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))

A brief explanation of the different data fields is presented hereafter:

• Preamble. It is a unique 24–bit field, distributed over three successive words. The preamble sequence (in three consecutive words) is 01010011 (83) - 10011010 (154) - 11000110 (198). It is assured that the start of the preamble is synchronous with a 6-second GPS sub-frame. Thus, the preamble allows the receiver to achieve frame synchronisation.
• Message Type Identifier. It is a 6-bit field, which permits up to 64 different messages (identifiers 0 to 63). Depending on the message type, the information included in the Data field has different meaning. Even though 64 type messages are available, only 20 are currently defined.
• Data Field. It contains different corrections and integrity information that depends on the type of message. The specific content of this field will be addressed in the following sections.
• Parity Information. The 24 bits of the end of the message (CRC parity bits) provide protection against burst and random error. For more information on parity bits algorithm generator see MOPS.

Messages are interrelated using the Issue of Data parameters (IOD), which are present in the message data. Also satellite messages are related with satellite navigation services ephemeris via the issues of data.

The sequence of transmission of the messages is not fixed and is responsibility of the SBAS Service Provider (each second the provider decides which message is to be sent). There are requirements in MOPS concerning the refresh time for each type of information and about the alarm conditions (problems with one or more satellite information or with ionospheric information). Under an alert condition, the SBAS must repeat the message with the alert information up to three times (i.e. four messages in four seconds in total). Further details in the next sections.

Message types

SBAS messages have a 6-bit message type identifier, which informs the receiver about the information the message holds. Due to the limited size of the type identifier (6 bits), 64 types of messages are possible. Nowadays, only 20 of these messages are defined. The following table summarises the current message types and the contained information.

(Table 2: SBAS Message types)

As a rough approximation, there are three different categories of messages: messages related with satellite information, messages related with ionospheric information and other ancillary messages. Several of those messages are interrelated using the IOD parameters present in the message data.

Satellite information messages

Satellite related messages contain the differential corrections that shall be applied to each satellite to improve the satellite clock and satellite orbit provided by the existing satellite navigation services. Also GEO navigation message is broadcast as no external system provides the GEO ephemeris data.

The following table summarises the messages included in this section.

(Table 3: Satellite messages)

Message type 1

Message type 1 includes the PRN mask assignments, chosen among GPS, GLONASS and GEO satellites. Instead of sending for each correction the satellite PRN associated with, a mask is created to save space in the messages. This bit mask contains the i-th bit to 1 to inform that i-th satellite PRN is being used.

Although there are 210 slots (bits) in the mask, only a maximum of 51 can be set at a time due to constraint in the limited size available to broadcast information (message type 6 does only have enough free space to allocate UDREI figures for 51 satellites). Corrections are provided only for these satellites.

The user will read the mask and then each satellite correction will be related with the satellite via the mask contained in this message. IODP indicates the mask’s applicability to the corrections contained in the messages to which the mask applies. Message type 1 format and the PRN allocations for message type 1 (depending on the satellite navigation service provider) are defined the section A.4.4.2 of MOPS.

Message types 2 to 5

Messages of type 2, 3, 4 and 5 include satellite fast corrections and UDRE values (via the UDRE indicator, UDREI). Message type 2 includes the information related to the first 13 satellites in mask. Message type 3 contains information related to the 14th to the 26th satellite in the mask and so on. If the number of satellites in mask is less than 40, the message type 5 will not used. If the number of satellites in the mask is less than 26, the message type 4 will not be broadcast. Finally, if there are less than 6 satellites to be allocated in the last fast correction message, this message type 2 to 5 can be replaced by a message type 24.

Summarizing the above mentioned requirements, fast corrections messages that have to be broadcast depending on the number of configured satellites are reflected in the following table:

(Table 4: Different combinations of fast corrections messages)

The time of applicability of the fast corrections, which is used in fast correction computation, is defined as the start of the epoch of the SBAS Network Time (SNT) second that is coincident with the transmission of the first bit of the message block (bit belonging to the preamble) at the GEO satellite.

The message format is defined the section A.4.4.3 of MOPS, as well as the table that the SBAS user will consider to translate the UDREI to a variance σ²UDRE. Note that the status of the satellite is also included into the UDREI:

• Use: UDREIs from 0 to 13 indicates that the satellite is usable.
• Not Monitored: UDREI=14 indicates that the satellite does not appear in the mask or even appearing there are not corrections or UDRE values available for it.
• Don’t Use: UDREI=15 indicates that an inconsistency has been found for this satellite (alarm situation) or the estimated fast correction is greater than 256.0 m.

Message type 6

Message type 6 contains the integrity information for 51 satellites, which is the maximum number of satellites that can be present in the PRN mask. This message also includes IODF_j (j=2…5) to relate the UDREI to the fast corrections included in messages of type 2 to 5 or 24.

Message type 6 can be used in two different ways. On the one hand, it allows the fast corrections to be updated infrequently. In PA mode the UDREI values have a time-out of 12 seconds, while the time-out for fast corrections is between 12 and 120 seconds, depending on information sent in message type 7. On the other hand, message type 6 may be also used in case of satellite alert conditions (even if just one satellite is in alert mode). The message type 6 format is defined the section A.4.4.4 of MOPS. It has to be remarked that this message does not include an IODP and hence the link to the PRN mask is not provided within the message. The UDRE indicators included in message type 6 do apply to the satellites defined in the last received PRN mask.

Message type 7

Message type 7 includes the degradation factors in time for the fast corrections received in fast corrections messages (types 2 to 5, 24) as well as the system latency time.

The fast correction degradation factor indicators, ai_j, where j is the satellite in mask, are translated into fast correction degradation factors ai (in metres), used for fast correction degradation, and user time-out interval for fast corrections I_fc (in seconds) for the different phases of flight, following the Table A-8 of MOPS.

Message type 7 format is included in the section A.4.4.6 of MOPS.

Message type 9

Message type 9 contains the information about the GEO navigation. As GEO satellites do not belong to any satellite positioning service (GPS, GLONASS), ephemeris for those satellites are not externally available. Therefore, it is the SBAS that is in charge of providing the user with the GEO ephemeris. Keep in mind that all components are expressed in ECEF reference coordinates and the time offset is with respect to SBAS Network time (SNT).

The message format is included in the section A.4.4.11 of MOPS. In addition to the ephemeris data an URA (User Range Accuracy), as defined for GPS satellites, is also provided. An IODN used to link the GEO long-term corrections with the message type 9 ephemeris is included in the previous versions of MOPS, but it has been removed from MOPS. However EGNOS makes use of this parameter to match the long-term corrections broadcast for its GEO satellites with the appropriate navigation data broadcast through message type 9.

The GEO satellite will provide message type 9 with its own navigation (so for that no PRN nor PRN mask is included in the message). A receiver using more than a GEO will receive and decode the message type 9 for each GEO satellite from the corresponding broadcast lane.

Message type 17

Message type 17 contains the almanac for up to three GEO satellites (more than one message of this type can be broadcast if almanacs are provided for a higher number of SBAS GEO satellites). Almanacs only provide information about satellite health and status as well as its rough position. Unused slots are marked with the PRN set to zero. No IODP is needed as each satellite PRN number is included.

The message format is included in the section A.4.4.12 of MOPS. As it can be appreciated, the precision of message type 17 parameters is worse than the one defined for the parameters of message type 9. The information included in messages of type 17 does only inform the user about the existence of the GEO satellites, their location, the general service provided and heath and status for acquisition purposes. However, GEO almanac positions cannot be used in the computation of the user position: message type 9 parameters have to be considered.

Message type 25

Message type 25 includes estimations of slow varying satellite ephemeris and clock errors (in ECEF WGS-84) with respect to the ephemeris and clock parameters broadcast by the satellite navigation service. IODE is used to relate the long-term corrections with the ephemeris used to which the corrections are computed.

The satellites for which these long-term corrections are available are:

• GPS satellites
• GLONASS satellites
• GEO satellites that belong to another SBAS

Long-term corrections for GEO satellites that do belong to the SBAS will be included in message type 9. Note however that this is not EGNOS approach. In addition to this, for visible GEO satellites not belonging to the SBAS but providing long-term corrections in message type 25, these corrections in message type 25 have to be related with message type 9 coming from the other SBAS. EGNOS makes use of satellite IODN of message type 9 although this parameter has been removed from MOPS and SARPS.

The Data Field of the message type 25 (212 bits long, from bit 16 to bit 227) is divided into two parts of 106 bits each. The information contained in each message half depends on the first bit of the sequence of 106 (named Velocity Code). Only the definition of one half is included hereafter since the other part has exactly the same structure.

• If the Velocity Code is 0, no drift (orbital velocity components and clock drift) is included in this part of the message. Long-term corrections for up to two satellites are included in this half part of the message in this case.
• If the Velocity Code is 1, orbital velocity components and clock drift are included in this part of the message. As more space is needed (compared with the case in which the Velocity Code is set to 0), only long-term corrections for one satellite are included.

Message type 25 format is included in the section A.4.4.7 of MOPS.

Message type 24

Message type 24 contains both fast and long-term satellite corrections. Message type 24 can be broadcast if the number of satellites in the last fast correction message is less than or equal to 6.

Message type 24 format is included in the section A.4.4.8 of MOPS. The first half includes fast corrections for 6 or less satellites whereas the second half holds the same long-term information as each half of the message type 25 (long term corrections for one or two satellites, depending on the value of the velocity code parameter).

Message type 28

This is an optional message included in the last versions of the standards, but it is not considered in the baseline of EGNOS for the time being. Message type 28 may be broadcast to provide the relative covariance matrix for clock and ephemeris error. Each covariance matrix is updated on the same order as the long-term corrections. This is an expansion of the information contained in the 2UDRE in that it specifies the correction confidence as a function of user location. This way message type 28 provides increased availability inside the service area and increased integrity outside.

Each satellite covariance matrix is a function of satellite location, reference station observational geometry, and reference station measurement confidence. Consequently it is a slowly changing function of time and hence it is updated on the same order as the long-term corrections. Message type 28 definition is included in the section A.4.4.16 of MOPS.

Interrelations between satellite messages

The following Issues of Data (IODs) are defined in order to relate the information of previously issued messages. There is no IOD linking fast corrections to long-term corrections, as small jumps depending on the use of one or other long-term correction with the same fast correction are allowed by the SBAS.

IODP

The IODP (Issue Of Data PRN mask) relates messages 2 to 5, 7, 24, 25 and 28 with message type 1. IODP appears in each of the previous messages. It is used to connect the information contained in messages 2 to 5, 7, 24, 25 and 28 with the satellite mask defined in a message type 1 that contains the same IODP.

Each time the mask changes, which will be very infrequent and normally due to a satellite launch or a satellite that is permanently set out of service, the IODP is incremented in 1 modulo 4 (i.e. from 3 goes to 0). Satellites that are temporarily but not permanently set out of service (e.g. during a manoeuvre) will not be removed from the PRN mask but flagged as “Not Monitored” in the UDREI section. During a change in the PRN mask, and to avoid interruption of the service, the user equipment shall keep both masks in order to use information with the old and with the new IODP. In case the IODP changes in messages 2 to 5, 7, 24, 25 or 28 before the reception of the new mask, which is not the nominal situation, the information contained in them cannot be used until the reception of the new message type 1. These messages shall be stored to be used after the new PRN mask is received.

Message type 6 is related with messages of type 2 to 5 and 24 via the IODF, but it does not include an IODP and therefore the link to the PRN mask is not provided within the message. The UDRE indicators included in message type 6 do apply to the satellites defined in the last received PRN mask.

IODF

The IODF (Issue Of Data Fast Corrections) is used to link the data broadcast in messages of type 2 to 5 with the UDREI transmitted in the message type 6. There are four IODF parameters: IODF_j links message type "j" to message type 6, with j = 2, 3, 4, 5 (the IODF broadcast in message type 24 is one of these, depending on the fast corrections message that it replaces).

In the message type 6 the four IODFs are included. Every time a new message type 2 to 5 is sent, the IODF_j is incremented in 1 unit between 0 and 2. The value IODF_j = 3 is usable under alarm condition and means that UDREI values apply to all active data from the corresponding fast correction message type (message type "j").

IODE/IODC

For GPS satellites:

The IODE (Issue Of Data Ephemeris) included in message type 25 (and also in the long-term part of message type 24) links the long-term orbit and clock corrections contained in the SBAS message to the GPS satellite broadcast ephemeris with the same IODE. For GPS satellites the IODE is defined as the 8 least significant bits of the IODC defined for each ephemeris in the GPS ICD.

The user shall maintain at least two GPS ephemerides. If the GPS IODE does not match the long-term correction IODE, this is an indicator that a new GPS ephemeris is being broadcast. The user shall continue using the previous ephemeris until the reception of long-term correction with the new IODE.

For GLONASS satellites:

As no IODE is included in GLONASS ephemerides, an ancillary algorithm has been defined to link these ephemeris and the long-term corrections broadcast by the SBAS. This algorithm is defined in SARPS but not in MOPS (GLONASS constellation in not augmented by WAAS and there are no plans for doing it in the future).

For GEO satellites:

Long-term corrections have only to be broadcast for GEO satellites that do not belong to the SBAS but to another SBAS. The way to link the GEO long-term corrections to the GEO ephemeris (message type 9 broadcast by that satellite, but filled by another SBAS) is to use the IODN defined in message type 9 as a GPS-like IODE. Although the 8 bits of the IODN field have been left spare in the last versions of the applicable standards, EGNOS makes use of the IODN to match the GEO long-term corrections with the appropriate message type 9 ephemeris.

Ionospheric information messages

The ionospheric corrections that shall be applied to the GNSS measurements depend on the line of sight satellite–user (the path the signal uses during the transmission). A grid 350 km above the WGS-84 ellipsoid Earth approximation is defined, with ionospheric delay corrections broadcast for those special points, known as Ionospheric Grid Points (IGPs). Ionospheric delay corrections are broadcast as vertical delay estimates applicable to L1 signal (ionospheric delay depends on the frequency of the signal) at these points.

Knowing the location of these special points and the estimated ionospheric delay for them, the user can compute for each measurement the ionospheric delay interpolating among the IGPs located in the neighbourhood of the line of sight user-satellite corresponding to this measurement. The following table summarises the messages included in this section.

(Table 5: Ionospheric related messages)

The IGPs that constitute the interpolation grid are predefined, divided in 11 numbered bands (band 0 to band 10) on a Mercator projection map of the Earth surface. Bands 0 to 8 are vertical, while bands 9 and 10 are defined around the poles. A total of 2192 IGPs are considered. Because of the large variation in the ionosphere vertical delay due to the solar activity, IPGs are more densely defined at lower latitudes.

Within each band between 0 and 8, IGPs are numbered from 1 to 201, starting from the South-West corner up each longitude column of the band from South to North and continuing for each column from West to East from the bottom of each column (there is not an IGP 201 in band 8). Within bands 9 and 10, IGPs are numbered from 1 to 192. The IGPs are numbered counting eastward from the western corner closest to the equator along each latitude row of the band (from West to East) and continuing for each row towards the poles. IGP coordinates are defined in the Table A-14 of MOPS.

Message type 18

Messages type 18 include the ionospheric mask. Each message contains the mask information of a band. A bit set to 1 indicates that ionospheric correction information is being provided for that IGP.

Also the IODI is included in the message. IODI range is from 0 to 3, changing each time the IGP mask is modified, which is expected to happen rarely. The user will link the corrections in messages of type 26 with the band definition in message type 18 using the IODI. As SBAS is a wide area, but local system though, the system will broadcast vertical ionospheric delays only for a restricted set of IGPs. In this sense:

• Bands that are not used by the SBAS (i.e. band 0 in EGNOS) do not have to be broadcast in a dedicated message type 18 with all IGPs set to 0.
• If the ionospheric band contains less than 201 IGPs, the IGP mask slots corresponding to those bits that represent IGPs that do not exit are set to 0.

The receiver uses the parameter “Number of Bands being broadcast” to know if there are more bands to be acquired or all available data have been received yet. It is necessary to get all messages of type 18 (the complete IGP mask) prior to use the information broadcast through messages of type 26. The format of message type 18 is included in the section A.4.4.9 of MOPS.

Message type 26

Messages of type 26 provide the Ionospheric Delay Corrections (GIVD) and their accuracy (σ²_GIVE) in terms of GIVEI (GIVE Indicators) for the IGPs that are configured in the mask. In order to match the ionospheric information with the applicable IGP mask the IODI parameter is also included. The format of these messages is included in the section A.4.4.10 of MOPS. The table that the SBAS user will use to translate the GIVEI to a variance σ²_GIVE is also included in this section.

As only 15 IGPs fit in message type 26 meanwhile each ionospheric band has up to 201, the bands are divided into blocks. Each block holds 15 IGPs. Block 0 contains the corrections for the first 15 IGPs in the mask (not in the band), block 1 contains the correction for 16 to 30 IGPs in mask … Each band is therefore divided into a maximum of 14 blocks (it is possible and normal to be divided in less blocks as a SBAS is not able to observe a whole band).

The status of an IGP, like for a satellite, can be:

• Use: There are available IGP Delay Estimate and GIVEI.
• Not Monitored: IGP does not appear in mask or even appearing in mask there is not available Delay Estimate or GIVEI.
• Don’t Use: An inconsistency has been found for this IGP (alarm situation) or the estimated delay is greater than 63.750 m. The user has to be alerted of the strange behaviour in the IGP for not to use it. IGP Delay Estimate is always positive since the ionosphere produces a positive delay in signal code. An IGP Delay Estimate of 63.875 m (byte 11111111 in the message field) indicates an alarm situation for the IGP.

The algorithm to compute the ionospheric delay correction and the upper bound of the residual error for a given line of sight based on the corrections and error bounds that are broadcast in messages of type 26 is included in A.4.4.10 of MOPS.

Other messages

This section includes the messages that are not directly related with satellite corrections or with ionospheric corrections. The information included in these messages is ancillary SBAS information, useful to compute the user navigation position in PA operations, to compute precisely the UTC time, or to degrade the UDRE over selected regions.

The following table summarises the messages included in this section.

(Table 1: Other messages)

Message type 0

This message will be used during SBAS testing. After the reception of message type 0, all ranging and correction information obtained from the SBAS must be discarded for safety critical applications. The existence of a message type 0 indicates that the system integrity performances are not assured.

MOPS has introduced a new potential use for the message of type 0, which is optional. During SBAS testing, the contents of message type 2 can be included in message type 0 and in such case it is not necessary to transmit this fast correction message. This information can be used for non-safety critical applications.

Message type 10

Message type 10 contains degradation parameters. These parameters are not satellite or IGP dependent so only one message of this type will be needed for the SBAS. The specific format is defined in the section A.4.4.6 of MOPS.

These parameters are used in the computation of the degradation parameters for fast and long-term corrections σ²_flt and ionospheric corrections σ²_ionogrid (defined in sections A.4.5.1 and A.4.5.2 of MOPS) for Precision Approach operations.

Message type 12

Message type 12 contains information about time-offset parameters between different system times. The first 104 bits contains the UTC parameters in the format defined in the GPS ICD. Then GPS time is included (in Seconds of Week and Week Number format) and a bit to indicate if GLONASS time offset is provided or not.

In addition to this, a time offset parameter to steer GLONASS time into SBAS time has been included in SARPS. In MOPS this parameter is not yet defined. EGNOS service provider makes use of this offset for the computation of GLONASS long-term clock correction, and EGNOS users need it to combine GPS and GLONASS measurements in the determination of their position and time. Message type 12 format is defined in the section A.4.4.15 of MOPS.

Message type 27

This is an optional message not considered in the baseline of EGNOS. Messages of type 27 are used to increase the UDRE values that are broadcast through messages of type 2 to 5, 6 or 24 over several selected areas. This degradation is incompatible with the one defined in message type 28. The message contains the value of δUDRE factor (a multiplier factor) to be applied to integrity monitoring algorithms depending on the user location: inside any defined region or outside of all regions. Each message contains up to 5 regions. If more regions are defined in the SBAS, more than one message type 27 will be broadcast. Different messages of type 27 are linked between them via IODS parameter (Issue of Data Service Message). Each time a parameter in any message type 27 of the group is changed, the IODS is incremented, being the effective range is from 0 to 7. Two messages with the same IODS have the same value for δUDRE factor outside.

Priority code is used to allow the overlapping of the regions. A user situated in the intersection of two or more regions will use the δUDRE factor for the region with the higher priority code. In case of equality in priority code, the user will use the smallest δUDRE factor as this results in better performance. Messages type 27 format is defined in the section A.4.4.13 of MOPS.

Message type 62

EGNOS does not transmit this optional message, which can be broadcast for SBAS internal test only. Upon reception of this message, the user will continue using the GEO broadcast data and ranging capabilities, but no additional information is acquired.

Message type 63

EGNOS does not transmit this optional message, which can be broadcast in case no other message is available to be sent. Upon reception of this message, the user will continue using the GEO broadcast data and ranging capabilities, but no additional information is acquired.

Messages time-outs and alerts

Message time-outs

The following table (which is just a mimic of Table A-25 of MOPS) includes the maximum update interval requirements for the SBAS data broadcast through the different messages, not for the messages themselves. There are various types of phase of flight: En Route, Terminal, Non-Precision Approach (NPA) and Precision Approach (PA), being PA the most restrictive one. Time-outs for corrections, integrity and GEO navigation data corresponding to the different phases of flight are also included in the table (they limit the interval of applicability of SBAS data):

(Table 7: SBAS data broadcast intervals)

(*) Message type 0 must be sent only if the system is not usable for safety-critical applications. After the reception of a message type 0 the SBAS signal shall be de-selected and all data received for one minute shall be discarded.

(**) Message type 1 (PRN Mask) and Message type 18 (IGP Mask) should be repeated several times whenever the satellite or ionospheric mask is changed respectively. This will ensure that all users receive the new mask before it is applied maintaining high continuity (i.e. in EGNOS message type 1 is sent four times within one minute upon a change of PRN mask).

Alert conditions

An alarm situation can be defined as a non-expected behaviour of the SBAS corrections. Two types of alarms are possible:

• Satellite alert: the status of a satellite is set to "Don't Use" due to an integrity risk detected at pseudorange level or an out of range of its fast correction or UDRE figure.
• Ionospheric alert: the status of an IGP is set to "Don't Use" due to an integrity risk detected at pseudorange level or an out of range of its delay estimation or GIVE figure.

Ionospheric alerts are always broadcast in messages of type 26. Satellite alerts can be sent in fast correction messages (types 2 to 5, 24) or in the integrity message (type 6). The IODF for the block in which the satellite is included will be set to 3 whether the message used to send the alert is a fast correction message, but this is not mandatory if message type 6 is used instead. Every alert condition will be repeated three times after the notification of the alert condition, that is, during an alert situation the message with the alarm information must be sent four times in four seconds, with the same information in all these epochs. Subsequent messages can be broadcast at the normal update rate, as defined in the previous section.

Summary

The aim of this article is to provide a résumé of what the structure of the signal broadcast by EGNOS is, based on the Appendix A of the RTCA MOPS DO-229-C and the Appendix B of the ICAO SARPs. It goes towards those readers not familiarised with aviation equipment standards, especially with MOPS and SARPs.

It presents the data that are broadcast by EGNOS through the GEO satellites and how this information is distributed in their signals. In addition to the GEO L1 ranging signal (GPS-like), EGNOS does broadcast differential corrections for GPS/GLONASS/GEO, ionospheric delay estimations for a set of predefined points defined on a grid 350km above the WGS-84 ellipsoid Earth approximation (IGPs), and integrity information to inform about the goodness of the service provided. A model to obtain the tropospheric delays is used instead of any kind of broadcast data due to the local character of the troposphere.

EGNOS does provide all this information through messages (blocks of 250 bits) encoded in the GEO signal. These messages are composed of a preamble, a type identifier, the message body and finally CRC parity information. The maximum update intervals for the data contained in the different messages are predefined and EGNOS Service Provider accounts for them for transmitting all the information in due time. From a total number of 64 possible message types (8 bits), nowadays there are only 20 defined and some of them are optional (types 27, 28, 62 and 63). Defined messages can be roughly separated in three categories:

(a) messages related with satellite information (types 1, 2 to 5, 6, 7, 9, 17, 24, 25 and 28), which contain the differential corrections that shall be applied to each satellite to improve the satellite clock and satellite orbit provided by the existing navigation services and the corresponding integrity bounds. Also GEO navigation message is broadcast as no external system provides the GEO ephemeris (type 9),

(b) messages related with ionospheric information , and (types 18 and 26), which contain the vertical delay estimates for the IGPs (valid for the user to remove the ionosphere contribution from GNSS L1 measurements) and the corresponding integrity bounds.

(c) other ancillary messages (types 0, 10, 12, 27, 62 and 63), which provide other kinds of useful information, as for instance message type 12, which allows to EGNOS users to compute precisely the UTC time.