Galileo History?

The first stage of the Galileo program was agreed upon officially on May 26, 2003 by the European Union and the European Space Agency (ESA). But system studies were conducted well before. In 1999 the 4 different concepts (from France, Germany, Italy and The United Kingdom) for Galileo were compared and reduced to one concept by a joint team of engineers from all four countries. The system is intended primarily for civilian use, unlike the US system, which is run by and primarily for the US military. The US reserves the right to limit the signal strength or accuracy of the GPS systems, or to shut down public GPS access completely, so that non-military users cannot use it in time of conflict. The precision of the signal available to non-military users was limited before 2000 (a process known as selective availability). The European system will not (in theory) be subject to shutdown for military purposes, will provide a significant improvement to the signal available from GPS, and will, upon completion, be available at its full precision to all users, both civil and military.

The European Commission had some difficulty trying to secure funding for the next stage of the Galileo project. European states were wary of investing the necessary funds at a time of economic difficulty, when national budgets were being threatened across Europe. Following the September 11, 2001 Terrorist Attack, the United States Government wrote to the European Union opposing the project, arguing that it would end the ability of the US to shut down GPS in times of military operations. On January 17, 2002 a spokesman for the project sombrely stated that "Galileo is almost dead" as a result of US pressure.

A few months later, however, the situation changed dramatically. Partially in reaction to the pressure exerted by the US Government, European Union member states decided it was important to have their own independent satellite-based positioning and timing infrastructure. All European Union member states became strongly in favour of the Galileo system in late 2002 and, as a result, the project actually became over-funded, which posed a completely new set of problems for the ESA, as a way had to be found to convince the Member States to reduce the funding.
On March 20, 2003, the United States and three other countries began military operations in Iraq, further motivating the EU to develop a navigation system independent of US control.

The European Union and European Space Agency then agreed in March 2002 to fund the project, pending a review in 2003 (which was finalised on May 26, 2003). The starting cost for the period ending in 2005 is estimated at EUR 1.1 billion. The required satellites—the planned number is 30—will be launched throughout the period 2006–2010 and the system will be up and running and under civilian control from 2010. The final cost is estimated at EUR 3 billion, including the infrastructure on Earth, which is to be constructed in the years 2006 and 2007. At least two thirds of the cost will be invested by private companies and investors, the remaining costs are divided between the European Space Agency and the European Union. An encrypted higher bandwidth Commercial Service with improved accuracy will be available at an extra cost, while the base Open Service will be freely available to anyone with Galileo compatible receiver.

The European Union has agreed to switch to a range of frequencies known as Binary Offset Carrier 1.1 in June 2004, which will allow both European and American forces to block each other's signals in the battlefield without disabling the entire system.

How works Galileo?

Space segment

The Galileo space segment will comprise thirty satellites in a Walker constellation with three orbital planes at 56° nominal inclination. Each plane will contain nine operational satellites, equally spaced, 40° apart plus one inactive spare satellite to replace any of the operational satellites in case of failures.
The orbit altitude of 23 222 km results in a repeat a constellation repeat cycle of ten days during which each satellite has completed seventeen revolutions.

Constellation features

The altitude of the satellites has been chosen to avoid gravitational resonances so that it is hoped that, after initial orbit optimisation, station-keeping manoeuvres will not be needed during the lifetime of a satellite. The altitude chosen also ensures a high visibility of the satellites.
The position constraints for individual satellites are set by the need to maintain a uniform constellation, for which it is specified that each satellite should be within +/- 2° of its nominal position relative to the adjacent satellites in the same orbit plane and should be within 2° of the orbit plane.
The in-plane accuracy is equivalent to a relative tolerance of over 1 000 km but requires very careful adjustment of the satellite velocity to ensure that the orbit period of all the satellites is kept precisely the same. The across-track tolerance allows the inclination and RAAN of each satellite to be biased at launch so that natural drifts remain within the tolerance without the need for orbit plane changes requiring major expense of fuel.
The spare, non-operational satellite in each orbit plane, ensures that in case of failure the constellation can be repaired quickly by moving the spare to replace the failed satellite. This could be done in a matter of days, rather than waiting for a new launch to be arranged which could take many months.
The satellites are being designed to be compatible with a range of launchers providing multiple and dual launch capabilities.

Ground control segment

The core of the Galileo ground segment will be the two control centres. Each control centre will manage “control” functions supported by a dedicated Ground Control Segment (GCS) and “mission” functions, supported by a dedicated Ground Mission Segment (GMS). The GCS will handle spacecraft housekeeping and constellation maintenance while the GMS will handle navigation system control.
The GCS will use a global network of nominally five TTC stations to communicate with each satellite on a scheme combining regular, scheduled contacts, long-term test campaigns and contingency contacts.
The TTC Stations will be large, with 13-metre antennas operating in the 2 GHz Space Operations frequency bands. During normal operations, spread-spectrum modulation (similar to that used for TDRSS and ARTEMIS data relay applications) will be used, to provide robust, interference free operation. However, when the navigation system of a satellite is not in operation (during launch and early orbit operations or during a contingency) use of the common standard TTC modulation will allow non-ESA TTC stations to be used.

Mission control segment

The Galileo Mission Segment (GMS) will use a global network of nominally thirty Galileo Sensor Stations (GSS) to monitor the navigation signals of all satellites on a continuous basis, through a comprehensive communications network using commercial satellites as well as cable connections in which each link will be duplicated for redundancy. The prime element of the GSS is the Reference Receiver.
The GMS communicates with the Galileo satellites through a global network of Mission Up-Link Stations (ULS), installed at five sites, each of which will host a number of 3-metre antennas. ULSs will operate in the 5 GHz Radionavigation Satellite (Earth-to-space) band.
The GMS will use the GSS network in two independent ways. The first is the Orbitography Determination and Time Synchronisation (OD&TS) function, which will provide batch processing every ten minutes of all the observations of all satellites over an extended period and calculates the precise orbit and clock offset of each satellite, including a forecast of predicted variations (“SISA”, Signal-in-Space Accuracy) valid for the next hours. The results of these computations for each satellite will be up-loaded into that satellite nominally every 100 minutes using a scheduled contact via a Mission Up-link Station.
The second use of the GSS network is for the Integrity Processing function (IPF), which will provide instantaneous observation by all GSSs of each satellite to verify the integrity of its signal. The results of these computations, for the complete constellation, will be up-loaded into selected satellites and broadcast such that any user will always be able to receive at least two Integrity Messages.
The Integrity messages will comprise two elements. The first is as an “Integrity Flag”, which warns that a satellite signal appears to exceed its tolerance threshold. This flag will be generated, disseminated and broadcast with the utmost urgency, so that the Time-to-Alert, being the period between a fault condition appearing at a user’s receiver input and the Integrity Flag appearing there will be no more than six seconds, and will be re-broadcast a number of times. The second element of the Integrity Message comprises Integrity Tables, which will be broadcast regularly to ensure that new users or users who have missed recent signal (for example when travelling through a tunnel) will be able to reconstitute the system status correctly.
The OD&TS operation thus monitors the long-term parameters due to gravitational, thermal, ageing and other degradations, while the IPF monitors short-term effects, due to sudden failure or change.
The Galileo Global Component will also include a set of Test User Receivers.

Satellites from Galileo?
• 30 spacecraft
• orbital altitude: 23222 km (MEO)
• 3 orbital planes, 56° inclination (9 operational satellites and one active spare per orbital plane)
• satellite lifetime: >12 years
• satellite mass: 675 kg
• satellite body dimensions: 2.7 m x 1.2 m x 1.1 m
• span of solar arrays: 18.7 m
• power of solar arrays: 1500 W (end of life)
Galileo Satellite Test Beds
The ESA and GJU sucessfully launched the first of two Galileo In-Orbit Validation Element test satellites, GIOVE-A (GSTB-2A), on 28 December 2005 by Soyuz launch vehicle, 6:19 MEZ from Baikonur, Kazakhstan. It began transmitting as planned at 13:51 MEZ while circulating earth in a height of 23 222 km. GIOVE-A, built by Surrey Satellite Technology Ltd (SSTL), is basically a transmitter beacon. GIOVE-B, built by Galileo Industries, has a more advanced payload which includes two atomic clocks and is targeted for launch in the spring of 2006. For both test satellites, the primary objective is achieving the ITU frequency-filing requirements that require using the allocated transmission frequencies by the set deadline date. GIOVE-B also has clock and MEO environment characterisation objectives, as well as Signal-In-Space and receiver experiments. GIOVE-B will contain a rubidium atomic clock and the first space-qualified passive hydrogen maser atomic clock.
GSTB-V1 Technology Developments

The Galileo System Test Bed (GSTB) has been defined as an integral part of the Galileo Design Development and Validation Phase. Its primary purpose is to mitigate programme risks.
The main objective of the first step of the Galileo System Test Bed (GSTB-V1) is to reduce Galileo programme risk on ground segment development by anticipated experimentation on Orbit determination & Time Synchronisation and Integrity concepts.
Pre-developments in the Processing Facilities are conducted to support the refinement of the critical ground segment algorithms and the assessment of related performances based on realistic measurements from the GPS system, in collaboration with the International GPS Service community and UTC time community.

The various developments are described below:
Galileo System Test Bed Version 1 (GSTB-V1) reduces the risk on the Galileo ground segment development through early experimentation with the Orbit Determination & Time Synchronisation and Integrity techniques.
The GSTB-V1 Experimental Orbitography and Synchronisation Processing Facility (E-OSPF) is the element responsible for the generation of the Navigation and Signal In Space Accuracy (SISA) products within the GSTB-V1. It implements highly sophisticated prototype algorithms for the Galileo Orbit Determination and Time Synchronisation (OD&TS) function, aiming at the computation of accurate and reliable orbit and clock predictions.
The GSTB-V1 Experimental Precise Timing Station (E-PTS) provides the accurate and stable reference time for the GSTB-V1: the Experimental Galileo System Time (E-GST).
The GSTB-V1 Data Server Facility (DSF) is the central element of the Galileo System Test Bed V1 (GSTB-V1). Its main role inside the GSTB Processing Centre (GPC) is to collect, redistribute, store and make publicly available the scientific data used and generated by the system.
The GSTB-V1 Experimental Integrity Processing Facility (E-IPF) is one of the core elements within the GSTB-V1 Processing Centre. Its high flexibility allows integrity analyses at ground segment level as well as at user and system levels.

Galileo Technology Developments

Galileo is an initiative of the European Space Agency (ESA) and the European Commission (EC). GalileoSat is the name given to the complementary development programme being carried out by ESA.

The various developments are described below:
Clock Monitoring and Control Unit (CMCU) interfaces the onboard Atomic Frequency Standards.
Galileo Communications Network provides integrity flags which indicate the quality of the Galileo navigation signals.
Comparative Study on Software Development Environments and Implementation Technologies identifies the key properties of software components for the Galileo Ground Segment and the best technologies to implement these components.

Galileo Constellation Mission Control System (GMCS) Assessment investigates whether there is a suitable European Mission Control System (MCS) platform that can be used as the basis for the GMCS.
Galileo System Simulation Facility (GSSF) is a software simulation tool that reproduces the functional and performance behaviour of the Galileo system.
Galileo System Test Bed Version 1 (GSTB-V1) reduces the risk on the Galileo ground segment development through early experimentation with the Orbit Determination & Time Synchronisation and Integrity techniques.
A Ground Segment Data Model & Data Standard (GMXL) has been developed for the full Galileo Ground segment to ensure consistency, commonality and good operability between Ground Segment subsystems. This investigates the use of extensible mark up language (XML) and involves the development of software to convert navigation receiver data into the XML format.
Galileo Interference Measurement Campaign investigates how well Galileo receivers can operate in the presence of interference.

Navigation Signal Generation Unit (NSGU) now under development generates the truly precise signals that convey accurate satellite time and ephemeris required for Galileo.
The Passive Hydrogen Maser (PHM) with its excellent frequency stability performance has been chosen as the Master clock in the Galileo Navigation Payload.
The Rubidium Atomic Frequency Standard (RAFS) is today the most widely used clock. The RAFS is currently one of the two clock technologies that is to be used in the Galileo Navigation Payload.
Galileo Receiver Pre-Developments investigates the most critical User Receivers and Ground Reference Receivers issues.

The Galileo Solid State Power Amplifier (SSPA) onboard Galileo needs to generate four signals (carriers) in two different frequency bands. The output power for each individual signal is more than 50 Watts. Obtaining the required output power levels with a certain efficiency and within preset linearity requirements proves to be a key requirement for the optimisation of the payload and indeed, the Galileo satellites.
Dual Mode TT&C Transponder unit performs the following functions in both standard and spread spectrum mode: Up-link telecommand demodulation, Down-link telemetry transmission, Coherent frequency turn-around and Ranging turn-around function.