GAGAN: Building block by block
GAGAN and IRNSS pose complex and unique challenges
GAGAN, a joint project of Airports Authority of India (AAI) and Indian Space Research Organization (ISRO), is a Satellite Based Augmentation System (SBAS) to the GPS. It augments the “Raw” GPS signals with the necessary corrections for meeting the accuracy, integrity, continuity and availability requirements specifi ed for aircraft navigation over Indian fIight Information Region. IRNSS is an indigenous Satellite Navigation System that will provide autonomous geospatial positioning with regional coverage. With 7 satellites in GEO/GSO it will provide allweather navigation in the Indian Landmass and 1500 km beyond its geopolitical border. GAGAN seeks to enhance the accuracy of GPS system while IRNSS seeks to form an independent navigation system. GAGAN is safety of life system whose requirements are very stringent with availability target of 99.999%, accuracy and precision levels also to be very high. Since this was the first time such a system was setup in India, it posed unique and daunting challenges in the development of the ground segment. We shall look into a few of them.
In GAGAN, corrections to GPS in terms of ionospheric error, clock error and ephemeris error are provided by the ground segment. It also determines integrity of navigation and alarms the user within 6 seconds (of occurrence of HMI).This places stringent specification on robustness of subsystems and the equipment that need to have built-in redundancy at all levels.
Monitoring of GPS satellite signals and Ionosphere is done by a set of widely separated Reference station (INRES), whose choice of location is critical to the mission. Extensive simulation exercises were done to choose the number and location of the various INRES sites. Multi path signal at INRES is critical performance driver of GAGAN. Elaborate multipath and interference survey of each site was done using extensive measurements and special analysis tools. In order to ensure the availability, each INRES had to be built with three independent chains of receivers. RF interference of Radars and other equipment operating in these frequencies had to be detected using special equipment and mitigation measures were taken involving significant effort and interagency coordination.
The uplink station (INLUS), which interfaces with satellite to broadcast the correction signals, comprises of signal generation unit and RF unit with 11 m antenna. The 11m antenna and RF systems were indigenously developed and operationalized. Designing a combined feed system for INLUS was a major challenge, and required extensive experimentation and several iterations to arrive at the final design.
Master Control Center (INMCC) is the nerve center for the GAGAN project collecting endless data performing navigation software function with attendant computer system, well supported by reliable communication links. INMCC availability requirements placed stringent configuration and infrastructure constraints on various facilities. Fully redundant terrestrial communication links were implemented connecting most the INRES to INMCC. For INRES at some locations only satellite links could be used. Outages in communication links proved a bottleneck during technology demonstration phase reducing the availability of the system and the last mile problem at Bangalore was solved by implementing multiple layers of dedicated Fiber optic links.
IRNSS is designed to provide PNT (Position, Navigation and Time) services with position to an accuracy of 20 m and time to an accuracy of 10 Nano seconds. Critical technologies in the ground segment are
1. Reference receiver development
(S and L band)
2. Establishment of precision timing facility (<10 nano seconds)
3. CDMA/Laser ranging of the satellite (centimeter level accuracy at 36000 km)
Of these, IRNSS precision timing facility needed the establishment of an independent Time scale. With no prior experience of architecting such a system it was a major challenge to establish the design of this system. Through considerable research and analysis the system was finalized as an ensemble of Hydrogen maser and Cesium clocks, robust processing algorithms and other related hardware. Presently the facility is undergoing stability tests. A special feature of IRNSS ground segment is centralized monitoring and control of the diverse elements of the ground segment. High levels of automation were achieved using software that was conceived, designed and developed in-house. Using this all the elements of the ground segment can be controlled from a single console.
GAGAN and IRNSS are technologies that have been envisioned to leapfrog India into a select group of countries with their own navigation systems. The nature of the requirements from these systems required the implementation of cutting edge technology, posing its own set of complex and unique challenges. Overcoming these challenges required innovative thinking and team effort, pushing the limits of technological expertise and project management skills. Team GAGAN consisted of engineers from AAI and experts from ISRO working together as a well knitted unit, while IRNSS is being developed by ISRO solely. On a personal note it was a stimulating experience to lead the establishment of the ground segment in the initial phase. Currently with all the elements of ground segment ready and GSAT-8 already providing the signal in space, GAGAN is entering the certification phase. Most of the elements of IRNSS ground segment have been established and IRNSS is envisaged to be fully operational by 2015. These crown jewels of Indian Space implementation will serve as the guiding light to the country well into the future.
GAGAN-TDS is a path breaking project on many counts
GAGAN-TDS is a path breaking project on many counts. It is one of the first major projects in the country in the field of Satellite navigation. TDS represents the Technology Demonstration System referring to the first phase of the spiral deployment of the SBAS system in the region. GAGAN-TDS was taken up by ISRO in collaboration with Airports Authority of India.
It was in late 1999 that I was asked to lead the project as the Project Director at ISRO Satellite Centre, Bangalore. As I progressed into making the detailed project report it was evident that the project was challenging than what was understood because of the following:
• Need to Implement a whole complement of Satellite Navigation elements including the Ground and Onboard Infrastructure for the first time in the country with specifications meeting stringent International Standards for Civil Aviation
• Need to carry the TDS phase elements to the operational phase, To save time and money
• Somewhat complex behaviour of the Ionosphere compared to other regions
The drawing up of the detailed specifications itself was a daunting task. GAGAN-TDS project had to learn from the experience of the US and the European teams who had implemented similar systems earlier in order not to repeat costly mistakes. A lot of efforts went in selection of the State of the Art equipment which will not become obsolete for many years. The services of independent International consultants were utilised particularly for ensuring that the system is suitable for eventual certification. A combined team of workforce drawn from both ISRO and AAI was created for execution. Managing diverse working teams proved to be difficult in the beginning but the related problems were overcome due to the support available from the management of both ISRO and AAI.
The development of the Ground infrastructure at so many locations for the reference stations presented many difficulties. A number of hurdles had to be crossed even before the work could start at the respective locations. An elaborate site survey had to be conducted for meeting stringent criteria for RF Interference and Multipath. Similar hurdles had to be crossed for the establishment of 18 TEC stations across the country for understanding the behaviour of the Ionosphere. The maintenance of these stations, collection of the data, and the subsequent analysis by various institutions threw enormous challenges hitherto not attempted. Volumes can be written about the hassles faced during the transportation and installation of sensitive equipment like the Atomic Clock at remote Reference Station locations. Ensuring continuous operation in these unattended stations was not easy. The establishment of Mission Control Centre and the Uplinking Station at Bangalore was unique and presented its own set of problems. Thanks to the skill and the dedication of the project team, the work was completed meeting all the objectives. This Infrastructure, in fact, proudly showcases the GAGAN-TDS achievements to the outside world drawing praise from the visiting dignitaries.
The development of the L-Band L1/L5 Navigation transponder for GAGANTDS was taken up at ISRO. This task proved to be tough given the fact that the latest FAA specifications were made applicable. The SAW filters required for the Transponder had to be indigenously developed for the first time as the project faced export restrictions for the supply of the item. When the development was completed it marked a big milestone since this system had the best specifications so far for any SBAS Navigation Transponder. As the launch of the satellite took more time than earlier planned, the project had to think of alternate means to qualify the Ground system. The project could get the services of the INMARSAT SBAS L1/L5 Transponder on temporary basis for the integration of GAGAN-TDS Ground system and establishment of the Signal in Space. During the GEO satellite integration, a number of technical problems were faced which at times was very frustrating. The combined efforts of the project team and the vendors of the equipment made it possible to solve the problems and to fine tune the system for performance. This was a real learning experience for the entire team. The availability of dedicated communication links linking all the stations to the Mission Control Centre, on a continuous basis meeting the stringent SBAS standards is very vital and it was found to be difficult to achieve on a continuous basis. The RF Interference and Multipath environment can change and it caused some problems. The successful GEO Integration culminated with the System Acceptance Test. The results were very encouraging. The subsequent test using a user receiver onboard a special aircraft also yielded good results. This marked the successful completion of the TDS phase.
The lessons learnt from GAGANTDS have been the basis for the Operational Phase which is presently in progress. Experience gained in GAGAN-TDS for the creation of the Infrastructure, development of the Navigation Software and the handling of the Ionospheric corrections will be very valuable for the Operational Phase of GAGAN as it clearly spells out the improvements required in the system configuration as well as the Ionospheric Corrections strategy.
As I look back, it is a matter of great pride and satisfaction for me to have been associated with such a path breaking project like GAGAN-TDS which in my opinion is far more than a Technology Demonstration System. I salute the GAGAN-TDS team members who made it possible.
Coordination of spectrum in GNSS is of utmost importance
Satellite Navigation is, by its very nature, a global program. The Indian Space Research Organization (ISRO) started the Satellite Navigation program in the early nineties. The International Civil Aviation Organization (ICAO) had prepared a report on Future Air Navigation System (FANS). The FANS report prepared by the member States recommended Satellite Navigation as a primary FANS technology for civil aviation for Communications, Navigation and Surveillance. The US with GPS and Russian federation with GLONASS had by then launched their global systems. By 1996, GPS had completed its constellation with 24 satellites in MEO orbit. GLONASS had more than 22 satellites by then but the number of satellite went down subsequently until 2002 when Russia revamped the GLONASS system. Today GPS has 31 satellites and GLONASS 24 and 3 spare. Europe realized that Satellite navigation is an important satellite based service and started filing for spectrum in the mid nineties for Galileo. Galileo has 4 satellites and expects to reach to 22 satellites in the next 2-3 years. For a good global coverage 24 satellites in Medium Earth Orbit (MEO) are required. The Chinese followed suit and India started the GAGAN and IRNSS programs about the same time.
The L band spectrum for Radio Navigation Satellite Service (RNSS) is very crowded. The US, Russia, Europe, China have filed for Global Navigation Satellite System (GNSS) principally in the L band in the eighties and nineties. India also filed for required spectrum in the L and S-band(2483.5 to 2500 MHz). Now, internationally, we have 5 global filings – US, Russia, Europe, China and India, 4 GPS augmentation filings by US, Europe, Japan and India and 3 Regional filings- China, Japan and India. If all systems are up and operating in the next decade, we may have nearly 200 satellites in the GNSS arena. Filings made by any country have to be “Coordinated” with all the countries who file for spectrum in that band. This calls for bi-lateral and multi-lateral co-ordination for spectrum, under the ITU umbrella.
Most of the satellites in GNSS are in the Medium Earth Orbit (MEO), ie the satellite altitude is about 25,000 to 27,000 Km from the centre of the Earth. Co-ordination of spectrum in GNSS is of utmost importance as otherwise there would be harmful interference from transmissions of one system into another which is detrimental to the service to be provided. Hence, any person leading the SATNAV program of the country has to interact with other system operators on a regular and sustained manner.
Designing the Communication network needs a very careful approach
Ihave enjoyed my work thoroughly all through my carrier and I owe my gratitude to our Department, which has allowed us to freely express and execute. I was in communications department from the day one of my carrier and involved in all the launch/satellite missions and also in many projects including GAGAN and IRNSS. My sincere thanks go to Mr S K Shivakumar, then Director, ISTRAC for assigning me the roles to play in GPS Aided Geo Augmented Navigation (GAGAN) and Indian Regional Navigation Satellite System (IRNSS) projects. Also I shall be grateful to the ISRO/AAI team that was great, with real stalwarts like Mr S Pal, Mr Kibe, Mr P Soma, Mr K N S Rao, Mr Ganesan, Mr Ramalingam (AAI), Mr Saraswathi (AAI), Mr Khan (AAI) and many more, who has made the toughest jobs to look simple without relenting on any issues.
Data Communication Network
The communication network requirements differ from a launch mission to a satellite mission and to projects like GAGAN/ IRNSS. While the launch/satellite mission networks are mission critical, the GAGAN and IRNSS are linked to the safety of the passengers in air. The Data Communication Network of GAGAN and IRNSS projects play an important role of transporting the navigation and measurement data from all the reference stations spread across the country to the Control Centre and also several internal communication back bone for various purposes. Designing the communication network needs a very careful approach in understanding the requirements of the project, the available solutions, various communication interfaces and the standards followed in different countries involved in similar projects.
The data communication requirements of GAGAN and IRNSS projects are very stringent and are similar in many aspects, one defined by M/S Raytheon and the other one of our own. The requirements were classified into the following categories
• Functional Requirements: Data flow requirements received from the project
• Technical Requirements: Defines the bandwidth, interfaces, latency etc.,
• Operational Requirements: Availability, Alarm, Escalation matrix etc.,
• Performance Requirements: Bit Error Rate, Mean Down Time etc.,
Though many of the requirements were critical, the challenging parameters are the availability and reliability factors. The availability number is 99.999% (one hour failure in 11years and 5 months). Hence, we were bound to work more on redundancies in terms of
• Link Level Redundancy
• Equipment Level Redundancy
• Changeover strategies or fail safe mode
Two different transmission media were suggested for link level redundancy, 1) Optical fibre back bone and 2) Satellite based backbone. End to End optical fibre connectivity was suggested for all the nodes and to a greater extent that was achieved also with the help of service providers. In the satellite communication segment, the equipment redundancies were built up with auto changeover configuration.
Still, achieving the target was looking as a distant dream and so to meet the availability requirements multiple links were proposed between two given stations. Involving multiple links between two given stations has its own problem of selection and processing that was resolved by network team.
It was also ensured that there is no single point failure at any given point including the cable entry, the UPS (different UPS for different systems), Communication Earthing etc., and the special needs of a communication rooms were addressed.
For a communication network, establishing the network is the first phase and maintenance becomes the next phase that needs to be given highest priority and AAI is in right track in tackling the issues and the project will be yielding fruitful results soon.
Optimizing the Antenna (outdoor) and Receiver (indoor) distance to get maximum signal
Ihad a very nice experience during the last one decade while establishing the reference stations for navigational systems (both GAGAN & IRNSS) across the country as per the Project requirement. I have started my first site survey visit in 2003 for GAGAN at Thiruvananthapuram airport and my last visit was at Jodhpur, RRSC Campus in 2011 for IRNSS before my retirement. Here I am sharing my views on antenna and Station requirements only.
GAGAN Started with 8 Reference stations for Technology Demonstration System (TDS) and increased to 15 stations for the Final Operation Phase (FOP). 10 Stations were established during the phase-I of IRNSS and activities started for another 7 stations to meet the FOP requirement of 17 Reference Stations.
Site selection criteria/site survey requirements
Outdoor requirement of Antenna location & Indoor requirement for accommodating receiver and other supporting equipment The main requirements are as below:
1) Site Foundation for Antenna (Hard bedrock)
• Radomes for environmental protection
• Lightning Protection
• Antenna Phase Centre measurements
•Antenna height & horizon mask (Elevation profileminimum 10 deg.)
• Antenna phase centre measurements
2) Site security, Ownership and permission for long term
3) Multipath (away from reflecting surfaces)
4) Radio Frequency Interference of intended frequencies
5) Site infrastructure requirements & Continuous Power (UPS)
6) Proper earthing equipment, building etc.
7) Met. Sensor provision
8) Local data logging & Storage
9) Data communication to Control Centre
• Accommodating the Reference stations within Airport Authority of India (AAI) and Indian Space Research Organisation (ISRO)
• Existing buildings/Rooms were utilised where ever possible for indoor equipments
– Wherever required, New constructions were also made
• Transportable containers were planned in few locations which can be re-located as per Project Requirement
• Coordination with WMO of DOT for RFI
• Few stations were re-located in view of RF interference for both GAGAN & IRNSS
• Planning suitable indoor equipment like Reference receiver and supporting units like communication etc.
• Receiver antennae were installed both on ground as well as on roof-top depending upon the site suitability
• Optimizing the Antenna (outdoor) & Receiver (indoor) distance to get maximum signal
• Periodical reviews on the sites – and suggestions were implemented