Image: Point One Navigation

GNSS has transformed the way both individuals and machines navigate across the globe, leading to a growing number of organizations utilizing positioning data in the development of products and applications. GNSS technology plays a crucial role in enabling autonomous vehicles, robots, logistics fleets, and emergency response systems to accurately determine the precise locations of places, people, and things on Earth’s surface. As a result, routes are not only more accurate and efficient but also safer.
As a satellite-dependent navigation system, various atmospheric and technological factors can impact the accuracy and precision of GNSS signals. These signals often need to be corrected by receivers before they can be used for positioning, and various correction methods exist today to achieve this. Each method has its own advantages and disadvantages, catering to diverse accuracy requirements and application scenarios.

Five causes of GNSS signal inaccuracies

When choosing the best GNSS correction method for a specific project, it is important to comprehend signal errors and their underlying causes. GNSS errors result from a combination of elements, such as ephemeris inaccuracies, disparities in satellite clocks, conditions in the ionosphere and troposphere, and inconsistencies between various satellite systems. Each signal correction method addresses these elements differently, resulting in pros and cons that must be weighed before selecting and implementing a solution.

1. Inaccurate ephemeris data

To calculate their position on Earth, GNSS receivers need to know the exact position in space of the satellites they use. Satellite positioning and orbital parameters are represented in ephemeris data, but sometimes this data is incorrect. Ephemeris inaccuracies cause the receiver to not know the satellites’ exact positions, thereby degrading the accuracy of their position calculation.

2. Differences in satellite clocks

Even the highly accurate atomic clocks on GNSS satellites can introduce errors in the timestamps receivers use to calculate positions. The exceptionally high speed at which GNSS satellites travel in space (approximately 7,000 mph) adds another layer of complexity to these calculations because even a nanosecond of difference can lead to substantial positioning errors.

3. Conditions in the ionosphere

The ionosphere, the outermost layer of Earth’s atmosphere, consists of charged particles that can affect the speed of light and radio signals as they pass through it. Fluctuations in solar radiation and other ionospheric conditions can result in delays or distortions in GNSS signals, introducing measurement errors that require correction for precise positioning. Although the influence of the ionosphere can result in significant signal interpretation errors, correction methods can effectively model and account for them.

4. Conditions in the troposphere

Weather, which occurs in the troposphere, the innermost layer of Earth’s atmosphere, also impacts GNSS signals as they travel from satellites in space to receivers. Temperature, humidity, and pressure can affect the speed of light and radio signals much like the charged particles of the ionosphere, leading to even more delays and distortion in GNSS calculations. However, because weather is highly localized, tropospheric errors must be modeled and corrected from a relatively close distance to achieve the level of accuracy needed for precise positioning.

5. Group delay (code bias)

Different countries and organizations around the world operate GNSS satellites. While they are generally aligned, minor discrepancies in time references and frequencies exist that can impact the accuracy of GNSS positioning. This is known as group delay or code bias and must also be corrected to ensure that signals are interpreted correctly.

Types of GNSS corrections

Understanding the origin of errors is critical when selecting the optimal GNSS signal correction method for a particular product or application. Each method has advantages and disadvantages ranging in importance depending on the application of GNSS positioning.
Real-time kinematic positioning (RTK) correction is widely regarded as the best method for achieving precise GNSS signal correction. It requires setting up a base station with a GNSS receiver at a very well surveyed location near the target area (usually within 30-50 kilometers), which transmits corrections to the end user’s GNSS receiver (called the rover). The proximity between the base station and the rover mitigates the impacts of signal errors. Any signal disparities that do exist can be analyzed to measure positional differences between the base and the rover, enabling the latter to calculate its position very precisely.

Real-time kinematic positioning (RTK) yields efficient and highly precise GNSS corrections but requires an extensive network
of base stations to support receivers across a large geographic area. (Image:…