问题
Is there a way to synchronize the timestamps in values meassured by Sensors (e.g. acceleration) (SensorEvent.timestamp) and GPS (Location.getTime()).
SensorEvent.timestamp are the nanoseconds since the device has been booted (local time). Location.getTime() are the miliseconds since1970 (global time).
One solution I had in mind was to calculate the offset between the local time and the global (GPS) time. But the problem is, that the time in Location is the time when the GPS-message arrived at the device, but it takes some time until the GPS-unit (don't know how it is called) of the device calculates the position and forwards it to my application, so I can't calculate at what local time the message arrived. I've read somewhere (can't remember where) that the delay from arrival of the message until it has been processed can be several 100 miliseconds, so I will always have a high offset between GPS-data and data from other sensors, if I algin the local time when the app gets the location-message and the timestamp in the location.
Is there a way to figure out how long it takes exactly to process the GPS-message? Or is there another way to synchronize the meassurements?
The reason why I want to synchronize the values is, that i want to apply a filter (e.g. Kalman-Filter) on the data. For this I want to calculate the offset between a sensor value and a gps-value as exact as possible.
回答1:
Usually Pulse Per Second (PPS) signals are used to link the time of measurement to the local clock. The GPS receiver toggles a pulse at the moment of measurement. This pulse is then timestamped using the local clock. On Android phones, this is not available.
In addition to the delay caused by processing and transmission time, there might be additional delays caused be filtering in the GPS software. This delay may even be depending on the speed or number of satellites used.
You might be able to 'calibrate' the delay between GPS data and your sensor data by comparing signals from both. E.g.: If you go from accelerating to decelerating, you will see this clearly in you phone's accelerometer and the GPS speed. However, I doubt if you can get a good accuracy for the delay. You could even add a state in you Kalman filter which estimates the delay.
来源：https://stackoverflow.com/questions/11461274/android-synchronize-timestamps-of-sensors-and-gps