High Efficiency Road Status Monitoring System

This paper presents an application where an integrated GPS/IMU system is used for land survey (mounted on a van), providing a significant improvement on the efficiency of the road status monitoring process. Being initially developed for aircraft navigation, the system integrates one (or more) GPS receiver with an Inertial Measurement Unit (IMU), assembled with an industrial PC that stores the data and registers the various events that the operator introduces during the survey. Some surveys have already been performed with this system where, along with the road profile, some attributes where gathered, such as: • traffic and road signs position; • traffic and road signs type; • traffic and road signs conservation state; • pavement signs type and positioning; • type of pavement material; • pavement conservation state; • characterization and location of city infrastructures. The resulting data is processed a posteriori and the results are presented in a CAD environment with association to an object database. This final interface is very useful for road management, significantly reducing the time spent in the office analyzing the its status, and the use of several database utilities, such has queries and association by attribute, allows immediate and diverse analysis to be performed. This paper describes the implemented system and also the obtained results of different surveys, in dense urban areas and in more than 550 km of intercity roads, focusing on the usefulness of such system and its resulting analysis environment. A video camera was also used to collect image information while surveying which is useful in the processing stage (and, with proper image processing tools, it can improve this process’ efficiency).

Position+Attitude from GPS+INS for RTK Airborne Sensoring

Telmo Cunha, Phillip Tomé

This is a presentation of the techniques used to process inertial and GPS data collected during an airborne gravimetric and altimetric survey in order to obtain absolute position and attitude estimates of the sensors mounted on board the aircraft. For high precision gravimetry and altimetry, these navigational parameters are required, in the order of a decimeter for position and better than one arc-minute for attitude. The survey took place in the Portuguese Azorean area in October 1997. It consisted of flying a series of profiles with a set of instruments mounted in a military aircraft, covering a large area around the islands, mainly over the mid-Atlantic ridge. The instruments whose position and attitude had to be determined were a gravimeter and two altimeters (one laser and another radar). For this purpose, three GPS receivers and an Inertial Measurement Unit (IMU) were used. The basic idea was to have a relatively low cost system that would be able to supply position and attitude with a level of accuracy and reliability similar to other more expensive systems, like gimbaled and ring laser gyro based inertial systems. One other requirement was that it should be able to work in real time. This feature could be useful for repeating flight tracks and for many other applications. For this purpose, a radio link was established with the aircraft to supply GPS data from a ground reference station. The methodology for processing GPS and IMU raw data was implemented with an integrated feedback loop. The position determination is based on real time DGPS with L1 and L2 carrier phase ambiguity fixing, using online IMU processed data to validate and increase the positions reliability, especially during GPS outages and when the aircraft is far away from the nearest GPS reference station. For the attitude angles, the IMU raw data is processed using the absolute GPS position measurements for continuous alignment. The baseline between a pair of GPS antennas is also computed (without resorting to a reference station data), using L1 and L2 carrier phase and doppler measurements. This baseline is very helpful for the continuous alignment of the strapdown analytical platform in heading, since this angle estimate is weakly coupled to position updates, especially in the absence of high horizontal accelerations. The position and attitude values were obtained from all the measurements using an Extended Kalman filter. Besides the normal navigation states, the filter also includes extra states to account for the inertial sensor errors and the vertical gravity anomaly.

1999

Real Time Aircraft Position+Attitude of Airborne Sensors

Telmo Cunha, Phillip Tomé

The determination of instantaneous positions with high accuracy using differential GPS techniques is of great importance for several applications, like airborne topographic mapping, airborne gravimetry and altimetry. In some of these applications long baselines are involved creating additional problems in the correct determination of the position of the sensors. The determination of the attitude of the aircraft is also of great importance in this context. This paper presents a technique that, taking advantage of the integration of GPS and inertial measurements, produces real time positions. The demand for real time positioning introduces some more constraints, which must be taken into account from the start of the algorithm development to the end of its implementation. A methodology for processing GPS and raw data from an inertial measurement unit (IMU) was implemented in a feedback integrated structure. The position determination is based on real time DGPS with L1 and L2 carrier phase ambiguity fixing, using on-line IMU processed data to validate and increase the positions reliability, especially during GPS outages and when the aircraft is far away from the nearest GPS reference station. By processing L1 and L2 carrier phase measurements in differential mode between a pair of the aircraft GPS antennas (without resorting to ground reference station data), a partial estimate of the aircraft attitude is obtained. This is critical to align the low cost inertial platform in heading either when the aircraft is stopped or in flight. In addition, the DGPS absolute positions are used to update the IMU data in real time. By using the proposed methodology it was possible to explore the entire potential of a low cost IMU/GPS system. The position and attitude values are obtained from all the measurements through an extended Kalman filter. Decimeter accuracy was achieved for the positions. For the orientation of the aircraft an accuracy of 0.01º (1) for roll and pitch and 0.1º (1 sigma) for heading was achieved. The performance and the results obtained with this system using data from an airborne campaign that took place in Portugal are discussed.

1999

Evaluation of a DGPS/IMU Integrated Navigation System

Telmo Cunha, Phillip Tomé

The development of efficient systems for precise attitude and position determination of airborne sensor platforms is having a huge impact in different remote sensing applications, resulting in higher productivity levels and lower exploitation costs. This technology is also being successfully applied to different mobile mapping systems. Being aware of this fact, the authors have developed a navigation system that can be used either in real time or in post-processing mode, depending on the requirements of the user. The system is modular and can have different configurations. In the case reported here the airborne segment consists of a low cost IMU, two double frequency GPS receivers, an industrial PC and a radio communication link to allow real time DGPS. As a complement, a mini TV is used to display graphical information, such as the position of the aircraft over a map and a simulated Horizontal Situation Indicator (HSI) that the pilots use to follow pre-programmed flight profiles. The ground segment is based on a GPS reference station, a PC that compresses the GPS corrections (and also displays data that is being sent by the aircraft) and a set of radio transceivers that are spread over the surveying area. In July/August 1999 the system was subject to final testing on board a Portuguese Air Force aircraft. In order to evaluate the attainable accuracy, a photogrammetric camera was also installed and rigidly fixed to the aircraft's structure. More than 400 photos were taken and the triggering instants registered. Ground control was performed for image georeferencing, which allowed the determination of the position and attitude of the camera at the instant the shot was taken. Because the camera is coupled to the aircraft body, this information can be compared to the obtained with the navigation system and its accuracy assessed. In this paper, besides a general description of the system implementation and algorithmic structure, the real time and post-processing results obtained during these test flights are presented and compared to the photogrammetric results. To conclude, general remarks regarding the system performance and its sootiness to different remote sensing applications are discussed.

2000

Real Time Aircraft Position+Attitude of Airborne Sensors

Telmo Cunha, Phillip Tomé

1999

Evaluation of a DGPS/IMU Integrated Navigation System

Telmo Cunha, Phillip Tomé

2000

Position+Attitude from GPS+INS for RTK Airborne Sensoring

Telmo Cunha, Phillip Tomé

1999