The EGNOS SBAS Message Format Explained

EGNOS
Title	The EGNOS SBAS Message Format Explained
Author(s)	Daniel Porras Sánchez & César Pisonero Berges, GMV S.A., Spain.
Level	Basic
Year of Publication	2006

Introduction

This article contains a brief summary of EGNOS signal structure as described in RTCA MOPS DO-229-C “Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation System airborne equipment” (particularly in its Appendix A “Signal characteristics and format”) just to allow the reader to have a first contact with the specification of the SiS that is applicable for Satellite Based Augmentation Systems (SBAS), in particular for the European EGNOS.

It is worthwhile to highlight that the SBAS SiS specification is also detailed in the ICAO SARPs “Standards and Recommended Practices”, Appendix B “Detailed technical specifications for the Global Navigation Satellite System (GNSS)”. These two standards are nearly aligned in what regards to the SBAS SiS specification but still some differences remain. In fact, some topics such as the augmentation of GLONASS constellation by EGNOS is covered by the ICAO SARPs but not by the MOPS (WAAS does not provides augmentation to GLONASS satellites) and hence both MOPS and SARPs will be referenced along the following paragraphs.

For a more precise knowledge of EGNOS SiS specification the reading of the aforementioned annex of MOPS is recommended. The purpose of the material presented hereafter is just to summarise the contents of MOPS Appendix A in order to make easier the first touch with it for those readers that are not familiarised with aviation equipment standards.

Summary of EGNOS SiS

SBAS broadcast data

Every satellite-based wide area augmentation system, as the European system EGNOS, does provide ranging signals transmitted by GEO satellites, differential corrections on the wide area and additional parameters aimed to guarantee the integrity of the GNSS user:

• GEO Ranging: transmission of GPS-like L1 signals from GEO satellites to augment the number of navigation satellites available to the users.
• Wide Area Differential (WAD): differential corrections to the existing GPS/GLONASS/GEO navigation services computed in a wide area to improve navigation services performance.
• GNSS/Ground Integrity Channel (GIC): integrity information to inform about the availability of GPS/GLONASS/GEO safe navigation service.

SBAS shall provide the following information:

• Satellite orbit and clock corrections to the existing satellite navigation services (GPS, GLONASS and GEO), as well as the estimation of errors associated to satellites (UDRE).
• Ionospheric corrections for a given grid of points, as well as the estimation of errors associated to ionosphere (GIVE).
• Tropospheric corrections. Satellite orbit/clock corrections and ionospheric corrections are dynamically modelled. The SBAS shall communicate the user the corrections that are available to be used by the receiver. The information of the models is packed on messages to be sent to the user. On the other hand, tropospheric corrections are statically modelled, which means that corrections are tabulated and the information does not depend on any external behaviour but the user position (a mean troposphere is assumed). The algorithm for computing the tropospheric correction is available to the global community (section A.4.2.4 of MOPS).

In addition to this, navigation data for each GEO satellite supporting ranging service is also transmitted through SBAS. SBAS interacts with the user via the Signal in Space (SiS). The way the SBAS delivers to the user the aforementioned corrections and integrity data as well as some ancillary information (timing, degradation parameters, etc.) is through messages encoded in the signal. These messages are sent each second with a data rate of 250 bits, as it is explained in the following section.

Signal data structure

The raw navigation message of the SBAS contains 500 bits. These raw data are ½ convolutional encoded with a FEC code, which means that 250 bits of information are available every second at user level. The 250-bit message has different parts, including an 8-bit preamble and 24 ancillary bits to include redundancy and error checking within the message. The following table and figure summarise the message format. Bit 0 is considered the most significant bit, i.e. the bit that is transmitted and received first.

(Table 1: SBAS Message format (components))

(Figure 1: SBAS Message format (lengths in bits))