GPS World, December 2009

WIRELESS Receiver Design Å FIGURE 10 Before top and after bottom MOPP guided tuning blue 256 trajectories green truth tions as well First the IF replay process is not perfect so a small amount of error is introduced with respect to the true underlying trajectory as a result of the post processing itself Provided this error is small compared to those caused by any corner cases of interest it does not signifi cantly affect the usefulness of the analysis but it must be kept in mind Second the accuracy of the replay and therefore the detection threshold for anomalous artifacts may depend on the RF environment and on the hardware profi ling used during post processing ideally this threshold would be constant regardless of the environment and post processing settings Third the replay process operates on a single IF fi le so it effectively presents the same clock and front end noise profi le to all replay trajectories In a real world test including a large number of nominally identical receivers these two noise sources would be independent though with similar statistical characteristics As with the imperfections in the replay process this limitation should be negligible provided the errors due to any corner cases of interest are relatively large Conclusions and Future Work The multiple offset post processing method leverages the unique features of software GNSS receivers to greatly improve the coverage and statistical validity of receiver testing compared to traditional hardware based testing setups in some cases by an order of magnitude or more The MOPP approach introduces minimal additional error into the testing process and produces results whose statistical independence is easily verifi able When corner cases are found the results can be used as a targeted tuning and debugging guide making it possible to optimize receiver performance quickly and effi ciently Although these results primarily concern continuous navigation the MOPP method is equally well suited to tuning and testing a receivers baseband as well its tracking and acquisition performance In particular reliably short time to first fix is often a key figure of merit in receiver designs and several specifications require acquisition performance to be demonstrated within a prescribed confidence bound Achieving the desired confidence level in difficult environments may require a very large number of starts the statistical method described in the 3GPP 34171 specification for example can require as many as 2765 start attempts before a pass or fail can be issued so being able to evaluate a receivers acquisition performance quickly during development and testing while still maintaining sufficient confidence in the results is extremely valuable Future improvements to the MOPP method may include a careful study of the baseline detection threshold as a function of the testing environment open sky deep urban canyon and so on Another potentially fruitful line of investigation may be to simulate the effects of physically distinct front ends by adding independent identically distributed swaths of noise to copies of the raw IF file prior to executing the multiple offset runs ALEXANDER MITELMAN is GNSS research manager at Cambridge Silicon Radio He earned his M S and Ph D degrees in electrical engineering from Stanford University His research interests include signal quality monitoring and the development of algorithms and testing methodologies for GNSS JAKOB ALMQVIST is an M Sc student at Luleå University of Technology in Sweden majoring in space engineering and currently working as a software engineer at Cambridge Silicon Radio ROBIN HÅKANSON is a software engineer at Cambridge Silicon Radio His interests include the design of optimized GNSS software algorithms particularly targeting low end systems DAVID KARLSSON leads GNSS test activities for Cambridge Silicon Radio He earned his M S in computer science and engineering from Linköping University Sweden His current focus is on test automation development for embedded software and hardware GNSS receivers FREDRIK LINDSTRÖM is a software engineer at Cambridge Silicon Radio His primary interest is general GNSS software development THOMAS RENSTRÖM is a software engineer at Cambridge Silicon Radio His primary interests include developing acquisition and tracking algorithms for GNSS software receivers CHRISTIAN STÅHLBERG is a senior software engineer at Cambridge Silicon Radio He holds an M Sc in computer science from Luleå University of Technology His research interests include the development of advanced algorithms for GNSS signal processing and their mapping to computer architecture JAMES TIDD is a senior navigation engineer at Cambridge Silicon Radio He earned his M Eng from Loughborough University in systems engineering His research interests include integrated navigation encompassing GNSS low cost sensors and signals of opportunity GPS World December 2009 www gpsworld com 34