GPS World - January 2008 - (Page 16)

SYSTEM DESIGN & TEST | Receiver Design Apples to Apples Standardized Testing For High-Sensitivity Receivers Alexander Mitelman, Per-Ludvig Normark, Martin Reidevall, and Stuart Strickland Cambridge Silicon Radio R ecent improvements in GNSS receiver technologies have driven demand and increased user expectations for positioning, navigation, and timing performance in challenging signal environments. As more and more manufacturers integrate GNSS receivers into their consumer products, a method for accurate, standardized testing of these devices becomes increasingly important. To date, several protocols have been published for testing GNSS receivers in speciﬁc situations. For example, the Institute of Navigation publishes a recommended set of tests to measure typical live-sky performance, and the 3rd Generation Partnership Project (3GPP) speciﬁes a test suite to validate the performance of assisted GPS receivers. Beyond these documents, however, there are few widely accepted speciﬁcations for general-purpose testing of highsensitivity receivers in a controlled environment. To address these shortcomings, we propose a simple standardized methodology for testing high-sensitivity GNSS 16 GPS World | January 2008 receivers. In designing this procedure, we ﬁrst deﬁne the quantities of interest as explicitly as possible, paying particular attention to certain assumptions that commonly underlie the numbers reported in receiver data sheets. We must also allow for the possibility that receivers can have diﬀerent algorithms for selecting and tracking satellites, and diﬀerent thresholds for determining signal lock and position validity. We therefore seek to ensure that our tests truly measure the deﬁned quantities in a meaningful, realistic way, and that our experimental design does not place any one particular implementation at an unfair advantage above another. Also, GNSS signal processing typically involves signals well below the thermal noise ﬂoor, so most receiver operations are fundamentally noisy processes with some element of randomness; where relevant, our approach produces statistically meaningful results by design. Finally, we brieﬂy discuss several oftenignored subtleties that can materially aﬀect experimental results and quantify those eﬀects. Sensitivity This key quantity is deﬁned as the lowest signal power, in dBm, detectable by a receiver under test (RUT). For GPS civil signals, the power level refers to the de-spread sinusoidal carrier at L1, that is, with the pseudorandom spreading code and navigation data modulation both removed. We discuss the proper mapping between signal power, in dBm, and the more familiar carrierto-noise ratio, in dBHz, in the next section. Sensitivity is generally measured in the context of a speciﬁc mode of receiver operation. e following modes are most commonly encountered in receiver testing. Acquisition Sensitivity. e lowest level at which a receiver can ﬁnd satellite signals. is metric is further broken down into two variants: Signal acquisition sensitivity: the lowest level at which a receiver can correctly identify the presence of a particular satellite’s modulation in the incoming RF signal within a given time-out interval; and 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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