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This page contains an overview of some of the features in the TDoA 3 implementation. For details, please see the code in the Crazyflie firmware and the LPS node firmware.
The different clocks in the system are drifting and it is important to keep track of the drift of other clocks, we call this clock correction. The clock drift is caused by temperature changes in the clock crystals and is a slow process, and this is a property that we explore to detect errors.
We try to measure the ratio of the frequency of the remote clock VS the local clock. This is done by looking at two packets from the remote anchor and dividing the delta of the receive timestamps (local clock) with the delta of the transmit timestamps (the remote clock). The assumption is that the distance to the remote anchor has note changed and that the time of flight of the radio waves therefore is constant. This is not entirely true for a moving Tag, but if the packet rate is high enough, the error will be small and can be ignored.
There are many other error sources (packet loss, reflections, delays through objects and packet collisions for instance) that will effect the clock correction significantly though but since we know that the clock drift is changing slowly, we can remove any abrupt changes. There is a mechanism that discards any values that are “too far away” from the current estimated clock correction. The accepted values are low pass filtered to smooth the estimated clock correction.
Finally there is also a “leaky bucket” (a counter) that is used to reset the clock correction mechanism if there have been too many outliers lately. This might indicate that the current clock correction is wrong and we have to start over.
TDoA 3 is designed to handle a dynamic number of anchors. Since we must store data about anchors in the system, both in the anchors as well as in the Tags, there is dynamic storage functionality. It has a limited number of slots for anchor data and the anchor id is used as a key to access the data. This sets a limit on how many remote anchors that can be handled by an Anchor or Tag. The assumption is that there should not be more than 15-20 anchors visible to a device at one time.
The algorithms used to decide if the data of an anchor should be replaced by a new anchor differs a bit between Anchors and Tags. Anchors are assumed to have a static position and should only see the same anchors over time, while a Tag is moving around in space and will be exposed to new anchors.
The first line of defence is the outlier detection in the clock correction step. It keeps the clock correction tidy but also indicates whether a packet is suspicious or not. Data in suspicious packets is not used in the system.
The second line of defence is to look at sequence numbers. If sequence numbers are not consecutive in received packets, special action has to be taken to handle it. It is for instance important when calculating TDoA to make sure we use timestamps from the same packets in all steps.
The third line of defence is to check that the TDoA value we have calculated is physically possible. The TDoA value can never be greater than the distance between the two anchors used for the measurement and value that are larger are discarded.
The Final check is to verify that a TDoA value fits the current estimated position. If the TDoA value is too far away it is likely that there has been an error and the value is discarded, if it is deemed to be good it is fed into the position estimator for further processing. In this step there is also a mechanism that tries to determine if there are “too many” outliers, which could indicate that the estimated position is wrong. This makes the outlier detector less picky and accepts larger errors (letting more TDoA samples through) to enable the position estimator to converge again.