GNSS Principles and Usage

GNSS Explained: Principles and Architecture

GNSS navigation relies on three core segments:

• Space Segment: Satellites in medium Earth orbit transmit time-stamped signals.
• Control Segment: Ground stations monitor satellites, update orbital data, and ensure accuracy.
• User Segment: Aircraft receivers process satellite signals to calculate position, velocity, and time.

All GNSS systems work on the principle of measuring the time delay between satellite signal transmission and reception. By comparing signals from at least four satellites, the receiver uses trilateration to determine its three-dimensional position and precise time.

The Four Main GNSS Systems

• GPS (USA): The original and most widely used system.
• GLONASS (Russia): Similar in function but with its own satellites and reference data.
• Galileo (Europe): Offers high accuracy and is fully interoperable with other systems.
• BeiDou (China): The latest to achieve global coverage, with unique features and reference frames.

Each system uses different reference ellipsoids and orbital parameters, so multi-constellation receivers must account for these differences.

GNSS Accuracy and Augmentation

While GNSS provides global coverage, its basic signals lack the integrity and precision required for critical aviation phases. Augmentation systems address this:

• GBAS (Ground Based Augmentation System): Measures GNSS errors at a ground station near the airport and broadcasts corrections to aircraft. This enables high-precision GLS (GBAS Landing System) approaches.
• SBAS (Satellite Based Augmentation System): Provides wide-area corrections and integrity via geostationary satellites.

GNSS in Aviation Operations

GNSS supports area navigation (RNAV) and performance-based navigation (PBN), allowing flexible routing independent of ground-based navaids. It is essential for modern SIDs, STARs, en-route navigation, and both non-precision and precision approaches.

GNSS vs GPS

'GNSS' refers to the global concept, encompassing all satellite navigation systems, while 'GPS' is just one (American) implementation. Modern receivers often use multiple systems for redundancy and improved accuracy.