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Static Initialisation of Dual-Antenna Systems advances technology for Autonomous Vehicles

Product News August 12, 2006

300_GrayKat5Autonomous vehicles need to have their inertial navigation system running before they can move; there are other applications where this is true too, like aircraft and boats. Dual-antenna RT3000 systems can calculate the heading even when stationary and this allows them to initialise before they move.

In the RT-Config software we have added a new checkbox to “enable static initialisation”. When this is checked the RT3000 will try to determine its heading using GPS alone. Searching for the correct solution can be as fast as 5 seconds but can also take over 5 minutes; satellite geometry and multipath have a huge influence on the search time and reliability. Once found, the RT3000 will initialise and start outputting all its data without having to move.

Because the dual-antenna search is not 100% reliable, it is not recommended that this is used unless it is required. GPS only systems can tolerate a lower reliability and then switch to the correct solution in one jump. The RT3000 cannot do this: it has a gyro that knows that the heading had not suddenly changed and it cannot easily jump from one solution to another. Typically the RT3000 algorithm is over 99% reliable in open sky.

Dual-antenna systems work by comparing the carrier wave of the GPS at the two antennas: there are many solutions to the phase of the carrier wave and a search is required to find the correct vehicle heading from carrier wave phase (known as the “carrier phase”). In the diagram below the “Primary” GPS antenna is in the centre. At the “Secondary” GPS antenna, wherever the carrier wave has the same phase, there is a possible solution to heading. This diagram only shows one satellite and possible solutions where the antennas are horizontal. In real applications the antennas can have different heights and there are lots of satellites which help figure out which position is correct. Typically there are 20,000 solutions in the dual-antenna search; these reduce to 20 very quickly but the final 20 take longer to discount. Eventually only one solution is left and it is the correct one.

When running, the RT3000 does not need to solve the carrier phase problem. We have an approximate value of heading and, from this, an approximate position for the secondary antenna can be computed. The GPS then provides a second measurement of heading, which is used to update the inertial navigation system.

Making the static initialisation feature available is an important strategic step for us. We are seeing more and more demand for autonomous vehicles. It is possible that there will be over 10,000 autonomous vehicles by 2015; by providing features like static initialisation of heading, we are expanding the opportunities of the RT3000 for this large, future market.

Closer to our core market in vehicle testing, autonomous vehicles are being talked about by nearly all manufacturers. Dangerous tests, such as rollover, and durability, particularly on Belgian block, are two areas where autonomous vehicles will become important. This new feature will help these autonomous vehicles get started while they are in zones where drivers may not enter.

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