GPS Positioning – Glossary & In-Depth Explanations

What is GPS Positioning?

GPS Positioning is a satellite-based method for determining a precise location anywhere on Earth. It relies on the U.S. Global Positioning System (GPS), a constellation of satellites transmitting synchronized signals. By measuring how long it takes signals from at least four satellites to reach a receiver, GPS uses trilateration—a geometric technique—to compute the receiver’s 3D position and synchronize its clock to GPS system time.

GPS positioning underpins navigation for aviation, maritime, and land transport, as well as surveying, mapping, geodesy, precision agriculture, asset tracking, and scientific research. Modern GPS can achieve meter-level accuracy for consumers and centimeter- or even millimeter-level accuracy for professionals using advanced correction methods. The technology’s core strengths are global coverage, real-time capability, and integration into compact, affordable devices.

Space Segment: The GPS Satellites

The space segment comprises a constellation of at least 24 operational GPS satellites in six orbital planes at about 20,200 km altitude. These satellites orbit every 11 hours 58 minutes, ensuring at least four are visible from any location at any time. Each carries multiple atomic clocks, broadcasting signals on several frequencies (L1, L2, L5) with encoded information about satellite position, time, and health.

Key points:

• Each satellite transmits unique PRN codes for identification.
• Atomic clocks ensure nanosecond-level timing precision.
• Signals include ephemeris (precise orbit), almanac (coarse constellation data), and clock corrections.

Modern blocks (IIR, IIF, III) offer enhanced accuracy, integrity, and anti-jamming features. The constellation is maintained for redundancy, so more than 30 satellites may be operational at a time.

Control Segment: Ground Infrastructure

The control segment monitors and manages the satellites. It includes:

• Master Control Station (Schriever Space Force Base, Colorado)
• An alternate control station
• A global network of monitor stations (Hawaii, Kwajalein, Diego Garcia, Ascension, Cape Canaveral, Colorado Springs)
• Ground uplink antennas

Monitor stations track satellite signals, collecting orbit and clock data. The Master Control Station computes corrections, uploads updates, and ensures all satellites remain within tight tolerances for position and timing. The segment operates 24/7, supports anomaly resolution, and updates software and security features.

User Segment: GPS Receivers and Users

The user segment includes all GPS receivers, from phone chips to survey-grade instruments. Receivers:

• Acquire and track satellite signals.
• Decode navigation data.
• Measure pseudoranges (apparent distances).
• Compute position, velocity, and time.

Modern receivers support multiple frequencies and GNSS constellations (GLONASS, Galileo, BeiDou), improving accuracy, reliability, and availability. Professional gear employs algorithms for carrier phase tracking, error correction, and data storage for post-processing.

Applications range from navigation and mapping to asset tracking, aviation, autonomous vehicles, scientific research, and more.

Trilateration: The Core Principle

Trilateration is the geometric technique GPS uses to determine a receiver’s position. Each distance measurement to a satellite defines a sphere. The intersection of three spheres yields two points; a fourth measurement resolves the correct one and corrects clock bias.

Mathematically, the receiver solves four nonlinear equations (one per satellite):

ρi = sqrt[(x - xi)^2 + (y - yi)^2 + (z - zi)^2] + cΔt

Where:

• ρi = pseudorange to satellite i
• (xi, yi, zi) = satellite coordinates
• (x, y, z) = receiver coordinates
• c = speed of light
• Δt = receiver clock bias

Receivers use iterative methods (least squares, Newton-Raphson) to solve for position and time.

Satellite Signal Timing

Satellites transmit signals modulated with precise timing codes and navigation messages. The receiver generates matching PRN codes and slides them in time to find alignment. The offset gives the signal’s travel time.

Key data in the navigation message:

• Ephemeris: Precise orbit for the transmitting satellite.
• Almanac: Approximate orbits for all satellites.
• Clock correction: Satellite clock offset and drift.
• Health flags: Satellite and signal integrity.

Timing is critical—1 microsecond error equals ~300 meters position error. Relativistic effects (due to gravity and motion) are corrected so GPS time remains accurate to nanoseconds.

Why Four Satellites?

Four satellites are needed because there are four unknowns: latitude, longitude, altitude, and receiver clock bias. The clock in GPS receivers is not as precise as the satellites’, so the fourth measurement allows the receiver to solve for its own clock error and position.

Tracking more than four satellites improves accuracy and allows detection of measurement anomalies. Survey-grade receivers routinely use 10 or more satellites for redundancy and error checking.

From Pseudorange to Position

Pseudorange is the measured distance to a satellite, including errors from clock bias, atmospheric delays, and multipath. The receiver forms equations representing spheres centered at satellite positions, with radii equal to the pseudoranges.

By solving these equations (typically via least squares), the receiver estimates its 3D position and clock bias. This process repeats many times per second to track movement.

High-precision applications use carrier phase tracking for millimeter-level accuracy, storing raw data for post-processing with external corrections.

Clock Errors

Clock errors arise from drift in satellite atomic clocks and especially receiver quartz clocks. The control segment continuously monitors and corrects satellite clocks; correction parameters are broadcast in the navigation message. The receiver’s clock bias is solved for as part of the position solution.

Advanced receivers and correction methods (DGPS, RTK) mitigate clock errors, especially important in aviation and surveying.

Atmospheric Effects

GPS signals are delayed by the atmosphere:

• Ionosphere (charged particles above 60 km): Frequency-dependent delay, corrected using dual-frequency receivers or models like Klobuchar.
• Troposphere (lower ~10 km): Delay depends on pressure, temperature, humidity, modeled using Saastamoinen or Hopfield models.

Uncorrected, these delays can cause errors of several meters. Correction networks and advanced receivers reduce atmospheric error impact.

Multipath Effects

Multipath occurs when signals reflect off surfaces before reaching the receiver, introducing errors. It is worst in urban, forested, or reflective environments.

Mitigation techniques:

• Choke ring or ground plane antennas
• Careful site selection
• Signal processing algorithms to reject multipath-contaminated measurements
• Carrier phase observations for surveying

In aviation, multipath must be strictly bounded for safety.

Satellite Geometry and Dilution of Precision (DOP)

Satellite geometry affects position accuracy, measured by Dilution of Precision (DOP):

• GDOP: Geometric (position + time)
• PDOP: Position
• HDOP: Horizontal
• VDOP: Vertical
• TDOP: Time

Lower DOP values mean better geometry and higher accuracy. Widely spaced satellites yield optimal DOP. High DOP (satellites clustered or low on the horizon) amplifies errors.

Professional receivers display DOP values, and standards define maximum DOP for safety-critical applications.

Other GPS Error Sources

• Ephemeris errors: Slight inaccuracies in broadcast orbit data, typically <1 m.
• Selective Availability (SA): Intentional error added pre-2000, now disabled.
• Receiver noise: Random errors from electronics, minimized in high-quality receivers.
• Interference/jamming: Accidental or deliberate RF sources; mitigated with filtering and resilient receiver design.
• Spoofing: Fake GPS signals to mislead receivers; countered by authentication and anomaly detection.

Differential GPS (DGPS)

Differential GPS (DGPS) uses a stationary base station at a known location to compute real-time corrections for errors common to nearby receivers. The base transmits these corrections, allowing roving receivers to improve accuracy from several meters to sub-meter or decimeter level.

DGPS corrects for satellite, clock, and atmospheric errors, and is widely used in marine navigation, agriculture, and surveying.

Real-Time Kinematic (RTK) and CORS

RTK GPS uses carrier phase measurements and real-time corrections from a base station to achieve centimeter or even millimeter-level accuracy. It requires a data link (radio, cellular, or internet) between base and rover.

CORS (Continuously Operating Reference Stations) networks provide real-time and post-processed correction data, supporting high-precision GPS nationwide.

Modernization and Multi-GNSS

GPS modernization adds new signals (L2C, L5) for improved accuracy, availability, and integrity. Receivers can also use signals from GLONASS, Galileo, and BeiDou (collectively GNSS), increasing the number of satellites, improving geometry, and enhancing reliability.

Applications of GPS Positioning

• Navigation: Aviation, maritime, automotive, personal devices
• Surveying and Mapping: Land, construction, cadastral, geodetic
• Precision Agriculture: Automated guidance, yield mapping
• Aviation: En-route, approach, landing (meeting ICAO standards)
• Timing: Synchronizing networks, power grids, financial systems
• Scientific Research: Earthquake monitoring, plate tectonics, meteorology
• Asset Tracking: Fleet management, logistics, wildlife tracking
• Autonomous Systems: Drones, robotics, driverless cars

Summary

GPS Positioning is a foundational technology for the modern world. By leveraging a constellation of satellites, advanced timing, trilateration, and robust correction methods, GPS provides accurate, reliable, and global positioning. Continuous improvements in signals, algorithms, and integration with other GNSS systems ensure its ongoing evolution and broadening applications.

References

• ICAO Annex 10, Volume I – Aeronautical Telecommunications: Radio Navigation Aids
• ICD-GPS-200 – GPS Interface Control Document
• U.S. Department of Defense – GPS.gov
• Kaplan, E.D. & Hegarty, C.J. (2017). Understanding GPS/GNSS: Principles and Applications
• Leick, A., Rapoport, L., & Tatarnikov, D. (2015). GPS Satellite Surveying

For authoritative and detailed information, always consult official GPS and GNSS documentation, standards, and scientific literature.