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Global Navigation Satellite Systems (GNSS) in Maritime Navigation

Introduction

Global Navigation Satellite Systems (GNSS) have revolutionised modern maritime navigation by providing accurate, real-time positioning, velocity, and timing information anywhere on Earth. Modern vessels depend on GNSS for route monitoring, voyage planning, collision avoidance, electronic chart systems, and safe navigation in restricted waters.

The major global satellite navigation systems currently in operation are:

• GPS (United States)
• GLONASS (Russia)
• Galileo (European Union)
• BeiDou (China)

Together, these constellations provide worldwide coverage and improved navigational reliability.

What is GNSS?

GNSS is a satellite-based navigation system designed to determine:

• Position
• Velocity
• Precise Time (PVT)

The system works by receiving signals from multiple satellites and calculating the vessel’s location using precise distance measurements and mathematical algorithms.

IMO Requirements for Maritime GNSS

According to IMO performance standards, a shipborne GNSS receiver should:

• Detect GPS and GLONASS satellites.
• Provide position information referenced to WGS-84 datum.
• Automatically select suitable satellites.
• Provide outputs to other bridge equipment.
• Have a static accuracy of approximately 35 m in normal mode.
• Have a static accuracy of approximately 10 m in differential mode.
• Acquire position within 30 minutes without valid almanac data.
• Acquire position within 5 minutes when valid almanac data is available.
• Output a new position at least once every second.

Components of GNSS Signals

Each GNSS satellite transmits signals consisting of three important components:

1. Pseudo Random Noise (PRN) Code

Each satellite has a unique PRN code that identifies it. The receiver generates the same code internally and compares it with the incoming signal. When both match, the receiver successfully acquires the satellite.

2. Carrier Signal

The carrier signal acts as a transport medium for navigation data from the satellite to the receiver. By itself, it carries no navigational information.

3. Navigation Message

The navigation message contains:

• Ephemeris Data
• Almanac Data
• Satellite Health Status
• Time Information

GPS C/A Code and P-Code

C/A Code (Coarse Acquisition Code)

The C/A Code is the standard civilian ranging code transmitted on the GPS L1 frequency (1575.42 MHz).

Functions:

• Identifies satellites.
• Enables receiver acquisition.
• Used for civilian navigation.
• Forms the basis of standard GPS positioning.

How Does the Receiver Acquire the C/A Code?

The receiver generates an identical PRN code internally and continuously compares it with incoming signals.

Process:

Satellite Transmits C/A Code

↓

Receiver Generates Same PRN Code

↓

Code Correlation (Matching)

↓

Satellite Acquired

↓

Signal Transit Time Measured

↓

Pseudorange Calculated

When both codes match perfectly, the receiver knows which satellite is being received and can measure the travel time of the signal.

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P-Code (Precise Code)

The P-Code is a high-precision ranging code primarily designed for authorised users.

Characteristics:

• Higher accuracy than C/A code.
• Transmitted on L1 and L2 frequencies.
• Very long code sequence.
• Used for Precise Positioning Service (PPS).

When encrypted, the P-Code becomes the Y-Code, preventing unauthorised access.

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Comparison Between C/A Code and P-Code

Feature	C/A Code	P-Code
Full Name	Coarse/Acquisition Code	Precise Code
Users	Civilian	Authorised Users
Frequency	L1	L1 and L2
Accuracy	Standard	Higher
Encryption	No	Encrypted as Y-Code
Main Purpose	Satellite Acquisition	Precision Positioning

Ephemeris and Almanac Data

Ephemeris Data

Ephemeris data provides highly accurate orbital information for individual satellites. Once a receiver acquires a satellite, it downloads the ephemeris information required for precise position calculations.

Almanac Data

Almanac data contains lower-accuracy orbital information for the entire satellite constellation. It helps receivers determine which satellites should be visible above the horizon and significantly reduces acquisition time after startup.

The Three Segments of GNSS

1. Ground Segment

The Ground Segment consists of:

• Monitoring Stations
• Master Control Stations

Functions include:

• Monitoring satellite health
• Tracking satellite orbits
• Calculating orbital corrections
• Correcting satellite clock errors
• Uploading updated navigation data

2. Space Segment

The Space Segment consists of satellites orbiting the Earth.

Functions include:

• Receiving updates from ground stations
• Broadcasting navigation signals
• Providing global positioning coverage

3. User Segment

The User Segment consists of GNSS receivers installed onboard ships and other navigation platforms.

Functions include:

• Receiving satellite signals
• Calculating position
• Determining speed
• Providing precise timing

How GNSS Determines Position

GNSS determines position using a technique known as trilateration.

The receiver measures the travel time of a signal between the satellite and the receiver. Since radio waves travel at the speed of light, the distance can be calculated.

Pseudorange Formula

PR = c × (tr − ts)

Where:

• PR = Pseudorange
• c = Speed of light (3 × 10⁸ m/s)
• tr = Time of reception
• ts = Time of transmission

The calculated distance is called a pseudorange because it contains small errors caused by receiver clock inaccuracies.

Trilateration Equation

The receiver uses pseudoranges from several satellites to calculate its position.

PR = √[(x₁ − x)² + (y₁ − y)² + (z₁ − z)²] + cΔt

Where:

• PR = Measured pseudorange
• (x₁, y₁, z₁) = Satellite coordinates
• (x, y, z) = Receiver coordinates
• c = Speed of light
• Δt = Receiver clock correction

Unknown values are:

x, y, z and Δt

Therefore:

• 3 satellites are needed to determine position.
• A 4th satellite is required to correct receiver clock error.

Modern GNSS receivers typically track many satellites simultaneously for higher accuracy.

Differential GPS (DGPS)

DGPS provides greater positional accuracy than standard GNSS.

A reference station with a precisely known location calculates the difference between:

• Actual surveyed position
• GPS-derived position

The error is transmitted to nearby users who apply corrections to improve accuracy.

DGPS Correction Formula

Error = GPS Position − True Position

Corrected Position = GPS Position − Error

Using these corrections, position accuracy can improve to approximately 10 metres.

Satellite Based Augmentation System (SBAS)

SBAS operates on the same principle as DGPS but broadcasts correction data through geostationary satellites.

Benefits include:

• Wider coverage
• Better integrity
• Improved accuracy
• Enhanced reliability

Examples include WAAS, EGNOS, and GAGAN.(General industry examples)

Speed Determination Using Doppler Shift

GNSS receivers determine vessel speed using the Doppler Effect.

Doppler Principle

Relative Motion

        ↓

Frequency Shift

        ↓

Velocity Calculation

Satellites move at approximately 3900 m/s in orbit. The receiver measures slight frequency changes in the received signal caused by relative motion between the satellite and the vessel.

These measurements are used to determine:

• Speed Over Ground (SOG)
• Course Over Ground (COG)

Common Sources of GNSS Error

Ionospheric Delay

Changes in the ionosphere can slow signal propagation and affect position accuracy.

Tropospheric Delay

Atmospheric moisture and pressure can delay transmitted signals.

Receiver Clock Error

Shipboard receiver clocks are not as accurate as satellite atomic clocks.

Satellite Clock Error

Although satellites use atomic clocks, very small timing errors still exist.

Multipath Error

Signals reflected by water surfaces, ships, cranes, or structures can create inaccurate range measurements.

Poor Satellite Geometry (GDOP)

Unfavourable satellite positioning can reduce navigation accuracy.

Satellite Orbit Error

Deviation of a satellite from its predicted orbital path can introduce position errors.

Receiver Autonomous Integrity Monitoring (RAIM)

RAIM continuously checks the reliability of GNSS position solutions.

Modern receivers compare measurements from multiple satellites to identify inconsistencies.

RAIM SAFE

Position accuracy is within acceptable limits.

RAIM UNSAFE

Calculated accuracy exceeds preset limits.

RAIM CAUTION

Insufficient satellites are available to verify the integrity of the position solution.

RAIM is an important safety feature because it warns navigators when GNSS information may not be reliable.

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Conclusion

Global Navigation Satellite Systems form the backbone of modern maritime navigation. By combining multiple satellite constellations, advanced position-fixing algorithms, differential corrections, and integrity monitoring systems, GNSS provides mariners with accurate and reliable navigation information. Understanding the principles, formulas, and limitations of GNSS is essential for every deck officer, navigator, and maritime professional operating in today’s technology-driven shipping industry.