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Klobuchar Ionospheric Model

2026-06-17T07:03:39Z

Jalbert: Fixed typo error in point 5: Changed latitude to longitude.

{{Article Infobox2
|Category=Fundamentals
|Authors=J. Sanz Subirana, J.M. Juan Zornoza and M. Hernández-Pajares, Technical University of Catalonia, Spain.
|Level=Advanced
|YearOfPublication=2011
|Title={{PAGENAME}}
}}
GPS satellites broadcast the parameters of the Klobuchar ionospheric model for single frequency users. The Klobuchar model was designed to minimise user computational complexity and user computer storage as far as to keep a minimum number of coefficients to transmit on satellite-user link.

This broadcast model is based on an empirical approach [Klobuchar, 1987] <ref name="Klobuchar"> [Klobuchar, 1987] Klobuchar, J., 1987. Ionospheric Time-Delay Algorithms for Single-Frequency GPS Users. IEEE Transactions on Aerospace and Electronic Systems (3), pp. 325-331.</ref> and is estimated to reduce about the 50% RMS ionospheric range error worldwide. It is assumed that the electron content is concentrated in a thin layer at <math>350</math> kilometres in height. Thence, the slant delay is computed from the vertical delay at the Ionospheric Pierce Point (IPP) <ref group="footnotes">That is the intersection of the ray with the ionospheric layer at <math>350</math> kilometres in height.</ref> multiplying by a obliquity factor, i.e., the mapping function, see figure 1 and equation (9).

::[[File:Klobuchar_ IPP_slide.png|none|thumb|520px|'''''Figure 1:''''' Ionospheric Pierce Points (IPPs), Vertical and Slant delay illustration. The IPP's trajectories for a receiver in Barcelona, Spain are shown in the map. The figure at bottom right shows the obliquity factor variation with the elevation of ray.]]

'''''Klobuchar Algorithm equations'''''

The Klobuchar algorithm to run in a single frequency receiver is provided as follows (from [Klobuchar, 1987]<ref name="Klobuchar"/>):

Given the user approximate geodetic latitude <math>\displaystyle \phi_u</math>, longitude <math>\displaystyle \lambda_u</math>, elevation angle <math>\displaystyle E</math> and azimuth <math>\displaystyle A</math> of the observed satellite and the coefficients <math>\displaystyle \alpha_n</math> and <math>\displaystyle \beta_n</math> broadcasted in the GPS satellite navigation message:

:'''1. Calculate the earth-centred angle (elevation <math>\displaystyle E</math> in semicircles).'''

::<math>
\psi =\frac{0.0137}{E+0.11}-0.022 \qquad\mbox{(semicircles)}
\qquad\mbox{(1)}</math>

:Note: All values expressed in semicircles (such as latitudes and longitudes) must be converted to radians before applying trigonometric functions. This conversion is crucial to avoid slight inaccuracies in the computed results. The conversion from semicircles to radians is performed by multiplying the value in semicircles by <math>\pi</math>.

:'''2. Compute the latitude of the Ionospheric Pierce Point''' (IPP)<ref group="footnotes"> That is, the latitude of the earth projection of the ionospheric intersection point with a mean ionospheric height layer at <math>350</math> kilometres.</ref>.

::<math>
\phi_I=\phi_u+\psi \cos A \qquad\mbox{(semicircles)} \qquad\mbox{(2)}</math>

:If <math>\displaystyle \Phi_I> +0.416</math> the <math>\displaystyle \Phi_I=+0.416</math>. If <math>\Phi_I< -0.416</math> the <math>\Phi_I=-0.416</math>.

:'''3. Compute the longitude of the IPP.'''

::<math>
\lambda_I=\lambda_u+\frac{\psi \sin A}{\cos \phi_I} \qquad\mbox{(semicircles)} \qquad\mbox{(3)}</math>

:Note: The input longitude values are in the range [0, 2π] (semicircles), not in the common range of [-π, π] (radians).

:'''4. Find the geomagnetic latitude of the IPP.'''

::<math>
\phi_m=\phi_I+0.064 \, \cos (\lambda_I-1.617) \qquad\mbox{(semicircles)} \qquad\mbox{(4)}</math>

:'''5. Find the local time '''t' ''' (in seconds) at the ionospheric pierce point.'''

:<math>
t' = (43200 \cdot \lambda_I + t_{\text{GPS}}) \mod 86400
</math>

:where:
:* <math>t_{\text{GPS}}</math> is the '''GPS time in seconds of the week''' (0 to 604800 s).
:* <math>\lambda_I</math> is the geodetic longitude of the ionospheric pierce point (in semicircles).
:* The '''modulo operation ensures''' that <math>t'</math> remains within a 24-hour cycle, which is required for the Klobuchar model to correctly compute the ionospheric delay.

:'''6. Compute the amplitude of ionospheric delay.'''

::<math>
A_I=\displaystyle\sum_{n=0}^{3}{\alpha_n \phi_m^n} \qquad\mbox{(seconds)}
\qquad\mbox{(6)}</math>

:if <math>A_I<0</math>, thence <math>A_I=0</math>.

:'''7. Compute the period of ionospheric delay.'''

::<math>
P_I=\displaystyle\sum_{n=0}^{3}{\beta_n \phi_m^n}\qquad\mbox{(seconds)} \qquad\mbox{(7)}</math>

:if <math>P_I<72\,000</math>, thence <math>P_I=72\,000</math>.

:'''8. Compute the phase of ionospheric delay.'''

::<math>
X_I=\displaystyle \frac{\displaystyle 2\pi(t-50\,400)}{P_I} \qquad\mbox{(radians)} \qquad\mbox{(8)}</math>

:'''9. Compute the slant factor (elevation <math>E</math> in semicircles).'''
::<math>
F=1.0+16.0\,(0.53-E)^3 \qquad\mbox{(9)}</math>

:'''10. Compute the ionospheric time delay.'''

::<math>
I_{_{L1_{GPS}}}=\left \{
\begin{array}{ll}
\left [5\cdot 10^{-9}+ \sum_{n=0}^{3}{\alpha_n \phi_m^n}\cdot \left ( 1- \frac{X_I^2}{2}+\frac{X_I^4}{24} \right) \right ] \cdot F &; |X_I|\leq 1.57\\[0.5cm]
5\cdot 10^{-9}\cdot F &; |X_I|\geq 1.57
\end{array}
\right .
\qquad\mbox{(10)}
</math>

:The delay <math>I_{_{L1_{GPS}}}</math> is given in seconds and it is referred to the GPS L1 frequency.

:To convert this delay into range units (meters), multiply the ionospheric delay by the speed of light \( c \):

::<math>
\Delta R = c \cdot I_{L1, GPS}
</math>

:Where:
::* <math>\Delta R</math> is the ionospheric delay in meters.
::* <math>c = 299,792,458</math> m/s is the speed of light in vacuum.

:This conversion is essential as the delay is initially calculated in time units (seconds) and must be expressed in distance units (meters) to be used in range calculations.

Although this algorithm is provided to estimate the ionospheric delay in the GPS L1 frequency signal, it can be also used to estimate the ionospheric time delay in the GPS L2 frequency signal or for the GLONASS and Galileo signals, as well. Indeed, taking into account that the ionospheric delay is inversely proportional to the square of the signal frequency, the delay for any GNSS signal transmitted on frequency <math>f</math> is given by:

::<math>
I_f= \left ( \frac{f_{_{L1_{GPS}}}}{f} \right )^2\,I_{_{L1_{GPS}}} \qquad\mbox{(12)}</math>

Figure 2 is a layout of the Klobuchar model algorithm, showing that the vertical delays are based on a constant value at night time and a half-cosine function in daytime, which amplitude and period are given as a function of the eight parameters (<math>\alpha_i,\, \beta_i,\,i=0,\dots,3</math>) broadcast in the GPS navigation message. A map with the geographical variation of the vertical delay is also depicted in this figure.

Figure 3 illustrates with an example the values of the GPS delays from the Klobuchar model and shows the effect of neglecting such delays in single point positioning for the horizontal and vertical error components.

::[[File:Klobuchar_Klobychar_slide.png|none|thumb|520px|'''''Figure 2:''''' Klobuchar ionospheric model. Algorithm layout.]]

::{|
|+align="bottom"|''Figure 3: Ionospheric correction: Range and position domain effect''
| [[File:Klobuchar_H_Nov_ion.png|none|thumb|400px|frameless]]
| [[File:Klobuchar_V_Nov_ion.png|none|thumb|400px|frameless]]
|-
| [[File:Klobuchar_Nov_iono_cor.png |none|thumb|400px|frameless]]
| '' First row shows the horizontal (left) and vertical (right) positioning error using (blue) or not using (red) the Klobuchar ionospheric correction defined in 1. The variation in range is shown in the second row at left ''
|}

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

==References==
<references/>

[[Category:Fundamentals]]
[[Category:GNSS Measurements Modelling]]

GPS Navigation Message

2026-06-01T08:31:33Z

Jalbert: Small fix in LNAV sub-frames 4 and 5.

{{Article Infobox2
|Category=Fundamentals
|Authors=J. Sanz Subirana, JM. Juan Zornoza and M. Hernandez-Pajares, University of Catalunia, Spain.
|Level=Basic
|YearOfPublication=2011
|Title={{PAGENAME}}
}}
The GPS monitor stations continuously track each satellite in GPS constellation to estimate its precise orbit and clock errors, then the Master Control Station uses those solutions to generate the GPS navigation messages. These data is uploaded to the GPS satellites through S-band ground antennas during passes and stored onboard. The satellites then rebroadcast the message to users on the L-band navigation signals (L1/L2/L5).

The navigation message provides all the necessary information to allow the user to perform the positioning service. As a minimum, each GPS navigation message includes the ephemeris parameters, needed to compute the satellite coordinates with enough accuracy; the time parameters and satellite clock corrections, to compute satellite clock offsets and time conversions; service parameters such as satellite health flags; ionospheric parameters, used by single frequency receivers; and the almanacs, which allow the computation of the position of "all satellites in the constellation" with a reduced accuracy (1 - 2 km of 1-sigma error) in order to support receiver signal acquisition.

Under nominal conditions, the GPS Control Segment uploads new ephemeris data approximately every two hours, with longer intervals of six hours or more during non-nominal conditions. The almanac changes much more slowly, being updated at least every six days but usually refreshed daily.

Besides the "legacy" L1 C/A navigation message (LNAV), four additional new messages were introduced during the so called GPS modernisation: L2-CNAV, CNAV-2, L5-CNAV and MNAV. The LNAV, L2-CNAV, CNAV-2, L5-CNAV are civil messages, while the MNAV is a military message. In modernised GPS, the same type of contents as the legacy navigation message (NAV) is transmitted but at higher rate and with improved robustness.

The messages L2-CNAV, L5-CNAV and MNAV have a similar structure and data format which allows more flexibility, better control and improved content. The MNAV includes new improvements for the security and robustness of the military message.
The CNAV-2 is modulated onto L1C signal, sharing the same band as the "legacy" LNAV navigation message.

Further information on the different navigation messages can be found in the respective GPS Interface Control Documents (ICD) available at [https://archive.gps.gov/technical/icwg/ GPS Official Website] <ref name="GPS GOV Webpage">[https://archive.gps.gov/ Official U.S. government information about the Global Positioning System (GPS)]</ref>.

== LNAV ==

The current “legacy” Navigation Message (LNAV) is modulated on both L1 C/A and L2 P(Y) carriers at 50 bps. The whole message contains 25 pages (or "frames") of 30 seconds each, forming the master frame that takes 12,5 minutes to be transmitted. Every frame is subdivided into 5 sub-frames of 6 seconds each; in turn, every sub-frame consists of 10 words, with 30 bits per word (see figure 3).
Every sub-frame always starts with the telemetry word (TLM), which is necessary for synchronism. Next, the transference word (HOW) appears. This word provides time information (seconds of the GPS week), allowing the receiver to acquire the week-long P(Y)-code segment.

[[File:Navigation Message.png|none|thumb|450px|alt=navigation message|'''''Figure 1:''''' "Legacy" Navigation message]]

The content of every sub-frame is as follows:
* Sub-frame 1: contains information about the parameters to be applied to satellite clock status for its correction. These values are polynomial coefficients that allow converting time on board to GPS time. It also has information about satellite health condition.
* Sub-frames 2 and 3: these sub-frames contain satellite ephemeris.
* Sub-frame 4: provides ionospheric model parameters (in order to adjust for ionospheric refraction), UTC information (Universal Coordinate Time), part of the almanac, and indications whether the Anti-Spoofing, A/S, is activated or not (which transforms P code into the encrypted Y code).
* Sub-frame 5: contains data from the almanac and the constellation status. It allows to quickly identify the satellite from which the signal comes. A total of 25 frames are needed to complete the almanac.
Sub-frames 1, 2 and 3 are transmitted with each frame (i.e., they are repeated every 30 seconds). Sub-frames 4 and 5 contain different pages (25 pages each) of the navigation message (see figure 1). Thence, the transmission of the full navigation message takes 25 × 30 seconds = 12.5 minutes.
The content of sub-frames 4 and 5 can be different between satellites. Furthermore, within the GPS LNAV navigation data structure, these subframes can differ depending on whether the PRN numbers belong to the lower set (1–32) or the upper set (33–63).

Further information on the LNAV message can be found in IS-GPS-200 ICD document <ref name="GPS ICD 200">[https://archive.gps.gov/technical/icwg/IS-GPS-200N.pdf GPS ICD-200 Revision N, "Navstar GPS Space Segment/Navigation User Segment Interfaces"]</ref>.

== L2-CNAV ==

The initial L2C broadcast consisted of a default message (Message Type 0) that did not provided full navigational data. Initially the plan was to keep the dummy transmission until the new [[GPS Future and Evolutions#New Control Segment|Operational Control Segment (OCX)]] would be operational. However the Air Force decided to anticipate the provision of the L2C navigation message with the aim of helping the development of compatible user equipments as well facilitate the CNAV Operations Concept. The message-populated broadcast started on April 2014 with reduced data accuracy and update frequency compared to the legacy GPS signals in wide use today. From December 2014 is planed that L2-CNAV data updates will increase to a daily rate, bringing L2C signal-in-space accuracy on par with the legacy signals. However, derived position accuracy cannot be guaranteed during the pre-operational deployment of the frequencies and its use must be used only for testing and research activities despite the health bit set “healthy”<ref name=early_cnav_message_populated>[http://gpsworld.com/dod-announces-start-of-civil-navigation-message-broadcasting/ DOD Announces Start of Civil Navigation Message Broadcasting], GPS World, GPS Staff, April 29, 2014</ref>. On December 2014, the CNAV navigation message started to be updated on a daily basis just like the legacy message but must be still considered as pre-operational data and its use must be restricted to testing purposes<ref>[http://gpsworld.com/cnav-messages-now-transmitted-daily/ CNAV Messages Now Transmitted Daily], GPS World, GPS Staff, January 2, 2015</ref>. Operational declarations for L2-CNAV will require implementation of new monitoring and control capabilities in Block 1 of the Next Generation Operational Control System (OCX).

Its design replaces the use of frames and sub-frames of data (repeating in a fixed pattern) of the original “legacy” NAV by a packetised message-based communications protocol, where individual messages can be broadcast in a flexible order with variable repeat cycles as represented in figure 2. Moreover, Forward Error Correction (FEC) and advanced error detection (such as a CRC) are used to achieve better error rates and reduced data collection times.

[[File:L2c.png|none|thumb|450px|alt=L2c|'''''Figure 2:''''' L2-CNAV Navigation message]]

Each message is composed by fixed data such as a Preamble, Message Type ID, Alert Flag, Message TOW count and CRC which lets 238 bits to be filled with other navigation related data. It is possible to define up to 63 different message types, but currently only the messages types 10-14 and 30-37 are defined. The remaining undefined and unused message types are reserved for future use.
Broadcast of messages is completely arbitrary, but sequenced to provide optimum user performance.

Further information on the L2-CNAV message can be found in IS-GPS-200 ICD document <ref name="GPS ICD 200"></ref>.

== L5-CNAV ==

Like L2-CNAV, the L5 message-populated broadcast started on April 2014 but set “unhealthy,” but as greater experience with the L5 broadcast and implementation of signal monitoring is achieved, this status may change upon review<ref name=early_cnav_message_populated/><ref>[http://insidegnss.com/nanu-alerts-gps-users-to-start-of-l2c-l5-cnav-messages/ NANU Alerts GPS Users to Start of L2C/L5 CNAV Messages], Inside GNSS, April 24, 2014</ref>. Operational declarations for L5-CNAV will require implementation of new monitoring and control capabilities in Block 1 of the Next Generation Operational Control System (OCX).

The L5-CNAV is modulated onto L5I signal component, containing basically the same information data as L2-CNAV. The message structure is exactly the same but its content may vary slightly.

[[File:L5c.png|none|thumb|450px|alt=L5c|'''''Figure 3:''''' L5-CNAV Navigation message]]

As in L2-CNAV, it is possible to define up to 63 different message types, but currently only the messages types 10-14 and 30-37 are defined. The remaining undefined and unused message types are reserved for future use.

Further information on L5-CNAV message can be found in IS-GPS-705 ICD document <ref name="GPS ICD 705">[https://archive.gps.gov/technical/icwg/IS-GPS-705J.pdf GPS ICD-705 Revision J, "Navstar GPS Space Segment/User Segment L5 Interfaces"]</ref>.

== CNAV-2 ==

The message CNAV-2 consists of sub-frames and frames and is modulated onto the L1C signal. Each frame is divided into three sub-frames of varying length being required multiple frames to broadcast a complete data message set to users.

* Subframe 1 (9 bits) provides Time of Internal.
* Subframe 2 (600 bits) provides clock and ephemeris data.
* Subframe 3 (274 bits) provides other navigation data which is commutated over multiple pages.

[[File:Cnav-2.png|none|thumb|450px|alt=Cnav-2|'''''Figure 4:''''' CNAV-2 Navigation message]]

Further information on CNAV-2 message can be found in IS-GPS-800 ICD document <ref name="GPS ICD 800">[https://archive.gps.gov/technical/icwg/IS-GPS-800J.pdf GPS ICD-800 Revision J, "Navstar GPS Space Segment/User Segment L1C Interfaces"].</ref>.

==References==
<references/>

[[Category:Fundamentals]]
[[Category:GPS]]
[[Category:GPS Signal Structure]]

GPS Navigation Message

2026-06-01T08:29:46Z

Jalbert: Small fix in LNAV sub-frames 4 and 5.

{{Article Infobox2
|Category=Fundamentals
|Authors=J. Sanz Subirana, JM. Juan Zornoza and M. Hernandez-Pajares, University of Catalunia, Spain.
|Level=Basic
|YearOfPublication=2011
|Title={{PAGENAME}}
}}
The GPS monitor stations continuously track each satellite in GPS constellation to estimate its precise orbit and clock errors, then the Master Control Station uses those solutions to generate the GPS navigation messages. These data is uploaded to the GPS satellites through S-band ground antennas during passes and stored onboard. The satellites then rebroadcast the message to users on the L-band navigation signals (L1/L2/L5).

The navigation message provides all the necessary information to allow the user to perform the positioning service. As a minimum, each GPS navigation message includes the ephemeris parameters, needed to compute the satellite coordinates with enough accuracy; the time parameters and satellite clock corrections, to compute satellite clock offsets and time conversions; service parameters such as satellite health flags; ionospheric parameters, used by single frequency receivers; and the almanacs, which allow the computation of the position of "all satellites in the constellation" with a reduced accuracy (1 - 2 km of 1-sigma error) in order to support receiver signal acquisition.

Under nominal conditions, the GPS Control Segment uploads new ephemeris data approximately every two hours, with longer intervals of six hours or more during non-nominal conditions. The almanac changes much more slowly, being updated at least every six days but usually refreshed daily.

Besides the "legacy" L1 C/A navigation message (LNAV), four additional new messages were introduced during the so called GPS modernisation: L2-CNAV, CNAV-2, L5-CNAV and MNAV. The LNAV, L2-CNAV, CNAV-2, L5-CNAV are civil messages, while the MNAV is a military message. In modernised GPS, the same type of contents as the legacy navigation message (NAV) is transmitted but at higher rate and with improved robustness.

The messages L2-CNAV, L5-CNAV and MNAV have a similar structure and data format which allows more flexibility, better control and improved content. The MNAV includes new improvements for the security and robustness of the military message.
The CNAV-2 is modulated onto L1C signal, sharing the same band as the "legacy" LNAV navigation message.

Further information on the different navigation messages can be found in the respective GPS Interface Control Documents (ICD) available at [https://archive.gps.gov/technical/icwg/ GPS Official Website] <ref name="GPS GOV Webpage">[https://archive.gps.gov/ Official U.S. government information about the Global Positioning System (GPS)]</ref>.

== LNAV ==

The current “legacy” Navigation Message (LNAV) is modulated on both L1 C/A and L2 P(Y) carriers at 50 bps. The whole message contains 25 pages (or "frames") of 30 seconds each, forming the master frame that takes 12,5 minutes to be transmitted. Every frame is subdivided into 5 sub-frames of 6 seconds each; in turn, every sub-frame consists of 10 words, with 30 bits per word (see figure 3).
Every sub-frame always starts with the telemetry word (TLM), which is necessary for synchronism. Next, the transference word (HOW) appears. This word provides time information (seconds of the GPS week), allowing the receiver to acquire the week-long P(Y)-code segment.

[[File:Navigation Message.png|none|thumb|450px|alt=navigation message|'''''Figure 1:''''' "Legacy" Navigation message]]

The content of every sub-frame is as follows:
* Sub-frame 1: contains information about the parameters to be applied to satellite clock status for its correction. These values are polynomial coefficients that allow converting time on board to GPS time. It also has information about satellite health condition.
* Sub-frames 2 and 3: these sub-frames contain satellite ephemeris.
* Sub-frame 4: provides ionospheric model parameters (in order to adjust for ionospheric refraction), UTC information (Universal Coordinate Time), part of the almanac, and indications whether the Anti-Spoofing, A/S, is activated or not (which transforms P code into the encrypted Y code).
* Sub-frame 5: contains data from the almanac and the constellation status. It allows to quickly identify the satellite from which the signal comes. A total of 25 frames are needed to complete the almanac.
Sub-frames 1, 2 and 3 are transmitted with each frame (i.e., they are repeated every 30 seconds). Sub-frames 4 and 5 contain different pages (25 pages each) of the navigation message (see figure 1). Thence, the transmission of the full navigation message takes 25 × 30 seconds = 12.5 minutes.
The content of sub-frames 4 and 5 can differ between satellites. Furthermore, within the GPS LNAV navigation data structure, these subframes can differ depending on whether the PRN numbers belong to the lower set (1–32) or the upper set (33–63).

Further information on the LNAV message can be found in IS-GPS-200 ICD document <ref name="GPS ICD 200">[https://archive.gps.gov/technical/icwg/IS-GPS-200N.pdf GPS ICD-200 Revision N, "Navstar GPS Space Segment/Navigation User Segment Interfaces"]</ref>.

== L2-CNAV ==

The initial L2C broadcast consisted of a default message (Message Type 0) that did not provided full navigational data. Initially the plan was to keep the dummy transmission until the new [[GPS Future and Evolutions#New Control Segment|Operational Control Segment (OCX)]] would be operational. However the Air Force decided to anticipate the provision of the L2C navigation message with the aim of helping the development of compatible user equipments as well facilitate the CNAV Operations Concept. The message-populated broadcast started on April 2014 with reduced data accuracy and update frequency compared to the legacy GPS signals in wide use today. From December 2014 is planed that L2-CNAV data updates will increase to a daily rate, bringing L2C signal-in-space accuracy on par with the legacy signals. However, derived position accuracy cannot be guaranteed during the pre-operational deployment of the frequencies and its use must be used only for testing and research activities despite the health bit set “healthy”<ref name=early_cnav_message_populated>[http://gpsworld.com/dod-announces-start-of-civil-navigation-message-broadcasting/ DOD Announces Start of Civil Navigation Message Broadcasting], GPS World, GPS Staff, April 29, 2014</ref>. On December 2014, the CNAV navigation message started to be updated on a daily basis just like the legacy message but must be still considered as pre-operational data and its use must be restricted to testing purposes<ref>[http://gpsworld.com/cnav-messages-now-transmitted-daily/ CNAV Messages Now Transmitted Daily], GPS World, GPS Staff, January 2, 2015</ref>. Operational declarations for L2-CNAV will require implementation of new monitoring and control capabilities in Block 1 of the Next Generation Operational Control System (OCX).

Its design replaces the use of frames and sub-frames of data (repeating in a fixed pattern) of the original “legacy” NAV by a packetised message-based communications protocol, where individual messages can be broadcast in a flexible order with variable repeat cycles as represented in figure 2. Moreover, Forward Error Correction (FEC) and advanced error detection (such as a CRC) are used to achieve better error rates and reduced data collection times.

[[File:L2c.png|none|thumb|450px|alt=L2c|'''''Figure 2:''''' L2-CNAV Navigation message]]

Each message is composed by fixed data such as a Preamble, Message Type ID, Alert Flag, Message TOW count and CRC which lets 238 bits to be filled with other navigation related data. It is possible to define up to 63 different message types, but currently only the messages types 10-14 and 30-37 are defined. The remaining undefined and unused message types are reserved for future use.
Broadcast of messages is completely arbitrary, but sequenced to provide optimum user performance.

Further information on the L2-CNAV message can be found in IS-GPS-200 ICD document <ref name="GPS ICD 200"></ref>.

== L5-CNAV ==

Like L2-CNAV, the L5 message-populated broadcast started on April 2014 but set “unhealthy,” but as greater experience with the L5 broadcast and implementation of signal monitoring is achieved, this status may change upon review<ref name=early_cnav_message_populated/><ref>[http://insidegnss.com/nanu-alerts-gps-users-to-start-of-l2c-l5-cnav-messages/ NANU Alerts GPS Users to Start of L2C/L5 CNAV Messages], Inside GNSS, April 24, 2014</ref>. Operational declarations for L5-CNAV will require implementation of new monitoring and control capabilities in Block 1 of the Next Generation Operational Control System (OCX).

The L5-CNAV is modulated onto L5I signal component, containing basically the same information data as L2-CNAV. The message structure is exactly the same but its content may vary slightly.

[[File:L5c.png|none|thumb|450px|alt=L5c|'''''Figure 3:''''' L5-CNAV Navigation message]]

As in L2-CNAV, it is possible to define up to 63 different message types, but currently only the messages types 10-14 and 30-37 are defined. The remaining undefined and unused message types are reserved for future use.

Further information on L5-CNAV message can be found in IS-GPS-705 ICD document <ref name="GPS ICD 705">[https://archive.gps.gov/technical/icwg/IS-GPS-705J.pdf GPS ICD-705 Revision J, "Navstar GPS Space Segment/User Segment L5 Interfaces"]</ref>.

== CNAV-2 ==

The message CNAV-2 consists of sub-frames and frames and is modulated onto the L1C signal. Each frame is divided into three sub-frames of varying length being required multiple frames to broadcast a complete data message set to users.

* Subframe 1 (9 bits) provides Time of Internal.
* Subframe 2 (600 bits) provides clock and ephemeris data.
* Subframe 3 (274 bits) provides other navigation data which is commutated over multiple pages.

[[File:Cnav-2.png|none|thumb|450px|alt=Cnav-2|'''''Figure 4:''''' CNAV-2 Navigation message]]

Further information on CNAV-2 message can be found in IS-GPS-800 ICD document <ref name="GPS ICD 800">[https://archive.gps.gov/technical/icwg/IS-GPS-800J.pdf GPS ICD-800 Revision J, "Navstar GPS Space Segment/User Segment L1C Interfaces"].</ref>.

==References==
<references/>

[[Category:Fundamentals]]
[[Category:GPS]]
[[Category:GPS Signal Structure]]

NAVIC

2026-05-21T15:06:04Z

Jalbert: Fixed errors in IRNSS Space Segment point

{{Article Infobox2
|Category=IRNSS
|Editors=GMV
|Level=Basic
|YearOfPublication=2011
|Logo=GMV
|Title={{PAGENAME}}
}}
The Indian Regional Navigational Satellite System (IRNSS) is a regional satellite navigation system owned by the Indian government. The system is being developed by Indian Space Research Organization (ISRO).

In April 2016, with the last launch of the constellation's satellite, IRNSS was renamed Navigation Indian Constellation (NAVIC) by India’s Prime Minister Narendra Modi.<ref> [http://gpsworld.com/with-irnss-1g-launch-india-completes-and-renames-its-navigation-constellation/ With IRNSS-1G launch, India completes and renames its navigation constellation]</ref>

==IRNSS Introduction==

[[File:Isrologo.jpg|ISRO logo|thumb|100px]]

IRNSS is an independent and autonomous regional navigation system aiming a service area of about 1500 kilometers around India. The system will be under complete Indian control, with the space segment, ground segment and user receivers all being built in India.<ref name="IRNSS_WIKI"> [http://en.wikipedia.org/wiki/Indian_Regional_Navigational_Satellite_System IRNSS in Wikipedia]</ref> It will have a range of applications including personal navigation.

==IRNSS Architecture==

IRNSS has 7 satellites complemented with the appropriate ground infrastructure as a minimum<ref name=IRNSS_ICD>[https://www.isro.gov.in/SatelliteNavigationServices.html Indian Space Research Organisation, Department of Space, Satellite Navigation Services]</ref>.
As it is traditional in GNSS systems, the architecture is described next in three different segments: the space segment, the ground segment and the user segment.

*The '''IRNSS Space Segment''': 3 of the 7 satellites are geostationary orbit (GEOs) and they are located at 32.5º East, 83º East and 129.5º East longitude<ref name=IRNSS_ICD/>. There are 4 geosynchronous satellites (GSO) in orbits of 24,000 km apogee and 250 km perigee inclined at 29 degrees. Two of the GSOs cross the equator at 55º East and the other two at 111.75º East (two satellites in each plane)<ref name=IRNSS_ICD/>. The life span of the GEOs is 9.5 years and 11 years in the case of the GSOs.<ref>[https://www.jagranjosh.com/general-knowledge/irnss-1606397318-1 What is the Indian Regional Navigation Satellite System (IRNSS-NavIC)? in Jagran Josh website]</ref> The Constellation Design Considerations have been mainly:
** Minimizing the Maximum DOP
** Minimum number of satellites
**Orbital slots for India for a continuous visibility with the control stations

[[File:IRNSSArchitecture.PNG|IRNSS Architecture|thumb|300px]]

*The '''IRNSS Ground Segment''' consists of:<ref name="ICD">[https://www.isro.gov.in/IRNSSSignal.html IRNSS SIGNAL IN SPACE ICD FOR STANDARD POSITIONING SERVICE]</ref>
**ISRO Navigation Centre
**IRNSS Spacecraft Control Facility
**IRNSS Range and Integrity Monitoring Stations
**IRNSS Network Timing Centre
**IRNSS CDMA Ranging Stations
**Laser Ranging Stations
**Data Communication Network
The ground segment is in charge of estimating and predicting IRNSS satellites position, calculation of integrity, ionospheric and clock corrections and running the navigation software.

*The '''IRNSS User segment''': the IRNSS user segment is made of the IRNSS receivers. They will be dual-frequency receivers (L5 and S band frequencies) or single frequency (L5 or S band frequency) with capability to receive ionospheric correction. They will be able to receive and process navigation data from other GNSS constellations and the seven IRNSS satellites will be continuously tracked by the user receiver. The user receiver will have a minimum gain G/T of -27 dB/K.
==IRNSS Services and Performances==

There will be two kinds of services:<ref name="ICD"/>

*Special Positioning Service (SPS)
*Precision Service (PS)
[[File:IRNSS_IGP.PNG|IGP GRID to be used in IRNSS|thumb|160px]]
Both services will be carried on L5 (1176.45 MHz) and S band (2492.028 MHz). The [[IRNSS_Signal_Plan|navigation signals]] would be transmitted in the S-band frequency and broadcast through a phased array antenna to keep required coverage and signal strength.

The data structure for SPS and PS takes advantage of the fact that the number of satellites is reduced -7 instead of the 30 used in other constellations- to broadcast ionospheric corrections for a grid of 80 points to provide service to single frequency users. The clock, ephemeris, almanac data of the 7 IRNSS satellites are transmitted with the same accuracy as in legacy GPS, GLONASS & Galileo.

The Performances expected for the IRNSS system are: Position accuracy around 20 m over the Indian Ocean Region (1500 km around India) and less than 10 m accuracy over India and GSO adjacent countries.<ref>[https://link.springer.com/article/10.1134/S2075108717020109 Vasudha, M.P., Raju, G. Comparative evaluation of IRNSS performance with special reference to positional accuracy. Gyroscopy Navig. 8, 136–149 (2017)]</ref>

==IRNSS Development==

The Indian government approved the project in May 2006, with the intention of the system to be completed and implemented by 2015.

The first satellite of the proposed constellation was successfully launched on the 1st of July 2013<ref>[http://www.gpsworld.com/india-launches-first-navigation-satellite/ India Launches First Navigation Satellite, GPS World]</ref>. It is IRNSS-1A one of the three Geosynchronous satellites that will is compose the entire constellation<ref>[https://www.isro.gov.in/irnss-programme IRNSS Navigation Satellites]</ref>. Despite the first launch was executed slightly later than the planned, at that time India has announced the deadline of 2015-2016 to launch the remaining six satellites<ref> IRNSS Press Release, July 2, 2013</ref>. As in the first launch the forthcoming will place in orbit only one satellite at a time. For that it is scheduled regular launches in every six months<ref>[http://tribune.com.pk/story/570847/india-to-launch-satellite-navigation-system/ India launches its first dedicated navigation satellite, The Express Tribune]</ref>.
As of 18 July 2013 the Indian Space Research Organisation (ISRO) announced that the satellite successfully reached its defined inclined geosynchronous orbit and that the verification tests would start one week after. Before that, in 23 July the German Aerospace Center was able to receive a signal transmitted in the L5 band from the IRSNSS-1A satellite. From the analysis of the received signal researchers from German Aerospace Center concluded that the signal structure is consistent with what was announced as [[IRNSS Signal Plan]] by ISRO<ref>[http://www.gpsworld.com/indian-regional-gnss-satellite-starts-signal-transmissions/ Indian Regional Navigation Satellite Starts Signal Transmissions, GPS World, 25 July 2013]</ref>.
On October 16, 2014, India’s Indian Space Research Organisation (ISRO) successfully launched its third navigation satellite IRNSS-1C abord a Polar Satellite Launch Vehicle (PSLV) rocket from Satish Dhawan Space Centre, Sriharikota.
The fourth IRNSS-1D satellite was successfully placed in orbit onboard the Polar Satellite Launch Vehicle (PSLV-C27), on March 28, 2015. The ISRO's Master Control Facility took over the control of the satellite and after that conducted several maneuvers in order to position the satellite in the geosynchronous orbit at 111.75 degrees East longitude with 30.5 deg inclination<ref>[https://insidegnss.com/indian-launches-fourth-irnss-spacecraft/ Indian Launches Fourth IRNSS Spacecraft, Inside GNSS, March 28, 2015]</ref>. The satellite reached its intended orbit slot on April 9th, 2015.<ref>[http://gpsworld.com/all-systems-go-with-a-spring-into-space/ The System: All Systems Go, with a Spring into Space]</ref><br>

The fifth satellite of the IRNSS constellation was launched on January 20, 2016. <ref>[http://gpsworld.com/indias-fifth-navigation-satellite-launched/ India’s fifth navigation satellite launched]</ref> That launch was closely followed by the 6th launch of a IRNSS satellite on March 10, 2016<ref>[https://insidegnss.com/india-successfully-launches-irnss-1f-into-orbit/ India Successfully Launches IRNSS 1F into Orbit]</ref>. The seventh and final satellite was launched on April 28, 2016 <ref>[https://insidegnss.com/india-completes-irnss-constellation/ India Completes IRNSS Constellation]</ref>.An eighth satellite was launched in 2018<ref>[https://www.isro.gov.in/Spacecraft/irnss-1i Department of Space, Indian Space Research Organisation website]</ref> to replace the failed IRNSS-1A.

In 2020 IRNSS was recognized as a component of IMO’s World Wide Radio Navigation Systems<ref>[https://insidegnss.com/indias-irnss-now-part-of-world-wide-radio-navigation-system/ Inside GNSS: India’s IRNSS Now Part of World Wide Radio Navigation System]</ref>, which enables merchant vessels to use IRNSS for obtaining position information.

==References==
<references/>

[[Category:IRNSS]]