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).

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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

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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

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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

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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.

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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

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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]

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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.

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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.

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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.

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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.

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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]

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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