The Importance of Groundplanes in Improving GNSS Reception and Accuracy
How Signal Reflections Affect GNSS Reception and the Role of Groundplanes in Improving Accuracy
One of the biggest challenges in GNSS (Global Navigation Satellite System) signal reception is signal reflection, also known as "multipath." This occurs when satellite signals bounce off obstacles such as buildings, trees, people, and the ground before reaching the antenna. The result is that the antenna receives multiple signals: one direct from the satellite and others reflected from different surfaces, such as buildings or the ground. These reflections distort the signal and reduce the accuracy of the reception.
Understanding Signal Reflections and Their Effect on GNSS Reception
The most significant signal reflection typically comes from the ground below the antenna. To address this issue, engineers developed a solution known as a "groundplane." A groundplane helps prevent signals from the lower hemisphere (the ground and surrounding reflections) from reaching the antenna. In its simplest form, a groundplane can be a flat surface, such as a CD disc, with the antenna placed in the center. More advanced designs use choke ring antenna, a set of concentric rings placed around the antenna.
Groundplane Solutions: From Simple CD Discs to Advanced Choke Ring Antenna
A well-designed choke ring antenna, weighing about 7.5 kilograms (16.5 pounds), can cost several thousand dollars. However, for less demanding applications, inexpensive groundplanes, like old CD discs, can significantly improve signal reception.
The Ideal Size of a Groundplane for GNSS Patch Antennas
For instance, when a satellite compass is used to measure the vectors between two antennas, placing the antennas just a few centimeters above a car roof (which acts as a signal reflector) will result in inaccurate measurements. In the photo titled "That Doesnβt Work" this setup causes signal fluctuations of up to 1-2 centimeters, instead of the required 5mm RMS (Root Mean Square) accuracy. However, when the antennas are placed on a proper groundplane, such as a CD disc, the accuracy improves drastically. This is illustrated in the photo titled "That Works" There is some debate regarding the ideal size of a groundplane for patch antennas. Some experts suggest that the groundplane should be slightly larger than half the wavelength, which is about 10 centimeters for L1 and 13 centimeters for L2 frequencies. Further details on this can be found in the "GNSS-Antennas_AppNote_UBX" document. It's important to note that patch antennas generally perform poorly without a groundplane. In contrast, other types of antennas are less sensitive to the absence of a groundplane.
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