Transmediterranean balloon flights: Lessons learned and perspectives

Available online at www.sciencedirect.com

Advances in Space Research 45 (2010) 498–506 www.elsevier.com/locate/asr

Transmediterranean balloon flights: Lessons learned and perspectives F. Caballero a,*, D. Spoto b, R. Ibba c, L. del Barrio d, F. Amaro b, M.D. Sabau a, A. Cardillo e, I. Musso e a

Instituto Nacional de Te´cnica Aeroespacial, Torrejo´n de Ardoz, Madrid 28850, Spain b Base di Lancio ASI Trapani-Milo SS 51, Trapani, Sicilia 91100, Italy c Agencia Spaziale Italiana, viale Liegi, 26 Roma 00198, Italy d Centro de Experimentacio´n de El Arenosillo INTA, Mazago´n, Huelva 21130, Spain e Istituto di Scienza e Tecnologie dell´Informazione CNR via Moruzzi, 1, Pisa 56124, Italy Received 24 October 2008; received in revised form 29 October 2009; accepted 10 November 2009

Abstract Transmediterranean balloon flights are made with zero-pressure stratospheric vehicles, that contain scientific experiments on board. The balloons are released during summer in Sicily, Italy. Eastern stratospheric seasonal winds transport them over the Mediterranean and the flight termination by telecommand and recovery is accomplished in Spain. The paper addresses a thorough description of the program, which was initially conceived to extend the flight time over the existing national options (Air-sur-l’Adour, Gap Taillard, Trapani, Leo´n), considers several flight support subsystems and gives hints for their future improvement, though the main focus is on the balloon operations. Ó 2009 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Zero-pressure stratospheric balloons, Scientific balloons operation

1. Introduction Transmediterranean flight activities were initiated by the Centre National d´E´tudes Spatiales (CNES France) in cooperation with the Comisio´n Nacional de Investigacio´n del Espacio (CONIE Spain) and the Consiglio Nazionale delle Ricerche (CNR Italy) in 1977 and continued since 1993, by similar Italian and Spanish institutions (ASI and INTA). Seventy successful transmediterranean crossings were made between 1977 and 2002. The balloons used for the flights, covered a wide range of volumes, from 330,000 to 1000,000 m3. They were fabricated by Winzen and Zodiac companies and all were flown from Trapani Launching Base in Sicily. A summary of these flights is provided in Fig. 1 (a flight duration histogram with sample size = 28) and Fig. 2 (a *

Corresponding author. E-mail address: [email protected] (F. Caballero).

multiple flight of recovered payload histogram with sample size = 17 and coverage of 49 different real operations). The mean flight duration of 45 flights between 1977 and 1992 (Sadourny, 1994) was 20 h 20 m and the longest flight was 27 h 05 m (Nausicaa flight 1979) (see Table 2). Other campaign aspects are presented in Table 1 (the annual number of flights during the period considered) and Table 2 (a summary of experiments conducted on 28 balloon flights, classified by different scientific disciplines). Table 2 also includes experiment objectives and details of payloads mentioned in the text. Fig. 3 shows the launch vehicle and spool truck together with the flight chain, carrying the gondola during the countdown for flight Birba 2001 (see Table 2). The subject of Fig. 4 is the payload preparation for flight Baby 2001 (see also Table 2). This work is organized as follows: Section 2 deals with the impact of additional vertical soundings realized from ground and space. Section 3 discusses the cartography improvements and connected air safety issues over European skies. Section 4 consists of the ways to exercise con-

0273-1177/$36.00 Ó 2009 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2009.11.003

F. Caballero et al. / Advances in Space Research 45 (2010) 498–506

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Fig. 1. Flight duration histogram.

that caused in 2004–2006 the cancellations of already scheduled campaigns. 2. Improvement of the vertical wind profiles by ground and space sounding

Fig. 2. Multiple flight of recovered payload histogram.

trol over the trajectory in latitude and longitude. Section 5 deals, among others, with the telemetry/telecommand equipment, precursor of future polar flights. Section 6 gives some hints to improve the power subsystem. Section 7 presents the means to have better precision in payload descent, using as guidance input the EGNOS (European Geostationary Navigation Overlay System) receivers. EGNOS itself is a satellite-based augmentation for GPS (Global Positioning System). Section 8 explains parachute recovery details. Finally, some concluding remarks for the future contained in Section 9, to try to avoid budget problems

Numerical weather prediction (NWP) deals with the numerical solutions of the governing equations of atmospheric processes. A square 1° longitude by 1° latitude is used inside the models to build up the tentative grid (Floury and Fuchs, 2001; Poiares Baptista and Leibrandt, 2001). The data we adopt come from the National Centre for Environmental Prediction (NCEP), the European Centre for Medium-range Weather Forecast (ECMWF) and the Fifth Generation Mesoscale Model (MM5) models. The first two cover a large area, high altitudes but with a low resolution in time and space (between one and a half degree and between 3 and 6 h). MM5 increases the resolution of the above models evaluating an accurate ground digital elevation model. The MM5 model we applied has higher resolutions (9 km and 1 h) but with a reduced coverage and lower maximum altitude (30 km). Previous soundings, releasing balloons up to 40 km, are a daily routine at Trapani and it could be increased when needed at El Arenosillo (normally they perform soundings three times a week). Regular Weather Service radiosounding stations, located in Sicily, Sardinia (Italy) and Palma de

Table 1 The number of Transmediterranean flights for each year from 1977 to 2003. 1977 2

1978 4

1979 3

1980 3

1981 5

1982 5

1983 4

1984 5

1985 5

1986 4

1987 5

1989 4

1990 6

1991 2

1992 1

1993 1

1995 2

1997 2

1998 2L

1999 3

2000 1

2001 1 + 1L

2002 2 + 2L

2003 1L

L designates flights that, due to campaign circumstances, performed only a local trajectory.

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Table 2 Experiments flown in Transmediterranean campaigns by scientific disciplines L indicates flights shifted during campaign to local. Flight

Institution

Scientific discipline

Experiment objectives

Poker Cesar Cleopatra Phoswich

IAS Frascati (I) CESR Toulouse, CEN Saclay/GRBS (F) ITESRE Bologna (I) IAS Frascati (I) IAF Milan, IAS Frascati (I), Uni Southampton (UK) ITESRE Bologna, IAS Frascati (I), CESR(F) IROE Florence (I)/Italian Grp. Cosmic Physics Palermo (I) Cosmic Rays Sci. Grp.CONIE (E)/ GRBS (F)/Atmos. Sci.Grp.(E) GRBS, Globe Physics Institute(F)/ Atmos.Sci.Grp. CONIE,Uni Auto. Barcelona(E) ICG/CNR Torino (I) Cosmic Rays Research Institute Tokyo (JP) IFCAI Palermo, IASF Bologna CNR (I)

X Astronomy X Astronomy/ biology X Astronomy

Hard X-ray radiation study 20–200 KeV range Study Crab, Cygnus, Hercules X-rays Drosophila,artemia,tobacco exposition to cosmic rays X-ray astronomy range 20–200 KeV

X Astronomy X Astronomy

X-ray astronomical study Hard X-ray radiation study in the range 10–300 KeV

X Astronomy

Observation of galactic/extragalactic sources in the range 20–300 KeV, specially Pulsar X

Cosmic rays/X Astronomy Cosmic rays/ biology Biology/ atmosphere/ cosmic rays Cosmic Rays

Spectral measurement of the temperature of cosmic radition/study Xrays sources: Crab, Cygnus Sensitive emulsion to measure cosmic origin particles/exposure of tobacco, grains to cosmic rays/ozone measurements Behavior living entities exposed to cosmic radiation/great wavelength anomalies in the Earth magnetic field/ozone, UV and T measurements/ heavy ions study w. plastic detectors Strange Quark Matter

Cosmic rays/X Astronomy

UV nocturnal atmospheric background for further Extreme Energy Cosmic Rays observation/spectroscopic imaging and polarimetry hard X/soft gamma rays Medium energy study 150 KeV–6 MeV Radiation study in the range 50–300 KeV

Enea Pallas Lapex

PAF Clytonee Alios

SQM

Baby Cactus Figaro Elena

IFCAI, Uni Palermo, IAS Frascati (I)/CEA, CESR (F) ITESRE Bologna (I)

Flight

Institution

Aglae

CESR, CEN, IRS Meudon, LPSP Verrie`res (F) IROE Florence (I) CESR, CEA, IAP, Obs. Meudon (F) Physics Institute Rome, IROE Florence (I)

Ulysse Tifani Argo

Gamma astronomy Gamma astronomy Scientific discipline IR astronomy IR astronomy IR astronomy IR astronomy

Epeos

ITESRE Bologna (I)

IR astronomy

Arome

CESR Toulouse LPSP, Obs. Meudon, IAP(F) Uni La Sapienza, CAISMI Florence, IROE, IAS (I) Aeron. Serv. CNRS (F) Atmos Sci.Grp. (E)/GRBS (F) Atmospheric Optics Lab Lille/Pronaos team, Balloons Div. CNES (F) Ebro Obs., Atmos. Sound.Sta.(E), Uni Washington (USA) IROE Florence (I)

IR astronomy

Mini-Tir Nausicaa Themis Iliada Safire B Birba

Trans med (L) Boome Rang (L) HASI (L)

IR astronomy

Experiment objectives Study of the galaxy disk in the infrared Infrared isotropometry Spectrophotometry of diffuse galactic background Cosmic background polarization/ Galactic plane observation and sky background anisotropy study in several bands Cosmic background measurements local/extragalactic 2–5 microns band Aromatic policyclical molecules near IR emission on the Galaxy plane

Uni La Sapienza, Uni Naples, Uni Naples, INFN Torino (I) RMC Ontario (CA) ASI

Life sciences

Technology

Galactic plane study/Background anisotropy in the wavelengths 400, 1200 and 2200 microns Ozone content/Behaviour unicellular organisms, grains, drosophila, artemia frogs under cosmic rays Radiative balance at balloon altitude/thermal stratospheric balance/ ascent behavior monitoring Terrestrial electric field components determination/lightning detection device Vertical detection of atmospheric minor constituents/validation MIPAS instrument on board Envisat Effects cosmic radiation on gene expression in human cells/cosmic radiatiom biodosimetry/correlation ozone distribution-neutronic flux intensity/TEPC rad. measurements S band TM/TC system qualification

ASI

Technology

Flight extension technique

Uni Padova CISAS (I)

Planetary science

Huyghens atmosphere structure instrument

Atmosphere/life sciences Atmosphere Atmosphere Atmosphere

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Fig. 3. Flight chain/launching vehicles Birba 2001 (see Table 2).

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more detailed technical tracking, at the beginning of TM acquisition by Palma station 1:50.000 is employed. The usual flight zone for the balloons over Southern Spain is contained in a single CD-ROM. Population density data are published by the Spanish National Statistics Institute encompassing a set of 9000 villages. The aeronautical information data that include drawings of air corridors and Terminal Manoeuvring Areas, incorporated to a digital layer, should be a part of an updated Geographical Information System. Air Cartographic Service in Spain superimpose these data in the 1:1000,000 and 1:25.000 maps handled by helicopter recovery crews. Several Transmediterranean flight trajectories, compiled by the Italian Trapani Launching Base for 1977 and 1978 campaigns, with 2 and 4 flights respectively are presented in Fig. 5, with a superimposed longitude–latitude grid also including the tracking stations coordinates. Fig. 6 compiled by ISTI Pisa, show the same features for the late flights 1999–2002 (8 flights). The trajectories show a sinusoidal pattern. Wind speed zonal component rises from very low values at the beginning of June to maximum absolute value in the beginning of August, then decreases. Wind speed meridional component does not show a clear uniformity like the zonal component, in fact there is a periodic oscillatory motion in the meridional direction composed with the rectilinear motion in the zonal direction. 4. Trajectory control: exercising it on latitude/altitude 4.1. Latitude control

Fig. 4. Payload Baby 2001 (see Table 2).

Mallorca (Spain), can furnish additional wind profiles supplementing stratospheric balloons campaigns. Short-range wind vector predictions will be soon possible. Direct wind measurement from space will be performed in 2011 (up to 30 km profiles) by ESA Atmospheric Dynamics Mission (ADM) Aeolus equipped with Doppler Wind Lidar (www.esa.int/esapub/br/br236/ br236/pdf), Processing Level 1 products (wind profile data direct from satellite) will be available, within 3 h from observation. NASA 3D Winds, also equipped with Doppler Lidar to measure winds, is one of the 15 missions proposed in the U.S. Earth science decadal survey.

3. Cartography in balloon operations: from paper to layered digital maps For balloon tracking, two scales now in digital cartography are recommended. For general picture and coarse technical track, when the balloon is at several hundreds of km from the relevant tracking station, 1:1000,000 is used. For

A risk element exists: if the trajectory goes down the 37° parallel, due to a North component, the landmass decreases with latitude. For sea recovery, an active flotation system deploys itself, triggering the inflation of a set of balloons, 570 l each, in less than 3 s. With the present TM configuration with 3 ground stations (Trapani 38°010 N–12°380 E, Palma 39°360 N–02°420 E, El Arenosillo 37°060 N–06°440 W), beyond a certain difference in latitude, whether higher or lower, problems of telemetry reception and eventually telecommand radioelectrical limit, could arise. Due to a South component over Spain, an incident occurred during Cesar-Cleopatra flight in 1978 and was successfully solved sending a telecommand in the uprange direction, more than 400 km from the flight tracking station showing the robustness of our systems. Another resource is to send the cutoff telecommand from Palma station, realized sometimes in the 80’s. El Arenosillo Center with long-range radar tracking capability, detected Transmediterranean balloons at 450 km, over Valencia region, in Spain (Cardillo et al., 2001; Musso et al., 2005). A possible device for latitude control, implements a wing and a rudder with its actuator, both attached to a tethered boom below the gondola. Near Real Time (NRT) wind profile, data that ADM Aeolus will deliver, permits to calculate the difference in wind speed at different

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Fig. 5. The balloon trajectories of Transmediterranean flights in 1977–1978.

Fig. 6. The balloon trajectories of Transmediterranean flights in 1999–2002.

altitudes. This difference inducts a horizontal force on the balloon, which is possible to apply in different magnitudes and directions, through control of the wing´s angle of attack. A further possibility is to place control vanes or control sails, at the balloon apex. Better control changes, imply small increments of velocity over a long time interval. 4.2. Altitude control The purpose is to cross the stratosphere with controlled velocity, allowing stops at some levels. The balloon system dynamics depends on the convection processes originated by the helium-skin and air-skin interactions and also on

the thermal balance driven by radiation of the balloon envelope. To exercise the control, valve actuations and ballast releases are used. In a programmed descent hypothesis, an efficiently commanded valve is better than other proven techniques like inserting a sleeve laterally to the balloon, a passive method depending of weather conditions or to fully open the valve on the balloon envelope, using high quantities of ballast to neutralize the opening effect. A zero-pressure stratospheric balloon becomes fully inflated at cruise altitude. One snapshot of the inflation process for Birba 2000 flight, with the balloon still in the launching pad, is illustrated in Fig. 7. Radiation controlled balloons (RACOONS) use the IR flux, coming up from the Earth surface, for altitude conser-

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Fig. 7. Inflation process Birba 2000 (see Table 2).

vation during the day/night pass. Another proposed implementation included a pressure vessel on board the gondola with a reasonable amount of the balloon filling gas, to replenish the gas leakages produced by cooling or envelope permeability. Payload flight at different altitudes is possible, with a winch below the balloon from which the gondola is tethered, through a rope or sling. On CNES initiative, the quoted winch, not strictly an altitude control system, was flight tested in Transmediterranean balloon Nausicaa 1985. It consist of a double flight chain whose elements are, in the sense from the balloon hook down to the gondola: winch flight termination device, winch parachute, winch box including TM/TC module and beacon, gondola flight termination device, main parachute, gondola equipped with TM/TC module, Omega receiver and beacon, ballast reservoir with equilibrium and piloting ballast allocations. At balloon release, the sling was rolled on a cylinder inside the winch box. After unblocking the cylinder in flight, through a TC order and sending a further TC order to command sling unrolling and descent, the test gondola (800 kg), with its flight chain hanged at a 700 m distance, below the balloon and winch box. Cut-off took place in two stages. First, the gondola cut-off drove to the main parachute opening and second, the winch cutoff from the balloon, drove to the winch parachute opening and descent of the winch box, its own beacon and the previously deployed sling to be recovered in this same condition. Operation of the device by CNES although performed flawlessly in 1985, did not follow in later years. 5. Telemetry and telecommand subsystem: standard and innovative features An UHF band TM/TC subsystem was employed since late Seventies for Transmediterranean flight operations. The characteristics of the S band TM/TC system (ASI concept 1993) included: S band links for TM and TC, full digital transmission, and experiment rate up to 1 Mbit/s. All experiments and housekeeping were supported by a sin-

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gle TM carrier. The TM was based in an ESA packet design, being the TM data transparent (the experimenter sees on ground the same data obtained on board). The TC was an ESA packet design, with full redundancy. A GPS is incorporated on board, for experiment time tagging. Monopulse tracking technique, to gather angle information with a single pulse, is used as a backup by ground antenna. There was a high link margin, up to limit visibility range. International Consultative Committee on Radiocommunications (ICCR) power flux limits were fulfilled (Spoto et al., 1995). Mini Telemetry on UHF band (further ASI concept 2000) was flight qualified and remains in use. Its corresponding ground station has a central unit with CPU, NALCO (Nacelle Alle´gue´e Controˆle´e/Controlled Lightened Gondola) specific software, four RS-232 ports, descrambler interface, audio and video cards, monitor, loudspeakers, keyboard and TC module. Easy connections with uplink (TC amplifiers) and downlink (TM receivers) are provided through RS-232. A French electronics company, under contract to ASI, prepares an state-of-the-art UHF band TM/TC system, for its use in future flights. Besides, a new TM for Long Duration Balloons (LDB) of circumpolar class, using Iridium constellation will be possibly tested in a Transmediterranean flight. IASF/ CNR Bologna, Laboratorio Elettronica Nucleare/Istituto Nazionale di Geofisica e Vulcanologia Rome, Dept. of Physics Univ. of Rome, develop STRAtospheric IriDIUM (STRADIUM). A NRT continuous link will be in place, with a primary TM/TC module for housekeeping, plus a secondary TM/TC module for science. To fly the LDB balloons at circumpolar trajectories, a new mixed method of downlink data dumping will be used. This system works with a low bit rate, when the payload is out of Line of Sight (LOS), using Iridium and performs a full data dump, when payload enters in the TM/TC ground station LOS. Iridium terrestrial full coverage is essential for polar flights. Previous flight from Trapani permits to apply the experience gained in future polar operations. 6. Harnessing electricity from batteries, solar panels and fuel cells Focusing on mature technologies, we must evaluate duration, power needs and cost/efficiency for zero-pressure stratospheric balloons over the Mediterranean. Power Rough Order of Magnitude (ROM) is a few kilowatts and flight operation duration ROM is one or two days. In past campaigns, primary Li batteries were used. A ground incident, during return handling of the 2001 payload, provoked a fire and damaged a Li battery. The new batteries should provide a great number of charge-discharge cycles, avoid leaks of electrolyte and generated gases. The new battery concepts present high specific energy (AH/kg) and good operation at flight profile temperatures. Mass should be reduced to the order of one quarter also allowing higher depth of discharge. Specific

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energy, specific power (J/kg), life cycle and cost per AH are key points. An operational technology is Li-Ion. First time in space, on board UK satellite Space Technology Research Vehicle, STRV1d launched in 2000 (Spurrett et al., 2002). An alternative is Nickel-Metal Hydride, NiM-H. First time in space, on board Japanese probe Nozomi launched in 1998 (Iwabuchi et al., 2002). Present work on solar panels intends: (1) To increase specific power from 60 W/kg to 1000 W/ kg. A trend exists to reduce thickness of cell cover, plate and accessories. Worldwide top in conversion efficiency is up to now 39, 7 percent, obtained by Dimroth and collaborators at the Fraunhofer Institute, Freiburg, Germany with multijunction III–V semiconductor solar cells (Troy, 2008). Efficiency better than 40% is pursued by an U.S. company under NASA contract, to develop nanostructured solar cells, made of indium gallium phosphide material and a single p–n junction device. (2) To obtain lightweight structures, although there are drawbacks on flight deployment, folding and interconnection with the outcome of reliability descent.

(3) To fabricate inflatable solar arrays. The idea has been explored in other concurrent space technologies. Fuel cells types suitable for balloons are: (1) Proton exchange membrane: Solid polymer electrolyte, reduced maintenance and risk of corrosion. Quick start-up. High cost of catalyzer. Impurities sensitive, if present in combustible (hydrogen, others). Operational T = 60–100 °C. (2) Alkaline: KOH aqueous electrolyte. The cathode presents quicker reaction inside the electrolyte. Better efficiency. Impurities sensitive. Operational T = 90– 100 °C. (3) Phosphoric acid: Cumbersome weight and volume. Low current and power. Impurities tolerant (hydrogen). Operational T = 175–200 °C (www.fuelcellsworks.com/Typeoffuelcells.html). Reactants are stocked exterior to the fuel cell, to cope with volume limitations. Electrodes and electrolyte are not consumed in the process. Design drivers are electrode structure and catalyzer selection (Cost and utilisation time

Fig. 8. European Geostationary Navigation Overlay System.

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of the cell are influential in a decision). Regenerative fuel cells combine with solar panels for use in water electrolysis, being this water a fuel cell product. Micro fuel cells, when available in large series to be the power supply in mobile phones, portable computers, etc., could be embarked at reasonable cost. 7. Localization: standard and innovative subsystems description In the 70s and 80s, Omega was the primary localization system for balloon operations. Its replacement by GPS, occurred after full deployment of Navstar constellation in 1991. Parameters transmitted through a TM channel were: latitude, longitude, altitude, time and the three instantaneous velocity components (vertical, N–S, E–W). Also radar tracking is currently used, in skin (primary radar which detect objects by reflected waves) and transponder modes A (identification and direct localization in distance and azimuth) and C (gives codified answers with barometric altitude). This codified altitude corresponds to the existing pressure in the Standard Atmosphere and is expressed in hundreds of feet, Flight Level form. Localization systems to be used possibly are: Argos can locate in NRT the balloon, using Doppler measurements with an accuracy of few hundreds meters. Argos transmitters are used on the gondola and balloon apex for accurate localization during flight and after separation. Data can be retrieved through a simple Internet connection. The signal sent to one of the Argos-3 equipped satellites (Metop, NOAA-N launched 2009, and SARAL (Satellite with ARgos3 and ALtika), an Indian-French satellite to be launched in 2010), can be received also directly on ground in real time, sending data of internal sensors or GPS receiver. A homing receiver, on the recovery helicopter, gives accurately the bearing. EGNOS is a GPS/SBAS (Satellite Based Augmentation System) with Mediterranean coverage (Gauthier et al., 2001, 2005). Positions can be determined with an error of less than 3 m horizontally and 5 m vertically for 95% of the time. A diagram of EGNOS system is presented in Fig. 8 including the GPS constellation, the Inmarsat III geostationary satellite transmitting the augmented signal to Europe and different types of ground stations: ranging and Integrity Monitoring Stations, Navigation Land Earth Stations sending the data to Inmarsat III, EGNOS Wide Area Network and different types of support installations. 8. Payload recovery: standard and innovative subsystems outline In the past, the parachutes used were the single canopy type or the raceme of canopies 3  200 and 3  270 m2 (parachutes of the later type were flown in Nausicaa 1980 and Poker 1981 respectively) with no failures reported.

Fig. 9. Payload recovery by helicopter.

Future innovative techniques contemplate a drogue parachute deployment at first, from 40 km to 10 km altitude. Below, a parasail equipped with an EGNOS receiver conveys its relevant data, as guidance input to an autonomous type guidance unit situated in the balloon gondola. Aerodynamic decelerators exist with capability to land, within 75 m of the desired point, a payload of 50–400 kg. The corresponding distance for a payload of 500–l000 kg is 200 m (http://www.airborne-sys.com). The recovery of a payload by helicopter illustrated in Fig. 9 represents the final stage of an average balloon operation. A previous disassembly in case of heavy payloads occurs, due to the weight limitations in helicopter manoeuvres with sling (a few hundred of kg). 9. Concluding remarks The return to Transmediterranean flights is possible with the three initial countries active presence and the rest of Europe co-operation. It requires an update of the support flight equipment existing on tracking stations, i.e. power, telemetry, telecommand, localization, recovery. The flights should be a tool for the scientific community specially in the fields of atmospheric science over the Mediterranean, biological research, future planetary balloons (on the path of Vega flights over Venus), proof and evaluation of new concepts and instruments. A “boomerang” Transmediterranean flight increases the total distance flown in 600 + 600 = 1200 km over the 2000 km presently

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flown, manoeuvring the balloon to divert it to a return path at a lower altitude, provided Western winds are blowing at this altitude. It requires Portuguese participation and evaluation of air navigation safety topics. Acknowledgments Thanks are due to Ms. H.V. Watson and Dr. J.A. Mor´ın˜igo for their contributions to the manuscript improvement with regard to the English usage and the technical matters respectively.The authors would like to thank the anonymous reviewers for the great amount of time spent in paper insight and the useful comments expressed. References Cardillo, A., Cosentino, O., Leone, R.M., et al. A software development plan for managing a stratospheric balloon flight. In: 15th ESA Symposium on European Rocket and Balloon Programs and Related Research, Biarritz, France, ESA SP-471, Noordwijk, Holland, pp. 519–523, 2001. Floury, N., Fuchs, J. WATS Water Vapour in Atmosphere and Stratosphere Report for Assessment ESA SP-1257(3), Noordwijk, Holland, pp. 1–38, 2001. Gauthier, L., Ventura-Traveset, J., Toran, F., et al. EGNOS Operation and their Planned Evolution ESA Bulletin 124, Noorwijk, Holland, pp. 57–61, November 2005. Gauthier, L., Michel, P., Ventura-Traveset, J., et al. EGNOS: the first step in Europe´s contribution to the Global Navigation Satellite System, ESA Bulletin 105, Noordwijk, Holland, pp. 35–42, February 2001.

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