Tethered aerostat

Abstract

A tethered aerostat positioned at an elevated location in the atmosphere, the aerostat comprising sufficient lighter-than-air gas such that the aerostat is substantially buoyant in air, the tether comprising a first length of tether, connecting the aerostat to an anchor located at and resting on the surface of a first substantially ground level location, and wherein the anchor has a weight such that, in a first anchoring condition, the anchor remains in place on the surface of the first substantially ground level location, but also such that it can adopt a second relocating condition, wherein the anchor is also attached to a relocating aircraft, the anchor being lifted above the surface of the first substantially ground level location, and can thereby be relocated to a second substantially ground level location by the relocating aircraft; and a method of transporting at least one aerostat as described herein, from the first substantially ground level location to the second substantially ground level location, the method involving attaching a relocating aircraft to the anchor in its first anchoring condition, the relocating aircraft lifting the anchor from the surface of the first substantially ground level location, transporting the anchor, and thereby also the tethered aerostat, to a second substantially ground level location and depositing the anchor on the surface thereof, followed by disconnecting the relocating aircraft from the anchor.

Description

[0001] Tethered Aerostat

[0002] Technical Field

[0003] The invention relates to tethered aerostats, typically of a low drag aerofoil design, and to a method of transporting the tethered aerostats from a first to a second substantially ground level location.

[0004] Background to the invention

[0005] Low latency access to information services is becoming vital for economic and social wellbeing. The Covid Pandemic has accelerated this process. New applications (for example, autonomous driving, remote medical support) and more generally the Metaverse, require reliable, low latency and high bandwidth mobile communications.

[0006] Compared with satellites, aerostat-supported platforms have several advantages, primarily because the distance from a transmitter to a receiver on Earth can be much less, with geostationary satellites typically at 36,000 km altitude and around 1000 km altitude for a “Low Earth Orbit” or LEO satellite.

[0007] This relative nearness of tethered aerostat platforms can result in much stronger signals relayed to Earth and avoid the expense of rocket launches as well as providing shorter development times, and allows power and backhaul connection via the tether.

[0008] Recently-developed, lightweight, very large-capacity phased array antennas have the potential to transform global mobile and fixed line connectivity e.g. by delivering cellular telephone services, including linking to the internet. This is dependent upon them being positioned appropriately, and at typically between 200m and 2500m over most geographies. Suitable tethered aerostats are therefore needed to support such antennas, that are reliable in all weathers at moderate elevations, with power and fibre optic cables as well as lighting and lightning protection being part of the tether. Such aerostats need to be situated below commercial aviation traffic, but sufficiently elevated to provide links of up to 80 km range for low population densities, up to 30 km for moderate rural densities and 5 km in urban areas.

[0009] Similarly, there are a range of earth observation, meteorological data collection, and astronomical data collection systems which could substantially benefit from being supported by a suitable tethered aerostat system that is reliable in all weathers at elevations at typically between 200m and 22000m.

[0010] A tethered aerostat has the potential to be far cheaper than satellite systems with similar functionality: power supply and fibre optic cable links can be supported by or be an integral part of the aerostat tether, avoiding the expensive backhaul systems and power systems required for satellites or aircraft. Furthermore data latency effects are important to many applications (including but not exclusively, augmented reality, autonomous driving, health care, interactive video games, video conferencing, remote control of UAVs etc.); there are significant problems with latencies offered by satellites, even low earth orbit satellites.

[0011] The need for improved mobile connectivity has prompted a resurgence of interest in using alternative delivery technologies rather than ever large numbers of mobile communication masts e.g. low earth orbit satellites, stratospheric platforms and tethered aerostats. All of these solutions have issues; respectively: data capacity and latency, technology readiness level, and wind stability.

[0012] Current tethered aerostat systems include tethered rigid and non-rigid airships (blimps) and hybrid balloon / kite systems. Airship-type designs are usually inclined to the horizontal to provide both aerodynamic lift as well as buoyancy. The principle of a hybrid balloon / kite system is that the balloon provides lift in low wind speed conditions and the kite provides lift in high wind speed conditions. Hybrid balloon / kite systems are usually less expensive than airships. However, for such balloon / kite systems in high wind speed conditions the horizontal drag on the balloon is high and the kite therefore needs to be large, incurring considerable drag forces to provide sufficient lift to maintain altitude. These large drag forces then require a very strong and hence, heavy, tether, which has its own associated drag that requires a still larger balloon / kite system to support it, thus reducing payload carrying capacity.

[0013] Current tethered airship designs and balloon / kite systems do not survive strong winds of more than 50 knots for smaller systems and for very large systems winds of more than 70 knots or exceptionally 100 knots. For continuous operation at useful elevations of typically over 400m, high wind speeds of over 100 knots will have to be sustained. With high winds, existing systems become unstable, moving uncontrollably and are ultimately blown over.

[0014] Because of these effects it has not been possible hitherto to design and build a reliable tethered aerostat system to deliver a high availability service because very high wind conditions will be encountered almost everywhere from time to time, particularly at suitable altitudes needed for many different applications. That, in turn, has meant that any potential use of a tethered kite / balloon aerostat carrying a system that requires high availability and reliability has not been possible.

[0015] Lighter than air balloon / kite systems are described in US 2011 / 0222077A1 with the following examples: US 2,398,745 and US 2,431,938 to Jalbert, US 4,029,273 to Cristofel Jr., in US 6,016,998 to R. Allsopp, and in U.S.Pat.No.6,499,695 to Talamo. In US 6,555,932 to Mizzi, there is described a combined buoyant aerofoil for use in generating electricity with wind power or for aerial advertising. Such combination balloon / kite systems are also available commercially for surveillance and advertising use, such as the SkyDoc TM Aerostat, supplied by Floatograph Technologies LLC, of SilverSpring Md., or the Helikite, supplied by Allsopp Helikites Ltd of Fordingbridge, Hampshire, UK. Such systems have been used for military and civilian use, and the data generated has been described as being conveyed to and from the ground station by means of wireless, cable or optical fiber.

[0016] US 2018 / 0050797 Al discloses a conceptual tethered lighter-than-air unmanned aerial vehicle that has a wing and a fuselage. However, no consideration is given to how to maintain such an arrangement so that it is stable in the atmosphere. KR 10-2016-0081328 discloses a buoyant aerostat that is tethered to ground level. However, no consideration for how such an apparatus can be kept stable in the atmosphere is given. US 2016 / 0122014 Al discloses a conventional blimp attached to the ground by two tethers. US 3,620,292 discloses a known type of wing balloon.

[0017] An aerostat system highly stable in extreme weathers for a wide variety of locations with the ability to withstand meteorological challenges including: extreme gusts, lightning strikes; prevention of snow and ice build-up; and safety features to prevent accidents with low flying aircraft, is therefore likely to be valuable in a whole variety of applications including but not limited to those described above.

[0018] However, such aerostats have to be large to provide the buoyancy required to lift suitable payloads. Furthermore, significant buoyancy is required to support the tether which has to contain lighting conductors required for all-weather operation at suitable altitudes (above 200m preferably 1000m and most preferably 1500m). Typical required aerostat volumes are at least 300 m3, more preferably above 1000 m3. An example of a vertical buoyant aerofoil is given in international patent application no. PCT / GB2022 / 053290.

[0019] Conventionally large aerostats are transported when uninflated and folded. However, the foldable materials currently in use suffer from significant helium permeability if they are to be suitably lightweight and strong and such aerostats typically need to be topped up with helium every two to six weeks by winching them down to the ground or some other means, which is highly undesirable since the launch and recovery of large aerostats is a significant undertaking requiring suitable weather conditions (moderate wind gusts) and careful management. The time taken will also lead to significant interruptions to communications service.

[0020] Fibre composite materials with suitable metal barrier technology and jointing technologies as aerostat skins allow aerostats to be operated for far longer without helium top-up. Furthermore, such aerostat skins can also allow many years of UV exposure without deleterious consequences, but these materials are not foldable.

[0021] As a result, large aerostats made from such unfoldable materials cannot be deployed on sites accessible only by road which inhibits the use of large aerostats made from unfoldable skins from many useful geographies.

[0022] In this context there is a need for additional means of transport of tethered aerostats, especially ones with unfoldable skins.

[0023] Summary of the Invention

[0024] In a first aspect, the invention relates to a tethered aerostat positioned at an elevated location in the atmosphere, the aerostat comprising sufficient lighter-than-air gas such that the aerostat is substantially buoyant in air, the tether comprising a first length of tether, connecting the aerostat to an anchor located at and resting on the surface of a first substantially ground level location, and wherein the anchor has a weight such that, in a first anchoring condition, the anchor remains in place on the surface of the first substantially ground level location, but also such that it can adopt a second relocating condition, wherein the anchor is also attached to a relocating aircraft, the anchor being lifted above the surface of the first substantially ground level location, and can thereby be relocated to a second substantially ground level location by the relocating aircraft.

[0025] In a second aspect, the invention relates to a method of transporting at least one aerostat as described herein, from the first substantially ground level location to the second substantially ground level location, the method involving attaching a relocating aircraft to the anchor in its first anchoring condition, the relocating aircraft lifting the anchor from the surface of the first substantially ground level location, transporting the anchor, and thereby also the tethered aerostat, to a second substantially ground level location and depositing the anchor on the surface thereof, followed by disconnecting the relocating aircraft from the anchor. As used herein, the term ‘aerostat’ includes any pay load and equipment attached to the aerostat other than the tether.

[0026] Thus, the tethered aerostat can be transported by a relocating aircraft whilst it is in inflated and buoyant form, being conveniently moved from a first to a second ground level location by use of a suitable anchor.

[0027] Preferably the tether comprises a second length of tether in coiled form and secured by the anchor. Preferably, the length of the first length of tether can be increased by uncoiling the second length of tether by passing it through or over a suitable winch or capstan. It can be decreased by a reverse process.

[0028] The first length of tether may therefore be conveniently located above a first ground level location, with the anchor resting on the ground surface holding the tethered aerostat in place. From this position, the anchor can release a length of the coiled second length of tether, thus allowing the tethered aerostat to increase its elevation due to the increase in the length of the first length of tether. In this condition, the second length of tether may also be connected to a ground station, capable of providing electrical power and a data connection. This can allow the aerostat payloads and operating equipment to be fully tested before deployment.

[0029] The tether may therefore include fibre optic cables for data transmission to the aerostat and its payloads, power cables for the transmission of power, and lightning conductors to allow for protection against lightning strikes. One or more of the power cables may serve as lightning conductors.

[0030] The first substantially ground level location may be situated close to a manufacturing location where the aerostat is fabricated and inflated e.g. in such a manner that the skin does not require folding. The aerostat can then be readily transported by a ground vehicle, fully inflated and tested from the manufacturing location to a local launch site, and launched up to a modest elevation in moderate weather typically when windspeeds are less than 15 knots, preferably less than 10 knots. Such weather conditions allow aerostat launches to be carried out with a minimal chance of damage to the aerostat.

[0031] As used herein, the term “unfoldable” means that the aerostat cannot be folded without damaging the structure of the aerostat or its skin.

[0032] Preferably the aerostat, in its first anchoring condition, has an elevated location at an altitude of from 100m to 10,000m, more preferably from 200m to 5,000m, most preferably from 250m to 3000m. For example, it may have an altitude of from 80 to 200m when the aerostat is in a parked state, and may have an elevation of from 100 to 10,000m when in an active state, e.g. providing telecommunications services to a region. In the first anchoring condition, the elevation will be essentially provided by the first length of tether. When in its second relocating condition, the first length of tether will be much shorter, e.g from 20 to 150m.

[0033] For example, when the aerostat has been fully tested after launch and the aerostat is required for operation, a further portion of the first length of tether may be winched in, the aerostat, with a first length of tether of typically at least 50m and preferably over 100 m and the anchor are then transported to the second substantially ground level location e.g. to an aerostat park or the aerostat operational site by a relocating aircraft.

[0034] An “aerostat park” is an intermediate location between the manufacturing site and the aerostat operational site where aerostats can be flown an at intermediate altitude but safely away from ground turbulence. Such a site can allow aerostats to be more readily available to far-flung operational sites located at considerable distances from the aerostat manufacturing site. Intermediate “aerostat parks” can be positioned to allow aerostat movement distances to be manageable and offering the possibility of having helicopter or fixing wing fuelling arrangements. Such intermediate parks can be used to set up efficient logistics operations. Thus, preferably the first and / or second or subsequent substantially ground level location is a tethered aerostat park before deployment, capable of storing a plurality of tethered aerostats according to the present invention.

[0035] After arriving at a substantially ground level location for operational use, the second length of tether may be connected to power and data feeds and any lightning conductor may be connected to ground and the aerostat is elevated to an operational or parking elevation.

[0036] When it is desired to relocate the tethered aerostat, a relocating aircraft can be attached to the anchor, lifting the anchor from the ground, so that the tethered aerostat adopts its second relocating condition, and can be relocated to a second ground level location.

[0037] The anchor operates primarily by means of its weight, so that it can operate to anchor the location of the tethered aerostat. Accordingly, the weight of the anchor (including any second length of coiled tether) exceeds the vertical component of tension in the tether. Preferably, the weight exceeds the vertical component of tension by at least 50%, more preferably by at least 100%, e.g. from 100 to 300%.

[0038] However, preferably the anchor is additionally secured to the first ground level location by an additional mechanical securing means, such as being releasably within a frame bolted to the ground. That ensures that the weight of the anchor is kept within reasonable bounds and does not approach the considerable weights that would otherwise be needed to keep the anchor on the ground at high wind speeds. At the operating sites the aerostat may encounter very high winds and the first length of tether experiences forces of at least 10 tonnes, and possibly over 20 or even 30 tonnes.

[0039] As will be discussed, the anchor typically controls the distribution of the length of the tether between the first and second length of tether. Preferably the ratio of the length of the second length of tether to the first length of tether, when being relocated or tested, is at least 10: 1, preferably at least 50:1, more preferably at least 200: 1. This allows for the aerostat to have a relatively low altitude when it is being tested, being transported or in an aerostat park, and a relatively high altitude when in service at an operational ground level location. Preferably, the anchor is attached to the relocating aircraft by a relocating tether. This may be attachable to the anchor in a variety of ways, such as by a simple hooking arrangement, and preferably electronically controllable from the aircraft for quick release in case of emergency. The relocating aircraft is therefore attached to the anchor by means of a relocating tether, lifting the anchor from the ground, so that the tethered aerostat adopts its second relocating condition, and can be relocated to a second ground level location which may be an aerostat park or an aerostat operational site.

[0040] The anchor can further include a winch to unreel or reel in the tether and aerostat at its deployment location. It has been discovered that it is desirable that the drag coefficient of the anchor typically comprising the winch, spooled tether and supporting arrangements should be low, preferably below 0.3 to ensure aerodynamic drag is minimised. This can be achieved by the design of open structures for the supporting arrangements, such as an open cage and / or by suitable streamlining or shrouding.

[0041] In this application, the drag coefficient of a tethered aerostat is defined as CD = F / (’ / 2u2A) , where F is the horizontal aerodynamic drag, p is the air density , u is the horizontal component of wind velocity and - as is conventional in aerofoil theory, well known to those skilled in the art, A is the plan area when seen looking at the aerostat horizontally normally to the wind. The leading edge at any vertical level, is defined by the point which first meets oncoming air, and the trailing edge at any level, the point which last meets oncoming air.

[0042] The drag coefficient of the anchor is similarly defined, with all variables defined as in the paragraph above. So, for example, if the anchor consisted of a solid cuboid of aligned to the wind with long edges L parallel to the wind and short edges 1 at right angles to the wind, then A would be equal to 1 x L.

[0043] The aerostat may be of a wide variety of designs, as the present invention can apply equally irrespective of aerostat shape or arrangement. One possible design are so-called vertical buoyant aerostats, which have an elongate body having a length extending between an upper end and a lower end in use. Examples of such a vertical aerostat is given in PCT / GB2022 / 053290. The elongate or tall body may be of any desirable length according to needs of the aerostat, that can be suitably manufactured. Preferably the distance between the upper end and the lower end is from 5 to 500m.

[0044] A substantive range of wind speeds, over which the tethered aerostat can remain approximately vertically oriented includes from 0 m / s to max operating wind strength typically between 40 m / s and 100 m / s, preferably as much as 120 m / s.

[0045] A low drag aerostat is highly preferred for such an aerostat to operate in high winds without an excessively strong and therefore heavy tether. If the aerostat has a higher drag then the aerostat needs to be larger to carry the weight of the heavy tether and pay load at high winds. A larger aerostat is more costly and less economic. It has been discovered that for practical utility in being able to carry substantive payloads in high winds of more than 30 metres per second, a drag coefficient of less than 0.35, preferably less than 0.2, or even less than 0.06.

[0046] For economic operation and to allow operation in adverse winds, the aerostat must be able to be ‘flown stably’ at speeds of at least 40 knots, preferably 60 knots and most preferably above 100 knots. The definition of ‘flown stably’ being a flight trajectory that does not cause significant oscillations of the aerostat or transverse movement of the relocating aircraft to cause a safety hazard to the aircraft flight.

[0047] It has been found that to allow such operation without requiring excessive helicopter or aircraft power requires a low drag coefficient of the aerostat and stability in high wind speeds. For example, if an aerostat of an approximately spherical shape is used, at even moderate airspeeds a tethered aerostat of this shape will exhibit a relatively high drag coefficient of >0.2 and oscillate from side to side causing flight stability problems for the towing aircraft. An aerostat of this shape having a volume of 2000 m3and a diameter of approximately 16m will have a drag at 60 knots of approximately 23 kN and require 1.1 MW of engine power at approximately 60% engine efficiency. If a low drag aerostat 50m length in the direction of the wind, with a hemispherical nose and a maximum diameter of 8 m with a suitable weight distribution to provide aerodynamic stability and having a drag coefficient of 0.26 is towed at 60 knots, the drag will be approximately 5 kN and require 250 kW of engine power at approximately 60% engine efficiency.

[0048] The lighter-than-air gas may be helium or hydrogen, or a mixture of these gases and other gases. In the prior art, lighter-than-air aerostats inevitably suffer leakage of the buoyant gas and typically need to be refilled every 6 weeks or so, or even more frequently. It has been discovered that an effective means of achieving improved containment of such gases is by the addition of one or more layers of lightweight metal foil (e.g. having a thickness of from 6 to 50 microns), such as, but not limited to, aluminium. That protection if applied externally also provides UV protection for the materials used in the elongate body, as known to those skilled in the art.

[0049] The aerostats according to the present invention are particularly suitable for providing telecommunications services to a wide area. Thus, preferably the aerostat comprises an antenna, preferably below the aerostat. This is because such antennas usually have a substantial mass, and therefore a low position contributes to the lowering of the centre of mass, which provides increased stability. A preferred design of antenna is a phased array antenna. Such antennas may be free to rotate relative to the aerostat to allow vertical orientation to be held independently of that of the aerostat (e.g. to within 10°).

[0050] Preferably the antenna is located below the lowest point of substantive portions of the aerostat envelope that has a metal surface. The antenna is preferably free to rotate relative to the aerostat to allow vertical orientation to be held independently of that of the aerostat.

[0051] The relocating aircraft may be a rotary-wing aircraft or a fixed-wing aircraft. When the relocating aircraft is a rotary-wing aircraft (i.e. a helicopter), it is vitally important that the aerostat tether cannot impact on any of the helicopter rotor systems. Such a helicopter can include a spar attached to the relocating tether linking the anchor to the helicopter, which extends significantly outside the radius of the main rotor (when viewed from above the helicopter) or to ensure that the anchor has a weight greater than the buoyancy of the aerostat with the first length of tether being significantly shorter than the length of the relocating tether attached to the helicopter.

[0052] In many countries there is a limit to the maximum length of relocating tether that an underslung load can be transported by helicopter - often around 150m. Preferably the relocating tether is longer than the first length of tether, preferably such that the relocating aircraft is greater than 20m above the upper end of the aerostat, preferably at least 40m, more preferably at least 80m. This ensures that ‘downwash’ from the helicopter does not significantly affect the flight stability of the aerostat.

[0053] By such means the aerostat always flies significantly below the helicopter. Such arrangement is preferred since it allows the helicopter pilot to jettison the aerostat if flight problems arise by severing the connection of the relocating tether with the anchor. In such an event, since the weight of the anchor is greater than the aerostat buoyancy, the aerostat will descend from the helicopter, which relieved of its load, will rise increasing the aerostat - helicopter separation, and allowing the helicopter to disengage from flight problems safely.

[0054] In a similar manner a fixed wing aircraft can tow the anchor coupled allowing the anchor to be jettisoned if emergencies arise with a descending aerostat and a rising aircraft resulting on aerostat release.

[0055] It is also desirable that aerostats are relocated from one substantially ground level location to another in relatively benign wind conditions with wind gusts not exceeding 40 knots, preferably below 30 knots and most preferably below lOknots. Such benign conditions reduce the risk of significant impact of aerostat surfaces on launch and recovery systems. Typically, the coiled second length of tether provides sufficient length to operate the aerostat when deployed at height thereby allowing the tethered aerostat to be tested and launched e.g. at a central base under benign conditions, kept either at the central base or transported by air utilising one of the methods described previously to a holding field before finally being transported to its deployment location.

[0056] This mode of operation has very significant advantages: after testing the aerostat coupled to its payload at ground level, with its tether attached and on a spool with its winch, the aerostat can be launched and maintained at a modest altitude typically of around 80 to 200m. At that altitude it is removed from the worst of ground turbulence and with a suitable low drag aerostat can be positioned there for months or even years at a time, even in the most severe weather, awaiting a time when it is required for deployment. At that time, it can be winched down to a suitable altitude - typically 100m - and a helicopter or fixed wing aircraft can be attached by a relocating tether to the anchor, which now includes the coiled second length of tether, the winch and supporting structure.

[0057] The tethered aerostat can then be transported by air to its operational location at a second substantially ground level location, where the anchor is lowered to a suitable base, attached and then a quick high integrity attachment can rapidly be made with minimal site work. Then the aerostat tether can be paid out by the winch until the aerostat is at its desired operating altitude and payload operation can begin. These operations, not requiring an aerostat launch and recovery from ground level, as reported previously, can be undertaken in more severe weather conditions at wind gusts of preferably less than 40 knots, preferably less than 30 knots allowing aerostat replacement under a much greater window of opportunity.

[0058] For long distance transport it is possible to have groups of aerostats connected in line by helicopter or more usually to fixed wing aircraft. A preferred method of flying is to have the aerostats each connected to their own anchor, connected to each other in line astern to the fixed wing aircraft. The anchors are lined up at the downwind end of a runway, the fixed wing aircraft is attached to the leading anchor, and the anchors are on sleds, minimising drag resistance with the ground during aircraft take-off. For landing, the aircraft operates at low speed over an aerostat park or airfield and releases the anchors at modest altitude typically 200 to 300m and the anchors land at a modest velocity onto an inflated jacket or with a drogue if necessary. From such a park, the aerostats can then be deployed by helicopter or taken to ground and fitted with an antenna or refurbished.

[0059] Tethered aerostats and their tethers and their anchors may have illumination lights and radio transponders to alert and warn other airspace users of their presence.

[0060] The invention will now be illustrated, by way of example with reference to the following figures.

[0061] Figure 1 is a side view of a tethered aerostat according to the present invention in its second relocating condition where the aircraft is a rotary wing aircraft, with a close-up perspective view of the anchor.

[0062] Figure 2 is a schematic representation of a tethered aerostat according to the present invention in place at a second substantially ground level location, shortly after the relocating aircraft has detached from the anchor, with a perspective of the transporting helicopter and its released attachment to the anchor.

[0063] Figure 3 is a perspective view of a tethered aerostat park comprising three tethered aerostats according to the present invention.

[0064] Figure 4 is a side view of three tethered aerostats according to the present invention in their second relocating condition, where the aircraft is a fixed-wing aircraft.

[0065] Figure 5 is a side view of three tethered aerostats according to the present invention shown in figure 4, in the initial process of transitioning (take - off) from their first anchoring condition to their second relocating condition, where the aircraft is a fixed-wing aircraft. Turning to the figures, figure 1 shows a tethered aerostat 101 being of low drag and made from an unfoldable skin, having a payload 102 and lifting surfaces 103 in its second relocating condition just after take-off or just before landing on a substantially ground level location 111.

[0066] The lifting surfaces 103 allow additional lift to be provided to the aerostat 101 both when the aerostat is deployed minimising change of angle of tether 104 with wind strength and allow additional lift reducing aircraft power requirements in high-speed transit. A first length of tether 104 containing one or more strength elements, fibre optic cable, power supply cables, and lightning current cables connects the aerostat system (aerostat, payload, and lifting surfaces) to an anchor 105 which includes a winch 108, a winch support structure 109, and a tether store 110 in which a second length of coiled tether is stored.

[0067] The anchor is also attached to a helicopter 107, which may also be a fixed wing aircraft, acting as the relocating aircraft, by a relocating tether 106. This tether 106 is under the control of the aircraft pilot and the anchor can be released from the relocating tether 106 by an electrically operated release (not shown) connecting the relocating tether 106 to the anchor 105.

[0068] The anchor 105 comprises an open cage winch support structure 109 made from metal or fibre reinforced plastic struts, in a generally geodesic arrangement. This allows air to pass through the anchor, reducing its drag coefficient. The anchor comprises a winch 108 near the top, which controls the length of tether that is coiled within the anchor.

[0069] Figure 2 shows this same tethered aerostat 201, in place at a second substantially ground level location 204, shortly after the relocating aircraft has detached from the anchor 203. The aerostat system 201 is attached by a first length of tether 202 to the anchor 203 which is bolted or otherwise securely attached to a plinth or base 204 which has foundations sufficient to withstand significant tether forces. The anchor is connected to a lightning grounding system 205 and a tether extension 206 is connects the coiled second length of tether contained within the anchor 203 to a high integrity quick coupling system 207 which allows the tether connections (typically fibre optic cables , electric power cables etc) to be connected securely and quickly to separate or combined cables 208 and 209 that lead to a local ground station 210 which is in turn connected to a power source and internet connections (not shown).

[0070] The system is shown just after helicopter 212 delivery or just before helicopter removal with the aerostat at a modest altitude of from 50 to 200m. The relocating tether 211 is detached from the anchor 203. When it is desired to move the aerostat 201 again, e.g. in moderate or low winds, the relocating tether 211 would be guided by ground crew and attached to the anchor 203 after the coupling system 207 was disconnected and after the anchor attachment to the plinth 204 was decoupled.

[0071] Figure 3 shows a “field” 300, acting as a tethered aerostat park, in which a number of tethered aerostats 301, 302 are positioned at a moderate altitude (e.g. 50 to 200m) prior to recovery or deployment. The tethered aerostat 301 is being tested prior to deployment with fibre optic cable and power cable 305 links to the tether extension 303 via a quick release secure connector 304. Such an arrangement can also carry out a complete payload and aerostat system check before helicopter removal.

[0072] Figure 4 shows flight transport of a number of tethered aerostats 401 by a fixed wing aircraft 407 acting as the relocating aircraft. The aircraft 407 is attached by relocating tether 406 to a plurality of anchors 403 attached to one another in line astern by joining tethers 405

[0073] Figure 5 shows the launch of such a system with detachable trolleys 505, 506 or skids minimising roll resistance for take-off. In the illustration anchor 507 has been lifted off the runway and its associated trolley 506 has detached. Such systems allow long distance transport economically from airfield to airfield. On arrival at the receiving airfield the aircraft is detached from the anchor at a low altitude and they fall to the ground for recovery e.g. retarded by parachute.

Claims

Claims1. A tethered aerostat positioned at an elevated location in the atmosphere, the aerostat comprising sufficient lighter-than-air gas such that the aerostat is substantially buoyant in air, the tether comprising a first length of tether, connecting the aerostat to an anchor located at and resting on the surface of a first substantially ground level location, and wherein the anchor has a weight such that, in a first anchoring condition, the anchor remains in place on the surface of the first substantially ground level location, but also such that it can adopt a second relocating condition, wherein the anchor is also attached to a relocating aircraft, the anchor being lifted above the surface of the first substantially ground level location, and can thereby be relocated to a second substantially ground level location by the relocating aircraft.

2. A tethered aerostat according to claim 1, wherein the tether further comprises a second length of tether, joined to the first length of tether and in coiled form and secured by the anchor.

3. A tethered aerostat according claim 2, wherein the ratio of the length of the second length of tether to the first length of tether is at least 10:1, preferably at least 50:1, more preferably at least 200: 1.

4. A tethered aerostat according to claim lor claim 2, wherein the anchor is in its first anchoring condition.

5. A tethered aerostat according to claims 2 and 3, wherein the second length of tether is connected to a ground station, providing electrical power and data connection.

6. A tethered aerostat according to claim 1 or claim 2, wherein the anchor is in its second relocating condition, wherein the anchor is also attached to a relocatingaircraft, the anchor being lifted above the surface of the first substantially ground level location.

7. A tethered aerostat according to claim 3, wherein the elevated location in the atmosphere is from 80 to 200m.

8. A tethered aerostat according to claim 1 or 2, wherein the elevated location in the atmosphere is from 100m to 10,000m9. A tethered aerostat according to claim 5, wherein the first length of tether is from 20 to 150m in length.

10. A tethered aerostat according to any one of the preceding claims, wherein the aerostat is made from unfoldable material.

11. A tethered aerostat according to any one of the preceding claims, wherein the anchor has a weight that exceeds the vertical component of the tension in the first length of tether.

12. A tethered aerostat according to any one of the preceding claims, wherein the anchor is secured to the first substantially ground level condition by a securing means.

13. A tethered aerostat according to any one of the preceding claims, wherein the drag coefficient as defined by the ratio of the aerodynamic drag compared to the dynamic pressure over the aerostat plan area when viewed horizontally normally to the wind direction is less than 0.35, preferably less than 0.2, most preferably less than 0.06.

14. A tethered aerostat according to any one of the preceding claims, wherein the anchor comprises a winch, operable to alter the length of the first length of tether and also of the length of the second length of tether if present.

15. A tethered aerostat according to any one of the preceding claims, wherein the anchor has a drag coefficient of less than 0.3.

16. A tethered aerostat according to any one of the preceding claims, wherein the anchor comprises an open cage, within which the second length of tether is coiled if present.

17. A tethered aerostat according to any one of the preceding claims, wherein the aerostat skin is constructed from materials that are unfoldable or cannot be reduced in size when the lighter-than-air gas is removed therefrom without a significant performance impairment.

18. A tethered aerostat according to any one of the preceding claims, which comprises lifting surfaces, to provide lift from the flow of air past the aerostat.

19. A tethered aerostat according to any one of the preceding claims, wherein the aircraft is a rotary wing aircraft.

20. A tethered aerostat according to any one of the preceding claims, wherein the anchor is attached to the relocating aircraft by a relocating tether.

21. A tethered aerostat according to claim 20, wherein the relocating tether is detachable from the anchor and is controllable from the aircraft.

22. A tethered aerostat according to claim 20 or claim 21, wherein the relocating tether is longer than the first length of tether, preferably such that the relocating aircraft is greater than 20m above the upper end of the aerostat, preferably at least 40m, more preferably at least 80m.

23. A tethered aerostat according to any one of the preceding claims, which comprises an antenna, preferably a phased array antenna.

24. A method of transporting at least one aerostat according to any one of the preceding claims, from the first substantially ground level location to the second substantiallyground level location, the method involving attaching a relocating aircraft to the anchor in its first anchoring condition, the relocating aircraft lifting the anchor from the surface of the first substantially ground level location, transporting the anchor, and thereby also the tethered aerostat, to a second substantially ground level location and depositing the anchor on the surface thereof, followed by disconnecting the relocating aircraft from the anchor.

25. A method according to claim 24, wherein the first and / or second substantially ground level location is a tethered aerostat park, capable of storing a plurality of tethered aerostats according to any one of claims 1 to 23.

26. A method according to claim 24 or claim 25, wherein the relocating aircraft is attached to a plurality of anchors, so as to relocate a plurality of tethered aerostats simultaneously.

27. A method according to claim 26 where the anchors of a plurality of tethered aerostats are mounted on detachable skids.

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