to delays in the signal traveling through our atmosphere. There’s also the risk of interference at the receiver. Whether it’s unintentional or intentional, some influences dilute the signal’s clarity or completely jam the receiver with noise.
Let’s go through each of these factors.
Satellite Timing or Orbital Errors
Though they are incredibly precise, satellite clocks are still at risk of drifting forward or backward by nanoseconds. Even the slightest drift in time can result in positioning errors.
Satellites generally follow predictable orbits around the Earth, but the orbital prediction isn’t perfect. There is some difference between where our models tell us the satellite is and where the satellite actually is located. When satellites are at a different orbital position than the position broadcast in its signal, we are calculating our position based on false information.
The satellite timing and position drift are monitored and broadcast by GNSS ground control. However, for high accuracy applications, more precise information is needed. This can be made available through Space-Based Augmentation Systems or Precise Point Positioning products. Additionally, differential computation techniques like those used in Real-Time Kinematic systems mitigate the need for precise clock and orbit information.
We explained in episode one that the basic principle of positioning is calculating your distance from several satellites. But activity in the atmosphere can distort and delay satellite signals, leading to errors in the calculation of these distances.
The ionosphere, a layer of atmosphere 80 to 600 kilometers above the Earth, is filled with ions charged by the Sun. These ions vibrate and affect GNSS satellite signals as they travel through. The distortions due to the ionosphere can be stronger or weaker depending on the time of day and the location on the Earth.
The troposphere is the layer of atmosphere closest to the Earth. Changes in its humidity, temperature, and atmospheric pressure affect GNSS signals, radio and even cellular frequencies.
There are systems and algorithms designed to help compensate for distortions due to the ionosphere and troposphere and we’ll touch on these technologies later.
Sometimes, GNSS signals will bounce off reflective objects as they make their way to the antenna. This is known as multipath because the signal may take multiple paths on its way to the antenna. Multipath is a major problem in providing highly accurate positions from GNSS because it is difficult to detect or model.
Signal interference can be unintentional or intentional. Unintentional interference happens when there is other electronic equipment emitting signals near the GNSS antenna. These other signals interfere with the true GNSS signal. This can reduce the clarity of the GNSS signal and even prevent the receiver from being able to find the signal at all.
Intentional interference usually takes the form of spoofing – giving false signals – or jamming. In both cases, your position is unreliable. Spoofing means that a false signal is provided to the receiver. This happens most often in a military scenario where an agent intentionally tries to generate a wrong position. Jamming can occur in military scenarios as well as in civilian life. Even though they are illegal, inexpensive jammers exist and can wash out GNSS signals altogether.
Engineers have developed algorithms to mitigate the effects of interference and multipath. Additional equipment like anti-jamming antennas can protect your signal from interference, whether that interference is malicious or not.
Here’s a snapshot into some of the technologies developed to protect and resolve positioning errors.
Additional infrastructure like precise point positioning, satellite-based augmentation systems, or real-time kinematic systems provide additional data to correct errors on the original satellite signal. Anti-jamming antennas protect your signals from interference. Specialized algorithms have been developed to make GNSS receivers more robust against these error sources. GNSS positi