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GPS Heading

Application Notes November 28, 2014

True Heading can be a difficult quantity to measure accurately, especially in dynamic conditions. The earth’s magnetic field can rarely be relied on; gyro-compasses are expensive and can be upset by high dynamic motion. By using two antennas, GPS Compasses can measure true heading but GPS alone has a low bandwidth, high noise and is unreliable near trees or buildings.

The RT2502 Inertial+GPS Navigation system combines gyros, accelerometers and two GPS receivers in one small package to overcome the technical difficulties associated with heading measurement.

Higher accuracy units in the RT series, such as the RT3202, RT3102 and RT3003 products are better at rejecting multipath and therefore give more accurate heading in built-up areas. 250Hz versions of the products exist for applications that require very high update rates, such as antenna steering and control.

The dual-antenna RT products have several features that make them very attractive as a GPS Heading solution.

  • Ambiguity resolution coupled to the inertial navigation system means that the reacquisition after a GPS outage is fast and robust.
  • Using the output of the inertial navigation system for heading, rather than the GPS, means that heading is output even when GPS is not available; the bandwidth is high; the update rate is fast and the latency or delay is very low.
  • Because a low-cost inertial navigation system is used, the price of the RT2502 compares very favourably with L1/L2 versions of GPS Heading, yet has many performance advantages.

The RT products have been designed for use in land, marine and air based applications. The high dynamics associated with racing cars and the low dynamics associated with steady flight or ships have no influence on the accuracy that can be achieved.

Many other measurements are output at the same time as heading, such as roll and pitch. Position, required for antenna steering to satellites or fixed land antennas, is also computed, even when GPS is not available. Other quantities include velocity, acceleration and angular rates.

The RT products represent great value, saving engineers time and effort in the installation phases of projects. The RT is very easy to use and is provided with comprehensive but simple configuration software. Flexible outputs on Ethernet, RS232 and CAN bus make it quick to interface. The RT can self-calibrate to its environment and this calibration can be saved and reused for faster alignment on future power cycles.

In use the RT products are autonomous and require little or no interaction with the user. Turn it on and it will start. The initial heading can be found statically (using ambiguity resolution) or dynamically (using the motion of the vehicle to prime the inertial navigation system).

Overall the RT products make excellent, cost effective GPS Heading devices and are packed with advantages over pure GPS-alone solutions.

Note: The definitive specifications are held in the RT2000 User Manual. Specifications are subject to change without notice.

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