Assessment of Multipath of the Hibernia Oil Platform for the Aviation DGPS System to be used for Helicopter Services (Fredericton/Canada).
Prof. R. B. Langley
Prof. R .B. Langley
Prof. Dr.-Ing. B. Heck
- Bearbeitung:Albrecht, Y.
GPS is currently being considered as a possible replacement for the Instrument and Microwave Landing Systems. The Helicopter industry, providing transport services such as personnel and supplies to oil platforms, and the owners of off-shore platforms are very interested in improving the reliability of helicopter en-route and final approach navigation. In the environment close to the platform various potential reflectors may cause multipath errors such as decks, support towers, rigs, derricks, cranes and containers. Once or even more times the GPS signal is bounced back off the ground, reflected or scattered from the aforementioned objects and arrives at the antenna along multiple paths.
DGPS measurements were undertaken on the Hibernia Oil Platform in Bull Arm Channel, Newfoundland (Canada) at the time when the construction was almost completed and before it was supposed to be towed out to its offshore location. The diploma thesis classifies and summarises the multipath phenomenon in general, presents the impact on the observables and different techniques of multipath mitigation in the receiver as well as in the proximity of the antenna. The data were processed with the Quality Control (QC) software to estimate C/A code pseudorange multipath and Ashtech PNAV software to estimate baseline lengths. The C/A-code pseudorange multipath represented by the QC output shows rms values of 15 m (Rig3), 7 m (Rig2), 5 m (Rig1) and 2 m (Base). The error range reaches 300 m (Rig3), 190 m (Rig2), 100 - 150 m (Rig1) and 15 m (Base). The more precise carrier phase baseline solution is used and has to be differenced from the pseudorange solutions to remove most of the movement. By applying that method the high multipath level of the Rig3 baseline (r.m.s. solution differences 8.8 m, max range in position 72 m) and the successive lower multipath of the Rig2 and Rig1 baseline is illustrated. The impact of multipath on position determination using C/A-code observations shown by these errors corresponds to the error level for an individual reference station due to multipath. Furthermore other phenomena such as trends, patterns, ...etc can be observed by decomposing the baseline solutions into components.
Performing cross-correlation analysis for two consecutive days an attempt was made to reveal the reproducibility of multipath. The computations were carried out successfully for the base station on the shore being an easier describable multipath environment than the oil rig. The permanent movement of Hibernia during the data acquisition was regarded to be the major problem and precluded the conclusive proof of the presence and the actual contribution of multipath to the error budget. However, day-to-day comparisons revealed that significant signatures could be observed for the reference and the platform stations.
The Carrier-to-Noise Ratio and its variation over time is used to detect multipath effects. Various investigations are made by applying spectral analysis methods to extract periodic constituents which can be assigned to dominant multipath frequencies. A geometrically derived multipath model is introduced and the environment at the Base and on the platform is assessed in terms of multipath reflecting signals and potential reflectors. As a result of these investigations some points can be computed with an accuracy of 0.5 - 1 meters (perpendicular distance) on a reflecting surface in the proximity of the antennas. In the case of Rig3 data, reflections from the Helideck and the Weather Deck are predominately the reason for multipath errors.
The fact that the reflecting points are gridded by azimuth/elevation lead to the idea of inverting the data to map out the discrete reflecting objects. Mesh plots of the surrounding area of the reference station and Rig3 are provided.
I did the research and composed the main part of my diploma thesis in Fredericton at the Department of Geodesy and Geomatics at the University of New Brunswick (UNB). As a member of my supervisor's, Prof. Langley's GPS research group, I was allowed to attend lots of courses, presentations and discussions about interesting topics in the field of GPS and Navigation. It was a great experience in science and life and I can actually recommend a stay in Eastern Canada.
Whoever is interested in writing a thesis, studying or doing some research (Praktikum) at UNB, please feel free to contact me (firstname.lastname@example.org) or Prof. Heck.