ION NTM-2001 / Long Beach, CA / January 22-24, 2001 1 Evaluation of Multiple-Reference DGPS RTK Using a Large Scale Network Paul Alves, Gerard Lachapelle, M.Elizabeth Cannon, Junjie Liu Department of Geomatics Engineering University of Calgary, Canada Bryan Townsend, Roberton Enterprises, Canada BIOGRAPHIES Mr. Paul Alves received a BSc in Geomatics Engineering from the University of Calgary in 2000 and is currently enrolled in the MSc program where he is focussing his research efforts on the use of multiple reference stations for RTK GPS. Dr. Gérard Lachapelle is Professor and Head of the Department of Geomatics Engineering where he is responsible for teaching and research related to positioning, navigation, and hydrography. He has been involved with GPS developments and applications since 1980. Dr. M. Elizabeth Cannon is Professor of Geomatics Engineering at the University of Calgary. She has been involved in GPS research and development since 1984, and has worked extensively on the integration of GPS and inertial navigation systems for precise aircraft positioning. Dr. Cannon is a Past President of the ION. Mr. B Townsend received his MSc in 1993 from the Department of Geomatics Engineering, University of Calgary. Since then he has worked in several areas of GPS including GPS surveying, GPS receiver design and wide area reference systems. Currently he is working in the area of Network RTK. Mr. Junjie Liu received a BSc in Geodesy from Wuhan Technical University of Surveying and Mapping in 1998. He is currently enrolled in the MSc program in the Department of Geomatics Engineering at the University of Calgary where he is involved in GPS related research. ABSTRACT The multiple reference station approach developed at the University of Calgary for RTK DGPS, namely MultiRef™, is being tested using the large scale Swedish reference network that has an inter-receiver distance of 200 to 400 km in its southern part. The methodology, software and real-time communication links used are described. The test took place in the Winter of 2000 under an exceptionally high level of ionospheric activity. Both integer and float ambiguity solutions are tested. The advantages and limitations of a multiple reference station approach for RTK positioning under the above conditions are discussed, based on the results obtained. INTRODUCTION The ability of a system to perform effective real-time kinematic (RTK) carrier phase DGPS positioning will depend on the magnitude of the remaining orbital, ionospheric, and tropospheric differential errors. The presence of these errors will affect the available positioning accuracy, whether float (real number) or fixed ambiguities are considered. The double differenced carrier phase equation shows these errors: !   !   "! "  ambiguity integer noise multipath iono tropo 1 N m Ion Trop ∆ ∇ + ö ç ç è æ ∆ ∇ + ∆ ∇ − ∆ ∇ + ∆ ∇ + ∆ ∇ = ∆Φ ∇ ε ρ λ The magnitude of the tropospheric, ionospheric, and orbital error will change as the inter-receiver distance changes. Namely, these errors will decrease as the baseline distance decreases. For short baselines the effect of these errors can become negligible: !   ambiguity integer noise multipath 1 N m ∆ ∇ + ö ç ç è æ ∆ ∇ + ∆ ∇ − + ∆ ∇ = ∆Φ ∇ ε ρ λ The effect of the remaining errors is on the order of a few centimetres. This situation is ideal for ambiguity resolution and RTK DGPS positioning. In order to achieve the ideal ambiguity resolution situation, a dense network of reference receivers is required. Fotopoulos & Cannon [2000] have shown that tropospheric, ionospheric, and orbit errors are spatially and temporally correlated. This dense network will ensure that at any point, in the area of interest, there is a