GPS World - January 2008 - (Page 29)

Receiver Design | SYSTEM DESIGN & TEST 160 140 120 Time to first fix (seconds) Mean 50th %ile 95th %ile position accuracy (of the initial ﬁx) versus signal level. ese results are shown for a sample RUT (in cold start mode) in FIGURE 6. 100 80 60 40 20 -130 -131 -132 -133 -134 -135 -136 -137 -138 Signal power on all satellites (dBm) -139 -140 -141 -142 p FIGURE 6 Time to first fix versus signal power (cold start) is reached. e power level is then reduced (1–2 dB steps are reasonable at modest signal levels, but for best accuracy the steps should be reduced to 0.5 dB below about –137 dBm or wherever the RUT’s acquisition threshold is expected to be) and the batch of starts is repeated. e test proceeds in this fashion until the receiver fails to meet the desired conﬁdence bound; the lowest power level where it does is then declared to be the acquisition sensitivity threshold. Having performed an adequate number of starts across the range of signal levels and determined the acquisition sensitivity threshold, it is informative to construct plots of TTFF and Two points are worth noting. First, the statistical derivation in the 3GPP standard extends to tests involving up to 2,765 trials for a 95 percent conﬁdence bound. at is, following the derivation to the letter, it is possible that an impractically large number of trials would be needed to establish the required level of performance with 95 percent conﬁdence. (It is, of course, possible to establish the conﬁdence bound with far fewer trials if the success rate is relatively high.) Whether to proceed with such a large number of trials, or to re-test with a slightly lower conﬁdence bound, is a strictly pragmatic decision for the test administrator. A reasonable alternative is to start with the amount of time available for testing, estimate the number of trials that can be performed within that time, and apply the 3GPP derivation in reverse to calculate a resulting achievable conﬁdence bound. Second, if an automated batch script is used to run multiple starts at each power level, as suggested, it is essential to introduce a random delay of at least several seconds between successive starts. Otherwise, it is possible that the receiver will always initiate acquisition at the same point in the navigation data message, resulting in highly correlated TTFF values. is www.gpsworld.com January 2008 | GPS World 29 http://www.hemispheregps.com http://www.hemispheregps.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

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