon needs to carry is 2.54kg, the sum of the weights of whip antenna
(0.100kg) and 8mm-thick cable (2.43kg for 10m).
As load that balloon carries balances with the net static lift, substitute 8 ;1 into equations (3.1) and (3.2).
3 1 8
3 1 8 1 1 2 1 8
r=1.02m
1 3 3
Appendix D: Workings for length of AUV module
As shown on Figure 7.1 below, off-the-shelf 20L helium gas canisters have a filling pressure
of 200bar, uncompressed gas capacity of 4.0m3, length of 0.91m and diameter of 0.21m.
Figure 7.1 [11]
15
The minimum number of canisters needed to inflate a balloon is 4 45
4 1 11 .
The minimum number of canisters to inflate the two balloons in the Disposable Helium
Balloons design (Figure 7) is 2 4 45
4 . Thus, the length of the module will have to
be at least 1 since the canisters, with a large diameter of 0.21m, can only
be placed end to end with one another.
As for the Reusable Helium Balloon (Figure 8), the AUV needs a corresponding number of
canisters to be placed on board depending on the amount of times required to elevate the
AUV antenna throughout a mission.
Appendix E: Additional Details of UAV-AUV Integration
Figure 9.3: Top view of UAV-AUV Integration
Figure 9.4: Mini waterproof UAV