What are the sources of errors in GPS surveying?

Short Answer:

The main sources of errors in GPS surveying include satellite clock errors, atmospheric disturbances, multipath effects, receiver noise, and orbital errors. These factors can affect the accuracy of position measurements, sometimes causing deviations of several meters.

GPS errors occur because the signals from satellites pass through the Earth’s atmosphere and are affected by various natural and technical factors. Using methods like Differential GPS (DGPS) or Real-Time Kinematic (RTK), many of these errors can be reduced for more accurate surveying results.

Detailed Explanation:

Sources of errors in GPS surveying

In GPS surveying, accuracy is very important for mapping, layout, and construction work. However, GPS signals can be disturbed by various conditions between the satellite and the receiver. These errors can lead to inaccurate position readings. Understanding these sources helps surveyors apply corrections and improve the quality of their results.

1. Satellite Clock Errors

Each GPS satellite has an atomic clock that keeps precise time, but even small errors in this timing can cause a mistake in distance measurement. Since the GPS system calculates location based on signal travel time, a timing error of just a few nanoseconds can lead to position errors of several meters.

2. Orbital Errors (Ephemeris Errors)

Satellites orbit the Earth, and their exact positions must be known to calculate accurate locations on the ground. If the satellite’s reported position (ephemeris data) is slightly off, the receiver will compute an incorrect position.

3. Ionospheric and Tropospheric Delays

As the GPS signal travels through the Earth’s atmosphere, it slows down:

  • The ionosphere, which is full of charged particles, causes delays especially during high solar activity.
  • The troposphere, which contains water vapor and weather effects, also slows the signal. These delays can lead to incorrect distance calculations.

4. Multipath Errors

Multipath occurs when GPS signals bounce off buildings, trees, or the ground before reaching the receiver. These reflected signals take a longer path, which causes errors in distance measurement. This is common in urban areas or places with many structures.

5. Receiver Noise and Quality

Each GPS receiver has its own internal noise and limitations in processing signals. A low-quality receiver may produce less accurate results, especially in weak signal conditions.

6. Satellite Geometry (DOP – Dilution of Precision)

The arrangement of satellites in the sky affects how well a GPS receiver can determine position. If satellites are grouped close together (poor geometry), the calculated position becomes less accurate. This is called a high DOP value.

7. Selective Availability (Historical)

In the past, the U.S. government intentionally degraded GPS signals for security reasons, known as Selective Availability. This is no longer active but historically caused large errors.

How to reduce GPS errors:

  • Use Differential GPS (DGPS) or Real-Time Kinematic (RTK) systems for correction.
  • Avoid areas with tall buildings or heavy tree cover.
  • Use dual-frequency receivers to reduce ionospheric errors.
  • Survey at times when satellite geometry is favorable (low DOP).
  • Use high-quality GPS receivers and antennas.
Conclusion

Errors in GPS surveying can come from several sources such as atmospheric delays, clock inaccuracies, satellite positions, and environmental obstructions. Understanding these sources helps surveyors choose the right equipment and correction methods to reduce errors and get more accurate results in fieldwork.