Basic Principles of Global Navigation Satellite System | CUET PG Geography | Notes

Introduction

The Global Navigation Satellite System (GNSS) is a generic, umbrella term referring to any satellite-based positioning, navigation, and timing (PNT) system that provides autonomous geospatial positioning with global coverage, allowing electronic receivers on or near the Earth’s surface to determine their precise location (latitude, longitude, and altitude) and accurate time by processing signals transmitted from a constellation of orbiting satellites. GNSS represents one of the most transformative technological developments in modern surveying, cartography, and geographical science, having revolutionized fields ranging from precision surveying and navigation to disaster management, agriculture, and everyday consumer applications. The most well-known GNSS is the American GPS (Global Positioning System), though several other independent systems now exist, including Russia’s GLONASS, the European Union’s Galileo, and China’s BeiDou (BDS), along with regional augmentation systems such as India’s NavIC (Navigation with Indian Constellation).

Basic Principles of GNSS

The Core Concept: Trilateration

GNSS positioning is fundamentally based on the geometric principle of trilateration (sometimes loosely called “triangulation,” though it is technically distinct, since it relies on distance measurements, not angles). The basic principle works as follows:

1. Each satellite in the GNSS constellation continuously transmits a radio signal containing information about its precise orbital position and the exact time the signal was transmitted (derived from a highly accurate onboard atomic clock).
2. A ground-based GNSS receiver picks up signals from multiple satellites simultaneously, and calculates the time delay between when each signal was transmitted and when it was received.
3. Since radio signals travel at the known, constant speed of light, this time delay is converted into a precise distance (range) between the receiver and each satellite: \(\text{Distance} = \text{Speed of light} \times \text{Time delay}\).
4. If the receiver’s distance from one satellite is known, the receiver could be located anywhere on the surface of an imaginary sphere centred on that satellite. By determining its distance from a second satellite, the receiver’s possible location is narrowed down to the circle formed by the intersection of two spheres. Adding a third satellite’s distance measurement narrows the possible location down to just two points in space (the intersection of three spheres), one of which can usually be readily discarded as an unrealistic location (e.g., far out in space or deep underground).
5. In practice, a fourth satellite is required, not to further narrow the spatial position geometrically, but to solve for and correct the receiver’s own internal clock error/offset — since, unlike the satellites, standard consumer receivers do not carry extremely expensive, highly precise atomic clocks, and even a tiny timing error would translate into a very large positional error given the enormous speed of light. By simultaneously solving four equations (for X, Y, Z position and time offset) using signals from at least four satellites, the receiver can determine both its precise three-dimensional position and correct time.