How-to Maximum Age and Skewing

When the last data from an observation is older than its maximum age, the data is not used anymore.

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Maximum Age

In QINSy, a computation cycle is triggered by an update from the primary positioning (GPS) system. This computation needs, besides the position data, data from other observations. For every observation, a maximum age is defined. When the last data from an observation is older than its maximum age, the data is not used anymore. This is especially significant for attitude data.

For heading, pitch, roll and heave, more than one observation can be used. These observations can be sorted by priority. When the maximum age of one observation has expired, QINSy will continue to calculate with the next in the list. Unless, of course, that observation has also expired. Note that pitch, roll and heave observations coming from the same system have the same maximum age.

Skewing

Skewing data means shifting data in time. For example, QINSy receives attitude data, but the trigger time (where QINSy performs all calculations) is two seconds later. Skewing the attitude data will 'predict' the value of the attitude data through a process called 'linear extrapolation'. See the figure below.

On the trigger time, there is no measurement. When skewing, from the last two measurements, the trend is determined. This trend is continued in time until the trigger time, as can be seen from the figure. When skewing is enabled for an observation, QINSy is allowed to skew up to the maximum age of that observation.

When skewing is disabled however, the last available measurement is used (if it is not older than the maximum age, of course). This usually gives the best results. But sometimes, when the update rate of a system is low and the measured signal exhibits no large changes from the trend, skewing can be enabled. For example, your ship has a gyro with a low update rate and you only make regular, constant turns, skewing can be enabled to improve results.

During replay

When QINSy is replaying, more data is available than online. Since all data is recorded, QINSy can 'look into the future', so to speak. On a trigger, there is always newer attitude data available. This data can be used to calculate a very accurate sample to be used in the calculation, by using 'linear interpolation'. See the figure below for an example.

During replay, the first attitude measurement before and the first after the trigger time are used to determine the trend. On that trend line, QINSy calculates the value of the observation, which is the best representation of reality you can get with the provided sensors.
Of course: both data points should be within the defined maximum age for that observation, so be sure not to enter a maximum age smaller than the time between updates.