Related webinar: Multi-domain GNSS Challenges. 26 June, 2:00 pm EDT.
If you are using a global navigation satellite system (GNSS) receiver, you are probably aware of the issues that can cause trouble determining your position. Things like foliage, city buildings, and mountains all cause issues in receiving signals from GNSS satellites. Electronic interference, whether intentional or not, can also keep your receiver from getting navigation signals.
Other less-known GNSS issues may also cause errors in your calculated position and velocity. Understanding those errors and how to mitigate them will help you keep your position updates coming, even in adverse conditions.
Though individual systems differ in their technical details, all have similar characteristics; and two effects, signal availability and DOP, are the two main factors that affect positioning accuracy.
- Signal availability. GNSS systems send encoded signals to receivers. If something gets in the way of that signal or modifies it, the receiver may not provide the correct position.
- Dilution of Precision (DOP). DOP is essentially a multiplicative factor in your position error – the higher the DOP, the more error in your position determination.
Understanding DOP is straightforward; DOP is determined solely on the positions of the GNSS satellites. The best way to drive your DOP numbers down is to use many different GNSS constellations by employing a multi-constellation GNSS chip in your receiver.
Now, let’s consider signal availability. GPS produces signals whose power is below the noise floor. This means the receiver has to integrate noise (at the right frequency, biased by doppler) until a correlation peak shows up that defines the navigation signal – for each satellite it thinks is in view. The receiver knows what satellites are in view by using the almanac it has stored in memory. If that almanac is old, it will take the receiver longer to get a fix on the satellites. This is known as Time to First Fix.
Signal travel times through the atmosphere and ionosphere can be lengthened but mitigated by mathematical models and two-frequency corrections. What happens to the position estimate when a connection drops on one of the frequencies? Can the receiver reacquire?
Signals can also be occluded by physical or electronic means. Understanding the effects of terrain, buildings, jammers, and other signal disruption sources is imperative in knowing how the receiver is working.
If signals are arriving at the receiver, it’s important to understand the receiver antenna gain profile, and how it moves over time. For example, if the receiver’s antenna is on a vehicle that rotates – what happens to GNSS signal lock?
Finally, while GNSS receivers are becoming more sophisticated as technology advances, they still inject noise into the communications stream, causing additional errors in position estimates. Understanding a receiver’s noise model is critical to understanding the navigation system as a whole.
AGI has investigated all of these issues and we’ve developed strategies and tools for our users that allow them to understand how these issues impact the mission.
Join me on June 26th when I’ll go more in-depth on each of these issues during my webinar (2:00 PM, EDT, click here to register).