What’s causing your RTK GNSS Receiver to Lose Lock?
The time taken to achieve convergence (often called lock or fix) is a key specification for a quality RTK GNSS Receiver. Achieving it quickly, as well as staying in convergence, requires satellite signals that are consistently strong, of high quality and unobstructed. Navigation and surveying equipment are measured by their up-time, and it is critical that as design engineers, we are doing everything within our power to ensure the RTK receiver is provided with a GNSS signal that is pristine and robust. But there are many factors that impact the satellite signal that your antenna will see and can be broadly categorised into two groups.
Group 1: Environmental factors
These are external conditions that can weaken or disrupt the signal as it travels from the satellite to the antenna. Many of these are well understood, and techniques can be employed to mitigate their impact on signal strength. But largely, we are often not able to mitigate the impact of these in the field. They include:
Ionospheric disturbances- the ionosphere is a layer of the atmosphere with charged particles that can cause signal delays and inaccuracies
Obstacles - physical barriers like buildings, trees, mountains, or even your own body can block the view of a satellite, significantly reducing signal strength, often by as much as 15dB
Satellite location - When a satellite is closer to the horizon, its signal strength can be much lower than if it was directly overhead, often reducing by as much as 6dB
Group 2: GNSS Antenna Implementation Factors
The implementation and quality of the antenna itself can also affect signal reception. The effort we expend here to optimise the gain will serve us well when our GNSS system is exposed to the environment factors outlined above (where we may have a limited ability to influence). Some key factors to consider for implementing your GNSS antenna;
Antenna gain - refers to the antenna's ability to convert a satellite signal arriving from a specified direction into electrical power at the output. A higher gain antenna will be more sensitive to signals from the desired satellites
Ground plane - For some antenna types, and where space allows, having a ground-plane can improve the gain of the antenna by as much as ~5dB
Antenna placement - ensure the antenna sits proud of any other obstruction of the vehicle that it is mounted on. Even a single obstruction could reduce the gain by 5-10dB
By understanding these factors, you can take steps to improve the quality of your GNSS signal. For example, using a high-quality antenna mounted on a ground-plane in a clear, open area will generally provide the best results. And given the link budget constraints imposed on a GNSS receiver, building as much headroom into the antenna as possible will assure that your RTK GNSS Receiver can converge quickly and stay converged longer.
