JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV Year GLOBAL POSITIONING SYSTEM Previous Question Paper (July - 2021 - R16)

GAGAN (GPS Aided GEO Augmented Navigation) is an Indian Satellite Based Augmentation System (SBAS) designed to improve the accuracy, integrity, and availability of GPS signals over the Indian Flight Information Region. The architecture consists of the following key segments:

• Ground Segment: Comprises Indian Reference Stations (INRES) that collect GPS data, Indian Master Control Centers (INMCC) that process the data to generate correction messages, and Indian Land Uplink Stations (INLUS) that transmit these messages to the geostationary satellites.
• Space Segment: Consists of geostationary satellites (GSAT series) that receive the correction data from INLUS and broadcast it to users on the L1 and L5 frequencies.
• User Segment: Includes GAGAN-enabled GPS receivers that process the augmented signals to provide precise positioning and navigation information.

The flow involves INRES stations tracking GPS satellites, sending data to INMCC for integrity and differential correction calculation, and INLUS uplinking this to GEO satellites for broadcast to users.

GPS Block II and Block IIA satellites represent the operational core of the GPS constellation. Their salient features include:

• Block II Satellites: These were the first full-scale operational satellites launched starting in 1989. They were designed for autonomous operation for up to 14 days without ground control intervention and featured redundant atomic clocks (two cesium and two rubidium) to ensure high timing precision.
• Block IIA Satellites: The 'A' stands for Advanced. These satellites introduced improved cross-link capabilities, allowing satellites to communicate with each other to update navigation data. They also featured enhanced survivability and the ability to perform more complex orbital maneuvers.
• Common Features: Both blocks utilized the L1 and L2 frequencies for civilian and military use, incorporated Selective Availability (SA) capabilities (now discontinued), and were designed to provide global coverage with a 24-satellite constellation architecture.