GIOVE-B Satellite & Payload Overview

Maktar Malik, Giuliano Gatti, Valter Alpe, Martin Johansson, ESA/ESTEC

Raoul Kieffer, Astrium GmbH, Ottobrunn, Germany

Gordon Robertson, Astrium Ltd, Portsmouth, United Kingdom

BIOGRAPHY

MAKTAR MALIK is a Payload System Engineer providing technical support to the ESA Galileo Project Office on navigation payload performance. He has been working on the Galileo program since 2000 and as Payload System Manager was leading design activities for GIOVE-B at EADS Astrium UK. At ESTEC he was overseeing all navigation payload activities from system engineering through to launch and commissioning for the GIOVE-A satellite. He is currently engaged in the GIOVE-A, B & IOV programs. He graduated with an MSc in Radio Frequency Communications from Bradford University in 1990 and a PhD in Electronic Engineering from the University of Kent in 1996, the research was in the field of meteor burst communications. GIULIANO GATTI received his Laurea in Electronic Engineering from Politecnico di Milano, Italy in 1979 and a Master in Business Administration from The Open University, UK in 2000. Since 2008 he is the GALILEO Space Segment Procurement Manager in the European Space Agency, European Space Research and Technology Centre (ESTEC), Noordwijk, The Netherlands and is responsible for the GALILEO FOC satellites. From 1999 he

was the GALILEO Space Segment Manager, and is responsible for the GIOVE and IOV satellites and for the GALILEO launchers procurement. Earlier, he was heading a section in the Technical Directorate of ESTEC dealing with microwave payload and technologies. His work experience include several years of microwave engineering work for space equipment at SPAR Aerospace (Canada) and GTE Telecom (Italy). Giuliano Gatti is a member of the IET, IEEE and ION, he has published several papers and he owns 3 patents. VALTER ALPE has been with ESA since 2002 and currently is the Satellites Infrastructure Manager for the Galileo Project, focusing on the operations of the GIOVE-A and GIOVE-B spacecrafts, after having been involved in the integration and testing of both and acting as procurement manager for the GIOVE-B one. Previously he has been working in space industry (mainly scientific satellites and space station projects) in the integration and testing domain and in the automation industry in the software development domain. He holds a degree in physics with honours obtained from university of Torino in 1984. MARTIN JOHANSSON received a Masters degree in Electrical Engineering at Chalmers University of Technology in Gothenburg, in 1984. Since 2004 he is working in the Galileo Project in the European Space Agency, European Space Research and Technology Centre (ESTEC), Noordwijk, The Netherlands office, first as the GIOVE-B Procurement Manager, and presently as the Galileo Payload Manager. Earlier, he was heading the Space Segment Technology Section in the Directorate of Applications dealing with telecommunications payload and platform technologies. His work experience include several years of microwave engineering work for space equipment at Ericsson Radio Systems (Sweden) and telecommunications

Payload engineering and management at ESA and Eutelsat (France). RAOUL KIEFFER was the Satellite Technical manager for GIOVE-B at Astrium GmbH and was responsible for the overall spacecraft design and the integration and test programme. Prior to GIOVE-B, he was in charge of a number of advanced system engineering studies and proposals, particularly in the area of LEO/MEO satellite constellations. Raoul Kieffer obtained degrees from the École Supérieure d’Ingénieurs en Électronique et Électrotechnique (France), the Technical University of Karlsruhe (Germany) and the University of Essex (UK) GORDON ROBERTSON was the Payload Engineering Manager for GIOVE-B at Astrium throughout the project. He led the team of systems and test engineers responsible for the design, integration and test of the payload in Portsmouth. He subsequently participated in Satellite testing and supported the in-orbit commissioning and test of the payload. He holds degrees in Theoretical Physics from the University of St Andrews and Satellite Communications Engineering from the University of Surrey, and is a Senior Member of the IEEE.

ABSTRACT

The second Galileo In-Orbit Validation Element satellite, GIOVE-B was launched on the 27th April 2008 into Galileo’s Medium Earth Orbit (MEO) at an altitude of 23,222 km by a Russian Soyuz launcher with a Fregat upper stage. Following a two week long platform commissioning on the 7th May 2008, the first Galileo navigation signal successfully transmitted was the composite BOC (CBOC) version of the MBOC signal.

The GIOVE-B main objectives are maintaining the frequency filings for the GALILEO programme, characterising the orbit radiation environment, evaluating critical payload technologies of the Galileo

Payloads units, specifically the Passive Hydrogen Maser (PHM) and Solid-State Power Amplifiers (SSPAs), and enabling early signal experimentation. Developing and launching this satellite has brought very valuable information/lessons learned in many satellite development area such as manufacturing, assembly, integration and testing, but also in other areas, namely mission analysis, launch and early operations, and in-orbit testing. All these will be reused in the frame of the on-going GALILEO development.

Maintaining the frequency filing objective, in particular, is a prime objective for GIOVE-B since GIOVE-A has already reached its nominal End of Life, even though it is still operating normally and transmitting its signal in space. The characterisation of the environment in the GALILEO Medium Earth Orbit (MEO), approximately 24000 km of altitude, was a second important objective of the GIOVE-A & B missions. In fact no previous European missions have flown in this orbit that is considered quite demanding as far as the radiation environment is concerned. To achieve this mission objective a radiation monitoring equipment (SREM) has been embarked on GIOVE-B. Its full characterisation will allow validating and proving the existing models and, in particular, it will enable an improved assessment of the level of shielding necessary for the GALILEO operational satellites to guarantee their flawless performance over the 12 years operational lifetime.

In addition GIOVE-B will provide in-orbit experience for equipment which has no flight heritage, in particular the Passive Hydrogen Maser (PHM), Solid-State Power Amplifiers (SSPAs) and navigation signal generator (NSGU) with the latest MBOC signals. The good performance of these equipments is the essence of the service quality that will be provided by the future GALILEO operational constellation and therefore their full validation in-orbit is fundamental. The spacecraft is controlled by

the control centre in Fucino, Italy supported by TTC stations in Fucino and Kiruna, Sweden and the signals were monitored using two stations one in Redu, Belgium and the other in Chilbolton, UK using a 25 metre dish. The signals were also tracked by a Septentrio Galileo receiver installed at Chilbolton. The paper will describe in detail the satellite design, highlighting the manufacturing and qualification aspects.

INTRODUCTION

Following a successful launch on 27 April 2008 from Baikonur, of the second Galileo In-Orbit Validation Element satellite, GIOVE-B began transmitting navigation signals on the 7th May and has continued to this day. This is a truly historic step for satellite navigation since GIOVE-B is now, for the first time, transmitting the GPS-Galileo common signal using a specific optimised waveform, MBOC (multiplexed binary offset carrier). These GIOVE-B signals are locked on-board to the most accurate clock in space, the highly stable Passive Hydrogen Maser clock, which will provide higher accuracy in challenging environments where multipath and interference are present and deeper penetration for indoor navigation.

GIOVE B Satellite

THE MISSION

The GIOVE-B main objectives are maintaining the frequency filings for the GALILEO programme, characterising the orbit radiation environment, evaluating

critical payload technologies of the Galileo Payloads units and enabling early signal experimentation.

Maintaining the frequency filing objective is of particular importance especially since the GIOVE A spacecraft is now 12 months beyond its End of Life Phase (EOP), even though it is still operating normally. The characterisation of the environment in the GALILEO 23200 km Medium Earth Orbit (MEO) is another important objective, since the radiation environment is very much dependant upon the 11 year solar cycle. Therefore GIOVE-B will continue to collect valuable data using the Standard Radiation Environment Unit (SREM) leading to a better assessment of the shielding for the IOV & FOC satellites which will have a 12 year EOP.

In addition GIOVE-B will provide in-orbit experience for equipment which has no flight heritage, in particular the Passive Hydrogen Maser (PHM), Solid-State Power Amplifiers (SSPAs) and navigation signal generator (NSGU) with the latest MBOC signals. The good performance of these equipments is the essence of the service quality that will be provided by the future GALILEO operational constellation and therefore their full validation in-orbit is fundamental.

The final objective is to enable signal experimentation, this has already proven useful with an early assessment of the improvement of the CBOC (Composite Binary Offset Carrier) signal (a particular implementation of MBOC) compared to the BOC(1,1). This signal redistributes 10% of the BOC (1,1) power to a BOC(6,1) component which is then linearly added to the BOC(1,1). A comparison of the composite CBOC with BOC(1,1) using the code multipath error performance measure shows a 20-25% improvement which should lead to a better indoor performance [1].

SATELIITE OVERVIEW

The GIOVE-B satellite was built by a consortium lead by Astrium GmbH. The spacecraft can be broken down into the payload and platform as shown in the Figure below.

GIOVE B Satellite Architecture

The payload provides the latest navigation signals at L-band and environmental monitoring capability in order to achieve the main mission objectives. The platform provides the power, TT&C data handling, attitude and propulsion capability. This 3-axis-stabilised satellite has stowed dimensions of 0.95 x 0.95 x 2.4 m and a launch mass of 500 kg. Its two solar wings, each 4.34m long, will supply up to 1100W. The propulsion system has a single tank carrying 28 kg of hydrazine.

PLATFORM DESIGN The platform structure has a 1 m sided cubic shape. All the equipment units are accommodated on four lateral panels; hydrazine mono-propellant units with a 40 litre tank and four thrusters are laid out on and under the lower plate.

GIOVE B AOCS Architecture

The avionics provides all satellite control and data handling functions, required for safe operation of the satellite during all mission phases including also non-nominal situations. The avionics is an integrated control and data system which consists of the Attitude and Orbit Control System (AOCS) with its sensors and actuators, the Data Handling (DH) and the Onboard Software (OBSW).

The electrical, on-board command and data handling architecture is centralised on one single computer, the Data Handling Unit (DHU). Accurate attitude determination is based on measurements using 2 earth sensors (nominal and redundant) and 3 internally redundant fine Sun Sensors. Both Earth sensors are accommodated on the payload module in an Earth sensor enclosure equipped with an autonomous thermal control. The Sun Sensors are distributed about the satellite in 3 positions that together give 360° coverage around the +/- Y axis. In-orbit platform attitude control is based on a gyro-earth-sun concept. Two accurate 3-axis gyros are used during Orbit Change manoeuvres where no Earth reference is available. Attitude acquisition is obtained using earth and solar measurements. Four small reaction wheels will generate torque for attitude command; momentum dumping being carried out using magneto-torquers or thrusters.

The thermal control subsystem is dimensioned to withstand the maximum thermal loads. The concept relies on passive radiators and active regulation with heaters, monitored by the central computer to ensure the safety and health of the satellite payload.

Electrical power is generated by two symmetric wing arrays attached near to the satellite centre of mass with two single-axis stepping motor-drives. Each wing is comprised of four 1.5 * 0.8 m panels covered with classical silicon cells. The power is distributed through a single electrical non regulated bus using a Li-Ion battery.

The S-band TT&C Sub-system including ranging capability provides the capability to command and control the spacecraft from ground, specifically by Control Centre in Fucino, Italy via the TT&C Ground stations in Fucino, Italy and Kiruna, Sweden

The spacecraft is also equipped with a Laser Retro-Reflector (LRR) consisting of 67 prisms. This enhances the characterisation of the on-board clock by use of Satellite Laser Ranging (SLR), a high precision technique for orbit determination that is independent of the navigation signal generation. By taking into consideration a more accurate orbit, the pseudorange error can be minimised.

PAYLOAD DESIGN GIOVE-B satisfies the mission requirements through its high performance payload, which includes a precision navigation signal generation and broadcast capability, as well as an environmental monitoring instrument.

At the heart of the payload is the timing section containing the atomic reference clocks and control and monitoring equipment. GIOVE-B includes three high precision clocks in a triple-redundant configuration: two Rubidium Atomic Frequency Standards (RAFS) and a Passive Hydrogen Maser (PHM). The PHM is the

first maser to orbit the Earth, and offers fundamental advantages in terms of its superlative accuracy (better than 1ns per day), and minimal drift over time.

GIOVE-B Passive Hydrogen Maser

Navigation signals are generated on-board in the Navigation Signal Generator Unit (NSGU). The on-board timing signal derived from the atomic clocks is incorporated into the navigation message by the NSGU together with various correction terms and other message data uploaded from the ground.

GIOVE-B Navigation Signal Generation Unit

GIOVE-B’s upgraded NSGU supports the latest navigation signal waveforms agreed between the EU and US authorities in July 2007, which are designed to allow the Galileo system to be compatible, and largely inter-operable, with GPS. GIOVE-B was the first satellite to transmit the MBOC (Multiplexed Binary Offset Coding) modulation standard from space, thus paving the way for its future roll-out on Galileo and GPS III satellites.

After leaving the NSGU, the channelized navigation signals are translated to L-band frequencies and then amplified by 50W solid state power amplifiers before being broadcast by the navigation antenna. The antenna is a 42-element planar array that provides an “isoflux” beam across the visible surface of the Earth.

GIOVE-B has three fully redundant channels operating in the allocated E5, E6 and E2L1E1 frequency bands. The flexibility to provide both single and dual-channel (E2L1E1+E5 or E2L1E1+E6) operation is possible, as operational needs dictate.

In addition to the navigation-related functions described above, the GIOVE-B payload carries the Standard Radiation Environment Monitor (SREM). This instrument records the various high-energy electron, proton, and cosmic ray fluxes found in the Satellite’s orbit, as well as the total accumulated radiation dose.

GIOVE-B Flight Payload under Test at Astrium Portsmouth

GROUND SEGMENT

After its successful launch by a Soyuz Rocket from Baikonur on 27 April and accurate insertion into its target orbit by the Fregat autonomous upper stage, GIOVE-B completed its Launch and Early Operations Phase (LEOP), followed by a successful In-Orbit Test Phase (IOTP) and is currently in its Nominal Operations Phase. A network of global ground stations were needed for LEOP since 24 hour coverage is required during the initial critical operations after launch.

Ground Segment Stations The main Mission Control Centre is Telespazio’s TT&C Ground Station at the Fucino Space Centre in Italy. This was/is used throughout the lifetime of the spacecraft to command & control it and assess its health. It is also supported by the TT&C ground station of Kiruna, Sweden operated by SSC.

Fucino Ground Station, Italy Two L-Band IOT stations were used, one at Redu, Belgium and the other in Chilbolton, UK. The Chilbolton site provided greater sensitivity since it had a 25 m dish as seen below.

Chilbolton IOT station, UK The very first L1 CBOC & E5 ALTBOC signals were picked up by the Chilbolton site on Wednesday 7th May 2008 at 02:41:37 UTC [2].

First L1 CBOC Signal From Space

PROJECT MILESTONES & EVENTS The GIOVE-B project commenced in July 2003 and was closely followed by a Qualification Status Review and Delta Preliminary Design Review.

In view of the novelty and criticality of the payload design, an Engineering Model (EM) payload was assembled and tested in the second half of 2004. The results obtained from the EM payload helped secure the design of the flight payload and provided much valuable experience in clock and signal test methodology.

EM Payload Testing

The flight payload assembly and test campaign took place in the first half of 2005 at Astrium in Portsmouth, UK. The payload was then transferred to Thales Alenia Space facilities in Rome for thermal vacuum testing prior to integration with the satellite platform.

Satellite testing took longer than originally anticipated due to the failure of a critical component in the on-board computer during thermal vacuum testing in August 2006. As well as repairing the on-board computer, components of the same type were replaced in other equipments as a precautionary measure.

The resumption of satellite testing allowed GIOVE-B to progress through the remaining test phases in Rome, and later on at ESA’s

ESTEC facilities in Noordwijk, The Netherlands. Test activities were essentially completed in November 2007, culminating in a successful Qualification Acceptance Review. By this time however, an upgraded NSGU, capable of providing the important MBOC signals, had become available from manufacturers RUAG (formerly Saab Space). ESA and the Industrial team took this opportunity to replace the NSGU on the satellite and repeat the necessary functional and performance tests.

GIOVE-B was airlifted to Baikonur Cosmodrome in Kazakhstan in March 2008. After an intensive 6-week launch campaign, the satellite was successfully launched at 04:16am (local time) on 27th April 2008 on a Soyuz launcher with a Fregat upper stage.

GIOVE-B Launch from Baikonur

Following a two-week commissioning period, the first navigation signals were successfully transmitted from GIOVE-B on the 7th May 2008, including the first ever broadcast from space of the composite BOC (CBOC) version of the MBOC signal.

Transmission of First Navigation Signals seen from Mission Control Centre in Fucino

The payload underwent a 2-month in-orbit test campaign which fully demonstrated the good health and excellent performance of the satellite. GIOVE-B entered routine operations after the successful In-Orbit Test Review on 3rd July 2008.

CONCLUSIONS The GIOVE-B mission has achieved its main objective of maintaining the GALILEO frequencies. The clock characterisation results assessed over the last 12 months confirm that Europe has the most accurate clock in space, the highly stable Passive Hydrogen Maser. For the first time the agreed interoperable GPS-Galileo composite L1 1575.42 MHz CBOC signal has been transmitted from space enabling early signal experimentation. Field experimentation has confirmed the CBOC signal is better than BOC(1,1) leading to a 20-25% improvement in multipath mitigation. The radiation monitoring results are promising and continue to enhance our understanding of this harsh environment. All in all this has been a highly successful mission.

ACKNOWLEDGEMENTS We recognise the outstanding cooperation with the spacecraft prime: Astrium GmbH (Ottobrunn, Germany); payload prime, Astrium Ltd (Portsmouth, UK); satellite

AIT, Thales Alenia Space Italy (Rome, Italy); Ground Control Operations, Telespazio (Fucino, Italy); In addition, the fundamental contribution of all the following parties is acknowledged:

• Redu IOT ( INDRA, Madrid ) • Chilbolton IOT station (SSTL,

STFC) • Payload unit suppliers (Spectratime,

Selex Galileo, Thales Alenia Space Spain, Norspace, RUAG (formerly Saab Space), EADS CASA Espacio, Astrium Ltd)

• GALILEO test receiver manufacturer (Septentrio Satellite Navigation)

• ARIANESPACE (previously STARSEM) launch team.

GIOVE B Teams (Partial)

REFERENCES [1] A. Simsky, D. Martens, J-M Sleewaegen, M. Hollreiser, M. Crisci, “MBOC vs. BOC(1,1) Multipath Comparison Based on GIOVE-B Data”, Inside GNSS, September/October 2008, pp. 36-39

[2] G. Gatti, M. Falcone, V. Alpe, M. Malik, T. Burger, M. Rapisarda “GIOVE-B Chilbolton In-Orbit Test: Initial results from the Second Galileo Satellite” Inside GNSS, September/ October 2008, pp. 30-35