Tech Chat | Understanding GNSS Positioning: From APC to Ground Target Point

In daily RTK measurement, we usually need to input the height of the ranging pole in order to convert the coordinates into a ground target point. Have you ever wondered the original position measured by the GNSS receiver are? Is it above, below or inside the GNSS receiver?

In this blog we will cover the position of the original coordinates measured by the GNSS receiver and how it is converted into our target point.

Key Concepts:

To grasp the fundamentals of GNSS positioning, it's essential to familiarize ourselves with several key points in positioning:

  • Marker: A marker serves as a ground reference sign, often attached to a tripod or directly affixed to an antenna. It can also be regarded as the ground target point pointed by the centering rod.
  • ARP (Antenna Reference Point): Antenna reference point refers to a well-defined point on the antenna. Generally, the intersection point between the bottom of the receiver antenna and the central axis of the antenna is selected as the reference point.
  • APC (Antenna Phase Center): The APC is the phase center of the antenna, representing the electrical center responsible for receiving signals. When the electromagnetic wave radiated by the antenna leaves a certain distance from the antenna, its equal phase surface will be approximately a sphere, and the sphere center of the sphere is the equivalent phase center of the antenna. It's worth noting that the APC might not coincide with the geometric center of the antenna. It's a theoretical point where signals are considered to emanate from.
  • PCO (Phase Center Offset): Antenna phase center deviation, the deviation between the average of APC and the ARP.
  • PCV (Phase Center Variation): PCV refers to changes in the phase center of an antenna. It accounts for the difference between the instantaneous phase center of an antenna and the mean phase center.

Understanding GNSS Measurement:

GNSS technology measures the distance from the phase center of a satellite transmitting antenna to the phase center of the receiver antenna. The coordinates obtained by the receiver represent the phase center coordinates of the receiver antenna. However, these coordinates aren't necessarily the surveyor's desired target point. To achieve this, it's necessary to configure the antenna height and rod height parameters during measurement.

Furthermore, the factory calibration values of different antennas may vary, necessitating the selection of the appropriate antenna type and applying the correct correction model for data processing.

How to set GNSS antenna parameters for SingularXYZ products?

- For Smart Antennas such as X1-series, Y1, etc.

The antenna parameters of the corresponding device can also be viewed when connected the RTK receivers to our field surveying software SingularPad or SingularSurv.

- For SV100-series GNSS receivers

If you are connecting an external antenna to SV100, you can select the corresponding antenna type in the Antenna Settings on the web interface​

GNSS measurement positioning involves several critical concepts, from ARP and APC to PCO and PCV. Understanding the interplay between these factors is vital for obtaining precise real-world coordinates. The choice of antenna type and proper parameter calibration is essential to ensure accurate GNSS measurements. This knowledge is key to harnessing the full potential of GNSS technology in various applications.

Learn more about SingularXYZ RTK products,

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