EGNOS Signal Structure

EGNOS
Title	EGNOS Signal Structure
Edited by	GMV
Level	Basic
Year of Publication	2011

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.

The next evolution of EGNOS; namely EGNOS V3, will provide dual-frequency signals on both bands L1 and L5 augmenting both GPS and Galileo constellation data.

EGNOS SIS Interface Characteristics

Coverage of EGNOS of GEOs with 5 degrees masking angle

The EGNOS Signal In Space format is compliant with the ICAO SARPs for SBAS as stated in the EGNOS Open Service Definition Document and the EGNOS Safety of Life Service Definition Document. The EGNOS SIS interface characteristics comprise carrier and modulation radio frequency, message structure, protocol and content of the EGNOS message (detailed in the 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; the footprint of the 3 Geostationary satellites can be observed in the illustration.

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 ground segment uplink stations.

EGNOS V3 will provide dual-frequency signals on both bands L1 and L5. It will provide augmented operational SoL services over Europe that improve the accuracy and availability of user positioning services. Also it will provide crucial integrity messages to EGNOS users with alerts within a few seconds in case of system degradation, consolidating EGNOS’ position as one of the leading edge GNSS Systems in the future. Will also ensure a full continuity of service for the next decade and will be the first operational SBAS implementing the dual frequency and multi constellation world standard, replacing EGNOS V2 which has been in operation since 2011. For this a new generation of user terminal will be needed according with the new standards for Dual Frequency Multi Constellation (DFMC).

EGNOS SIS RF Characteristics

As any other SBAS, EGNOS broadcasts its augmentation information in L1 band, at 1575.42 MHz, using right-hand circular polarization (RHCP). Each individual second, EGNOS transmit a navigation message containing 250 bits of information. 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).

For unobstructed view-lines with elevation angles greater than 5 degrees on the Earth surface, the EGNOS signal is received by 3dBi linearly polarised antennae with a power in the –161 dBW to –153 dBW range, assuming an orientation of the antenna orthogonal to the propagation direction. In addition, spurious transmissions are bounded at least 40dB below unmodulated carrier power. It is planned to enhance the signal-to-noise ratio in future replenishments of the EGNOS space segment, with the intention to improve the tracking performance at user level.

As stated in MOPS DO 229, the most relevant EGNOS signal characteristics are detailed hereafter:

• 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.

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.

In addition, there are other flexible platforms useful to generate test signals for development and evaluation of user SBAS receivers. 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.