Real Time Aircraft Position+Attitude of Airborne Sensors

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.

A GPS/INS System for Positioning and Orientation of Airborne Sensors: Results from flight tests in Portugal

Telmo Cunha, Phillip Tomé

Efficient systems for determining the precise position and orientation of airborne sensors will have a great impact in different remote sensing activities, with consequences in their productivity and therefore substantially increasing their cost effectiveness. In the scope of national and international projects, the authors have developed and tested an airborne system built with multiple GPS receivers and a low cost inertial unit. The methodology is based on the integrated processing of raw inertial measurements and GPS data collected from two receivers on board the aircraft and one, or possibly more, reference stations. The algorithm for processing GPS and IMU (Inertial Measurement Unit) raw data was implemented in an integrated feedback loop. The position determination is based on DGPS with OTF ambiguity fixing, using on-line processed inertial data to validate and increase position 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 absolute GPS position/velocity updates for continuous alignment. Also, by processing L1 and L2 carrier phase measurements in differential mode between two GPS antennas on the aircraft (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 stable flight. The inertial system also provides high rate (DGPS filtered) position and velocity estimates that can be valuable during GPS signal outages and for the robustness of the ambiguity fixing procedures. By using the proposed methodology it was possible to explore the entire potential of a low cost GPS/IMU system. The position and attitude values are obtained from all the measurements through an Extended Kalman Filter. Decimeter accuracy was achieved for the positions, while the estimated accuracy for the aircraft orientation is 0.01º for roll and pitch and 0.05º for heading (maximum values). The basic idea was to develop a low cost system that can supply position and attitude with a level of accuracy and reliability similar to more expensive systems, such us gimbaled and ring laser gyro based inertial systems. The final tests of this GPS/IMU system took place during this summer. The inclusion of a radio link between a reference station and the aircraft enabled the system to operate in real time. It proved to be successful in navigating the aircraft during all flight phases, including runway approach and landing. Furthermore, aerial photographs were taken in order to have an independent evaluation of the system accuracy. Results from the airborne campaigns and the performance of this system are presented. Its potential for different applications is discussed.

1999

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

Integrating Multiple GPS Receivers With a Low Cost IMU For Aircraft Attitude Determination

Telmo Cunha, Phillip Tomé

Many airborne applications, such as altimetry or photogrammetry without ground control points, strongly rely on the precise evaluation of the aircraft’s attitude. This paper presents an approach to the attitude determination problem, where a system combining Litton’s LN-200 Inertial Measurement Unit (IMU) with differential GPS (DGPS) is described. The methodology is based on the integrated processing of raw inertial measurements and GPS data collected from at least two receivers on board the aircraft and one, or possibly more, reference stations. It is well known that GPS and inertial navigation systems have complementary operational characteristics that can be exploited in an integrated system for continuous tracking of the aircraft’s navigational parameters (attitude, velocity and position). The incorporation of DGPS observations of one of the aircraft’s GPS antennas, specifically absolute position and doppler velocity readings, improves the overall system accuracy, including the estimates of roll and pitch angles that result from filtered raw inertial measurements, since it enables the continuous alignment of the strapdown analytical platform. By processing carrier phase measurements in differential mode between a pair of the aircraft’s GPS antennas (without information from any reference station), good attitude observations, especially true heading, are obtained. This is critical to align the low cost inertial platform in heading, since azimuth errors are weakly coupled with velocity or position errors. In fact, by this method alignment is achieved in motion and very quickly. The inertial system also provides high rate (DGPS filtered) position and velocity estimates that can be valuable during GPS signal outages and in DGPS ambiguity fixing procedures that occur whenever a new satellite becomes visible or cycle slips are detected. The estimated accuracy is 0.01º (1s) for roll and pitch and 0.1º (1s) for heading. Should the reference station data be absent and stand-alone doppler and position estimates be used instead, some degradation of the roll and pitch accuracy are to be expected. Besides a description of the relevant theory, this paper also includes post-processing results from one of a series of flights that took place in the Azorean area during a gravimetric and altimetric survey. The present methodology was successfully employed to process all data from this campaign.

1999

Integrating Multiple GPS Receivers With a Low Cost IMU For Aircraft Attitude Determination

Telmo Cunha, Phillip Tomé

1999

A GPS/INS System for Positioning and Orientation of Airborne Sensors: Results from flight tests in Portugal

Telmo Cunha, Phillip Tomé

1999

Position+Attitude from GPS+INS for RTK Airborne Sensoring

Telmo Cunha, Phillip Tomé

1999