What are the limitations of GNSS?

     GNSS. In general, are great. They are easy to use, they don't drift, and they can achieve high accuracy. However, they are not perfect. For starters, in order to utilize them to their full potential, they need a clear, undisturbed view of the sky. This is fine if you're working in a field, but if you're trying to survey city streets, or working under bridges and tunnels, at best you get reduced accuracy, and at worst you don't get a measurement at all.

    A common misconception is that GPS velocity measurements are made by measuring the time it takes to move between two consecutive position measurements. This is not how speed is measured - which is why we use GPS. Positional accuracy has no effect on velocity accuracy.

     To calculate velocity. GPS receivers actually use the Doppler shift. So they look at the frequency shift of each satellite's carrier and use that information to calculate a velocity vector. So it doesn't matter if your receiver is in SPS mode or RTK integer, the velocity measurement will be equally accurate.

     Also, while a GPS receiver is great for capturing position and velocity measurements, it's not quite as good if you're also interested in roll, pitch, and yaw.A dual-antenna system helps with some of these issues, but it still doesn't provide a complete picture. And for extremely dynamic maneuvers, it simply doesn't have the required resolution.

     When you take an inertial navigation system and create a GPS-assisted INS with GPS (which can also be written as GPS+INS), you solve the above problems, and you also solve the problems associated with an unassisted inertial navigation system (in particular, the drift problem). This combination is at the heart of OxTS' precision measurement solutions. It's also why our customers can measure with confidence - using OxTS' market-leading technology to measure position, orientation and dynamics in all environments.