Outages and Accuracy
There are two key things we need from our INS: angular accuracy and help during GNSS outages.
Looking at the orientation or angular accuracy we can reverse out the needs of our IMU by determing the accuracy required of our final product. For the moment let's say we need 1cm accuracy at 15m of range. Using the arc tan (1 / 1500) we get 0.04 degrees. You could then argue that we need 50% of that, or maybe 33% of that, but generally let's say that we need 0.04 degrees of accuracy in the tip/tilt (not heading). Ok, that is relatively easy. Now we can shop for an IMU that meets our needs.
Next up is the problem with outages. During an outage we have no external positioning information to guide our system. At this point the INS uses the last known position along with the orientation and velocity vectors to compute the positioning. The positions computed in this way are very accurate over short periods of time. As the time increases the solution drifts. The lower performing systems will drift much more rapidly than the more expensive systems. The drift rate is very important for a mobile system on the ground as it frequently encounters outages from overhead structures (urban canyon, trees, etc.).
Since even a high-end INS will drift during outages, if they drift beyond acceptable levels then control points must be used to improve the accuracy. If you need to maintain less than 5 cm accuracy and you can expect 10 cm of drift from a high-end system and 25 cm from a middle-of-the road system, then both require a correction, most likely using control points.
Looking at the orientation or angular accuracy we can reverse out the needs of our IMU by determing the accuracy required of our final product. For the moment let's say we need 1cm accuracy at 15m of range. Using the arc tan (1 / 1500) we get 0.04 degrees. You could then argue that we need 50% of that, or maybe 33% of that, but generally let's say that we need 0.04 degrees of accuracy in the tip/tilt (not heading). Ok, that is relatively easy. Now we can shop for an IMU that meets our needs.
Next up is the problem with outages. During an outage we have no external positioning information to guide our system. At this point the INS uses the last known position along with the orientation and velocity vectors to compute the positioning. The positions computed in this way are very accurate over short periods of time. As the time increases the solution drifts. The lower performing systems will drift much more rapidly than the more expensive systems. The drift rate is very important for a mobile system on the ground as it frequently encounters outages from overhead structures (urban canyon, trees, etc.).
Since even a high-end INS will drift during outages, if they drift beyond acceptable levels then control points must be used to improve the accuracy. If you need to maintain less than 5 cm accuracy and you can expect 10 cm of drift from a high-end system and 25 cm from a middle-of-the road system, then both require a correction, most likely using control points.
