GPS World, April 2014

Receiver Design GALILEO C N 0 for hot warm and cold start and for different constellation combinations exploiting hardwaresimulated GNSS data Good results are achieved especially when introducing Galileo signals FIGURE 4 reports the hot start TTFF for different C N 0 values in the range 25 53 dB Hz computed using the receiver The receiver connected to a signal generator LV FRQ JXUHG LQ GXDO FRQVWHOODWLRQ PRGH 36 DQG Galileo and carries out 40 TTFF trials with a random delay between 15 and 45 seconds In a standard additive ZKLWH DXVVLDQ QRLVH 1 FKDQQHO DQG LQ KRW VWDUW conditions the results mainly depend on the acquisition strategy and on the receiver availability of correlators and acquisition engines In an ideal case with open sky conditions and variable C N 0 the introduction of a second constellation only slightly improves the TTFF performance this result cannot be generalized since it mainly depends on the acquisition threshold of the receiver which can change using signals of different constellations In real world conditions the situation can vary Cold Start Secondly we analyze TTFF differences due WR WKH GLIIHUHQW VWUXFWXUH RI 36 DQG DOLOHR QDYLJDWLRQ PHVVDJHV 7KH 1 9 PHVVDJH RI WKH DOLOHR VLJQDO DQG WKH GDWD EURDGFDVW E 36 VLJQDOV FRQWDLQ data related to satellite clock ephemeris and GNSS time SDUDPHWHUV UHOHYDQW WR WKH SRVLWLRQ VLQFH WKH GHVFULEH the position of the satellite in its orbit its clock error and the transmission time of the received message TABLE 1 shows some results in the particular case of FROG VWDUW ZLWK DQ LGHDO RSHQ VN 1 VFHQDULR 7KH 77 LV VLJQL FDQWO ORZHU ZKHQ XVLQJ DOLOHR VDWHOOLWHV ZKLOH WKH PHDQ 77 ZKHQ WUDFNLQJ RQO 36 VDWHOOLWHV LV HTXDO WR DERXW VHFRQGV V LW GHFUHDVHV WR V when considering only Galileo satellites and to 225 s in the case of dual constellation Similarly the minimum and maximum TTFF values are lower when tracking Galileo VDWHOOLWHV 7KH SHUFHQW SUREDELOLW YDOXHV FRQ UP WKH WKHRUHWLFDO H SHFWDWLRQV JDLQ LQ WKH LGHDO FDVH ZLWK open sky conditions the results with two constellations are quite similar to the performance of the signal with faster TTFF However in non ideal conditions use of multiple constellations represents a big advantage and underlines the importance of developing at least dualconstellation mass market receivers Furthermore it is interesting to analyze in more detail WKH FDVH RI D 36 DQG DOLOHR MRLQW VROXWLRQ 36 DQG Galileo system times are not synchronized but differ E D VPDOO TXDQWLW GHQRWHG DV WKH 36 DOLOHR 7LPH 2IIVHW 72 KHQ FRPSXWLQJ D 397 VROXWLRQ ZLWK mixed signals three solutions are possible to estimate LW DV D IWK XQNQRZQ WR UHDG LW IURP WKH QDYLJDWLRQ PHVVDJH RU WR XVH SUH FRPSXWHG YDOXH Q WKH UVW FDVH it is not necessary to rely on the information contained min Max Mean 95 GPS 222 401 319 362 Galileo 186 366 247 323 GPS Galileo 196 354 225 319 TABLE 1 Comparison between TTFF in seconds in cold start for different constellation combinations in the navigation message eventually reducing the 77 RZHYHU YH VDWHOOLWHV DUH UHTXLUHG WR VROYH WKH YH XQNQRZQV DQG WKLV LV QRW DOZD V WKH FDVH LQ XUEDQ scenarios or harsh environments as will be proved below On the contrary in the second case it is necessary to obtain the GGTO information from the navigation message and since it appears only once every 30 seconds in the worst case it is necessary to correctly demodulate VHFRQGV RI GDWD RWK DSSURDFKHV VKRZ EHQH WV DQG disadvantages depending on the environment The receiver under test exploits the second solution in this case it is possible to see an increase in the average TTFF ZKHQ XVLQJ D FRPELQDWLRQ RI 36 DQG DOLOHR GXH WR the demodulation of more sub frames of the broadcast message www gpsworld com April 2014 GPS World 39