GPS World - January 2008 - (Page 17)

Receiver Design | SYSTEM DESIGN & TEST On paper, many products have similar performance specifications, but experience teaches that there can be a significant difference between specified performance and measured performance even under well-controlled conditions. Manufacturer data sheets can make it difficult to meaningfully compare receiver performance. A standard methodology precisely defines acquisition, tracking, and reacquisition, and uses specific input signals for testing each quantity. Position acquisition sensitivity: the lowest level at which a receiver, having identiﬁed a satellite signal as described above, can reliably decode the navigation data (almanac, ephemeris, etc.) from that satellite within a given time-out interval. is is directly related to the data decoding threshold (DDT) and denotes the lowest level at which the receiver can generate a position ﬁx in the (partial or total) absence of assistance data. e duration of the time-out interval has a signiﬁcant eﬀect on the observed position acquisition sensitivity, so this value must always be stated explicitly. Tracking Sensitivity. e lowest level at which a receiver, having produced a position ﬁx as described above, can track satellite signals reliably. is metric also has two variants: Position tracking sensitivity (or navigation senstivity): the lowest level at which a valid position ﬁx is available during at least a speciﬁed percentage of a given test interval; and Signal tracking sensitivity: the lowest level at which the receiver reliably tracks at least one satellite. www.gpsworld.com ese two variants are normally equivalent, but can diﬀer slightly due to the details of various ﬁltering and estimation algorithms implemented in a given receiver’s navigation ﬁlter (for example, dead-reckoning). Also, it is worth noting that the tracking sensitivity (in either variant) is typically much lower than the data decoding threshold, since it is possible to produce code-based pseudorange measurements even when the bit error rate is high enough to preclude robust decoding of the navigation data stream. Reacquisition Sensitivity. e lowest level at which a receiver, having produced a position ﬁx as described above, can directly regain signal tracking and navigation following a brief outage, using the existing timing and tracking parameters stored in memory. A typical example of the situation quantiﬁed by this metric is signal loss due to momentary blockage, such as would occur when driving under a highway overpass or past a building. As with acquisition sensitivity, this deﬁnition necessarily makes certain assumptions about timing, since there is no universally agreed-upon time-out interval after which a receiver reverts to sky search or some other acquisition mode. erefore, any assumptions should be stated explicitly in reacquisition performance measurements. dBm versus dBHz The quantity that most accurately characterizes receiver performance is C/N0 (in dBHz), not absolute signal power (in dBm). These two numbers are equivalent if, and only if, the correct assumption is made about noise-ﬂoor density. In practice, however, virtually all receiver manufacturers express sensitivity in terms of an absolute power level that corresponds to the true underlying C/N0, referred to an assumed thermal noise density of N0 = –174 dBm/Hz. ( is number is obtained by evaluating Boltzmann’s constant at a standard “room temperature” of 290 Kelvin.) is mapping is reasonable in most situations, but can be substantially incorrect if the RF chain contains an external lownoise ampliﬁer (LNA, as with an active antenna, for example). We discuss the January 2008 | GPS World 17 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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