View source for EGNOS Signal Structure

{{Article Infobox2
|Category=EGNOS
|Authors=GMV
|Level=Basic
|YearOfPublication=2011
|Logo=GMV
|Title={{PAGENAME}}
}}
[[EGNOS General Introduction|EGNOS]] (European Geostationary Navigation Overlay Service) is the European satellite-based augmentation service (SBAS) that complements the existing satellite navigation services provided by the US Global Positioning System (GPS). The EGNOS signal-in-space is broadcast by Geostationary Earth Orbit (GEO) satellites in the L1 frequency, centred at 1575.42 MHz, meeting stringent standards established by organizations like ICAO and RTCA.

==EGNOS SIS Interface Characteristics==
[[File:GEO FootPrints.JPG| Inmarsat and Artemis EGNOS geostationary broadcast areas  |340px|thumb|right]]

The EGNOS Signal In Space format is compliant with the ICAO SARPs for SBAS<ref>SARPS Amendment 77, Annex 10 to the Convention on International Civil Aviation, Aeronautical Telecommunications: International Standards and Recommended Practices, Volume 1, Radio Navigation Aids, November 2002.</ref> as stated in the [[EGNOS Open Service | EGNOS Open Service]] Service Definition Document and the [[EGNOS Safety of Life Service|EGNOS Safety of Life]] Service Definition Document.<ref name=" EGNOS OS SDD">[http://www.essp-sas.eu/service_definition_documents EGNOS Open Service Definition Document]</ref><ref name=" EGNOS SOL SDD">[http://www.essp-sas.eu/service_definition_documents EGNOS Safety of Life Service Definition Document]</ref> The EGNOS SIS interface characteristics comprise carrier and modulation radio frequency, message structure, protocol and content of the EGNOS message (detailed in the [[EGNOS Messages | EGNOS Message Format ]] definition).

EGNOS uses three GEO satellites at different longitude to broadcast its signals across the whole EGNOS service area, namely two Inmarsat-3 satellites, one positioned over the Atlantic and the other over Africa, and the ESA's Artemis satellite, located over Africa as well.<ref name=" EGNOS User Guide for Developers">[http://ec.europa.eu/enterprise/policies/satnav/egnos/files/brochures-leaflets/egnos-user-guide_en.pdf   EGNOS User Guide for Developers]</ref> Contrary to the case of core navigation space vehicles with active navigation payloads, the EGNOS GEO satellites carry transponders relaying the signal transmitted from the [[EGNOS Ground Segment|ground segment]] uplink stations.
The elements of the EGNOS satellite infrastructure deployed and their orbital location are detailed in the following table. 

{| class="wikitable" | align="center"
|-
!align="center" |Satellite || align="center" |  PRN   || align="center" |ID(NMEA) || align="center" |Position
|-
|align="center" |'''ARTEMIS''' || align="center" |  124   || align="center" |37 || align="center" |21.5E
|-
|align="center" |'''INMARSAT AOR-E''' || align="center" |  120   || align="center" |33 || align="center" |15.5W
|-
|align="center" |'''INMARSAT AOR-W''' || align="center" |  126   || align="center" |39 || align="center" |25E
|}

The PRN (Pseudo Random Noise) represents the 1023-bit pseudorandom code employed in the generation of the SIS corresponding to each geostationary satellite. Besides, the footprint of the 3 Geostationary satellites can be observed in the illustration.

It is noted that EGNOS does not provide GEO ranging.<ref name=" EGNOS OS SDD"/>

==EGNOS SIS RF Characteristics==
The EGNOS GEO satellites transmit right-hand circularly polarised (RHCP) signals in the L band at 1575.42 MHz (L1). 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. 

This raw navigation message is ½ convolutional encoded with Forward Error Correcting (FEC) Code resulting in a 500 symbol/second EGNOS data stream.

This data stream is added modulo-2 to a 1023-bit PRN code, which will then be biphase shift-keyed (BPSK) modulated onto the L1 carrier frequency at a rate of 1.023 Mega-chips/second (Mcps). <ref name=" RTCA MOPS DO 229">Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation System Airbone Equipment</ref>

The EGNOS SIS is such that, at all unobstructed locations near ground level from which the satellite is observed at an elevation angle of 5 degrees or higher, the level of the received RF signal at the output of a 3dBi linearly polarised antenna is within the range of –161 dBW to –153 dBW for all antenna orientations orthogonal to the direction of propagation. Besides, spurious transmissions will be at least 40dB below unmodulated carrier power over all frequencies.

It is intended that future geostationary satellites used for EGNOS replenishment will broadcast higher minimum signal power levels in order to improve the acquisition and tracking performance of the user receiver.

The most relevant EGNOS signal characteristics are detailed hereafter:<ref name=" RTCA MOPS DO 229"/>
* '''Carrier Phase Noise''': The phase noise spectral density of the unmodulated carrier is such that a phase locked loop of 10Hz one-sided noise bandwidth is able to track the carrier to an accuracy of 0.1 radians rms.
* '''Signal Spectrum''': The broadcast signal is at GPS L1 frequency of 1575.42 MHz. At least 95% of the broadcast power is contained within a +/- 12MHz band centered at the L1 frequency. The bandwidth of the signal transmitted by an EGNOS satellite is at least 2.2MHz. 
* '''Doppler Shift''': The Doppler shift, as perceived by a stationary user, on the signal broadcast by EGNOS GEOs is less than 40 meters per second (≈210 Hz at L1) in the worst case (at the end of life of the GEOs). The Doppler shift is due to the relative motion of the GEO.
* '''Carrier Frequency Stability''': The short term stability of the carrier frequency (square root of the Allan Variance) at the input of the user´s receiver antenna will be better than 5x10-11 over 1 to 10 seconds, excluding the effects of the ionosphere and Doppler.
* '''Polarization''': The broadcast signal is right-handed circularly polarized. The ellipticity will be no worse than 2 dB for the angular range of ±9.1o  from boresight.
* '''Correlation Loss''': Correlation loss is defined as the ratio of output powers from a perfect correlator for two cases: 1) the actual receiver EGNOS signal correlated against a perfect unfiltered PN reference, or 2) a perfect unfiltered PN signal normalized to the same total power as the EGNOS signal in case 1, correlated against a perfect unfiltered PN reference. The correlation loss resulting from modulation imperfections and filtering inside the EGNOS satellite payload is less than 1 dB.

==Navigation Payload Specifications==
The following tables show key performance parameters of the Inmarsat-3 and Artemis Satellite Navigation Payloads:<ref name=" Inmarsat-3 Satellite Navigation Payload ">Inmarsat-3 Satellite Navigation Payload; G.V. Kinal & O. Razumovsky (Inmarsat, London, United Kingdom)</ref> <ref name=" Artemis Satellite Navigation Payload ">The ESA Artemis Satellite-Navigation Mission: In Orbit Testing and Use in EGNOS; J.Ventura Traverset, P.Y.Dussauze, C.Montefusco & F.Torán (GNSS-1 Project Office, ESA, Toulouse, France); C.Lezy, F.Absolonne, B.Demelenne (ESA Redu Station, Redu, Belgium); A.Bird (Artemis Project, ESA/ESTEC, Noordwijk, The Netherlands)</ref>

{| class="wikitable" | align="center"
|-
! || align="center" | Inmarsat-3 || align="center" |  Artemis 
|-
| align="center" |  L-Band EIRP at edge of coverage (dBw)   || align="center" |27.5 || align="center" |>27
|-
| align="center" |  BandWidth   || align="center" |2.2MHz || align="center" |4MHz
|-
| align="center" |  Polarisation   || align="center" |RHCP || align="center" |RHCP
|-
| align="center" |  Frequency Stability (over 10s)   || align="center" |4.3x10-11(AOR-E), 1.1x10-11 (AOR-W)) || align="center" |2x10-12
|}

The measured parameters were tested after launch of the specified satellites and, in the case of Artemis, payload specifications were verified from ESA´s Redu ground station in Belgium during the In-Orbit Test (IOT) campaign.

==SBAS Signal Generators==
Nowadays, there exists operational commercial off-the-shelf equipment to generate the SBAS signal to be uplinked to the GEO for relay.<ref name=" Novatel GUS-Type 1 Signal Generation Product Sheet ">[http://webone.novatel.ca/assets/Documents/Papers/GUSSignalGenerator.pdf    Novatel GUS-Type 1 Signal Generation Product Sheet ]</ref>

In addition, there are other flexible platforms useful to generate test signals for development and evaluation of user SBAS receivers.<ref name=" Spirent GSS4100 Signal Generator Product Reference Sheet">[http://www.testequipmentconnection.com/specs/Spirent_GSS4100.PDF Spirent GSS4100 Signal Generator Product Reference Sheet]</ref> These products allow to select specific navigation satellite systems, to define influencing error sources, to choose signal frequency and modulations schemes as well as the RF Front end and filter characteristics.

==Notes==
<references group="footnotes"/>

==References==
<references/>

[[Category:EGNOS|Signal]]
[[Category:EGNOS Signal Structure|Signal]]