Intro to GNSS Episode 2 – From Satellite to Reception

Hexagon | NovAtel® Geomatics Designer Todd Richert has been working with GNSS technologies with NovAtel since 2015, and he shares his knowledge in the second episode of our Introduction to GNSS on-demand webinar series. This second of seven presentations follows the satellite signal as it travels through the atmosphere towards an antenna and reveals the disturbances that can cause positioning errors: satellite timing and orbital errors, atmospheric delays, multipath, and interference at the receiver.

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. Though satellites follow predictable orbits, the ways we model the orbits isn’t always accurate. When satellites are at a different orbital position than the position broadcast in its signal, we are calculating our position based on false information.

Errors can also occur due to atmospheric activity distorting or delaying satellite signals, leading to errors in the calculation of these distances. The ionosphere includes ions charged by the Sun that 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 where changes in its humidity, temperature, and atmospheric pressure affect GNSS signals, radio and even cellular frequencies.

Multipath, another factor that causes errors, occurs when GNSS signals bounce off reflective objects like buildings as they travel to the antenna. Multipath is a major problem in providing highly accurate positions from GNSS because it is difficult to detect or model.

There can also be interference at the receiver affecting positioning accuracy. Signal interference can be unintentional or intentional. Unintentional interference happens when there is other electronic equipment emitting signals near the GNSS antenna, reducing 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.

Additional infrastructure like GNSS ground control, space-based augmentation systems, or precise point positioning products, or real-time kinematic services can help correct for timing and orbital errors, as well as atmospheric delays. 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.

Future episodes of our Intro to GNSS webinar series will explore the satellite signal itself, positioning calculations, sensor fusion, and much more.

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