Especificación de materiales

GNSS is composed of existing constellations of navigation satellites and complementary satellite systems for improving performances like the augmentation system EGNOS in Europe or ground complements systems like Differential GNSS. They are presented in this part of the report.

3.1.3.1 Operational Satellite Positioning Systems (GPS/GLONASS) 3.1.3.1.1 GPS

Regarding principles of satellite positioning described in 3.1.1 that have an impact on the GPS structure concerning satellites, ground stations and timing. Therefore, the GPS is composed of three segments defined as Space Segment, User Segment and Control Segment.

The space segment [29] is the satellite part of the positioning system. The United States' Global Positioning System (GPS) [31] space segment consists of up to 32 (MEO) satellites in six different orbital places, with the exact number of satellites varying as older satellites are retired and replaced. GPS became operational in July 1995 with the following noteworthy modifications:

 Modernization of GPS II : GPS L2C since the end of 2005, GPS L5 in 2010  With the adding of L1C, GPS III could be operational in 2021

 New military signal M-code on L1 and L2

Each of the GPS signal is composed of one carrier to transmit the signal at the desired frequency, a navigation message which contains some useful data for the user such as orbit information, (explained in 3.1.2 which is a correction of the deviation between satellite clock and GPS time) and a PRN code which is a random binary sequence different for all satellite which enables to recognize them.

Today, several signals are available for civil aviation application named GPS L1 C/A and GPS L5 (in the future L1C will be available and is described only for information purpose in this part). These signals are the GNSS signals located in the specific frequency bands named Aeronautical Radio Navigation Services (ARNS). ARNS bands are reserved for aeronautical systems and particularly protected from in-band interference by regulation authorities.

The characteristics of these available GPS signals for Civil Aviation (frequency occupation, structure) are described in the following table extracted from [32].

Constellation Signal Modulation Code Length (ms) Chip rate (Mcps) Navigation Data (sps) Secondary Code Length GPS L1 C/A BPSK(1) 1023 1.023 50 No L1C-I TMBOC(6,1,4/33) 10230 1.023 100 No L1C-Q 10230 1.023 Pilot 1800 bits L5-I QPSK(10) 10230 10.23 1000 NH-10 (10bits) L5-Q 10230 10.23 Pilot NH-20 (20 bits)

Table 5-GPS signals for civil aviation

The User Segment [33] includes millions of GPS receivers (military or civilians). These receivers can be static on Earth, or mobile in a vehicle on Earth, in an aircraft or a spacecraft. They permanently collect GPS signals and process them to compute the position and velocity of the user.

The role of the Control Segment [33] is to ensure the surveillance of the received signal characteristics, to compute the ephemeris data and the satellites clock corrections, and to download the navigation message into the satellites payload. Therefore, the control segment is composed of 4 major subsystems: the Master Control Station (MCS, soon replaced by a New Master Control Station) located in Colorado which is responsible for constellation command and control, a Back-up Master Control Station (BMCS, soon replaced by an Alternate Master Control Station), a network of 4 ground antennas and a network of monitor stations globally-distributed.

As mentioned in introduction, under the GPS SPS specifications [33], the probability of failure is less than 10−5_{𝑝𝑒𝑟 ℎ𝑜𝑢𝑟 and per satellite and therefore an integrity monitoring function is needed to meet the integrity} risk requirement for aviation operations of the order of 10−7.Indeed, such standalone receivers (GPS, GLONASS or BeiDou) cannot be used without spatial, regional, local augmentation which improve accuracy and/or integrity. Also a RAIM algorithm could be used for providing integrity information for some approaches and landing operations such as NPA. These regional GNSS are briefly presented below and a more detailed section 3.1.4 concerns other augmentation techniques.

3.1.3.1.2 GLONASS [34]

The formerly “Soviet Union” , and now Global'naya Navigatsionnaya Sputnikovaya Sistema (Global Navigation Satellite System), or GLONASS, was a fully functional navigation constellation in 1995. After the Collapse of the Soviet Union, it fell into disrepair, leading to gaps in coverage and only partial availability, but the full orbital constellation has since been restored in 2011.

3.1.3.2 Navigation Satellite Systems in development (GALILEO/ BeiDou /QZSS/IRNSS) 3.1.3.2.1 GALILEO [35]

The European Union and European Space Agency agreed in March 2002 to introduce their own alternative to GPS, called the Galileo positioning system. At an estimated cost of EUR 3.0 billion, the system was originally scheduled to be operational in 2010 but the updated calendar is currently defined as:

 1999 : Definition of Galileo  2002 : Start of the Project

 2005 : First satellite was launched for testing  2011 : The 2 first Galileo Satellites was launched  2015 : 12 satellites are in orbit

 2020 : Constellation completed

Today, several signals are available for civil aviation application by similitude with GPS named Galileo E1 and Galileo E5a (in the future E5b will be available and is described only for information purpose in this part). These signals are the GNSS signals located in the specific frequency bands named Aeronautical Radio Navigation Services (ARNS). ARNS bands are reserved for aeronautical systems and particularly protected from in-band interference by regulation authorities.

The characteristics of these available Galileo signals for Civil Aviation (frequency occupation, structure) are described in the following table extracted from [32].

Constellation Signal Modulation

Code Length (ms) Chip rate (Mcps) Navigation Data (sps) Secondary Code Length GALILEO E1B CBOC(6,1,1/11) 4092 1.023 250 No

E1C 4092 1.023 Pilot Primary x 25

(100 ms)

E5A-I

QPSK(10) 10230 10.23 50

Primary x 20 (20 ms)

(100 ms)

E5B-I

QPSK(10)

10230 10.23 250 Primary x 4

(4 ms)

E5B-Q 10230 10.23 Pilot Primary x 100

(100 ms)

Table 6-Galileo signals for civil aviation

3.1.3.2.2 BeiDou Navigation Satellite System (BDS or BeiDou-2) [36]

China has expand their experimental regional navigation system known as BeiDou-1 which consists of four satellites (three working satellites and one backup satellite) into a global navigation system BeiDou Satellite Navigation Experimental System called BeiDou-2 (also known as COMPASS).

This new system is under construction as of January 2015 and is planned to be a constellation of 35 satellites by 2020.In-mid 2015, China started the build-up of the third generation BeiDou system (BDS-3) in the global coverage constellation. The first BDS-3 satellite was launched 30 September 2015 and in March 2016, 4 BDS-3 in-orbit validation satellites have been also launched.

3.1.3.3 Regional GNSS 3.1.3.3.1 QZSS [34]

The Quasi-Zenith Satellite System (QZSS), is a proposed regional positioning system. It is not required to work in standalone mode but to enhance GPS covering Japan. The first demonstration satellite was launched in September 2010. In 2011 the Government of Japan has decided to accelerate the QZSS deployment in order to reach a 4-satellite constellation by 2018 while aiming at a final 7-satellite constellation in the future.

3.1.3.3.2 IRNSS [35]

The Indian Regional Navigational Satellite System (IRNSS) is an independent regional satellite navigation system being developed by Indian Space Research Organisation (ISRO) which would be under the total control of Indian Government. Five satellites are already placed in orbit and the IRNSS constellation of seven satellites is expected to operate from June 2016.

These regional GNSS can therefore be used to enhance systems. Another technique to meet the stringent ICAO requirements for GNSS is the Differential GNSS. The following section details its principle.