GPS World - January 2008 - (Page 33)

Receiver Design | SYSTEM DESIGN & TEST 200 180 160 140 Mean TTFF (seconds) 120 100 80 60 40 20 -130 -131 -132 -133 -134 -135 -136 -137 -138 Signal power on all satellites (dBm) -139 -140 -141 -142 12 SV 10 SV 8 SV 6 SV algorithms, and those (unrelated) factors can have an undesirable eﬀect on the estimate of tracking sensitivity. Finally, the results shown in the ﬁgure underscore the importance of accurate calibration throughout the testing setup when measuring TTFF. While the measurement is completely insensitive to small variations in signal power at high power levels, a calibration error as small as 0.5 dB (whether at the simulator output or anywhere in the RF chain before the LNA) can dramatically aﬀect the measured TTFF values. ALEXANDER MITELMAN is GNSS Research Manager at Cambridge Silicon Radio. He earned his Ph.D. in electrical engineering from Stanford University. His research interests include signal quality monitoring and algorithm development for GNSS. p FIGURE 10 Mean cold start TTFF versus signal power as a function of PER-LUDVIG NORMARK heads the technical development of GPS at Cambridge Silicon Radio. He was founder of Nordnav Technologies, acquired by Cambridge Silicon Radio in January 2007. He received his M.Sc. from Luleå University of Technology. His interests are focused on embedded software receivers for GPS and future GNSS systems. number of satellites in view both parts are needed before a receiver can get a ﬁx. In either case, the ﬁrst number is important regardless of what type of receiver start is being tested, and it is reasonable to expect that the measurement results may depend on the number of satellites in view (more to choose from implies the sky search algorithm will ﬁnd them faster). e results may also be a function of how each receiver’s sky search algorithm is designed. A simple example of this eﬀect is shown in FIGURE 10. At relatively high signal levels, the number of satellites in the simulator scenario does not appreciably aﬀect average TTFF. At lower levels, however, the number of satellites in view signiﬁcantly inﬂuences the acquisition process, and thus TTFF. As the ﬁgure demonstrates, this eﬀect becomes particularly pronounced as the signal power approaches a given receiver’s acquisition sensitivity threshold. e conclusion is that the choice of scenario matters. For true best-of-breed testing (for example, when validating numbers on data sheets without speciﬁc guidance about the test setup), a full satellite constellation should always be used. For controlled but more realistic tests, the experiment should be performed over multiple scenarios with varying numbers of satellites in view. It is worth noting that this issue can also aﬀect the tracking sensitivity measurement, albeit typically less severely than the acquisition test. Speciﬁcally, as the simulator power level steps down and a RUT gradually begins to lose lock on tracked satellites, loss of valid position will occur more quickly if the receiver has fewer satellites to start with. Similarly, if the simulator swaps one or more satellites out of the RF stream due to hardware constraints (this can occur, for example, when the number of satellites in view according to the simulation details exceeds the available number of RF channels), and this occurs below the acquisition threshold, the RUT will again be forced to operate with a reduced number of satellites through no fault of its own. In either case, the receiver’s behavior will depend on the speciﬁcs of its acquisition, reacquisition, and sky-search www.gpsworld.com MARTIN REIDEVALL is business development manager at the LBS Strategies Unit of Cambridge Silicon Radio. STUART STRICKLAND leads the LBS Strategies Unit at Cambridge Silicon Radio. He previously served as director of the GPS product line at SiGe Semiconductor. He earned his Ph.D. from Harvard University. Lower Noise Antenna = Better C/N0 Wi-Sys Antennas Provide up to 30% Greater Accuracy! World’s Best Designs Perfectly tuned Lowest noise gure (0.5dB) Best Out-of-Band Filter Dual Anti-jamming SAW lter Industrial grade housing Precision Ag Military COTS Security Grade Ultra-compact Embedded Fleet-grade LBand + GPS Cell + GPS 2.4GHz + GPS Globalstar™ STU OEM Linear Patch Wi-Sys Communications Inc. Antennas & RF Modules Tracking & Timing Receivers Complete LBS Solutions Quality Wireless Engineering www.wi-sys.com 613.254.7386 January 2008 | GPS World 33 http://www.wi-sys.com http://www.wi-sys.com http://www.gpsworld.com

Table of Contents Feed for the Digital Edition of GPS World - January 2008

GPS - January 2008 Contents Out in Front Expert Advice The Money-Go-Round u-Nav Latest Acquisition Apples to Apples Global SBAS 2008 GPS Receiver Survey Advertisers Index & Company Directory The Manufacturer's Road Year of the Who Working Indoor Up and Down Good, Better, Best Marketplace Classifieds Seen + Heard

GPS World - January 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.