Global Navigation Satellite System (EGONS)

Introduction

Global Navigation Satellite System (GNSS) is an umbrella term that include vase range of satellite navigations system that provide geospatial and positioning data in a global range covering, systems such as: GPS (USA) , GLONASS (Russia), QZSS (Japan) ...etc.

Here well define the European Geostationary Navigation Overlay Service (EGONS) as an example of GNSS,its history, system segments, explore satellites names and their signals, navigation, Reference from and time and accuracy.

Definition

The European Geostationary Navigation Overlay Service (EGNOS) is a satellite based augmentation system (SBAS) developed by the European Space Agency and EUROCONTROL. It was intended for general Purpose application and freely accessible. also supply GPS, GLONASS and Galileo by sending report about their reliability and accuracy of their positioning data and provide corrections

History

The system started in 2009 after the European Commission signed a contract with the European Satellite Services Provider to run it. And the official start was on the 1st of October 2009.

In 2011 the system was certified to be used in safety of life applications. And In 2012 EGONS data service became available.

Under a project called EGONS Service Extension to South Africa (ESESA) there were efforts to extend EGONS Coverage to the south of Africa region.

Segments of the system

EGNOS consists of mainly four functional segments:

1. Ground segment

The ground segment consist of: 

1. A network of 40 Ranging Integrity Monitoring Stations (RIMS): its main function is to collect data from GPS and sent them every second to the Central Processing Facilities (CPF) of each (MCC).

2. 2 Mission Control Centres (MCC): Acts as EGNOS system’s brain, it receivers information from (RIMS) ns generate corrections.

3. 6 Navigation Land Earth Stations (NLES): for redundancy purposes there are two NILESs for each GEO, it transmit the messages received from CPF to the GEO.

4. The EGNOS Wide Area Network (EWAN): provides the communication network for all the components of the ground segment.

2. Support segment

It performs the activities of system operations planning and performance assessment, and they are:

1. The Performance Assessment and Checkout Facility (PACF): provides support in performance analysis, troubleshooting and operational procedures, as well as upgrade of specification and validation, and support to maintenance.

2. The Application Specific Qualification Facility (ASQF): provides civil aviation and aeronautical certification authorities with the tools to qualify, validate and certify the different EGNOS applications.

3. Space Segment

It consist of three geostationary satellites providing corrections and integrity information for GPS satellites in the L1 frequency band (1575,42 MHz). And at any where and time at least two of the three GEOs provide an operational signal.

4. User Segment

EGNOS receivers provide their users’ their positions with integrity and accurately,  and the user must use an EGNOS-compatible receiver to receive EGNOS signals.

Satellites

Satellites list

The satellites of the space segment are as follow:

Satellite	NMEA/PRN	Signals	Location	Status
Inmarsat 3-F2 (Atlantic Ocean Region-East)	#33 / #120	L1	15.5°W	active
ARTEMIS	#37 / #124	-	21.5°E	retired
Inmarsat 4-F2 (Europe Middle East Africa)	#39 / #126	-	64°E	testing
Inmarsat 3-F1 (Indian Ocean)	#44 / #131	-	64.5°E	retired
SES-5 (Sirius 5 or Astra 4B)	#49 / #136	L1 / L5	5.0°E	testing
Astra 5B	#36 / #123	L1 / L5	31.5°E	active

Table 1: Given data

Satellites signals

The satellites transmit right-hand circularly polarised (RHCP) signals in the L1 band (1575.42 MHz). The broadcast signal is a combination of a 1023-bit PRN navigation code of the GPS family and a 250 bits per second navigation data message carrying the corrections and integrity data elaborated by the EGNOS ground segment.

Navigation Message

The Navigation Data is composed of a number of different Message Types:

Message Type	Contents	purpose
0	Don’t Use (SBAS test mode)	Discard any ranging, corrections and integrity data from that PRN signal. Used also during system testing
1	PRN Mask	Indicates the slots for GPS and EGNOS GEO satellites provided data
2-5	Fast corrections	Range corrections and accuracy
6	Integrity information	Accuracy-bounding information for all satellites in one message
7	Fast correction degradation factor	Information about the degradation of the fast term corrections
94	GEO ranging function parameters	EGNOS GEO satellites orbit information (ephemeris)
10	Degradation parameters	Information about the correction degradation upon message loss
12	SBAS network Time/UTC offset parameters	Parameters for synchronisation of EGNOS Network time with UTC
17	GEO satellite almanacs	EGNOS GEO satellites Almanacs
18	Ionospheric grid point masks	Indicates for which geographical point Ionospheric correction data is provided
24	Mixed fast/long-term satellite error corrections	Fast-term error corrections for up to six satellites and long-term satellite error correction for one satellite in one message
25	Long-term satellite error corrections	Corrections for satellite ephemeris and clock errors for up to two satellites
26	Ionospheric delay corrections	Vertical delays/accuracy bounds at given geographical points
27	EGNOS service message	Defines the geographic region of the service
63	Null message	Filler message if no other message is available

Table 2: Message types (MT) and purposes.

Reference Frame and time

Position coordinates and time information are referenced to a reference systems established by the EGNOS system which is called the EGNOS Network Time (ENT) timescale and the EGNOS Terrestrial Reference Frame (ETRF) which is an independent realisation of the International Terrestrial Reference System (ITRS2).

References

1. EGNOS Open Service (OS) Service Definition Document.

2. https://en.m.wikipedia.org/wiki/European_Geostationary_Navigation_Overlay_Service

3. https://www.gsa.europa.eu/european-gnss/egnos/egnos-system