Galileo Navigation Message: Difference between revisions

Latest revision as of 11:14, 4 June 2021

Fundamentals
Title	Galileo Navigation Message
Author(s)	J. Sanz Subirana, JM. Juan Zornoza and M. Hernandez-Pajares, University of Catalunia, Spain.
Level	Basic
Year of Publication	2011

The Galileo satellites broadcast different types of data in four navigation messages: the F/NAV navigation, the I/NAV navigation message, the Commercial Navigation Message (C/NAV) and the Governmental Navigation Message (G/NAV).

The G/NAV navigation message does not belong to the public domain and the C/NAV is not yet defined. Please note that the formerly Commercial Service is now known as High Accuracy Service.

The details of the Galileo Signal Status are provided in the Galileo OS SIS Interface Control Document (ICD). ^[1] No other ICD documents have been published to date.

General Structure and Contents

The Galileo Signal-In-Space data channels transmit different messages type according to the general contents identified in the following table. The F/NAV types of message correspond to the OS and the I/NAV types of messages correspond to both OS and CS (current High Accuracy Service). The following table indicates the signal component associated to each navigation message type. Let us also note that the Return Link Message from Galileo SAR service is provided as part of I/NAV data.

Source: Galileo OS SIS ICD ^[1]

The complete navigation message data are transmitted on each data component as a sequence of frames. A frame is composed of several sub-frames, and each sub-frame is composed of several pages. The page is the basic structure for building the navigation message.

All data values encoded using the following bit and byte ordering criteria:

For numbering, the most significant bit/byte is numbered as bit/byte 0.
For bit/byte ordering, the most significant bit/byte is transmitted first.

According with the current published ICD ^[1] both F/NAV and I/NAV navigation messages are called super-frame and broadcast as a sequence of frames that are composed of several sub-frames which in turn is composed by several pages.

The general structure of the navigation messages are common but the number of sub-frames and pages as well as the duration of each page are not the same, as represented in the following table.

Message	Channel	Data rate (bps)	Page Duration (s)	Pages in a sub-frame	Sub-frames in a frame
F/NAV	E5a-I	25	10	5	12
I/NAV	E1B, E5b-I	125	2	15	24

This arrangement allows accomplishing the three different main categories of data to be transmitted at different rates:

Fast rate (for urgent data, such as integrity): page.
Medium rate (Ephemeris, Clock Correction): sub-frame.
Slow rates (Almanacs): frame.

The page starts with a Synchronisation Word (SW) followed by the interleaving FEC (Forward Error Correction) coded navigation data and ends with tail bits for the FEC decoding. In addition both navigation messages transmit a CRC data field in order to detect corrupted data. The size of this field is different in each navigation message. Three levels of error coding are applied to the Galileo Message Data Stream:

A Cyclic Redundancy Check (CRC) with error detection capabilities after recovery of the received data;
A one-half rate Forward Error Correction (FEC). Tail Bits (sequence of zeros) to allow Viterbi decoding;
Block Interleaving on the resulting frames: provides robustness to the FEC decoding algorithm since in presence of a burst of erroneous bits it is converted into small errors in several pages. This scheme allows reducing the bit error ratio in the increased data rates.

As referred, the FEC coded symbols are transmitted interleaved within the page and because of that the navigation data can only be decoded when the complete interleaving FEC coded part is received.

The following table summarizes the parameters transmitted by FNAV and INAV messages.

As can be seen in the table above, the parameters transmitted in F/NAV and I/NAV have the same size except for signal and message specific parameters, such as "Issue of Data (IOD)" and "Navigation Data Validity and Signal Health Status". In the case of "Satellite Almanac" set, the size is also different because the set includes the "Satellite signal health status". The Galileo ephemeris parameters are Keplerian-like orbital elements as in GPS. The nominal period update is 3 hours, being valid for a 4 hours time interval. The 1-hour overlap interval is intended to help against short outages or delays. The Galileo Almanac is also similar to the GPS and GLONASS ones.

F/NAV Structure

The F/NAV navigation message is transmitted on channel E5a-I at a rate of 25 bps. The F/NAV message structure is shown in the following figure, where the duration of each entity is indicated.

Each frame of the F/NAV message has a duration of 600 seconds and it is composed by 12 sub-frames. In turn, each sub-frame has a duration of 50 seconds and it is composed by 5 pages with a duration of 10 seconds. The page itself comprises 3 main fields:

Synchronisation pattern: it is no encoded and it has a length of 12 bits and is always 101101110000 being its purpose to allow the receiver to achieve synchronisation to the page boundary; it allows the receiver to achieve synchronization to the page boundary.
F/NAV word: has a length of 238 bits and it is the interleaved and FEC encoded part of the page that encodes the fields: Page Type (6 bits) enabling the page content identification;, Navigation Data (208 bits) and CRC (24 bits) to detect potential bit errors. The CRC is computed on the Page Type and Navigation Data fields.
Tail: has a length of 6 bits and consists of 6 zero-values that are used to enable the completion of the FEC decoding of each page.

F/NAV Navigation message structure

The Page Type field identifies the broadcast page which allows the user-receivers to react accordingly and grants the possibility of changing the pages sequencing, while keeping backward compatibility. The CRC is used as the most inner mechanism of errors detection because it is computed only on the Page Type and Navigation Data fields.

Please refer to the Galileo OS SIS ICD^[1] for detailed analysis on the F/NAV Frame layout and page contents.

I/NAV Structure

The I/NAV navigation message is transmitted on both E1B and E5b-I channels at a rate of 125 bps. The two versions of the I/NAV message are disseminated through the same structure being its content also the same but not aligned over the time.

The I/NAV message structures for the E1B and E5b-I signals use the same page layout since the service provided on these frequencies is a dual frequency service, using frequency diversity. Only page sequencing is different, with page swapping between both components in order to allow a fast reception of the data by a dual frequency receiver. However, the frame is designed to allow receivers to work also with a single frequency.

Each frame of the I/NAV message has a duration of 720 seconds and it is composed by 24 sub-frames. In turn, each sub-frame has a duration of 30 seconds and it is composed by 20 pages with a duration of 2 seconds. The page itself is composed by 3 fields:

Synchronisation pattern: has a length of 10 bits and is always 0101100000 being its purpose to allow the receiver to achieve synchronisation to the page boundary;
I/NAV page part: has a length of 114 bits and it is the interleaved FEC encoded part of the page where the navigation data is conveyed.
Tail: has a length of 6 bits and consists of 6 zero-value bits that are used to enable the completion of the FEC decoding of each page part.

The page part of the message can be even or odd and both parts are always broadcast one after the other. In channel E1B the even part is transmitted first while in channel E5b-I the even part is transmitted after. The combination of two page parts constitutes a valid set of data (Nominal Page) that has to be parsed together to get the Navigation Data. The advantage of this configuration is that a receiver decoding the I/NAV on both frequencies can decode the same pages in half the time of a single frequency receiver.

INAV Navigation message structure

Two types of I/NAV pages are defined:

Nominal pages: having a duration of 2 seconds transmitted sequentially in time in two parts of duration 1 second each on each of the E1B and E5b-I components. The first part of the page is denoted ‘even’ and the second one is denoted ‘odd’.
Alert pages: having a duration of 1 second transmitted in two parts of duration 1 second each at the same epoch over the E1B and E5b-I components. Again, the first part of the page is denoted ‘even’ and the second one is denoted ‘odd’. This transmission is repeated at the next epoch but switching the two parts between the components.

Please refer to the Galileo OS SIS ICD ^[1] for detailed analysis on the I/NAV Frame layout and page contents. Let us remark that the Galileo Search and Rescue (SAR) Return Link Message (RLM) is transmitted only in the E1-B component. The SAR field structure for the E1-B component in nominal mode is formatted according to the values stated in the Galileo SIS ICD^[1] and the Galileo SAR Service Definition Document^[2].

Galileo OS-NMA capability will be added in the Reserved 1 40 bits in E1 I/NAV message.

The Galileo OS ICD released in January 2021 ^[1] introduces new features in parts of I/NAV message transmitted on the Galileo E1-B signal component which were previously marked as spare or reserved. This means that these improvements will provide full backwards compatibility. The new features are:

Reduced Clock and Ephemeris Data (RedCED);
Reed-Solomon Outer Forward Error Correction Data (FEC2);
Secondary Synchronization Pattern (SSP).

As a result of these technical solutions, users will experience an improvement of the Galileo E1 Open Service performance in terms of Robustness and Timeliness; a significant Time To First Fix Improvement in challenging environments addressing both unassisted and assisted GNSS; backward compatibility guaranteed (no impact on legacy or non-participative receivers) and low complexity implementation within OS receivers.

The Galileo system is set to begin transmitting these new I/NAV capabilities by 2023. ^[3]

Notes

The information from this article has been updated by GMV based on the applicable Galileo OS Signal-In-Space Interface Control Document (ICD).

References

Hofmann-Wellenhof, B., Lichtenegger, H., K. and Wasle, E., 2008. GNSS - Global Navigation Satellite Systems. Springer-Verlag, Wien, Austria.
Powe, M., 2006. Introduction to Galileo. powerpoint presentation, progeny.

[GAL_SIS_ICD-1] ^ ^a ^b ^c ^d ^e ^f ^g Galileo OS SIS ICD

[SAR_SDD-2] Galileo SAR Service Definition Document

[3] Galileo Open Service Signal-in-Space Interface Control Document, v2.0

[1]

[2]

[3]

@@ Line 1: / Line 1: @@
 {{Article Infobox2
 |Category=Fundamentals
-|Title={{PAGENAME}}
 |Authors=J. Sanz Subirana, JM. Juan Zornoza and M. Hernandez-Pajares, University of Catalunia, Spain.
 |Level=Basic
 |YearOfPublication=2011
+|Title={{PAGENAME}}
 }}
-==Introduction==
+The Galileo satellites broadcast different types of data in four navigation messages: the F/NAV navigation, the I/NAV navigation message, the Commercial Navigation Message (C/NAV) and the Governmental Navigation Message (G/NAV).
-Each Galileo satellite transmits ten navigation signals in the frequency bands E1, E6, E5a and E5b, right-hand circularly polarised. These signals are designed to support four different services: Open Service (OS), Safety of Life (SoL) service, Commercial Service (CS) and Public Regulated Service (PRS). A brief description of these Galileo services is provided as follows:
-* '''OS:''' The Open Service is free of charge for worldwide users. Up to three separate signal frequencies are offered within OS. Single-frequency receivers will provide performance similar to GPS C/A. In general, OS applications will use a combination of Galileo and GPS signals, which will improve performance in severe environments such as urban areas.
+The G/NAV navigation message does not belong to the public domain and the C/NAV is not yet defined. Please note that the formerly Commercial Service is now known as High Accuracy Service.
-* '''SoL:''' The Safety-of-Life Service provides timely warnings to the users when them fails to meet certain margins of accuracy (integrity). The service will be certified for SoL users requiring integrity information (civil aviation...). Two signals are provided in the frequency bands reserved for Aeronautical Radio-Navigation Services (E1 and E5).
-* '''CS:''' The Commercial Service provides access to two additional signals protected through commercial encryption (ranging data and messages). Higher data rates (up to 500 bps) for broadcasting data messages are provided.
-* '''PRS:''' The Public Regulated Service is for security authorities (police, military, etc) requiring a high continuity of service, with controlled access. Under governmental control. Enhanced signal modulation/encryption is introduced to provide robustness against jamming and spoofing. Two PRS navigation signals with encrypted ranging codes and data will be available. The Galileo satellites will also be able to detect and report signals form Cospas- Sarsat Search and Rescue (SAR) beacons.
-As in the case of GPS, all satellites share the same frequencies, and the signals are differentiated by the code division multiple access CDMA<ref group="footnotes">Where the spread spectrum codes enable the satellite to transmit on the same frequencies simultaneously.</ref> technique. These signals can contain data and pilot channel. Both channels provide ranging codes, but the data channels include also navigation data. Pilot channels (or pilot tones) are data-less signals, so that no bit transition occurs, helping the tracking of weak signals. A diagram of the different Galileo signals is shown in figure 1 (from [Powe, M., 2006]) where the data and pilot channels are plotted in orthogonal planes to highlight that they are shifted by 90 degrees in phase, which allow for their separation in the receivers.
+The details of the Galileo Signal Status are provided in the Galileo OS SIS Interface Control Document (ICD). <ref name = "GAL SIS ICD">[https://www.gsc-europa.eu/sites/default/files/sites/all/files/Galileo_OS_SIS_ICD_v2.0.pdf Galileo OS SIS ICD]</ref> No other ICD documents have been published to date.
-[[File:Galileo Signals in Space.png|none|thumb|400px|alt=Galileo Signals in Space|'''''Figure 1:''''' Galileo Signals in Space]]
-A brief description of each signal is provided as follows<ref group="footnotes">As Mainly extracted from OS SIS ICD.</ref>
+== General Structure and Contents ==
-* E1 signal supports the OS, CS and SoL service and PRS. It contains three navigation signal components in the L1 band. The first one, E1-A, is encrypted and only accessible to authorised PRS users, and it contains PRS data at ''50 bps''. The other two components, E1-B and E1-C, are open access signals with unencrypted ranging codes accessible to all users. E1- B is a data channel and E1-C a pilot (or data-less). The E1-B data stream, at ''125 bps'', also contains unencrypted integrity messages and encrypted commercial data. '''Note:''' the band E1 is shared with GPS L1.
+The Galileo Signal-In-Space data channels transmit different messages type according to the general contents identified in the following table. The F/NAV types of message correspond to the OS and the I/NAV types of messages correspond to both OS and CS (current High Accuracy Service). The following table indicates the signal component associated to each navigation message type. Let us also note that the Return Link Message from Galileo SAR service is provided as part of I/NAV data.
-* E6 signal is a dedicated signal for supporting the CS service and PRS. It provides three navigation signal components transmitted in the E6 band. E1, the first one, E6-A, is encrypted and only accessible to authorised PRS users, carrying PRS data. The other two, E6-B and E6-C, are commercial access signals and include a data channel E6-B and a pilot (or data-less) channel E6-C. Its ranging codes and data are encrypted. A data rate of 500 bps allows the transmission of added-value commercial data.
-* E5a signal supports OS. It is an open access signal transmitted in the E5a band that includes two signal components. A data channel, E5a-I, and a pilot (or data-less) channel, E5a-Q. The E5a signal has unencrypted ranging codes and navigation data, which are accessible by all users. It transmits the basic data to support navigation and timing functions, using a relatively low ''25 bps'' data rate that enables more robust data demodulation. Note: the band E5a is shared with GPS L5.
-* E5b signal supports the OS, CS and SoL services. It is an open access signal transmitted in the E5b band that includes other two signal components: The data channel E5b-I and the pilot (or data-less) channel E5b-Q. It has unencrypted ranging codes and navigation data accessible to all users. The E5b data stream also contains unencrypted integrity messages and encrypted commercial data. The data rate is ''125 bps''.
-Note: the band E5b will be shared with GLONASS G3.
-The E5a and E5b signal components are modulated onto a single E5 carrier frequency 1191.795 MHz using a technique known as AltBOC. The composite of the E5a and E5b signals is denoted as the E5 signal and can be processed as a single large bandwidth signal with an appropriate user receiver implementation, which results in a low multipath and tracking noise signal.
+[[File:Message_types.png |none|thumb|400px| Source: Galileo OS SIS ICD  <ref name = "GAL SIS ICD">[https://www.gsc-europa.eu/sites/default/files/sites/all/files/Galileo_OS_SIS_ICD_v2.0.pdf Galileo OS SIS ICD]</ref> ]]
-Next table 1 shows a summary of the Galileo signals, frequencies and applied modulations. The ranging Code Rate and Data Rate are also given in the table.
-[[File:Galileo Navigation Signals.png|none|thumb|400px|alt=Galileo Navigation Signals|'''''Table 1:''''' Galileo Navigation Signals. The two signals located in the E5a and E5b bands respectively are modulated onto a single E5 carrier frequency 1191.795 MHz using the AltBOC technique: AltBOC(15,10).]]
-The Galileo ranging codes are generated from primary and secondary codes, by using a tiered codes construction. Tiered codes generate a very long code that protects against interference. They are generated by multiplying a fast code (primary) by a slow code (secondary), running one chip for each epoch of the primary code. That is, tiered codes consist of successive repetitions of a primary code modulated by the chips of a secondary code. The repetitive primary codes allow faster acquisitions while the entire code is used for tracking.
+The complete navigation message data are transmitted on each data component as a sequence of frames. A frame is composed of several sub-frames, and each sub-frame is composed of several pages. The page is the basic structure for building the navigation message.
-==Galileo Navigation message==
+All data values encoded using the following bit and byte ordering criteria:
-The Galileo satellites will broadcast five types of data in four navigation messages: The Freely accessible Navigation Message (F/NAV) and Integrity Navigation Message (I/NAV), the Commercial Navigation Message (C/NAV) and the Governmental Navigation Message (G/NAV). Next table 2 summarises the content of the Galileo messages, with an indication of the associated channels and services (table inspired in [Hofmann-Wellenhof et al., 2008]).
+* For numbering, the most significant bit/byte is numbered as bit/byte 0.
-[[File:Galileo Messages.png|none|thumb|400px|alt=galileo navigation messages|'''''Table 2:''''' Content of the Galileo message types]]
+* For bit/byte ordering, the most significant bit/byte is transmitted first.
-The Galileo ephemeris parameters are Keplerian-like orbital elements as in GPS (see section ??). The nominal period update is 3 hours, being valid for a 4 hours time interval. The 1-hour overlap interval is intended to help against short outages or delays. The Galileo Almanac is also similar to the GPS and GLONASS ones.
+According with the current published ICD <ref name = "GAL SIS ICD">[https://www.gsc-europa.eu/sites/default/files/sites/all/files/Galileo_OS_SIS_ICD_v2.0.pdf Galileo OS SIS ICD]</ref> both F/NAV and I/NAV navigation messages are called super-frame and broadcast as a sequence of frames that are composed of several sub-frames which in turn is composed by several pages.
-The ''Integrity'' data will provide time-alarms and parameters to compute the integrity risk to support Safety-of-Life applications.
+[[File:Galileo_Navigation_Message_Structure.png |none|thumb|400px]]
-The ''Supplementary'' data is expected to provide information to support different envisaged commercial services as differential corrections for High Precision Positioning Service, and different kind of information data as weather alerts, traffic information... The data is encrypted to control the access to authorised users by the service providers.
+The general structure of the navigation messages are common but the number of sub-frames and pages as well as the duration of each page are not the same, as represented in the following table.
-The ''Public Regulated'' data is under governmental control and it is devoted to the Public Regulated Service (PRS). The system will guarantee a high continuity of service with controlled access by encryption of data.
+{| class="wikitable"
+|-
+!|Message||Channel||Data rate (bps)||Page Duration (s)||Pages in a sub-frame||Sub-frames in a frame
+|-
+| align="center" | F/NAV
+| align="center" | E5a-I
+| align="center" | 25
+| align="center" | 10
+| align="center" | 5
+| align="center" | 12
+|-
+| align="center" | I/NAV
+| align="center" | E1B, E5b-I
+| align="center" | 125
+| align="center" | 2
+| align="center" | 15
+| align="center" | 24
+|}
-The ''Search and Rescue'' data will provide the capability to send acknowledgment SAR messages to a Beacon equipped with a suitable Galileo receiver.
+This arrangement allows accomplishing the three different main categories of data to be transmitted at different rates:
+* Fast rate (for urgent data, such as integrity): page.
+* Medium rate (Ephemeris, Clock Correction): sub-frame.
+* Slow rates (Almanacs): frame.
+The page starts with a Synchronisation Word (SW) followed by the interleaving FEC (Forward Error Correction) coded navigation data and ends with tail bits for the FEC decoding. In addition both navigation messages transmit a CRC data field in order to detect corrupted data. The size of this field is different in each navigation message.
+Three levels of error coding are applied to the Galileo Message Data Stream:
+# A Cyclic Redundancy Check (CRC) with error detection capabilities after recovery of the received data;
+# A one-half rate Forward Error Correction (FEC). Tail Bits (sequence of zeros) to allow Viterbi decoding;
+# Block Interleaving on the resulting frames: provides robustness to the FEC decoding algorithm since in presence of a burst of erroneous bits it is converted into small errors in several pages. This scheme allows reducing the bit error ratio in the increased data rates.
-The complete navigation message is transmitted on each data channel as a sequence of frames. A frame is composed of a certain number of sub-frames, and a sub-frame is composed of several pages. This arrangement allows accomplishing the three different main categories of data to be transmitted:
+As referred, the FEC coded symbols are transmitted interleaved within the page and because of that the navigation data can only be decoded when the complete interleaving FEC coded part is received.
-* Repeated at fast rate (for urgent data, such as integrity): page.
-* Medium rate (like data required for warm start) : sub-frame.
-* Slow rates (like data required for cold start): frame.
-The page starts with a Synchronisation Word (SW) followed by the data field. 5
+The following table summarizes the parameters transmitted by FNAV and INAV messages.
-After the data, a Cyclic Redundancy Check (CRC) parity bits are provided to detect the reception of corrupted data. The page ends with tail bits for the Forward Error Correction (FEC) encoding.
-Three levels of error coding are applied to the Galileo Message Data Stream:
+[[File:Params.png|none|thumb|400px]]
-# A Cyclic Redundancy Check (CRC) with error detection capabilities after recovery of the received data
-# A one-half rate Forward Error Correction (FEC). Tail Bits (sequence of zeros) to allow Viterbi decoding.
+As can be seen in the table above, the parameters transmitted in F/NAV and I/NAV have the same size except for signal and message specific parameters, such as "Issue of Data (IOD)" and "Navigation Data Validity and Signal Health Status". In the case of "Satellite Almanac" set, the size is also different because the set includes the "Satellite signal health status".
-# Block Interleaving on the resulting frames: provides robustness to the FEC decoding algorithm by avoiding packets of errors. This scheme allows reducing the bit error ratio in the increased data rates.
+The Galileo ephemeris parameters are [[GPS and Galileo Satellite Coordinates Computation|Keplerian-like orbital elements as in GPS]]. The nominal period update is 3 hours, being valid for a 4 hours time interval. The 1-hour overlap interval is intended to help against short outages or delays. The Galileo Almanac is also similar to the GPS and GLONASS ones.
-[[File:Frame Structure.png|none|thumb|400px|alt=frame structure|'''''Table 3:''''' Frame structure]]
+== F/NAV Structure ==
+The F/NAV navigation message is transmitted on channel E5a-I at a rate of 25 bps. The F/NAV message structure is shown in the following figure, where the duration of each entity is indicated.
+[[File:FNAV.png |none|thumb|400px]]
+Each frame of the F/NAV message has a duration of 600 seconds and it is composed by 12 sub-frames. In turn, each sub-frame has a duration of 50 seconds and it is composed by 5 pages with a duration of 10 seconds. The page itself comprises 3 main fields:
+* Synchronisation pattern: it is no encoded and it has a length of 12 bits and is always 101101110000 being its purpose to allow the receiver to achieve synchronisation to the page boundary; it allows the receiver to achieve synchronization to the page boundary.
+* F/NAV word: has a length of 238 bits and it is the interleaved and FEC encoded part of the page that encodes the fields: Page Type (6 bits) enabling the page content identification;, Navigation Data (208 bits) and CRC (24 bits) to detect potential bit errors. The CRC is computed on the Page Type and Navigation Data fields.
+* Tail: has a length of 6 bits and consists of 6 zero-values that are used to enable the completion of the FEC decoding of each page.
+[[File:Fnav.png | F/NAV Navigation message structure|none|thumb|400px]]
+The Page Type field identifies the broadcast page which allows the user-receivers to react accordingly and grants the possibility of changing the pages sequencing, while keeping backward compatibility. The CRC is used as the most inner mechanism of errors detection because it is computed only on the Page Type and Navigation Data fields.
+Please refer to the Galileo OS SIS ICD<ref name = "GAL SIS ICD">[https://www.gsc-europa.eu/sites/default/files/sites/all/files/Galileo_OS_SIS_ICD_v2.0.pdf Galileo OS SIS ICD]</ref> for detailed analysis on the F/NAV Frame layout and page contents.
+== I/NAV Structure ==
+The I/NAV navigation message is transmitted on both E1B and E5b-I channels at a rate of 125 bps. The two versions of the I/NAV message are disseminated through the same structure being its content also the same but not aligned over the time.
+[[File:INAV.png |none|thumb|400px]]
+The I/NAV message structures for the E1B and E5b-I signals use the same page layout since the service provided on these frequencies is a dual frequency service, using frequency diversity. Only page sequencing is different, with page swapping between both components in order to allow a fast reception of the data by a dual frequency receiver. However, the frame is designed to allow receivers to work also with a single frequency.
+Each frame of the I/NAV message has a duration of 720 seconds and it is composed by 24 sub-frames. In turn, each sub-frame has a duration of 30 seconds and it is composed by 20 pages with a duration of 2 seconds. The page itself is composed by 3 fields:
+* Synchronisation pattern: has a length of 10 bits and is always 0101100000 being its purpose to allow the receiver to achieve synchronisation to the page boundary;
+* I/NAV page part: has a length of 114 bits and it is the interleaved FEC encoded part of the page where the navigation data is conveyed.
+* Tail: has a length of 6 bits and consists of 6 zero-value bits that are used to enable the completion of the FEC decoding of each page part.
+The page part of the message can be even or odd and both parts are always broadcast one after the other. In channel E1B the even part is transmitted first while in channel E5b-I the even part is transmitted after. The combination of two page parts constitutes a valid set of data (Nominal Page) that has to be parsed together to get the Navigation Data. The advantage of this configuration is that a receiver decoding the I/NAV on both frequencies can decode the same pages in half the time of a single frequency receiver.
+[[File:Inav.png | INAV Navigation message structure |none|thumb|400px]]
+Two types of I/NAV pages are defined:
+* Nominal pages: having a duration of 2 seconds transmitted sequentially in time in two parts of duration 1 second each on each of the E1B and E5b-I components. The first part of the page is denoted ‘even’ and the second one is denoted ‘odd’.
+* Alert pages: having a duration of 1 second transmitted in two parts of duration 1 second each at the same epoch over the E1B and E5b-I components. Again, the first part of the page is denoted ‘even’ and the second one is denoted ‘odd’. This transmission is repeated at the next epoch but switching the two parts between the components.
+Please refer to the Galileo OS SIS ICD <ref name = "GAL SIS ICD">[https://www.gsc-europa.eu/sites/default/files/sites/all/files/Galileo_OS_SIS_ICD_v2.0.pdf Galileo OS SIS ICD]</ref> for detailed analysis on the I/NAV Frame layout and page contents. Let us remark that the [[Galileo Search and Rescue Service|Galileo Search and Rescue (SAR)]] Return Link Message (RLM) is transmitted only in the E1-B component. The SAR field structure for the E1-B component in nominal mode is formatted according to the values stated in the Galileo SIS ICD<ref name = "GAL SIS ICD">[https://www.gsc-europa.eu/sites/default/files/sites/all/files/Galileo_OS_SIS_ICD_v2.0.pdf Galileo OS SIS ICD]</ref> and the Galileo SAR Service Definition Document<ref name="SAR SDD">[https://www.gsc-europa.eu/sites/default/files/sites/all/files/Galileo-SAR-SDD.pdf Galileo SAR Service Definition Document]</ref>.
+[[Galileo Open Service Navigation Message Authentication | Galileo OS-NMA]] capability will be added in the Reserved 1 40 bits in E1 I/NAV message.
+The Galileo OS ICD released in January 2021 <ref name = "GAL SIS ICD"/> introduces new features in parts of I/NAV message transmitted on the Galileo E1-B signal component which were previously marked as spare or reserved. This means that these improvements will provide full backwards compatibility. The new features are:
+* Reduced Clock and Ephemeris Data (RedCED);
+* Reed-Solomon Outer Forward Error Correction Data (FEC2);
+* Secondary Synchronization Pattern (SSP).
-[[File:Galileo Navigation Message Structure.png|none|thumb|400px|alt=Galileo Navigation Message Structure|'''''Figure 2:''''' Galileo Navigation Message Structure (from [Powe, M., 2006]).]]
+As a result of these technical solutions, users will experience an improvement of the Galileo E1 Open Service performance in terms of Robustness and Timeliness; a significant Time To First Fix Improvement in challenging environments addressing both unassisted and assisted GNSS; backward compatibility guaranteed (no impact on legacy or non-participative receivers) and low complexity implementation within OS receivers.
-==User domain Galileo performances==
+The Galileo system is set to begin transmitting these new I/NAV capabilities by 2023. <ref>[https://www.gsc-europa.eu/sites/default/files/sites/all/files/Galileo_OS_SIS_ICD_v2.0.pdf Galileo Open Service Signal-in-Space Interface Control Document, v2.0]</ref>
-The performance requirements for the Galileo services are summarised in the next table (From [Hofmann-Wellenhof et al., 2008], based on Falcone (2006)).
-[[File:Performance requirements for the Galileo services.png|none|thumb|400px|alt=Performance requirements for the Galileo services|'''''Table 4:''''' Performance requirements for the Galileo services: (1) Single frequency accuracy deepens on which frequency is used. (2) Offset Galileo to UT over 34h. (3) Receiver autonomous integrity monitoring is possible.]]
 ==Notes==
 <references group="footnotes"/>
+The information from this article has been updated by GMV based on the applicable Galileo OS Signal-In-Space Interface Control Document (ICD).
 ==References==
+<references/>
 * Hofmann-Wellenhof, B., Lichtenegger, H., K. and Wasle, E., 2008. GNSS - Global Navigation Satellite Systems. Springer-Verlag, Wien, Austria.
 * Powe, M., 2006. Introduction to Galileo. powerpoint presentation, progeny.
+[[Category:Fundamentals]]
+[[Category:GALILEO]]