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Accuracy of the Global Positioning System (GPS)

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The accuracy of the Global Positioning System (GPS) is not a single value but spans a wide range depending on the equipment used, the techniques applied, and various environmental factors.

Here is a breakdown of the typical range of GPS accuracy:

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🛰️ Standard Consumer GPS (e.g., Smartphones, Car Navigation)

For typical consumer devices, you can expect an accuracy of approximately 7 to 15 meters (23 to 50 feet) under open skies with a strong signal. 

  • Altitude (Vertical Accuracy) fits into the comparison. This is the "Z" axis and, for a normal GPS, it is significantly less accurate than your horizontal (Latitude/Longitude) position.

    • What it is: Your height above (or below) the reference ellipsoid (a mathematical model of Earth), which is then often corrected to mean sea level.

    • How it's measured: It's calculated along with your Lat/Lon position from the same satellite signals.

    • "Normal" Accuracy (No WAAS): As a general rule, vertical error is about 1.5x to 2x worse than the horizontal error.

      • If your horizontal accuracy is 7-15 meters (23-50 ft), your vertical accuracy will likely be 10 to 25 meters (30 to 80 ft).

 

## Augmented and Differential GPS (DGPS)

These systems use ground-based reference stations to correct for errors in the satellite signals, offering significantly better accuracy.

  • WAAS (Wide Area Augmentation System): Commonly used in aviation and marine navigation in North America, WAAS improves accuracy to less than 1 meter (about 3 feet). Many modern consumer GPS receivers are WAAS-enabled.

  • DGPS (Differential GPS): This technique uses a stationary receiver at a known location to calculate and broadcast correction data to other receivers in the area. It typically achieves an accuracy of a few centimeters to a meter.

 

## High-Precision Survey-Grade GPS (RTK & PPK)

For professional applications like land surveying, construction, and precision agriculture, highly advanced techniques are used to achieve centimeter-level accuracy.

  • RTK (Real-Time Kinematic): This method uses a stationary "base station" that sends live corrections to a moving "rover." RTK is capable of achieving 1 to 2 centimeters (less than an inch) of accuracy in real-time.

  • PPK (Post-Processed Kinematic): In this method, data from the base and rover are collected and processed later on a computer. By analyzing the data after the fact, PPK can often achieve even more reliable accuracy than RTK, also in the range of 1 to 3 centimeters.

 

Key Factors Influencing GPS Accuracy

Several factors can affect the precision of a GPS reading:

  • Satellite Geometry: The position of the satellites in the sky. Better accuracy is achieved when satellites are widely spaced.

  • Signal Blockage: Anything that blocks the line of sight to a satellite, such as buildings, trees, or mountains.

  • Atmospheric Conditions: The satellite signal is delayed as it passes through the ionosphere and troposphere.

  • Multipath Error: The signal bouncing off surrounding objects before reaching the receiver.

  • Receiver Quality: The sensitivity and processing power of the GPS receiver itself.

 

In summary, while your phone can get you within a few feet of your destination, specialized equipment can pinpoint a location to within the width of a coin.

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