But India’s proximity to the equator has posed problems for its SBAS. Will ionospheric phenomena affect the use of GAGAN by airline aircraft? Narayan Dhital, a satellite navigation engineer and air navigation specialist at the Galileo Control Center in Germany, has published a study evaluating GAGAN services. APV-I and studied APV-II (Approach Procedure with Vertical Guidance) in Indian airspace. His research focused on critical integrity parameters such as satellite error bounds and ionospheric error bounds, which provide crucial information about the reliability and accuracy of satellite navigation systems in challenging environments. Dhital spoke
Manju V
reported the findings of his study.
What were the key findings of your GAGAN performance evaluation study, particularly with regard to the usefulness of the system in helping airlines implement the GAGAN-enabled vision in India?
It has long been discussed at expert meetings and aviation stakeholders’ meetings that equatorial regions pose a serious hazard. Ionospheric Challenges which hinders exploitation of full benefits of GNSS based flight procedures. Since majority of Indian FIRs fall in core equatorial region, it is not surprising to find that GAGAN service performance is not regularly meeting the APV-I requirement, which is being met by counterpart WAAS and EGNOS, mostly vertical guidance confidence requirement (ionospheric delays mostly affect vertical accuracy of GNSS). GAGAN system has to regularly increase the ionospheric error safety limit, daily occurrence of large ion content around 2 pm local time and difficult to predict occurrence of increased ion content triggered by solar activities.
The large period of spatiotemporal variation of the vertical confidence limit represents a serious challenge for implementing APV-I procedures across the entire Indian FIR. With an increase in the number of reference stations used to generate GAGAN corrections for satellite orbits, satellite clocks and ionospheric delays, such spatiotemporal variation can be reduced. The plan proposed by GAGAN is to include more reference stations in the outer region of India’s geography. This will definitely bring benefits, for which we have to wait and see.
Gagan is capable
Assistant
RNP 0.1 aircraft procedures and APV-I can be met to a great extent. But most importantly, 99.99% availability is not met for APV-I, let alone APV-II. Moreover, given the greater ionospheric activity in the equatorial region, the impact of ionospheric events on GAGAN is more severe than other SBAS systems. Keeping these factors in mind, how do you see the use of GAGAN system by airlines in India in the near future?
Given that the Indian aviation market has a promising forecast and there is news about a large contract of over 900 aircraft with Airbus and Boeing in early 2023, I still see a gradual increase in the use of GAGAN for flight procedures in the future. This will largely be driven by the Dual Frequency Multi Constellation (DFMC) GAGAN approach. In such an approach, the above-mentioned critical challenge of the ionosphere is largely eliminated and hence, the confidence in vertical guidance is improved. The issue of ionospheric scintillation will still remain which is even more prominent in the equatorial region. But scintillation, which is caused by rapid fluctuations in the amplitude and phase of the signals, mostly affects the continuity of service. DFMC Will provide reliable APV-I services. It is worth noting that until last year, only GPS was certified by ICAO for air navigation. Now, Galileo, which is also a global constellation, has also been certified, and the multi-constellation approach is moving forward.
Another important use case of GAGAN is support for PBN implementation and ADS B implementation. The market clearly exists, but it will grow slowly (as is always the case with aviation, changes to safety critical infrastructure are not easy).
Do you think GAGAN-enabled approaches will largely be restricted to smaller non-metro airports that are not equipped with ILS?
Of course, the GAGAN approach is best used at smaller airports. At larger airports, airlines are operating larger aircraft for long-haul flights. Reliability of smooth operations comes first and the existing ILS offers such a high level of reliability that GAGAN based APV-I cannot provide.
At larger airports, ILSs are providing reliable services even during all weather operations (including zero visibility). In such a context, the use of satellite navigation based procedures can be viewed in two ways:
A. One way to implement SBAS in such large airports is traffic flow optimization (ultimately, cost effective…environmentally friendly) where CAT-I type flying is done. This should be done during low solar cycle phase and very quiet solar activities.
B. Use of Ground Based Augmentation System (GBAS-F) instead of SBAS (though DF reduces ion error substantially, it cannot overcome the scintillation effect and hence continuity cannot be improved). And GBAS-F has the capability to carry out all weather operations up to CAT-III.
At present, the GAGAN system can be used by small aircraft flying to larger airports and aircraft flying primarily to regional airports. When smaller airports implement GAGAN-based LNAV/VNAV and APV-I procedures, there are many benefits in terms of operational costs, maintenance costs and environmental protection. There are even more benefits in hilly areas and regional airports.
Moreover, new airports are clearly the main enablers of GAGAN adoption. As in the case of African countries, where A-SBAS may hold promise for regional socio-economic development, GAGAN can greatly support UDAN- the “Regional Connectivity Scheme” (RCS) being developed by the Indian government.
It is also mentioned in the answer to the previous question that more fleets can be equipped with GAGAN in the future under the mandate of ADSB. Going forward, the PBN scheme also plays an important role.
Last year, IndiGo conducted a test in which an ATR aircraft took an LPV approach at Kishangarh airport. The minimum was 250 feet. Do you think the integrity performance of the GAGAN system meets the standards to allow such LPV approaches to be used on a routine basis? If not, please state the factors/conditions that may limit its use.
As mentioned above, the utility of LPVs is currently limited in India due to high ionospheric activity. ICAO has recommended India to regularly monitor and evaluate the performance of ionospheric monitoring systems. Continuous evaluation of performance is the first step to creating a plan for regular LPV approaches. After that, if the performance requirement can be met, design/validation of LPV flight procedures is important. It took some time for India to gain proficiency in the design and validation of SBAS procedures. That proficiency can now be used to speed up LPV implementation in other airports.
5) Airports in cities like Mumbai, Delhi, Bangalore, Chennai, Kolkata, Kochi etc. are equipped with ILS. How do you see the use of GAGAN system at these airports?
As far as Mumbai airport is concerned, I believe LNAV/VNAV approach is published by Airports Authority of India. But the minimum value of LNAV/VNAV is higher than ILS Cat-1.
So will you see the Gagan system being used to access such metro airports in the near future?
With respect to Mumbai airport, LNAV/VNAV is published but LPV is not published due to terrain and obstacle avoidance requirements. The main difference between LPV and LNAV/VNAV is the structure of protected areas around the flight approach path. If the geographical terrain permits, LPV based procedures can be implemented instead of LNAV/VNAV at any airport. If DFMC GAGAN is realized, LPV 250 is the most viable procedure. From the technology point of view, LNAV/VNAV and LPV procedures fall under the same approach method.
At these larger airports, I would see GAGAN procedures as a secondary navigation system to the installed ILS. Newer aircraft that are equipped with SBAS can fly GAGAN based procedures during the nominal ionospheric period and possibly during periods of traffic congestion. Smaller aircraft may find the GAGAN based approach more attractive in terms of operating and maintenance costs.
In short, I see good potential for GAGAN based approach in metro airports once the DFMC system is implemented. Till then, regional airports and small aircraft are the main customers who can take GAGAN forward.
(Narayan Dhital’s evaluation of GAGAN’s performance for non-precision aircraft approach and landing was published in the September 2023 issue of Coordinates, a specialized monthly magazine on positioning, navigation, related technologies and applications.
