GAGAN

Other SBAS
Title	GAGAN
Edited by	GMV
Level	Basic
Year of Publication	2011

The GPS Aided Geo Augmented Navigation system (GAGAN) is the SBAS implementation by the Indian government.

GAGAN Introduction

ISRO logo

In August 2001 the Airports Authority of India and the Indian Space Research Organization (ISRO) reached an agreement for the establishment of the GAGAN system.

The development plan consists of two different phases:

1. Technology Demonstration System (TDS);
2. Initial Experimental Phase (IEP);
3. Final Operational phase (FOP).

The TDS phase was completed in August 2007 using the navigation payload of the INMARSAT 4F1 satellite. The Initial Experimental Phase (IEP), initially planned to be finished by 2009, is currently being implemented concurrently with TDS Phase.

On 15 April 2010, it was attempted to launch the first GAGAN navigation payload. The equipment was installed in the satellite, GSAT-4, but, unfortunately, the launch failed and the GEO satellites never reached its nominal orbit. In May 2011, the GSAT-8 satellite carrying a GAGAN SBAS payload was successfully launched by the Ariane-V launch vehicle of Arianespace from Kourou, French Guiana. Later on 28 September 2012, another Ariane 5 rocket successfully launched the India's GSAT-10 satellite which carries the second GAGAN payload.

GAGAN Stability tests were successfully completed in June 2013. The overall performance of the systems was reviewed by the review committee of the DGCA (Director General of Civil Aviation).

GAGAN Architecture

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  GAGAN overview.

The main components of the GAGAN Architecture are:

• Space segment: three operational GEO satellites: The GSAT-8 and GSAT-10 satellites were successfully launched in March 2011 and April 2012, respectively. The remaining satellite is schedule to be launched during 2014 aboard an Arianne 5 launch vehicle.
• Ground segment: On the ground, the GPS data is received and processed in the 15 Indian Reference Stations (INRES), located at Ahmedabad, Bengaluru, Bhubaneswar, Kolkata, Delhi, Dibrugarh, Gaya, Goa, Guwahati, Jaisalmer, Jammu, Nagpur, Porbandar, Portblair, Trivandrum. The Indian Master Control Center (INMCC) composed by two sites and located in Bangalore, processes the data from the INRESs to compute the differential corrections and the estimate of its level of integrity. The SBAS message generated by the two INMCC is uplinked to the GEO satellites through its corresponding Indian Land Uplink Station (INLUS).
• User segment: GAGAN-enabled GPS receivers, with the same technology as WAAS Receivers, capable to use the GAGAN Signal-in-Space (SIS). User equipment for civil aviation shall be certified against several standards (see article SBAS Standards).

The company Raytheon was awarded in 2009 with the contract to modernize the Indian air navigation system. Raytheaon was responsible for building the Ground Stations being supplied by some ground equipments by ISRO.

GAGAN Signals and Performances

The GAGAN GEO satellites will broadcast SBAS navigation data using L1 and L5 signals, with Global Positioning System (GPS) type modulation. The specification of the SBAS message data format is contained in the ICAO SARPS Appendix B for the aspects related with the signal in space, as well as in the RTCA MOPS DO-229D for the minimum performance requirements applicable to the airborne SBAS receiver equipment. The format of the messages is thoroughly explained in the article The EGNOS SBAS Message Format Explained. (See the article SBAS Fundamentals for more information.)

The performance objectives of the Final Operational Phase of the GAGAN system are:

• RNP 0.1 en route navigation in India Flight Information Region (FIR).
• APV-2 over the land masses in India FIR.

On December 30, 2013 the GAGAN system was certified to RNP0.1 service level by the Director General of Civil Aviation (DGCA). This means, as of this date, aircrafts equipped with SBAS receivers will be able to use GAGAN SiS in Indian airspace for en route navigation and non-precision approaches without vertical guidance. The GAGAN signal availability filled the gap between EGNOS and MSAS coverage areas.

Ionospheric issues

One of the main concerns about an SBAS implementation in India is the ionospheric behavior at these latitudes, as India is located in the equatorial ionospheric anomaly belt. The ionosphere near the geomagnetic equator has physical process and features that rarely affect mid-latitudes. These include the Appleton geomagnetic anomaly, plasma bubbles, and scintillations.

Free from adverse ionospheric effects, current SBAS in the mid magnetic latitudes provide vertical guidance for the single frequency users. The ionosphere equatorial anomaly and the ionospheric phenomena typically found at equatorial latitudes, produce large spatial gradients and temporal gradients in the ionospheric delay. This significantly challenges SBAS to meet the stringent requirements associated to precision guidance. The macroscopic effects (equatorial anomaly) are not well approximated with the 5 x 5 degree grid thin shell model specified in the current SBAS Standards. Also, the microscopic phenomena (plasma bubbles) cause sharp gradients during a short period of time (less than 5 minutes). If these small scale features are not observed or alerted by the SBAS system, it would make difficult to ensure integrity compatible with the precision approach alert limits. Finally, the user equipment and the reference stations of the SBAS system might suffer from tracking and noise problems because of scintillation, which is a particularly likely problem in equatorial latitudes depending on the season and day time. All these problems are under study by several groups and different approaches for an SBAS implementation in equatorial magnetic regions have been presented.

In order to cope with the ionospheric perturbations at the equatorial level, the GAGAN system makes use of the ISRO GIVE Model – Multi Layer Data Fusion (IGM-MLDF). The model ensures that the broadcast GIVEs have a sufficiently high level of integrity so that the user ionosphere vertical errors (UIVEs) computed by user receivers will bound their vertical ionosphere errors with a very high probability. The algorithm provides the delay and the confidence values for the user, resulting in improved accuracy and availability.