How to improve GNSS accuracy in challenging environments?

The Global Navigation Satellite System (GNSS) has revolutionized the way we determine our position in space. However, its accuracy may be compromised in challenging environments such as dense urban areas, dense forests, or mountainous terrain. This article explores how to improve the accuracy of GNSS in these contexts, analyzing the challenges and techniques that can be implemented to overcome them.

GNSS Accuracy Challenges

The challenges faced by GNSS in challenging environments are varied. The main factors affecting accuracy include:

1. Signal obstruction: In urban areas, tall buildings can block satellite signals, creating what is known as shadowing. In dense forests, trees can interfere with signal reception, making position determination difficult.

2. Signal reflection: GNSS signals can bounce off surfaces such as buildings, streets and other objects, known as multipath. This can lead to errors in measuring the distance between the satellite and the receiver.

3. Atmospheric conditions: The atmosphere affects the propagation of GNSS signals. Phenomena such as the ionosphere and troposphere can cause signal delays, impacting the accuracy of the calculated position.

4. Clock errors: Synchronization between the satellite clock and the receiver is crucial. If there are discrepancies in the time, this can result in significant errors in the calculated position.

Weather conditions affecting GNSS accuracy

The accuracy of GNSS can be significantly influenced by weather conditions. Below are the best and worst conditions for GNSS accuracy:

Better weather conditions

• Clear Sky: The ideal conditions for GNSS accuracy are a clear sky with no clouds. This allows the receiver to have a clear line of sight to multiple satellites, minimizing obstructions.

• Stable Temperature: Moderate, stable temperatures help avoid abrupt changes in the atmosphere that could cause signal errors. Extreme temperatures can cause variations in air density, affecting signal propagation.

• Low humidity: High humidity can cause changes in the refraction of GNSS signals. A moderate or low humidity level is preferable to maintain accuracy.

Worst weather conditions

• Storms and heavy rain: Rain can cause attenuation of GNSS signals, while storms can generate turbulence that affects the transmission of radio waves.

• Fog and thick clouds: Although clouds do not block signals completely, they can cause some attenuation and dispersion of signals, affecting the quality of the received signal.

• Strong winds: In environments where mobile antennas are used, strong winds can destabilize the equipment, affecting signal reception and therefore accuracy.

• Low temperatures: Extremely low temperatures can affect the receiver electronics, and in freezing conditions, water can accumulate on the antennas, which could also cause interference.

Techniques to improve GNSS accuracy

To address these challenges and improve the accuracy of GNSS in challenging environments, various techniques and technologies can be implemented:

1. Use of advanced GNSS receivers

The most modern GNSS receivers are designed to improve signal reception in difficult conditions. These receivers can handle multiple satellite constellations, not only GPS, but also GLONASS, Galileo and BeiDou. By accessing more satellites, the probability of receiving clear signals increases, thus improving accuracy.

Additionally, many advanced receivers use phase code technology to increase accuracy. This technique allows the phase of radio waves to be measured, which can result in an accuracy of centimeters rather than meters.

2. Differential corrections

Differential correction is a technique that uses ground reference stations to improve the accuracy of GNSS. These fixed stations know your exact position and can calculate errors in the GNSS signals. They then send corrections to the mobile receiver, thus adjusting the calculated position.

Differential corrections can be received in a variety of ways, including:

• RTK (Real-Time Kinematic): This technique allows the receiver to receive corrections in real time, which significantly improves accuracy, especially in applications that require high accuracy.

• DGPS (Differential GNSS): A simpler form of differential correction that also improves precision, although it does not achieve the same accuracy as RTK.

3. Advanced antenna techniques

Antenna choice also plays a crucial role in GNSS accuracy. Modern antennas are designed to minimize the multipath effect by using technologies such as:

• Selective Design Antennas: These antennas are designed to filter out unwanted signals and reduce interference from reflected signals.

• Wideband Antennas: Wideband antennas can pick up a wider range of frequencies, making them more effective in harsh environments.

4. Signal processing improvements

Signal processing is essential to calculate position in a GNSS receiver. Implementing advanced algorithms can help mitigate the impact of signal errors. Some techniques include:

• Kalman filtering: This algorithm is used to estimate the state of a dynamic system from noisy measurements. In the context of GNSS, it can help smooth measurements and improve overall accuracy.

• Atmospheric correction models: Incorporating models that compensate for the effects of the atmosphere can help improve accuracy. These models use meteorological and physical data to predict how atmospheric conditions will affect the signal.

5. Use of complementary technologies

There are complementary technologies that can improve the accuracy of GNSS, especially in challenging environments:

• High Frequency GNSS: Using higher frequencies can help you better penetrate structures and obstructions. For example, the use of the L5 frequency, which is more resistant to interference and provides better accuracy.

• Inertial Navigation Systems (INS): Combining GNSS with an inertial navigation system can help provide more accurate position data. The INS uses accelerometers and gyroscopes to calculate movement and can provide information even when the GNSS signal is temporarily lost.

Basic recommendations to improve GNSS accuracy

To maximize GNSS accuracy in challenging environments, consider the following recommendations:

• Avoid placing devices under trees: Whenever possible, place GNSS receivers in open areas and avoid placing devices under trees or structures that could block or reflect satellite signals.

• Select a good location: Look for elevated or clear locations to place your receiver. Open areas minimize signal obstruction and reduce shadowing.

• Use quality antennas: Investing in good quality antennas that are designed for harsh environments can significantly improve signal reception.

• Take advantage of differential corrections: Whenever possible, use differential correction techniques such as RTK or DGPS to obtain more accurate data.

• Perform frequent calibrations: Be sure to regularly calibrate your GNSS equipment and review its performance. This can help detect problems and optimize accuracy.

• Monitor atmospheric conditions: Knowing the weather conditions can be useful to anticipate possible interruptions in the GNSS signal.