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THEME 3: MULTI-SENSOR INTEGRATION
Time Synchronisation
For a multisensor system, time synchronisation of measurements is an important task. There are 3 different methods to achieve integrated-sensor time synchronisation, namely:
(a) hardware,
(b) software, and
(c) a combination of the two.
The former two methods are either too expensive or inconvenient for many academic purposes. The third method is to utilise the time tagging 1PPS signal from a GPS receiver as the accurate time reference. This method is suited for researchers as it allows for the development of a low-cost, flexible, and reliable system. A flexible synchronised data acquisition system can be created using a multifunction DAQ (data acquisition) card. SNAP has developed a Cost Effective Synchronisation System (CESS) based on the third method.
The CESS consists of 3 primary components: GPS receiver, DAQ card and support software.
Setup of the CESS
The architecture of the CESS
GPS receiver (Jupiter) outputs a 1PPS signal and 10kHz pulse signal, together with GPS data raw measurements. The analogue data output from the sensor(s) is (are) connected to the interface box. The GPS receiver 1PPS signal is used as a trigger, while the 10kHz signal is used as a very accurate time base to generate a scan signal. When the first 1PPS signal from the GPS receiver is detected, sampling of the analogue input from the sensor(s) begins at a rate of one hundred times a second (in the case of 100Hz sample rate). The sample rate can be changed from 50Hz to 2500Hz (depending on the application and the specification of the multifunction DAQ card). LabVIEW based software (LBS) logs the sensor data to a file, and also can display the data so that it can be monitored. At the same time GPS data from the receiver is read by LBS, including a message containing UTC time. This is used to time tag the sensor data, by adding the message at the end of the sensor data every second. In the data file, there is a time tag every 100 records (100Hz sample rate) of sensorŐs data. The time tag appears approximately 40 records before the next 1PPS signal arrives. Therefore, the relationship between GPS time (or UTC time plus a number of leap seconds) and the sensor(s) is determined with the aid of the 1PPS signal.
The architecture of the CESS software
To evaluate the synchronisation accuracy of the CESS two experiments were conducted. The first was to use a reference hardware synchronised INS system (the C-MIGITS), and compare this with the CESS using a Crossbow IMU (Inertial Measuring Unit) as the test sensor. By moving the two units in the same manner, comparisons can be made between the data logged by the hardware synchronised C-MIGITS and the CESS-synchronised Crossbow IMU.
In the second experiment standard reference pulses were used to test the synchronisation accuracy of the CESS. Test 1 shows that the synchronisation error is below 1 second. In test 2, CESS can obtain the pulse with a width of 0.4 ms at the expected position, which means that the accuracy of the system must be better than 0.4 ms.
Experimental Setup of Vehicle-Mounted INS/IMU at Clovelly Bay, Sydney, Australia
On 10th April 2003, an experiment was conducted in the Clovelly Bay carpark, Sydney, to test the system. The figure shows the setup of the equipment on the roof of the vehicle. The trajectories of this experiment are shown in Figure 1, black line is the reference trajectory, while red one and green one are the trajectories utilising GPS data alone and the GPS/INS combination respectively. (An offset between the reference and other trajectories has been added to make the figure clearer.) There are almost no differences between these trajectories. However, if the system doesnŐt work well, i.e. the synchronisation was not good enough, the trajectories should be like those in Figure 2.
Figure 1: 2D Trajectories With No Synchronisation Errors
Figure 2: 2D Trajectories With Synchronisation Errors
LI, B., 2004. A cost effective synchronisation system for multisensor integration. 17th Int. Tech. Meeting of the Satellite Division of the U.S. Institute of Navigation, Long Beach, California, 21-24 September, 1627-1635. (Download PDF)
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