Flights from Raven4 production units are showing good results from the Raven4's new accelerometer. We'll want to collect data from more flights before passing final judgement, but it's looking like the Raven's long-standing issue of accelerometer accuracy is in the past. The following data came from Kevin Small's flight from a couple of weeks ago, using 10Hz GPS recorded data, which is a feature of the Featherweight GPS Trackers we are working on that is almost complete.
The difference between the accelerometer-based apogee altitude and the GPS AGL altitude was 1%, closer even closer than the baro-based altitude, which is about 2% low, due to flying in warmer conditions than the standard atmosphere model assumes.
The results were also good news for the GPS performance. The Featherweight GPS tracker only logs GPS points to the file when they pass the GPS receiver validity checks, so all of the points you see in the graph are valid data. 96% of the possible GPS measurements were valid in this flight. The graph below shows that the new U-Blox 8th-generation receivers are great at velocity measurement as well:
There is excellent agreement between the acclerometer-derived velocity and the GPS velocity. The GPS provides the velocity in 3 dimensions relative to the ground, while the accelerometer only knows what the velocity is in the direction of the rocket, wherever it's pointing (assumed upward until apogee). This is potentially a big advantage for using GPS for measuring velocity, since it also can be used to calculate the flight angle, as shown above. You can see that during the boost the flight angle was around 3 degrees, and stayed under 10 degrees until 16 seconds into the flight. At apogee, the flight angle is 90 degrees (by definition) At some point I'm going to make a combined GPS tracker/altimeter that uses the GPS-based flight angle as a safety check for airstarts.
