Small airports to be 1st to rely on GAGAN satnav for landings, at major Indian airports it would to be the second choice

Aircraft ordered by IndiGo and Air India last year —a total of 970 Boeing and Airbus planes—will come fitted with receivers that can capture ‘augmented satellite signals’ and thus carry out an approach and landing without any help from ground-based navigation aids. Unlike the Global Navigation Satellite System (GNSS) signals such as the basic GPS signals used by motorists, aircraft navigation, especially in low-visibility conditions demands augmented GNSS.Augmentation makes the GNSS/GPS signals reliable enough for aircraft navigation. Augmentation is done by SBAS or satellite-based augmentation system of geosynchronous satellites that move with the Earth to stay above a certain region, that is. India’s SBAS called GAGAN, stands for GPS Aided Geo Augmented Navigation. GAGAN satellites take the basic GPS signals and augment them to improve its accuracy, integrity, availability, and continuity.
But India’s proximity to the equator has posed problems for its SBAS. Will ionospheric events impact the use of GAGAN by airline aircraft? Narayan Dhital, satellite navigation engineer and air navigation expert at the Galileo Control Centre in Germany published a study assessing the GAGAN services for APV-I and APV-II (Approach Procedure with Vertical guidance) across Indian airspace. His research focused on critical integrity parameters such as satellite error bounds and ionospheric error bounds, providing crucial insights into the reliability and accuracy of satellite navigation systems in challenging environments. Dhital spoke to

Manju V

about the findings of his study.

What were the salient findings of your GAGAN performance assessment study, especially with respect to the system’s usability in carrying out GAGAN-enabled approaches in India by airlines?

It has been long discussed in experts meetings and aviation stakeholders meeting that equatorial regions pose severe ionospheric challenges that hinder the exploitation of full benefits of GNSS based flight procedures. As majority of Indian FIR falls under the core equatorial region, it is not surprising to find out that the performance of GAGAN service is not regularly meeting the APV-I requirement which is being met by counterpart WAAS and EGNOS, mostly the vertical guidance confidence requirement (ionospheric delay mostly effects the vertical accuracy of the GNSS). The GAGAN system has to regularly inflate the ionospheric error protection bound, daily phenomenon of larger ion contents around 2 PM local time and difficult to predict phenomenon of enhanced ion contents triggered by solar activities.
The large span of spatio-temporal variation of the vertical confidence bound reflects a severe challenge to roll-out APV-I procedures throughout Indian FIR. With the increase in number reference stations used to generate the GAGAN corrections for satellite orbits, satellite clocks and ionospheric delays, such spatio-temporal variation can be reduced. The proposed plan by the GAGAN is to include more reference stations in the outer zone of the India geography. This will definitely bring benefits, which we have to wait and see.

GAGAN is capable of

supporting

RNP 0.1 aircraft procedures and APV-I to a large extent. But most crucially, the 99.99% availability is not met for APV-I, let alone APV-II. Also, the impact of ionospheric events on GAGAN is more severe than, say, on other SBAS systems, given the greater ionospheric activity in the equatorial region. Considering these factors, how do you see the GAGAN system being used by airlines in India in the near future?

Given that the Indian aviation market has a promising forecast and the news earlier in 2023 regarding a large contract of 900 plus aircrafts with Airbus and Boeing, I still see gradual increment in the uptaking of GAGAN for flight procedures in the future. This will be largely 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 fluctuation in amplitude and phase of signals, impacts mostly the continuity of the service. Otherwise, the DFMC will allow reliable APV-I services. It is to be noted that, up until last year, only GPS was certified by the ICAO for air navigation. Now, Galileo, which is also a global constellation, has also been certified, and the multi-constellation approach is on the way.
Another crucial use case of GAGAN is the support towards PBN implementation and ADS B implementation. The market is obviously there but it will move slowly (as always is the case with aviation, changes are not easy in safety critical infrastructure).

Do you think GAGAN-enabled approaches would largely be limited to smaller non-metro airports that are not equipped with ILS?

Definitely, the GAGAN approaches are best utilized in the smaller airports. In the larger airports, the airlines are operating bigger aircraft for long haul flights. Reliability of smooth operations comes first and the existing ILS offers such a greater degree of reliability which the GAGAN based APV-I can not offer.
In the larger airports, the ILS are providing reliable services even during the All Weather Operations (including zero visibility). In such context, the uptake of satellite navigation based procedures can be foreseen in two ways:
a. one way for undertaking SBAS in such larger airports is the traffic flow optimization (ultimately, cost effective…environmental friendly) where a CAT-I like approach is flown. This has to be done during the reduced solar cycle phase and during very calm solar activities.
b. the uptake of Ground Based Augmentation System (GBAS-F) instead of SBAS (even though DF largely reduces iono error, it can not remove scintillation effect and hence the continuity can’t be improved). And the GBAS-F has the potential to meet all weather operation upto CAT-III
The GAGAN system for the time being can be used by smaller aircrafts flying in the larger airports and predominantly by aircrafts flying in regional airports. There are a plethora of benefits in terms of operational cost, maintenance cost and environmental protection, when smaller airports implement GAGAN based LNAV/VNAV and APV-I procedures. It has even more benefits in the mountainous regions and regional airports.
Other than that, the newer airports are obviously the main enablers of GAGAN uptake. As is the case of African countries, where A-SBAS can be promising for regional socio-economic development, the GAGAN can provide great support to the UDAN-“Regional Connectivity Scheme” (RCS) being developed by the Government of India.
It is mentioned in the answer to the previous question as well that the mandate of ADSB in future could bring more fleets to be GAGAN equipped. Going forward, the PBN plan also plays a critical role.

Last year, IndiGo carried out a trial in which an ATR aircraft carried out an LPV approach at Kishangarh airport. The minima was 250 feet. Do you think GAGAN system’s integrity performance meets the standards to allow for such LPV approaches to be used on a regular basis? If not, please explain the factors/conditions that might limit its use.

As mentioned above, the usability of LPV is currently limited due to the higher ionospheric activities in India. ICAO has recommended India to regularly monitor and evaluate the performance of ionospheric monitoring systems. A continuous assessment of the performance is a first step to have a plan for regular LPV approaches. After that, if the performance requirement can be met, the design/validation of LVP flight procedures is important. It took some while for India to gather competences in design and validation of SBAS procedures. That competence can now be used to speed up LPV implementation in other airports.

5) Airports in cities such as Mumbai, Delhi, Bangalore, Chennai, Kolkata, Kochi etc are equipped with ILS. How do you see GAGAN system being used in these airports?

As far as Mumbai airport is concerned, I believe, an LNAV/VNAV approach has been published by the Airports Authority of India. But LNAV/VNAV has a higher minima than ILS CAT-1.

In the near future then, do you see GAGAN systems being used to carry out approaches in such metro airports?

Regarding the Mumbai airport, the LNAV/VNAV is published but not the LPV due to the terrain and obstacle avoidance requirements. The main difference between LPV and LNAV/VNAV is the configuration of protected areas around the flight approach path. If the geographical terrain allows, LPV based procedures instead of LNAV/VNAV can be implemented in any airports. LPV 250 is the most feasible procedure if the DFMC GAGAN is realized. From a technology point of view, LNAV/VNAV and LPV procedures fall under the same approach method.
At these larger airports, I see GAGAN procedures as a secondary navigation system to the established ILS. The newer aircraft which are SBAS equipped can fly the GAGAN based procedures during nominal ionospheric period and probably during traffic congestion period. The smaller aircraft might find GAGAN based approach more attractive in terms of operational and maintenance cost.
In summary, I see a good prospect for GAGAN based approaches in metro airports after the DFMC system comes into play. Until then, the regional airports and smaller aircrafts are the main customers that can drive GAGAN further.

(Narayan Dhital’s assessment of GAGAN performances for non-precision aircraft approach and landing was carried in the September 2023 issue of Coordinates, an exclusive monthly magazine on positioning, navigation, associated technologies and applications.