GPS World - January 2008 - (Page 30)

SYSTEM DESIGN & TEST | Receiver Design 15 10 Generator power setting (dB) 5 0 -5 -10 -15 OFF 45 T0 T3 Reacquisition time (sec) 40 35 30 25 20 15 10 5 Receiver A Receiver B Receiver C Receiver D (N = 3) T2 T1 0 5 10 15 20 25 Scenario time (min) 30 35 40 45 0 -158 -157 -156 -155 -154 -153 -152 -151 -150 -149 -148 -147 -146 -145 -144 -143 -142 -141 -140 -139 -138 Power level (dBm) p FIGURE 7 Power profile used to measure reacquisition sensitivity p FIGURE 8 Time to reacquisition versus signal power will unfairly bias the results. Reacquisition. The reacquisition sensitivity test resembles a combination of the tracking and acquisition tests. Fortunately, because reacquisition implies an initially valid position ﬁx by deﬁnition, it is not necessary to divide the test into independent batches; rather, a single continuous run will sufﬁce. The power proﬁle for this test is shown in FIGURE 7. e labeled quantities have the following physical interpretations (reasonable default values are shown in parentheses): T0 = initialization to decode full almanac (15 min; not shown to scale) T1 = signal outage duration (15 sec) T2 = reacquisition interval (45 sec) T3 = reset interval before next attempt (60 sec) ∆ = power step size (0.5 dB) N = number of trials at each power level Note that the values suggested above represent a useful starting point, but may need to be adjusted according to speciﬁc end-user testing requirements. In any case, the ﬁrst reacquisition step (of duration T2) must be made at a power level at least 1–2 dB below the anticipated reacquisition threshold to ensure correct identiﬁcation of the threshold. Absent other speciﬁc guidance, a reasonable starting point is 3 dB above the tracking threshold found in the previous section. Once the RUT has been driven with a power proﬁle as shown above, its output (again, typically in the form of NMEA 30 GPS World | January 2008 logs) can be analyzed to determine the threshold of reacquisition and the position accuracy upon reacquisition at each power level. Example results of this process are shown in FIGURE 8 (demonstrating a comparative study of four receivers). ese curves illustrate several interesting aspects of receiver performance. For example, we see that Receiver A, which is fastest to reacquire at relatively high power levels, is the slowest at –150.0 dBm by a signiﬁcant margin, and has a worse (higher) reacquisition threshold than either Receiver C or Receiver D. us fastest reacquisition does not necessarily correspond to best overall performance. is is likely the result of a tradeoﬀ in receiver tuning. Also, note that Receiver D exhibits gaps in its reacquisition performance around –156.0 dBm and –154.5 dBm. is receiver’s curve was generated with a single trial at each power level (that is, N=1) to demonstrate an important, though often overlooked, aspect of the reacquisition measurement. As with acquisition, the estimate of reacquisition sensitivity is noisy, especially in the vicinity of the receiver’s performance threshold. To reduce the possibility that this noise signiﬁcantly aﬀects the experimental results, and to produce an accurate estimate of the true reacquisition threshold, it is necessary to repeat the reacquisition test multiple times at each power level, following the same methodology described in the previous section. While the 3GPP methodology does not contemplate reacquisition among its measured performance criteria, a comparable number of trials per power level should be used. Subtleties Several subtleties materially affect results and must be considered during testing. LNA, Gain, C/N0. If a RUT is normally designed to work with an active antenna, it is necessary to include an LNA in the testing setup, as shown in Figure 1. is ampliﬁer must be carefully chosen so that its gain and noise ﬁgure closely match the corresponding characteristics of the ampliﬁer in the active antenna. If the noise ﬁgure of the external LNA differs signiﬁcantly from that of the active antenna, the testing setup will deliver a diﬀerent C/N0 to the receiver’s front end than would be seen with the antenna in place. As noted earlier, this ratio is really the metric that determines receiver performance, so failing to account for this eﬀect can signiﬁcantly aﬀect the testing results. As for the LNA gain, it is critical to ensure that the total in-band power delivered to the front end of the receiver falls within the linear range of the receiver’s automatic gain control (AGC) circuitry, which in turn ensures that the analog-todigital converter (ADC) is driven within its speciﬁed input range. A simple way to visualize this eﬀect is to consider a histogram of RF (or IF) sample amplitudes. Since spread-spectrum signals of the type used in GPS and other GNSS systems typically have a power density www.gpsworld.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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