structure that supports these calculations.
Our next video will share what happens to the satellite signal when it leaves the satellite, the factors that contribute to positioning errors, or signal degradation, and more. But, if you’re ready to get a head start in learning these GNSS concepts, you can download our in-depth book An Introduction to GNSS on our website.
Our on-demand webinar series continues with Todd Richert, Geomatics Designer at Hexagon | NovAtel, sharing the technological and atmospheric influences that degrade or weaken signals resulting in positioning errors. We include a transcript below.
Hello, and welcome to episode two of Hexagon NovAtel’s on-demand webinar series on An Introduction to GNSS. My name is Todd Richert; I’m a geomatics designer in Hexagon’s Autonomy and Positioning division. Thanks for joining me!
This is the second of seven presentations designed to introduce you to GNSS basics. Today, we’ll explain what happens to the satellite signal as it leaves the satellite and moves through the atmosphere and is received by user equipment. As it travels, the signal faces technological and environmental disruptions that can cause positioning errors. We’ll review these disturbances and the technologies that help mitigate the errors.
This series builds off our existing expertise in the industry where we have literally written the book on GNSS. If you’re ready to dig deeper into these concepts, then download our Introduction to GNSS book.
As we covered in episode one, a satellite generates a signal and sends it through the atmosphere, where GNSS antennas sense that signal and convert it into raw measurements used by the receiver. These measurements are used to compute a position, which may also be combined with other sensors before being applied in user equipment. The signal is susceptible to degradations and distortions as it makes its journey from the satellite to the receiver.
Positioning errors can occur due to degradations in the data provided by the satellite itself. Errors arise due 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 eve