GPS World, April 2014

GALILEO Receiver Design min Max Mean HOT start 45 124 79 WARM start 315 672 363 COLD start 405 2657 1050 TABLE 2 TTFF in seconds exploiting GPS and Galileo constellations in harsh environments Sensitivity Performance in Harsh Environments Harsh environment is the general term used to describe those scenarios in which open sky and ideal propagation conditions are not fulfilled It can include urban canyons where the presence of high buildings limits the SV visibility and introduces multipath denied environments where unintentional interference may create errors in the processing or sites where shadowing of line of sight LoS path is present for example due to trees buildings and tunnels In these situations it is necessary to pay particular attention to the signal processing stage performance is in general reduced up to the case in which the receiver is not able to compute a fix UVW DWWHPSW WR PRGHO VXFK DQ HQYLURQPHQW KDV been introduced in the 3GPP standard together with WKH GH QLWLRQ RI 166 PLQLPXP SHUIRUPDQFH requirements for user equipment supporting other 166V WKDQ 36 RU PXOWLSOH 166V ZKLFK PD RU PD QRW LQFOXGH 36 7KH VWDQGDUG WHVW cases support up to three different constellations in dualconstellation case it foresees three satellites in view for each constellation with a horizontal dilution of precision 23 UDQJLQJ IURP WR 7R SHUIRUP 77 DQG VHQVLWLYLW WHVWV DSSO LQJ WKH 33 VWDQGDUG WHVW FDVH ZH FRQ JXUHG D 166 VLPXODWRU scenario with the following characteristics starting from the nominal constellation 6L 69V WKUHH 36 ZLWK 351 DQG WKUHH DOLOHR ZLWK FRGH QXPEHU 23 LQ WKH UDQJH QRPLQDO SRZHU DV SHU FRUUHVSRQGLQJ 66 user motion with a heading direction towards 90 D LPXWK DW D FRQVWDQW VSHHG RI NLORPHWHUV KRXU NP K In addition to limiting the number of satellites we LQWURGXFHG D QDUURZEDQG PXOWLSDWK PRGHO 7KH PXOWL 69 two states land mobile satellite LMS model simulator generated fading time series representative of an urban HQYLURQPHQW 7KH PRGHO LQFOXGHV WZR VWDWHV a good state corresponding to LOS condition or light shadowing D EDG VWDWH FRUUHVSRQGLQJ WR KHDY VKDGRZLQJ EORFNDJH Within each state a Loo distributed fading signal is assumed It includes a slow fading component lognormal fading corresponding to varying shadowing conditions of the direct signal and a fast fading component due to multipath effects In particular the last version of the two state LMS simulator is able to generate different but correlated fading for each single SV according to its elevation and azimuth angle with respect to the user position the angular separation within satellites is crucial since it affects the correlation of the received signals 7KLV DSSURDFK LV EDVHG RQ D PDVWHU VODYH FRQFHSW where the state transitions of several slave satellites are modeled according to their correlation with one master satellite while neglecting the correlation between WKH VODYH VDWHOOLWHV 7KH QXLVDQFHV JHQHUDWHG DUH WKHQ imported in the simulator scenario to timely control phase and amplitude of each simulator channel Using this LMS scenario the receivers performance in harsh HQYLURQPHQWV KDV EHHQ WKHQ YHUL HG ZLWK DFTXLVLWLRQ 77 DQG WUDFNLQJ WHVWV 7KH 77 ZDV HVWLPDWHG ZLWK DERXW WHVWV LQ KRW ZDUP DQG FROG VWDUW UVW XVLQJ ERWK 36 DQG DOLOHR satellites and then using only one constellation In WKH VHFRQG FDVH RQO WKH LV FRQVLGHUHG VLQFH according to the scenario described at maximum three satellites are in view TABLE 2 reports the results for the GXDO FRQVWHOODWLRQ FDVH LQ KRW VWDUW WKH DYHUDJH 77 LV DERXW V DQG LW LQFUHDVHV WR V DQG V UHVSHFWLYHO IRU WKH ZDUP DQG FROG FDVHV OHDUO WKH UHVXOWV DUH PXFK worse than in the case reported earlier of full open sky 1 FRQGLWLRQV Q WKLV VFHQDULR RQO VL VDWHOOLWHV are available at maximum moreover the presence of multipath and fading affects the results and they exhibit a larger variance because of the varying conditions of the scenario TABLE 3 shows similar results but for the GPS only case Q WKLV FDVH WKH UHFHLYHU ZDV FRQ JXUHG WR WUDFN RQO 36 VDWHOOLWHV 7KH PHDQ 77 LQFUHDVHV ERWK LQ WKH KRW DQG in the warm case whereas in cold start it is not possible FRPSXWH D ZLWK RQO WKUHH VDWHOOLWHV WKH DPELJXLW of the solution cannot be solved if an approximate position solution is not available It may seem unfair to compare a scenario with three satellites and one with six satellites However it can be assumed that this is representative of what happens in limited visibility conditions where a second constellation theoretically min Max Mean HOT start 47 380 118 WARM start 316 1097 519 COLD start N A N A N A 4 SVs required for cold start TABLE 3 TTFF in seconds exploiting only GPS constellations in harsh environments GPS World April 2014 www gpsworld com 40