High Altitude Long Endurance air vehicles have been an emerging and promising market in recent times. Though a lot of research and development has gone into this area, it is only of late that the implementation of this technology has bloomed into a thinkable reality. The practical applications of such a technology is immense. It has a lot of advantages when compared to terrestrial wireless communication. With the advancements in technology we have been able to experiment at higher operational altitudes, longer durations with greater payloads and increased autonomy. The applications for these UAV’s are immense, ranging from military applications to scientific data collection and 3G/4G services to being communication relays for satellites. Due to the advancements in technology and the future of innovation being at the brink of our generation, a steep increase in the global UAV global market has been predicted. The UAV market by 2025 is predicted to be worth close to 90 billion USD.
In our study, we analyze the potential of High Altitude platforms for Wireless communication and address the challenges that such a system would face. The HAPS system is a much more cost effective method of wireless communication than compared to satellites. It can make efficient use of radio spectrum resources, while demonstrating greater system capacity, higher transmission quality and lower operating risk. It is also a flexible, non-pollutant and cost effective alternative to terrestrial systems.
The reason why the UAV technology has been such a marvel of our time is because it reduces human interference in many situations and reduces human casualties in military applications. Countries are already investing large sums of money to strengthen their defense forces. For example, Britain is in the fore front with the introduction of their solar powered spy UAV called Zephyr. The testing of this HALE UAV was done in 2010 and had a record endurance of 336 hours. They claim that in the near future the ZEPHYR can achieve a 3-month endurance. This vehicle is mainly going to be used for long term surveillance purposes. Similarly, a New Mexico based startup company called Titan aerospace declared that they were working on a long endurance UAV that can stay in high altitude for years. The market is still young for this niche market but proves to be a very promising field.
1. Why use such a system?
People living in some of the remote and rural parts of the world do not have access to internet. It is surprising to know only 40% of the world’s population has access to internet. Currently if you need access to internet in remote areas it comes at an outrageous cost and low internet speed. The High-Altitude Platform is a very cost effective solution to this problem. The HAP’s system is deployed into the stratosphere (60,000ft-80,000ft) which is considerably below the location of satellites. The main parts needed for a HAP’s are ground receivers, broadcasting equipment and internet provider’s. The feature of the HAP’s is the speed of the internet and the product is the availability of wireless internet and the platform is the HAP’s itself. A HAP’s system can replace many terrestrial masts and wires, reduce the environmental impact, reduce cost drastically, resolve the site acquisition problem and reduce maintenance costs as well. The HAP’s system prove to be a valuable service for areas where existing ground infrastructure is lacking.
Primarily our interest is to implement cellular or high speed data networks using technological advancements in airborne platforms which raise the potential for combining the advantages of geostationary with terrestrial-systems-like coverage and signal delay. The use of this technology gives it a large coverage area giving it a competitive advantage over terrestrial alternatives. HAP’s are a much more cost effective and efficient method of wireless transmitting. Sending satellites are always a very costly affair and it is prone to failures. HAP’s have a much lower failure rate once the engineering is formulated. The success of direct broadcast satellite (DBS) in the US is indicative that there is a market for a much more efficient and cost effective method. These flights even if they do not have extended flight hours, once a failure is detected in the system it can easily be recovered for maintenance whenever necessary. When you consider the cost of the mission deploying the aircraft for such extended periods it would come up to a cost of 750 euros per flight hours where as a fleet of MALE UAV’s flying at low altitudes would cost about 3060 euros for two hours. An area of about 300km diameter would be covered if the onboard antenna irradiation is aptly chosen. When comparing the HAP’s system with GEO satellites it gives us a power advantage of up to 66 dB.
Listed below are the advantages of the HAP’s system:
1. Easy and flexible deployment.
2. Better line of sight coverage than terrestrial communication methods.
3. Easy to maintain, update and reconfigure the payload.
4. Lower cost of deployment and operation compared to satellite’s.
5. It can allow Low Probability of Intercept benefits compared with satellite or terrestrial alternatives.
6. Much closer range than satellite, with advantages in terms of link budgets and delay.
7. It facilitates the use of Extremely High Frequencies (EHF) with increased spectral availability.
8. Higher Capacity is facilitated by favorable link budgets compared to satellites.
9. Incremental Deployment- services provides can be expanded gradually to a wider network as and when pleased.
10. Environmentally Friendly.
11. The UAV being at 80,000ft does not have much activity in Civil Airspace. (Quintana, 2010)
With the availability of high energy density fuels these days we can have an efficient fuel system for long endurance. The use of renewable and regenerative methods are also ways to increase flying time.
The HAP’s system is seen as a Broadband-Fixed Wireless Access (B-FWA) providing very high data rates to the users. The frequency allocation at 47/48 GHz offers 2300MHz of bandwidth, which can be apportioned 50:50 to users and backhaul links, and again 50:50 to up and down links.
The downlink HAP power is about 1W per cell, and it can support rates of upto 60Mbit/s which is well within the bandwidth required per cell of 25MHz when using16-QAM or higher modulation schemes. There is also the possibility of using the frequency range of 18-32 GHz for fixed services.
The available bandwidth is one of the most important design considerations to look through. The use of the Shannon equation gives us the relationship of the carrier signal to noise.
R = Maximum Data rate
= Nyquist Bandwidth= samples/Sec
C = Carrier Power
N = Noise Power
The HAP’s system is not spectrally as efficient as terrestrial broadband systems due to the fact that the minimum size of their cell is limited by the maximum size of the antenna that can be accommodated on the platform. It has been proven that an increase in the transmitted power gives rise to an increase in bandwidth saving
This section focuses on the challenges that the HALE UAV is possibly going to face and address feasible solutions. There are numerous challenges that one faces while designing a HALE UAV, starting from the aero foil design of the wing.
2.1. Atmospheric Conditions
To design and operationally viable HALE to patrol at an altitude of about 80,000ft additional operational factors need to be considered. It is found out from the references that the pressure at 82,115ft would be about 25 millibar. It was found out that the jet stream speed at altitudes 17km-20km have low value. Although the wind does not affect the aircrafts power generation, it has a significant effect on the UAV’s drag and power consumption. It is found out that the instantaneous power required to counter the wind forces that is exerted on the UAV is
is the density of air
is the drag coefficient
is the cross-sectional area of the wing
is the wind speed
The choice of a flying altitudeconsiders two factors:
1. The solar radiation available from 45 to 37 latitude
2. The wind profile as a function of the altitude; at the 65,000ft to 80,000ft the jet stream speed has a relatively low value.
The turbulence in the stratosphere will lead to instability in the UAV (roll, pitch and yaw) but larger the aircraft greater the stability. Electronic steering of an array antenna and mechanically stabilized sub-platforms are possible solutions to this issue.
2.2. Effect of Icing
Icing has always been a massive issue concerning high flying UAV’s. The effects of icing on the aircraft can cause system failure if appropriate measures are not taken. With the buildup of ice on the airframe surface it can cause altitude loss. The reasons why loss of altitude can be experienced are:
1) The breakup of laminar flow of air over the leading edge can cause reduced lift.
2) An increase in power will be applied to counter act the reduced lift.
3) The lift eventually decreases as there is no more power available.
In addition to this equipment failure can be caused which will then result in a loss of altitude or incorrect propagation.
The best method to avoid icing conditions is to forecast the weather conditions and check for icing. Also, certain de-icing equipment or the use of thermos materials can be used to protect the sensitive on board equipment. The different types of ice formed is described in the table below:
Additionally, a multi-layer de-icing skin can be used in the UAV. An ice resistant structure is provided which includes a self-supporting, structural platform, a retaining and a subsurface anti-icing or de-icing layer. The retaining, protective layer is disposed over the self-supporting, structural platform. The subsurface anti-icing layer is located between the self-supporting, structural platform and the retaining, protective layer. The subsurface anti-icing layer is a functional layer such that an AI agent is released to a surface of the retaining protective layer by an activation mechanism responsive to a change in an environmental condition. (United States Patent No. 20150122947, 2015)
2.3. Energy management / Energy Balance
Designing a long endurance vehicle that operates majorly on the solar energy is quite challenging. The solar power available is not constant, which requires us to design a very efficient energy storage system. It was found out that the maximum specific solar power, for a short period, is of 1240 W/m2 and 1190 W/m2 at 36 and 45 N.lat (optimal) also 675 W/m2 and 475 W/m2 at 36 and 45 N.lat (worst). But when the solar panels are placed with its surface perpendicular to the earth’s surface, the solar power drastically increases to a maximum specific solar power of about 1200 W/m2 on the worst day.
One of the biggest technical hurdles of such a system is the performance of energy storage. The power balance to be able to store sufficient energy for station-keeping throughout the night is an issue that needs to be tackled. The objective of the UAV is to maintain a level flight once it attains the required altitude. The electric energy not required for propulsion and payload operation is pumped back into the fuel cells energy storage system and at night a geostationary position would be maintained by a level turning flight using the stored energy.
2.4. Wing Design
The endurance parameter must be fulfilled to minimize the power required for horizontal flight. The aerodynamic characteristics of the UAV needs to be optimized in order to account for endurance. Minimizing the parameter is essential. This is because due to the lower structural load, we must choose to reduce the drag coefficient instead of simply increasing the lift coefficient. The total drag coefficient is obtained from the sum of all the profile drag, induced drag and parasite drag. The Reynolds number of less than 700,000 is considered. The power per unit wing area that is required to maintain horizontal flight is computed by:
Where is the propulsion efficiency.
A wing profile with a high lift coefficient and low drag coefficient would be necessary to reduce the wing and horizontal drag.
Sondes are used to record temperature, dew point, ambient pressure and GPS derived wind speed. Usually sondes suffer from poor scalability. Their radio frequency telemetry is designed to transmit small packets and they are non-recoverable, the amount of observations they can make is usually limited to data space available.
2.6. Rain Effects
The rain attenuation effects are negligible at the range of 2Ghz but are prevalent at higher frequencies especially above 20GHz.The higher the frequency, the higher the attenuation and the impact on the Quality of Service. Rain effect is basically due to reflection, scattering or absorption of the radiations. It is proven that for HAP availability of 99.9% and above, rain is the dominant attenuation factor at frequencies of 28GHz and above. Other factors, such as clouds, water vapor, oxygen and scintillation do not contribute at availabilities above 99%. There is a need for extensive collection and analysis of rainfall attenuation and scattering effects.
Interference is caused by antennas serving cells on the same channel and arises from overlapping main lobes or side lobes. Two main kinds of interference can be said to happen in HAPS. Propagation in HAPS systems is achieved mainly through free space this the interference levels can be predicted and assumed easily and successfully.
The HAP schemes usually use multiple spot beams over the coverage area leading to greater capacity through frequency reuse. Thus, provision will have to be made for the possibility of handoff which may arise when platform motion leads to movement of the antenna beam. Using fixed antennas on the HAP and accommodating motion simply through some handoff technique is a possibility but it comes at the cost of delay and jitter limitations for future services.
To optimally utilize network capacity, suitable coding and modulation techniques will be required to support the broadband services within the specified Quality of Service and bit error rate requirements obtainable under different link conditions. Due to the uniqueness, channel assignment and resource allocation schemes tailored for multimedia traffic will have to be redeveloped to specifications.
3. Certification and Regulations
With the advancements in UAV technologies in recent times, the number of UAV’s that share airspace with commercial aircrafts has been increasing. There needs to be proper regulations and certification procedures that ensure safe and legal usage of the UAV’s. With this kept in mind the CAA has recognized the need to develop civil standards for UAV’s in the UK.
In this section, the airworthiness standards for setting design standards for HALE UAV’s is described. The method that is used compares the hazard presented by the new aircraft with that of the existing conventional aircraft to obtain an indication of the appropriate level of requirements which should be applied.
The main criteria taken into consideration is the capability of a vehicle to harm any third parties. This is broadly proportional to the kinetic energy of the vehicle on impact. There are two kinds of impact that are possible; one is either the impact that arises because of an emergency landing maneuver and when there is complete loss of control.
Emergency Descent Scenario:
The likelihood of the unpremeditated descent is usually dominated by the reliability of the propulsion system. When we calculate the K.E at impact in such a scenario the mass taken into consideration is the maximum take-off mass and the velocity used is the approach velocity (engine off).
For airships V= A combination of the terminal velocity resulting from the static heaviness and probable wind velocity.
Loss of Control Situation:
In this case when we consider the K.E at impact the mass considered is the maximum take-off mass and the velocity used is the probable terminal velocity.
For airships V= Terminal velocity when there is no propulsion/ thrust.
Method of Comparison:
In order to obtain the indication of the level of requirements appropriate to novel aircraft the following procedure is followed:
Step1: Calculating the K.E of the aircraft for each scenario.
Step2: Using the values obtained and comparing the values with Fig 123 separately and determine the code to applied keeping in mind the intent of preventing the occurrence of each scenario.
Fig1 will provide us with an indication of the standards to be applied to any feature of the design whose failure would affect the ability to maintain safe altitude above the surface.
Fig2 will provide an indication of the standards to be applied to any feature of the design whose failure would affect the ability to maintain the control.
If it is found out that the aircraft fits in more than one code, then it indicates that it may be appropriate to apply to a combination of standards.
Step3: We then construct a certification basis that addresses the same aspects of the design as the existing codes and to the level indicated by the K.E comparison. For a HALE UAV, special conditions will need to be considered for any novel features of the design that is not addressed by the existing codes.
In addition to this, operational requirements may dictate the inclusion of specific design features which may in-turn necessitate the inclusion of additional certification requirements. For example, the Rules of the Air specify that an aircraft operating over a congested area must be able to maintain a safe altitude following the failure of one power unit.
The FAA,DoD, NASA, as well as foreign civil aviation authorities are working together to study the changes required in government regulations and infrastructure to accommodate UAV’s (DeGarmo, 2004)
4. Operator Training
The HALE UAV that we take into consideration is a fully autonomous system that enables it to can also be a Remotely Operated Aircraft (ROA). This means that the operator needs training to develop the skills needed to control the UAV.
The first and basic document for UAV training guidance was the NATO STANAG 4670 PFP(NNAG-JCGUAV) D (2006)001-Rev2. It was issued with the request of ratification 13 September 2006, till December 2006. After all these years, a UAV designated operator training is still a matter of argue between many organizations. The latter version of this regulation is called as NATO STANDARD ATP-3.3.7 (Edition V, Version 1) from 22 April 2014.
The NATO STANAG 4670/ATP-3.3.7 training guidance is based on three documents namely:
1) Chairman, CJCSI 3255.1, Joint Aircraft Systems minimum training standards, originally dated 17 July 2009, Change 1, dated 31 October 2011, current version as of 4 September 2012;
2) AAP-03, Edition J, Version 1, dated November 2011;
3) AAP32)A), Change3, dated January 2002.
The operators of the HALE UAV must be trained to BUQ Level 4. The knowledge and skills required to operate under VFR and instrument Flight Rules (IFR) would be expected.
The best method to train operators is to make use of simulators. The UAV simulators are designed to provide training for UAV operators and pilots, image interpreters, and mission commanders. These simulators fabricate a generic aerodynamic model to provide mission- specific or real tie operational training for HALE UAV’S.
Simulators need to enable the operator to analyze real time effects like varying climatic conditions and terrains and have trajectory planning modes and simultaneous localization systems.
The FAA brought in a set of new norms in 2015 which deals with UAS operators training and operators responsibility. The basic principles defined by the FAA are s follows:
1) UAV operator must be at least 17 years old;
2) Pilots of small UAV would be considered for “operator” instead of “pilot” widely applied and used;
3) UAV operators would be required to:
i) Pass an initial aeronautical knowledge test at an FAA-approved knowledge test center;
ii) Be vetted by the transportation Security Administration;
iii) Obtain an unmanned aircraft operator certificate with a small UAS rating;
iv) Make available to the FAA, upon request, the small UAS for inspection or testing, and any associated documents/records required to be kept under the proposed rule;
v) Report an accident to the FAA within 10 days of any operation that results in injury or property damage;
vi) Conduct a preflight inspection, to include specific aircraft and control station systems checks, to ensure the small UAS is safe for operation.
ARCHITECHTURE OF HALE UAV
There is no doubt about the fact that aerial platforms will play an increasingly important role in the delivery of wireless services. With corporation from the providers of the platforms and due to the excessive demand of such a technology it is only a matter of ‘when’ before we see this come into play.
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