In the realm of precision navigation and positioning, Differential GNSS (DGNSS) often emerges as a critical yet misunderstood technology. Despite its pivotal role in enhancing GPS accuracy, several myths persist, clouding its true value. Today, let’s discuss some common misconception in the survey world related to DGNSS (or DGPS).

You may have heard people emphasize the importance of having 400, 800, or even 1200 channels in a DGNSS receiver. Have you ever thought deeper to understand what this actually means? Let’s clarify this with some simple mathematical calculations.

There are a total of 120 operational satellites: GPS (31), Galileo (24), GLONASS (24), BeiDou (30), NavIC (7), and QZSS (4). These satellites are distributed across the orbital plane and are not all visible from a particular point on Earth.

An elevation mask is a threshold angle that defines the minimum elevation above the horizon that a satellite must have to be considered in the positioning calculations of a DGNSS receiver. The purpose of this mask is to filter out low-angle satellite signals that are more likely to be degraded by atmospheric interference, multipath effects, and obstructions.

It is recommended to set the Elevation Mask to 15 degrees to use only high-quality satellites for positioning. However, even with an Elevation Mask set to 5 degrees, you wouldn’t be able to see even half of the satellites simultaneously.

With an Elevation Mask of 5 degrees, you would only be able to track a maximum of 14 GPS, 6 Galileo, 6 GLONASS, 11 BeiDou, and 4 QZSS satellites. Out of the 7 NavIC satellites, only 5 are operational, and many companies do not support NavIC.

Global Navigation Satellite Systems (GNSS) such as GPS, GLONASS, and Galileo are critical in providing accurate location information for a variety of applications ranging from navigation and surveying to agricultural and emergency services. However, the precision of GNSS positioning is susceptible to a variety of faults that can jeopardize the dependability of location data. In this blog article, we will look at various forms of GNSS errors and how they affect accuracy.

These are the factors that may make it difficult for a GNSS receiver to calculate an exact position and degrade precision.

GNSS satellites are equipped with atomic clocks to provide precise timing signals.

However, variations in the satellite clocks can introduce errors in the ranging measurements, affecting the accuracy of position calculations. Different global systems use different types of atomic clocks, and most satellites will have multiple types of clocks on board. For e.g. Rubidium and Caesium are standard on GPS , Galileo has Hydrogen maser and Rubidium.

These clocks are incredibly accurate, but they aren’t perfect, and experience ‘drift’. This means they lose or gain one nanosecond for every three hours of time.

For example, 10 nanoseconds of clock error results in 3 meters of position error.