Month: November 2011

At the heart of the RT3000 family of Inertial and GPS Navigation products is a six-axis Inertial Measurement Unit (IMU). The Inertial Measurement Unit uses three solid-state MEMS angular rate sensors and three servo (force-feedback) accelerometers. The three accelerometers are all mounted at 90 degrees to each other, and the three angular rate sensors are mounted at the same angles as their respective accelerometers. Together they give full 3D motion measurement of the body of the vehicle.

Careful calibration of the Inertial Measurement unit ensures that the normal bias and scale factor errors are reduced (the Kalman filter makes further corrections to them too). More critically, parameters such as misalignment are corrected. Misalignment parameters are critical to inertial measurements and are usually overlooked by people. (For example, if the roll gyro has a 1 degree misalignment angle with respect to the yaw direction and the vehicle rotates round a 360 degree circle then the integrated roll rate from the gyro will have an error of 6.3 degrees! 1 degree alignment is typical of how accurate the gyro is aligned inside its case and is irresective of how accurate you have aligned the case). In the RT3000 we align the gyros to better then 0.05 degrees, and the Kalman filter also removes errors as they accumulate; there is no degradation in performance while circling when using and RT3000.

A powerful, floating-point DSP provides 3D anti-aliasing filters for the inertial sensors. The algorithms on the DSP compensate for coning and sculling errors, allowing the products to work accurately in very high vibration environments. The DSP actually outputs changes in velocity and changes in angle, as is customary for Inertial Navigation Systems.

The survey-grade GPS receivers in the RT3000 use both L1 (civilian) and, optionally, L2 (military) frequencies to measure position, velocity and heading. The receivers have very fast reacquisition and, in the dual-antenna products, heading ambiguities are resolved by tight coupling with the solution from the inertial sensors. GPS is used to prevent the measurements from the Inertial Sensors from drifting.

The outputs of the Inertial Measurement Unit (IMU) are integrated in a WGS-84 Strap down Navigator. The use of Strap down Navigation makes the system immune to effects of earth rotation, transport rate and Coriolis accelerations. The outputs of the Strap down Navigator are the position, velocity and orientation of the system. Together with the accelerations and the angular rates they are output from the Inertial and GPS Navigation System.

The measurements from the Inertial Sensors and GPS receiver(s) are combined to form a blended navigation solution. A 24-state extended Kalman filter, running at 33Hz, corrects the navigation outputs and the Inertial Sensors (for bias and scale factor errors). The lever-arm to the GPS antenna and the angle of the GPS antennas is estimated; the user does not need to survey these accurately. The Strapdown Navigator provides the real-time outputs while the GPS receiver and the Kalman filter work in arrears. This enables very low-latency measurements to be output.

The measurements from the Inertial Sensors and from the GPS can be stored internally, up to 2 GB on current models. The measurements can then be post-processed. The advantage of post-processing is that it is not necessary to transmit the Differential Corrections from the GPS Base-Station in real-time. Instead, the GPS Base Station's information can be combined with the measurements after the mission. Waypoint software is used to post-process the GPS information before combining it with the inertial sensor's information post-mission.

Differential Corrections can be used to improve the performance of the RT3000 products in real-time. The RT3000 has been tested using Beacon, SBS/WAAS and OmniSTAR corrections. We can provide an special GPS Base-Station, the RT-Base, which provides exceptionally good differential corrections.

The RT3000 also includes several algorithm technologies, which are not normally found in competitive systems. These are described below and they help give the RT3000 huge advantages over comparable systems.

Advanced Slip

To improve the Heading and Slip Angle performance, the products include an Advanced Slip algorithm. This algorithm runs a compact car model in the Kalman filter to improve the Slip Angle.

Heading Lock and Dual-Antenna

When stationary for long periods of time, the heading of an Inertial Navigation System tends to drift. In the RT3000 family this can be solved using a Dual-Antenna product or by selecting Heading Lock. The Dual-Antenna products use two GPS antennas and measure position at two locations. The difference in the position of the two GPS antennas gives the Heading of the vehicle. Dual-Antenna means that the heading is free from drift, even when the vehicle has very low dynamics. Heading Lock monitors the speed of the vehicle. When the vehicle becomes stationary, the Heading is locked to its current value and cannot drift. The vehicle can remain stationary indefinitely without the variables of the Inertial Navigation System becoming too inaccurate to use.

Garage Mode

Many vehicle tests require the vehicle to return to the garage in between tests for changes and alterations. Sometimes these alterations can take many hours. During this time the Inertial Navigation System's variables will drift and they may not be able to recover when GPS becomes available again. To solve this, the RT3000 family includes a Garage Mode option. Garage Mode identifies when the product is in the garage and applies special corrections to keep the internal variables stable, even though GPS is unavailable. Then when the vehicle leaves the garage it can recover to full accuracy very quickly.

GPS Anomaly Rejection

GPS receivers often make measurements that have very large errors in them. If these errors are used to update the Inertial Navigation System then they cause havoc and can make the system unstable. The RT3000 family identifies GPS measurements that are incorrect and rejects them. This leads to a more robust positioning solution.

INS Drift Recovery

There are times when the Inertial Solution and the GPS measurements cannot be matched together. When this happens, the GPS measurements are rejected. But when too many GPS signals are rejected it is important to suspect that there is a problem with the Inertial Sensors instead. The RT3000 family is able to recover from these situations by monitoring the GPS and Inertial Solutions when they disagree. Special algorithms force the two back together if there is a mismatch for too long.

Wheel Speed Aiding

For survey vehicles, particularly road survey and rail survey, the drift from the RT3000 family can be significantly reduced by using wheel speed. When GPS is not available, wheel speed can keep the velocity stable and prevent it from drifting. The products are able to identify wheel slip and discard the wheel information when the wheel is slipping.

Improve your GPS with our Inertial+
Targeted at the survey market, the Inertial+ represents a big move in the Inertial+GPS navigation system market by being very cost-effective. The low price tag does not stop it from being highly accurate though. By combining an Inertial+ with your high accuracy RTK GPS receiver you can achieve 1cm precision, 0.03 degrees roll/pitch accuracy and have low drift rates when GPS is not available.

LIDAR Mobile Mapping

OxTS’s Inertial+ and Riegl’s Q120 scanner are perfect for cost effective mobile mapping applications.The Inertial+ is a low-cost inertial and GPS navigation system, ideal for correcting LIDAR systems. To read more and see some stunning data, click here…
 

Road Condition Assessment
The Swedish Road Administration is using OxTS' inertial and GPS navigation systems to assess the condition of its vast road network. To read more click here…

Inertial+ used for airborne LIDAR surveys
Scandinavian Laser Surveying Aps in Denmark has combined the Inertial+ with a Riegl laser scanner, giving them a low cost, precision airborne LIDAR system for surveying. To read more click here…

Example Road Survey Data in Google Earth

Want to see some example data on how the Inertial+ can improve the measurements from your GPS receiver? We have some Google Earth data that compares the GPS-only data with data from an Inertial+. Click here…
 

Finalised Trimble 5700 Integration

The integration of the Trimble 5700 receiver has now been finalised. The Inertial+ can be used in combination with many GPS receivers, providing a cost-effective solution to improve your GPS measurements even in difficult GPS conditions. To view a list of all integrated receivers click here…
 
 

Test the Inertial+ for free!

Demo systems are available in almost all countries. If you are interested in testing the Inertial+ systems, please contact us to arrange a suitable date for you.