A mobile mapping system (MMS) is a combination of different sensors used to capture useful information from our surrounding environment.
At the hears of a mobile mapping system, a set of navigation instruments are located that aim to find its position and orientation in a local or global coordinate system. Such instruments could be Global Navigation Satellite Systems (GNSS), Inertial Measurement Units (IMU), and Inertial Navigation Systems(INS).
The other important compartments of an MMS are multi-cameras, panoramic cameras, Light Detection and Ranging (LiDAR) instruments, infrared-camera, multi-spectral camera, gyroscope, altimeter, magnetometer etc.
There is an intrinsic need to synchronize different sensors in an MMS. A very useful way is to use electric signals to put synchronization labels on measurements captured from different sensors. A relatively easy way to use such a signal is to employ Pulse Per Second (PPS) signal from a GNSS receiver. Usually, an industrial GNSS has two outputs: GPRMC signal that contains locational information about a moment that is known with an accuracy less than 1 second, and a PPS signal that tells more about exact capturing moment.
A PPS signal is usually comes sooner than the GPRMC message. Its start is the exact point where the GPRMC message is referred. Very high time synchronizations are possible with using a PPS signal. Accuracies of nano second could be achieved in very high accuracy applications. In surveying application, however, an accuracy less than 0.1 micro second seems enough.
Different sensors are usually equipped to accept PPS +GPRMC messages as input, therefore, we can easily put time stamps on different bags of data. This is called central synchronization using GNSS device.
Sometimes, sensors accept PPS, but there is a need to send time update messages separately. This is done through standard interfaces such as RS232 or UART.
Another method to synchronize different sensors is to use local time servers. A laptop e.g., could be set up to act like a time server. In this way, all sensors grab time information by using a suitable data communication protocol such as UDP.
The latter approach has this nice benefit that it could be used in situations that we don’t access PPS and GPRMC, such as indoor positions.