GPS World, January 2011

RECEIVER DESIGN Vehicle Navigation FIGURE 13 Trajectories obtained using assisted HS GPS GLONASS processing 300 ms integration time To evaluate the impact of the coherent integration time on performance the data was re processed with a coherent integration time of 300 milliseconds ms instead of the 100 ms used for the data presented so far The resulting trajectories are shown in FIGURE 13 It is interesting to note that increasing the receiver sensitivity in this way does not yield better navigation performance In fact in the urban canyon environment the major issue is not the signal attenuation which can be overcome by increased coherent integration but rather the multipath effect By increasing the coherent integration time to 300 ms the receiver becomes more sensitive to dynamics resulting in poorer navigation performance Discussion High sensitivity processing in urban canyon environments is a very effective means of improving navigation performance Given the discussion above however it is clear that the performance is not limited by the strength of the received signal but rather by the effect of multipath and satellite geometry The advantage of high sensitivity processing in this case is two fold The fi rst advantage over standard tracking techniques is the open loop nature of HS processing The time varying nature of the multipath channel causes signifi cant variation in signal level This variation can cause traditional tracking loops to lose lock In fact the poor performance of the standard FIGURE 14 Sample correlation function showing two peaks wide correlator strategy in the above analysis can be explained by the fact that the receiver was unable to maintain lock on the satellites in view Hence no measurements were generated and no solutions computed The survey grade receiver used has advanced multipath mitigation technology which helped to avoid loss of lock but may have been tracking non line of sight signals during portion of the down town test leading to errors in the navigation solution The second advantage of HS processing is related to the coherent integration time and the vehicle dynamics As the receiver antenna moves through the multipath environment a different Doppler shift is observed on signals coming from different directions Thus the lineof sight and multipath components become separated in frequency A longer coherent integration time increases the frequency resolution of the correlator output due to the familiar sinc shape Thus if the lineof sight is present and the coherent integration time is long relative to the inverse of the Doppler difference between the line of sight and refl ected signals individual peaks become visible in the grid of correlators This effect can signifi cantly reduce the impact of multipath on the measurements FIGURE 14 gives an example of this Conclusions The addition of GLONASS capability can signifi cantly improve 10 percent improvements observed here the number of position solutions with redundancy available in the urban canyon With increasing GLONASS satellite availability the benefi ts of using GLONASS will even be greater It was shown that for the urban multipath environment the greatest benefi ts are seen when using a HS GNSS processing strategy with moderate extended coherent integration times 100 ms Future interesting applications include the use of dual frequency measurements as almost all current GLONASS satellites transmit civil signals at both L1 and L2 Acknowledgments The authors would like to kindly acknowledge and thank Defence Research and Development Canada DRDC for partly funding this work The authors also wish to thank Tao Lin PhD candidate in the PLAN group for his signifi cant contribution to the block processing and data aiding software Manufacturers The tests used a National Instruments PXI 5661 RF downconverter and digitizer the PLAN GSNRx as standard wide correlator receiver the u blox Antaris 4 standalone HS GPS NovAtel OEMV 3 survey grade GPS GLONASS and the PLAN group software receiver GSNRx as the assisted HS GPS GLONASS CILLIAN ODRISCOLL received his Ph D in 2007 from the Department of Electrical and Electronic Engineering University College Cork and is currently a post doctoral fellow in the PLAN Group of the University of Calgary GÉRARD LACHAPELLE is a professor of geomatics engineering at the University of Calgary where he holds a Canada Research Chair in wireless location and heads the Position Location and Navigation PLAN Group MOHAMED TAMAZIN is a M Sc candidate in the the PLAN at the University of Calgary He holds a M Sc in electrical communications from the Arab Academy for Science and Technology Alexandria Egypt GPS World January 2011 www gpsworld com 58