Important Questions For AAI ATC Interview | Article 3/5

Here is the 3rd article of the exclusive AAI ATC Interview Series which primarily focuses on the preparation of the JE (ATC) Interview. The series consists of Five Articles that cover non-technical as well technical questions that I suppose can be asked in the ATC interview.

• Read First Article of AAI ATC Interview Series (Questions 1 to 26)
• Read 2nd article of AAI ATC Interview Series (Questions 27 to 38 )
• Read 4th article of AAI ATC Interview Series (Questions 46 to 52)
• Read Last article of AAI ATC Interview Series (Questions 53 to 65)

This article covers GNSS, GPS and working of GAGAN and GPS along with other relevant probable questions.

Ques. 39: What is GNSS?

Ans. GNSS is an acronym for Global Navigation Satellite Systems. GNSS uses a cluster of navigation satellites to trace or pinpoint the user’s location. The signals received from GNSS navigation satellites can also be used to locate other objects, people, or goods at any time. Examples: GPS, GAGAN.

Ques. 40: What are various GNSS systems in the world?

Ans. Some of the GNSS in the world are GLONASS (Russia), GALILEO (Europe), GPS (USA), GAGAN (India), and BeiDou-2 (China).

Ques. 41: What is GPS?

Ans. GPS or Global Positioning System (GPS) is a satellite-based system owned and maintained by USA Govt. It is an all-weather system, used to provide locations and time information anywhere in the world. A simple example is a GPS receiver installed in radio cabs. GPS can provide real-time 3D positioning, navigation, and timing anywhere on or near the earth. GPS system is used extensively by the military, commercial and civil users worldwide.

Working: The GPS concept is based on time. The satellites carry very stable atomic clocks that are synchronized to each other and to ground clocks. GPS satellites continuously transmit their current time and position. A GPS receiver monitors multiple satellites and solves equations to determine the exact position of the receiver and its deviation from true time.

Ques. 42:Minimum Number of satellites required by GPS to determine the location of an object?

Ans. To accurately locate an object minimum of 4 satellites are required, however, three satellites can also serve the purpose but the located position will be less accurate. In case of 3 satellites the receiver approximates the locations by assuming that user is at mean sea level.

• The first satellite locates you somewhere on a sphere.
• The second satellite narrows your location to a circle created by the intersection of the two satellite spheres.
• The third satellite reduces the choice to two possible points
• Finally, the fourth satellite helps calculate a timing and location correction and selects one of the remaining two points as your position.

Ques. 43: What is the difference between GPS and GAGAN?

Ans. GPS is a satellite-based navigation system that is used to provide real-time position and time data to users having GPS receivers. As of now, the GPS system has 31 satellites in its constellations which transmit signal to locate any object on or near the earth.

On the other hand, GAGAN is an augmentation system that augments the already existing GPS constellation. GAGAN provides more accuracy than GPS. This accuracy is result of processing of GPS satellites data at GAGAN Master Control Centre and sending the corrected data to the intended user through geostationary satellites.

Ques. 44: What is GAGAN and how does it work?

Ans. GAGAN is the acronym for GPS Aided GEO Augmented Navigation. GAGAN is a joint venture of ISRO (Indian Space Research Organization) and AAI (Airports Authority of India). Let’s understand the working of GAGAN step by step.

1. GPS satellite data is collected by a network of precisely surveyed ground reference stations which are known as INRES(INdian REference Stations). These INRES are strategically positioned across the country so that the entire Indian Airspace can be covered. Total 14 INRES.
2. Data collected by INRESs is sent to Bengaluru-based INMCC (INdian Master Control Centre)which in turn generates messages to correct any signal errors.
3. These correction messages are then uplinked through INLUS(INdian Land Uplink Station) and broadcast through communication satellites (Geostationary) to the user (Aircraft).

Try to understand the working of GAGAN with help of this image:

Ques. 45: What are the benefits of GAGAN?

Ans. One of the prime benefits of GAGAN will be related to the Aviation sector where it will provide enhanced accuracy, availability, and integrity to GPS signals for all the phases of a flight. It will result in fuel savings and decreased carbon emissions. In addition, GAGAN will provide benefits in all modes of transportation like railways, highways, and maritime.

• Read Questions 1 to 26
• Read Questions 27 to 38
• Read Questions 46 to 52
• Read Questions 53 to 65

Edit: Read the latest post about important definitions and terms like ADS-B, SSR, SBAS, GBAS, Runway, Flight Level, etc for ATC Interview Preparation. Link is:
Important Terms and Definitions for JE-ATC Interview

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