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Real Time Kinematic GPS in an Urban Canyon Environment  Adeniyi Iyiade University of Plymouth, SEOES. Portland Square PL4 8AA. Plymouth. UK Email: aiyiade@plymouth.ac.uk Abstract As Global Positioning Systems (GPS) measurements like other survey measurements are not devoid of errors. In Real Time Kinematic (RTK) positioning in an urban canyon environments where the GPS signals are blocked by high buildings and there are not enough satellites signals in estimating the positions of the user, computation of the antenna positioning is a major problem. The research work uses an Extended Kalman Filter (EKF) as an attempt to reduce the effects of errors such as the atmospheric errors, multipath and receiver noise on GPS signals during transmission. However, some unmodelled biases still remain in the GPS observations, stochastic models were used as a quality indicator to identify and isolate such biases. The end result shows an improved coordinates solution. Introduction RTK GPS Technology has become a global utility in engineering survey activities. RTK GPS offers an efficient means of providing near instantaneous positions by employing differential GPS positioning whereby users can obtain centimetre/sub-centimetre level position in real time. In RTK GPS all algorithms using code information only are limited to range accuracy of about 0.5-1.0 metres due to code noise. However, range measurement using carrier phase information on the other hand is limited to only 0.5-3.0 millimetre by noise. Thus the use of carrier phase measurements in high precision positioning has become indispensable. Yet in order to use the carrier phase measurements in an urban canyon environment, the user has a couple of positioning errors to deal with. Researchers have focused on defining the different positioning error sources, studying their effects, and searching for methods to decrease, or possibly eliminate these errors in order to achieve better positioning accuracies in RTK set up in an urban canyon environment One of the major problems is the Multipath error which occurs when GPS signals are reflected from nearby objects before reaching the antenna. Also, electrical interference occurs when secondary sources or other transmitters and receivers distort the reception of the GPS signals or affect the receiver’s circuitry. These are particularly problematic for RTK GPS in an urban canyon environment, because they act as bias during the short location occupations and can prevent satellite tracking. Another problem is the sudden jumps in the carrier phase signal from one epoch to the next. This is called cycle-slips, cycle-slips must be detected and corrected before influencing the navigation solution and before changing filter-states in the code smoothing systems. Besides the afore-mentioned error sources with the use of carrier phase observables, other problems that are associated with RTK GPS positioning is the baseline length which tends to degrade the accuracy as the baseline length increases. This is predominantly due to atmospheric effects (ionosphere and troposphere), which become de-correlated and thus no longer cancel over longer distances through differencing algorithms.
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