Indian Regional Navigation Satellite System (IRNSS)

History

The Indian Regional Navigation Satellite System (IRNSS) with an operational name of NAVIC (“sailor” or “navigator” in Sanskrit, Hindi and many other Indian languages, which also stands for NAVigation with Indian Constellation) is an autonomous regional satellite navigation system that is being set up by India, that will be used to provide accurate real-time positioning and timing services over India and the region extending to 1,500 kilometres (930 mi) around India. The NAVIC system will consist of a constellation of 3 satellites in Geostationary orbit (GEO), 4 satellites in Geosynchronous orbit (GSO), approximately 36,000 kilometres (22,000 mi) altitude above earth surface, and two satellites on the ground as stand-by, in addition to ground stations. The system was developed because access to foreign government-controlled global navigation satellite systems is not guaranteed in hostile situations, as happened to the Indian military in 1999 when it was dependent on the American Global Positioning System (GPS) during the Kargil War. The Indian government approved the project in May 2006.

The constellation of seven NAVIC satellites is already in orbit and the system is expected to be operational from September 2016, after a system check. NAVIC will provide two levels of service, the standard positioning service will be open for civilian use, and a restricted service (an encrypted one) for authorized users (including the military).

Development

IRNSS is an autonomous regional satellite navigation system being developed by ISRO (Indian Space Research Organization). The government of India approved the project in May 2006, with the intention of the system to be completed and implemented in the timeframe 2016.

The objective of the project is to implement an independent and indigenous regional spaceborne navigation system for national applications. The IRNSS design requirements call for a position accuracy of < 20 m throughout India and within the region of coverage extending about 1500 km beyond. The system is expected to provide accurate real-time position, velocity and time observables for users on a variety of platforms with a 24 hour x 7 day service availability under all weather conditions. 1) 2) 3) 4) The IRNSS is being developed parallel to the GAGAN (GPS Aided GEO Augmented Satellite Navigation) program, the ISRO SBAS (Satellite Based Augmentation System) version of an overlay system for GNSS signal corrections. 5) 6) 7) The proposed IRNSS system will consist of a constellation of seven satellites and a supporting ground segment. Three of the satellites in the constellation will be placed in a geostationary orbit and the remaining four in a geosynchronous inclined orbit of 29º relative to the equatorial plane. Such an arrangement would mean all seven satellites would have continuous radio visibility with Indian control stations. ISRO has filed for 24 MHz bandwidth of spectrum in the L5-band (1164 – 1189 MHz) for IRNSS and for the second signal in S-band (2483.5 – 2500 MHz). The IRNSS constellation architecture consists of the following elements:

  • Space segment: The IRNSS satellites carry a navigation payload in a redundant configuration. A separate C-band transponder for precise CDMA ranging is included in the payload configuration. The important functions of the IRNSS payload are: Transmission of the navigational timing information in the L5 bands; transmission of navigation, timing information in S-band; generation of navigation data on-board, CDMA ranging transponder for precise ranging.

    The navigation payload will have the following subsystems: NSGU (Navigation Signal Generation Unit), Atomic clock unit, comprising of Rubidium atomic clocks, clock management and control unit, frequency generation unit, modulation unit, high power amplifier unit, power combining unit and navigation antenna.

    The IRNSS spacecraft are dedicated for navigation services and they are configured to be of a class that can be launched by the Indian launcher PSLV. The design incorporates most of the proven subsystems available indigenously tailoring it specifically for the navigation.

  • Ground segment: The ground segment is responsible for the maintenance and operation of the IRNSS constellation. It contains a whole complement of the elements required for a basic constellation and is mainly comprised of:
    – Master Control Center for spacecraft control and navigation, IRNSS tracking and integrity monitoring stations, CDMA ranging stations, uplinking and telemetry stations, communication links and network timing center.
  • User segment: Specially designed receivers and antennas are needed to receive the IRNSS signals. The receivers are also planned for receiving multi-constellation signals inclusive of GPS, GLONASS, Galileo and IRNSS. It is planned to broadcast the time difference between the IRNSS time and the time of the other constellations to enable the users to take advantage of the signals available to them.

Space segment

The space segment consists of seven satellites:

  • 3 satellites in GEO (Geostationary Orbit) at 32.5°, 83° and 131.5° East.
  • 4 satellites in geosynchronous orbit placed at inclination of 29° with longitude crossing at 55° and 111.75° East
  • Two spare satellites are also planned
  • The satellites are specially configured for the navigation. Same configuration for GEO and GSO which is desirable for the production of the satellites.
  • Plans call for the IRNSS satellites to be launched by the Indian launcher PSLV
  • The first satellite will be launched in the summer of 2013. The subsequent launches are planned once in six months. The
    full constellation will be operational by 2016.

User segment:

The user segment consists of IRNSS receivers operating in:

  • Single frequency (L5 at 1176.45 MHz or S-band at 2492.028 MHz)
  • Dual frequency (L5 and S-band)

The single frequency and dual frequency receivers shall receive both SPS (Special Positioning System), which is provided to all users, and RS (Restricted/Authorized Service) signals, which is an encrypted service provided only to authorized users.

The IRNSS user receiver calculates its position using the timing information embedded in the navigation signal, transmitted from the IRNSS satellites. The timing information being broadcast in the navigation signal is derived from the atomic clock onboard the IRNSS satellite.

The IRNWT (IRNSS Network Time) is determined from a clock ensemble composed of the cesium and hydrogen maser atomic clocks at the INC (Indian Navigation Centre) ground stations. As with UTC, IRNWT is also a weighted mean average time, but with two substantial differences. IRNWT will be made available in real time and is a continuous time without leap seconds. The IRNSS satellites carry a rubidium atomic frequency standard onboard. At INC through navigation software, these onboard clocks are monitored and controlled. The deviation between each of the satellite and IRNWT is modeled with a quadratic function of time, and the parameters of this model are calculated and transmitted as a part of the IRNSS broadcast navigation messages.

The parameters are often called as clock bias (A0) or the clock offset (in seconds), drift (A1) or the relative frequency instability (in seconds/second) and aging (A2), also referred to as relative frequency shift (in seconds/second2). Apart from these corrections, any IRNSS users should consider the necessary relativistic time adjustment. With these adjustment parameters, which are usually calculated once per day, are then transmitted to the satellites, thus the satellite clock errors are expected to be well within 5-10ns which fulfills the requirement.

The estimated accuracy is < 20 m over the Indian ocean region, and < 10 m over main land India.
Figure 14: Illustration of the IRNSS coverage which includes an area of ~1500 km around the Indian land mass (image credit: ISRO)

IRNSS signals:

The IRNSS constellation is expected to provide a position accuracy (2σ) of better than 20 m over India and a region extending outside the Indian land mass to about 1,500 km. The system will provide two types of services: (Ref. 4)
1) SPS (Standard Positioning Service)
2) RS (Restricted/Authorized Service)

Both of these services will be provided at two frequencies, one in the L5 band and the other in S-band.
SPS will use bi-phase shift keying BPSK (1) modulation, whereas the RS service will employ binary offset carrier (BOC (5, 2)) modulation. An additional BOC pilot signal is being provided for the RS Service in order to help provide better acquisition and performance. As each L5-band and S-band contains three signals, the IRNSS design adds an interplex signal in order to maintain the constant envelope characteristic of the composite signal.

The transmission is done using the L-band and S-band helix array antenna to provide global coverage in right-hand circularly polarized (RHCP) signals. Thus, user receivers can operate in single-and/or dual-frequency mode.
Timing group delay:

The time of radiation of the navigation signals on each carrier frequency and among frequencies is not synchronized due to the different digital and analog signal paths that each signal must travel from the IRNSS satellite signal generator to the transmit antenna. This hardware group delay is defined as a time difference between the transmitted RF signal (measured at the phase center of a transmitting antenna) and the signal at the output of the onboard frequency source.
Three different parameters comprise this group delay: a fixed/bias group delay, a differential group delay and a group delay uncertainty in bias and differential value.

The fixed delay or hardware group delay is a bias term included in the clock correction parameters transmitted in the navigation data and is, therefore, accounted for by the user computations of system time in the appropriate GPS interface specifications. More specifically, this delay represents the amount of time it takes the signal to start from the common clock, travel through each code generator, modulator, up-converter, transmitter, and finally emerge from the satellite antenna.
The hardware group delay uncertainty reflects the variability in the path delay due to changeable conditions in the operational environment and other factors. The effective uncertainty of the group delay will be in the range of few nanoseconds (on the order of 1-3 ns).

Each IRNSS navigation signal has two hardware paths — main and redundant. The hardware will be different for each path in terms of data generator, modulator, up converter, travelling-wave tube amplifier (TWTA), cable, and integration components.
In case of failure, the signal will be diverted from the main subsystem to the redundant subsystem. he delay of main and redundant subsystem will be different and thus cause a difference in the mean path delay based on the selected path for the navigation signal.

Differential group delay is the difference in delays between two navigation signals. It consists of random plus bias components. The mean differential is defined as the bias component and will be either positive or negative. For a given navigation payload redundancy configuration, the absolute value of the mean differential delay shall not exceed a few nanoseconds, i.e., on the order of 15 to 30 ns.

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