On the capabilities and limitations of high altitude pseudo-satellites

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Abstract

The idea of self-sustaining air vehicles that excited engineers in the seventies has nowadays become a reality as proved by several initiatives worldwide. High altitude platforms, or Pseudo-satellites (HAPS), are unmanned vehicles that take advantage of weak stratospheric winds and solar energy to operate without interfering with current commercial aviation and with enough endurance to provide long-term services as satellites do. Target applications are communications, Earth observation, positioning and science among others. This paper reviews the major characteristics of stratospheric flight, where airplanes and airships will compete for best performance. The careful analysis of involved technologies and their trends allow budget models to shed light on the capabilities and limitations of each solution. Aerodynamics and aerostatics, structures and materials, propulsion, energy management, thermal control, flight management and ground infrastructures are the critical elements revisited to assess current status and expected short-term evolutions. Stratospheric airplanes require very light wing loading, which has been demonstrated to be feasible but currently limits their payload mass to few tenths of kilograms. On the other hand, airships need to be large and operationally complex but their potential to hover carrying hundreds of kilograms with reasonable power supply make them true pseudo-satellites with enormous commercial interest. This paper provides useful information on the relative importance of the technology evolutions, as well as on the selection of the proper platform for each application or set of payload requirements. The authors envisage prompt availability of both types of HAPS, aerodynamic and aerostatic, providing unprecedented services.

Introduction

Further, quicker, longer, higher. From the beginning of the aviation era, those have been persistent concerns for manufacturers, pilots, operators and users. Most of the trials to reach new achievements faced the unavoidable limits of materials, aerodynamics and propulsion systems against the air drag and the everlasting gravity force.

But progress has been spectacular so far. Materials and manufacturing processes allow airplanes to carry more payload and fuel. Supersonic flight is mature and hypersonic velocities are manageable. Air-breathing engines are now equipped with powerful compressors, enabling high altitude cruise only formerly reachable by large balloons and rocket-assisted vehicles. Electronics performance and reliability improve flight control and unmanned operation.

In parallel, solar panels increase their efficiency every day and automotive industry boosts the development of electric power plants. In this context, the idea of self-sustainable air vehicles that excited engineers in the seventies [9] has become a reality, as proved by popular Solar Impulse [8,110] and other initiatives worldwide [9,17,140].

High altitude platforms, or Pseudo-satellites (HAPS), are those aerial platforms able to emulate satellite performance at local scale. That means enough altitude for the payloads to cover an interest area without interfering with current commercial aviation, and enough endurance to provide long-term services as satellites do. Communications, Earth observation, positioning, and astronomy, among other applications, could benefit from these platforms.

Both aerostatic and aerodynamic solutions are today in-vogue in the race for stratospheric commercial conquest. Whereas the first may be very large and difficult to handle on the ground, they can carry heavier payloads than airplanes, whose values are the simpler development effort and the more mature control mechanisms. Of course there are hybrid solutions trying to keep the best of both worlds. There is also controversy on considering balloons as HAPS, since they are hardly controllable. The same occurs with manned airplanes; whereas they are capable of cruising in the stratosphere, pilot presence advises against extremely long endurance. The balloon and the manned airplanes are not considered as pseudo-satellites in the present paper due to these limitations.

This paper provides a review of the technologies involved in stratospheric flight, their readiness level and their expected evolution to compare the two approaches for HAPS: aerostatic and aerodynamic. The performance analysis on typical operational scenarios will provide useful information on the capabilities and limitation of each solution. On-design and off-design comparisons estimate the impact of the different mission and vehicle design parameters on the global performance. The paper is organised along this logic: a historical review with the main requirements allocated to HAPS; an analysis of the atmospheric environment; a motivated analysis of involved technologies with emphasis on the key issues that may limit the mission achievements, such as aerodynamics, propulsion, power management, structures and materials, thermal control, ground assets and operational constraints; a comparison between airplanes and airships in terms of performances and the sensitivities of key figures. Lessons learned are then extracted to identify bottleneck technologies, future trends and challenges.

From the multitude of projects that have provided useful knowledge in the field, a short list has been selected to illustrate the type of initiatives throughout history, the solutions adopted to key issues and the major achievements.

The classic method to reach the stratosphere is through unmanned balloons. Although initially dedicated to in-situ atmospheric observations, they are currently affordable platforms for other scientific and technological disciplines such as astronomy, Earth observation and telecommunications and even planetary exploration [49]. Payload capacities move from few kilograms to several tons. Similarly, flight durations of up to several days are available. An illustrative example is the recent mission POGO+ from the Swedish Space Corporation, which demonstrated a 40-km, 7-day flight with 1728-kg on board [79]. In parallel, in 2016 the Loon Project managed to fly several balloons for 14 weeks around an area of interest in Peru, just by selecting the proper altitudes to drift on the wind in the desired directions; in 2017, the concept provided basic internet for 7 weeks to people suffering devastating floods in the same area [108].

Other limited-endurance stratospheric platforms are fast jets. Manned jets initially operated in military applications, these fast airplanes are also used today for scientific purposes. The ER-2 and the M − 55 Geophysica reach more than 20 km ceiling with a mission endurance of more than 6 h whereas the SR71 Blackbird was able to reach up to 27 km at supersonic speeds but only for 1.5 h. In essence, the dynamic pressure given by high velocities are used to compensate low air density while powerful propulsion plants keep the cruise conditions.

Using the same principles, unmanned operations and modern technologies allow these models to reach an endurance of several days. As a matter of example, from 1998 the Northrop Grumman Global Hawk can fly 35 continuous hours in the stratosphere before running out of kerosene. Today there are more competitors with similar performances.

But despite the above, solar airplanes and airships exhibit best conditions to serve HAPS requirements as they can offer station keeping at very low operational costs given their ‘unlimited’ endurance. Although hybrid solutions are possible, this paper is focused on the comparison between wing-based aerodynamic and buoyancy-based aerostatic flight options. The below information is mainly taken from the review documents [26], [154] and [113] as well as web pages from manufacturers. Priority has been given to active programs and those with relevant findings through flight tests.

The most relevant projects developing stratospheric solar airplanes have been:

  • HELIOS: The Environmental Research Aircraft and Sensor Technology (ERAST) Program was a NASA initiative started in 1994 to develop a flying wing stratospheric airplane. Two prototypes reached 21-km (Pathfinder) and 29-km (Helios) record-winning altitudes. The long wings suffered from aeroelastic instability due to turbulence, leading to a program closure in 2004 [98].

  • AEV-3: this 17.2 aspect ratio, 53-kg airplane has been developed and flown by the Korean Aerospace Research Institute in 2016 after successful first and second generation models in the former years. The AEV-3 requirement is to achieve 18-km altitude with 5-kg payload with a range of cruise velocities between 6 m/s (minimum energy) and 10 m/s [60].

  • AQUILA: this internet-aimed drone promoted by Facebook (initially developed by Ascenta in UK) intends to fly at an altitude between 18-km at night and 27-km in daylight. This solution is compatible with telecom applications and reduces the need of propulsion power when energy cannot be harvested from the Sun. The platform to be used is a flying wing aircraft with 42-m wingspan and 400-kg take-off mass. The mission life in the stratosphere, to which the airplane is injected by a balloon, will be 90 days. Up to now, a tropospheric flight of 96-min has been reported by Facebook in 2016 [155].

  • ZEPHYR: starting in 2000 at the Flemish Institute for Technical Research with the Pegasus project, the airplane developed by Qinetiq was finally transferred to Airbus in 2013. There is current evidence of activity in the project as several units have been sold for military applications in UK. The Zephyr-7 is the only solar-powered airplane that has demonstrated a unique mission duration of 14 days at more than 21-km flight altitude carrying a payload of 5 kg. The company plans to improve such a performance with Zephyr-S and even to develop a larger version with up to 20-kg payload capacity, to be operational around 2019. The use of Lisingle bondS batteries and light-weight structures free from harmful aeroelastic effects are considered the major key technologies on-board [149].

  • CAI HONG: meaning ‘Rainbow', this solar airplane has been developed by the China Academy of Aerospace Aerodynamics. In 2017, a video-recorded test proved stable flight at 20-km altitude. The airframe comprises a pair of slender fuselages that support high-mounted wings measuring 45-m in span [143]. The target payload size and mission endurance remain undisclosed.

The selected projects for stratospheric solar airship development have been:

  • HISENTINEL: it is a relevant research programme developed by the US Army from 1996 to 2012. The objective was to sequentially fly under propulsion 20, 50 and 80 lb of payload in the stratosphere by lighter-than-air vehicles for at least 30 days. There were important achievements such as the deflated balloon-like launching but problems with the propulsion system and gas leakage through seams avoided tests lasting more than a few hours [122].

  • SPF (Stratospheric Platform): developed by the National Aerospace Laboratory of Japan (today JAXA) from 1998, the program included several prototypes of growing size (up to a huge 245-m length model) and a hangar. In 2005, after successful tropospheric missions with a 68-m prototype, the program was cancelled due to financial restrictions. The main advances focused on the regenerative fuel cells, gas-bag management and light flexible structures (Zylon) [85].

  • Korean Stratospheric Airship Program: the project to obtain a lighter-than-air stratospheric platform stared in 2000 in the Korea Aerospace Research Institute. A 50-m length model was able to fly with 100-kg payload at 5-km altitude. There is little more information from 2005, although a huge 22-ton airship was under consideration.

  • HAA (High-Altitude Airship): in 2002, the US Army initiated the HAA program, with a long endurance 73-m length prototype (HALE-D) contracted to Lockheed Martin. The program was stopped in 2011 after an incident during a test flight due to the air management subsystem. The official report [41] summarises the operations and expenses of the several USA efforts in airship development, including stratospheric ones.

  • STRATOBUS: Thales Alenia Space started developing the Stratobus in 2015, aiming at a stratospheric airship with 250-kg payload able to keep its positon at 20-km altitude for one year. The Stratobus will be about 125 m long, with an envelope made of UV-resistant woven carbon fibre, and able to stand winds of 90 km/h thanks to its two fuel cell-powered prop motors. As the program advances some of the technical solutions are varying such as the position of propulsion plants and solar panels. The investment is active, with plans for qualification flights in 2019.

HAPS are capable of providing services that could complement, compete with or even replace those currently offered by airplanes, satellites and terrestrial networks. Most relevant services are germane to, among others areas, telecommunications, Earth observation, GNSS or scientific applications.

HAPS are promising platforms for the improvement of existing communication systems, both in capacity and coverage [51]. For example, terrestrial networks hardly provide a reasonable quality of service and data rates to most rural and remote locations even in developed countries. Meanwhile, satellite services have been traditionally focused on broadcasting applications and government communications. New initiatives such as the massive LEO constellations also pretend to provide robust and efficient universal communications for a large amount of private users [34]. But the irruption of HAPS systems within the communication networks can provide a series of advantages at different levels when compared to satellite or terrestrial solutions, as summarized in Table 1.

Based on HAPS capabilities listed in Table 1 and inspired by the analysis of HAPS communications systems included in Ref. [51], different telecommunications services can be envisaged to be soon offered from such platforms:

  • Direct-To-Home (DTH) broadband: Direct-To-Home (DTH) broadband: HAPS could be useful in unserved areas (i.e. areas with no infrastructure providing communication services) or underserved areas (i.e. areas with poor connectivity). In this case the HAPS could mimic a satellite or a terrestrial tower.

  • Trunking: A large numbers of users under the footprint of a HAPS can connect to it and share a single satellite connection. Users can benefit from the good balance between coverage and signal degradation provided by the HAPS solution, avoiding the requirement to have a dedicated satellite connection for each user.

  • Backhauling: HAPS can provide very high capacity backhaul links between nodes of a network (e.g. cell phone towers) and its backbone, avoiding then the deployment of costly optical fibre or terrestrial microwave links. Furthermore, they could also trunk several backhaul links and connect them to the core network via satellite.

  • High Throughput Services: Although current GEO satellites are capable to generate hundreds of spot beams, each of them has a relatively large size. Should too many users in the same spot intend to stablish a connexion the beam may be overloaded and some of them will have no service. HAPS could then help to offload the beam.

  • Tactical Communication: usually provided in UHF, HAPS based service is scalable, agile, reliable, affordable, defendable, rapidly deployable and requires minimum in theatre ground infrastructure [130].

  • Mobile Broadband: Currently, broadband services to mobile users are usually provided by terrestrial wireless networks. If no terrestrial coverage exists, service can be provided by already existing satellites (e.g. Iridium, Inmarsat, etc.). HAPS could provide a service equivalent to the satellite, but offering higher capacity thanks to the much more favourable link budgets.

  • 5G: HAPS can be part of the infrastructure needed to support 5G services, where a single platform can maintain not only a large number of connections, but a wide range of services and applications (e.g. DTH broadband, mobile broadband, trunking, backhauling, Internet-of-Things, etc.).

The deployment of such services could be notably hindered due to the limitations derived from the telecom bands assigned to HAPS by the World Radio Conference (WRC) when providing fixed services (according to Resolution 122 from WRC-07 [62] and Resolutions 145 and 150 from WRC-12 [63]). Regulation establishes several geographical limitations and conditions of operation of HAPS in all the assigned bands. Due to these reasons it is expected that during the following WRC meeting (to be held in 2019, WRC-19) appropriate regulatory actions for HAPS within existing fixed-service allocations can be taken (Resolution 809 from WRF-15 [64]).

HAPS are also a very interesting platform for Earth Observation (EO) payloads, providing useful capabilities for many services and complementing satellite and conventional aircraft (manned and unmanned) imagery.

While space sensors can map large areas worldwide, they offer a relatively coarse resolution for certain applications and suffer operational constraints due to the fixed-timing acquisitions and weather conditions (e.g. cloud cover). On the other hand, aircraft surveys can be planned more flexibly, but they can pose difficult and costly campaign organization efforts. When continuous monitoring of an area is needed, the relatively limited persistence of existing aircrafts (even Medium Altitude Long Endurance unmanned versions) requires the deployment of multiple platforms.

A large variety of EO-based services can be identified. A good systematic classification is offered by the European Association of Remote Sensing Companies (EARSC) [36]. All these services are likely to be provided by means of HAPS when the coverage area is local or regional. However, some of them are more suitable to HAPS as those can keep flying for very long periods, up to several months, and its capability for Earth Observation is similar to other conventional aircrafts as proven from manned vehicles (Fig. 1). Taking advantage of its superior endurance [129], estimates that permanent coverage of an area over a 21-day period using a single HAPS makes it possible to reduce by a 60% the required personnel for the operation of the system or having a cost per flight hour of just a 15% that of the conventional unmanned aircrafts. The services that have been identified as highly promising for HAPS are: security, maritime and emergency management [50]. Both passive and active sensors are under consideration, although given the low maturity of current platforms and associated services, the first are clearly the precursors.

The availability of HAPS platforms opens up opportunities for the provision of navigation services, either with a stand-alone service, with additional infrastructure to complement existing systems or with services which allow improving the performance provided with such systems. HAPS could provide functionalities such as:

  • Additional ranging sources to assist and improve positioning

  • Network node to provide data from an external source

  • Reference stations for network RTK (Real Time Kinematic) and PPP (Precise Point Positioning) types of services

  • Additional sensor platform to perform radio occultation and/or GNSS reflectometry measurements.

Finally, there are many other applications for HAPS. In-situ observations of atmosphere or other scientific disciplines can benefit from the high altitude of the HAPS in different ways. As a paradigmatic example, it is an ideal platform to perform astronomical observations because most of the atmosphere lies below the telescope. The concept has already been proven as successful thanks to the 12-km altitude Stratospheric Observatory for Infrared Astronomy (SOFIA), a joint USA-German space science project active from 1996 [45]. SOFIA mounts a 2.7-m telescope inside a modified Boeing 747. Unmanned HAPS can even provide a more stable platform with extended endurance.

Section snippets

Stratospheric environment

Traditionally, the atmosphere has been decomposed in layers, each one having a characteristic vertical temperature gradient. The layer of particular interest in the case of HAPS is called stratosphere which typically is considered to start at 20 km [61] although this depends on the latitude (in the poles it starts as close as 8 km). The maximum altitude of the layer is close to 50 km. This section is intended to provide a succinct overview of the stratospheric environment focusing on: its

Aerodynamics and aerostatics

The aerodynamics of the platform is a critical element not only for its performance, but also for the definition of the design of the control and propulsion system. Platform configuration is obviously quite different for airships and airplanes.

HAPS modelling

In order to develop simple budgets for mass and power that enable comparison between airships and airplanes in stratospheric flight, the many models found in literature [10,78,104,153] can be simplified in a common process as depicted in Fig. 15. Those more complex models normally include optimization algorithms that trade-off parameters among the subsystems to maximize interesting figures of merit. However, the model in this paper is thought to maintain its validity in both airplane and

Operational considerations

To make the commercial exploitation of any flight vehicle feasible there must be a regulatory framework supporting its activities. HAPS are not an exception.

Although this issue has many ramifications of technical, legal and economical nature, the conceptual scheme developed in Ref. [65] can provide some insight when considering the same problem for Large Hybrid Air Vehicles. In fact, it runs parallel to the difficulties raised by the International Civil Aviation Organization (ICAO) when

Conclusions

After several serious attempts worldwide in the last decades, high altitude pseudo-satellites (HAPS) are close to becoming operational. Aerodynamic versions will be first, following the wake of Airbus/Zephyr-S, cruising the stratosphere for long periods of time and carrying small instruments for precursor applications. Aerostatic counterparts will come afterwards, following the developments of Thales/Stratobus, with large payloads to provide unprecedented imagery or communications services.

The

Acknowledgements

The authors would like to express our deepest gratitude to the partners involved in the HAPPIEST project from the European Space Agency (AO 1–8464/15/NL/GLC), and in particular the Agency's Technical Officer. The authors also acknowledge the valuable advices of the anonymous referees that helped to enhance the manuscript.

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