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发表于 2007-10-23 12:33:11
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GPS Design Issues in handheld devices - Whitepaper
Abstract This document discusses common GPS implementation problems in portable devices. Typically the performance of GPS is highly reduced in PND, PDA, Feature Phone and Smart Phone devices due to lack of understanding of basic HW/RF design and EMI issues. The root cause arises typically from both poor antenna design/selection and internal EMI interference. GPS signal power level in open sky environment is nominally –125dBm. This is equivalent to the power level of a 60W light bulb lit in Japan and observed in Finland (if you could see it). The GPS signal level is also about 160dB lower than a GSM transmitter transmitting at 1W (+30dBm). We are talking about a very weak signal that leaves very little room for receiver design errors… -125dBm signal level corresponds roughly to about 50dBHz of SNR (or C/No). Poor antenna design and EMI problems usually reduces the maximum SNR up to 15-20dB. This means that maximum SNR is only 30-35dBHz. Such a low SNR has an impact on performance since data decoding from the satellite becomes difficult. The user sees this as long start-up times (TTFF) and poor performance in general, like position jumps or velocity drifts or worst, no position fix at all........... |
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