Galileo Receivers

GALILEO
Title	Galileo Receivers
Edited by	GMV
Level	Intermediate
Year of Publication	2011

The Galileo System will be an independent, global, European-controlled, satellite-based navigation system and will provide a number of guaranteed services to users equipped with Galileo-compatible receivers.

Basic elements of a generic GNSS Receiver are an antenna with pre-amplification, an L-band radio frequency section, a microprocessor, an intermediate-precision oscillator, a feeding source, some memory for data storage, and an interface with the user. The calculated position is referred to the antenna phase centre.

Galileo receivers

A Galileo Acquisition Receiver Architecture

The Galileo global navigation satellite system will employ many new methods and technologies to offer superior performance and reliability. Development of the advanced receivers required to make use of the system is continuing.

A GALILEO Receiver is a device capable of determining a navigation solution by processing the signal broadcasted by Galileo satellites. Once the signal is acquired and tracked, the receiver application decodes the navigation message. The navigation data contain all the parameters that enable the user to perform positioning service. The four types of data needed to perform positioning are:

1. Ephemeris which are needed to indicate the position of the satellite to the user receiver.
2. Time and clock correction parameters which are needed to compute pseudo-range.
3. Service parameters which are needed to identify the set of navigation data, satellites, and indicators of the signal health.
4. Almanac which are used to compute the position of all the satellites in the constellation with a reduced accuracy, so that the receivers improve the time needed for the initial satellite acquisition process.

For single frequency receivers, the Broadcast Group Delays and Ionospheric parameters are also needed.

Three receiver development activities have been initiated within the Galileo programme, addressing the different needs of the system development process and covering the range of signals and services that will be offered.

Activities in receiver development are in the following areas:

• test user segment;
• receivers for the signals transmitted by the first, experimental satellites;
• receivers for the Galileo receiver chain.

Particularities

Each GNSS system uses a specific Reference Frame; although a multi-constellation receiver is able to convert all information to the same common frame, a Galileo-only receiver uses the Galileo GTRF reference frame. In an analogous way, each system has its own time reference defined by the respective control segments; the time reference for Galileo is called “Galileo System Time” (GST).

Each GNSS System transmits its own navigation message, defined in the respective Signal In Space Interface Control Documents, SIS ICD. As an example, the satellites transmit information that allows the receiver to compute their positions. For the case of Galileo (in-line with GPS but unlike GLONASS), the satellites transmit the orbit parameters as updated by the Ground Segment and refreshed every 3 hours. The Galileo receiver then computes the satellite position based on these transmitted ephemeris parameters.

Another distinction regarding the transmitted navigation message with impact on the receiver is the ionospheric parameters transmitted to support the single frequency receiver in computing the ionospheric error; Galileo uses the NeQuick Ionospheric Model.

GNSS signals modulation, structure, navigation message contents and formats are often different among signals from the same system and from different systems. Most of these characteristics are easily implemented at the receiver (e.g. requiring only “software modifications”, such as the use of different PRN codes or the ability to cope with different message structures). The main difference among GNSS receivers falls into the specific characteristics that have impact at RF level, such as the Multiple Access Techniques employed. Galileo (as GPS and BeiDou) uses CDMA techniques allowing a simpler RF module (than for example GLONASS), since all signals in the same frequency band have a common carrier.

It should be noted that the current trend consists on facilitating the access of each system to the receivers, i.e. fomenting multi-constellation receivers. Hence, most discussions and agreements among the systems’ responsibles are conducted in the sense of taking this effort out of the user segment, focusing on compatibility and interoperability aspects in the system design.

Test user segment

The test user segment is being used for system validation and signal experimentation. Two parallel developments have been performed, with the aim of securing equipment availability and achieving the highest confidence in the results. The test user segment consists of:

• a test user receiver for the Open Service, Commercial Service and Safety-Of-Life Service;
• a test user receiver for the Public Regulated Service (PRS);
• Search and Rescue (SAR) test beacon equipment;
• test support tools, such as a simulator for the satellite constellation.

The receivers are based on a highly flexible software-defined concept implementing 14 different receiver configurations. They are able to emulate different receiver types and provide a variety of internal measurements when combined with an analysis sub-system running on an attached laptop computer.

Experimental satellite receivers

These receivers are being used to receive the signals that are transmitted by the first, experimental satellites GIOVE. They serve both as reference receivers for Galileo sensor stations and as experimental receivers for field tests.

Galileo receiver chain

Galileo sensor stations will be equipped with high-performance, ultra-reliable receivers. The stations provide measurement data to the Galileo system central processing facilities for establishing system integrity and performing satellite orbit determination and time synchronisation.

Receivers

GNSS Receivers manufacturers have already made multi-constellation receivers designed for a variety of applications. These receivers support a wide range of satellite signals, including Galileo signals in E1 and E5 bands.

Commercial Receivers

Among the companies developing multi-constellation commercial receivers, both in OEM Receiver Boards and/or Enclosures Receivers already on-sale, we can find (non exhaustive list):

• Javad
• Leica
• Maxim
• Novatel
• Septentrio
• TopCon
• Trimble

The corresponding datasheet for each receiver (available in the company website) claims Galileo receivers compliant to Galileo Open Service SIS ICD. Furthermore, there are studies showing the advantage of using a modelling language to facilitate the implementation of the navigation message decoding in the receivers, in-line with the Galileo SIS ICD.

Scientific/ Space Receivers

A few examples of scientific and space receivers (non-exhaustive list):

• BroadReach
• IFEN

There is more information on Space Applications in NAVIPEDIA.

First achievements

The objectives of the initial part of the design phase for the test user segment have been fully achieved. A prototype receiver has been constructed, which is capable of receiving all Galileo signal components on all carriers defined in the current specification. The feasibility of acquiring and tracking the new Galileo signals has been proven.

Prototype receivers have also been used for verification of the payloads on the experimental satellites, GIOVE-A and GIOVE-B. They are also being used for validation of the satellite constellation simulator, which emulates the signals as generated by the satellites and modified by environmental effects (such as atmospheric effects, multi-path reception and interference), resulting in a signal that a user receiver would see on the ground.

The Open Service Interface Control Document (ICD) for Galileo is available on the European Commission website , so receiver manufacturers can already work to prepare the acquisition of real Galileo data. Furthermore, the European GNSS Agency (GSA) has been supporting a number of activities within the user segment.