GPS World, December 2009

WIRELESS Receiver Design Å FIGURE 5 Broadcast ephemeris and long term orbits er for all phones once GPS evolved far enough thanks to the seven technology enablers A GPS faster longer higher Massive parallel correlation longer higher with coarse time High sensitivity cheap antennas Coarse time navigation fast TTFF without periodic wakeup Low TOW decode from weak signals Host based GPS together with RF CMOS single die Meanwhile as all this developed several important spin off technologies evolved to take this technology beyond the mobile phone The most significant of all of these was long term orbits LTO conceived on May 2 2000 and now an industry standard Long Term Orbits Why May 2 2000 Remember what happened on May 1 2000 the U S government turned off selective availability SA on all GPS satellites Suddenly it became much easier to predict future satellite orbits and clocks from the observations made by a civilian GPS network At Global Locate we had just such a network for doing A GPS as illustrated in FIGURE 5 On May 2 we said SA is off wow What does that mean for us And thats where LTO for A GPS came from Figure 5 shows the A GPS environment with and without LTO The left half shows the situation with broadcast ephemeris only An A GPS reference station observes the broadcast ephemeris and provides it or derived data to the mobile A GPS receiver in your mobile phone The satellite has the orbits for many hours into the future the problem is that you cant get them The blue and yellow blocks in the diagram represent how the ephemeris is stored and transmitted by the GPS satellite The current ephemeris yellow is transmitted the future ephemeris blue is stored in the satellite memory until it becomes current So frustratingly even though the future ephemeris exists you cannot ordinarily get it from the GPS system itself The right half of the figure shows the situation with LTO If a network of reference stations observes all the satellites all the time then a server can compute the future orbits and provide future ephemeris to any A GPS receiver Using the same color scheme as before we show here that there are no unavailable future orbits as soon as they are computed they can be provided And if the mobile device has a fast enough CPU it can compute future orbits itself at least for the subset of satellites it has tracked Beyond Phones This idea of LTO has moved A GPS from the mobile phone into almost any GPS device Two of most interesting examples are personal navigation devices PNDs in cars and smartphones themselves that continue to be useful gadgets once they roam away from the network Now of course people were predicting orbits before 2000 all the way back to Newton and Kepler in fact Its just that in the year 2000 accurate future GPS orbits werent available to mobile receivers At that time the International GNSS Service IGS had as it does now a global network of reference stations and provided precise GPS orbits organized into groups called Final Rapid and Ultra Rapid The Ultra Rapid orbit had the least latency of the three but in 2000 Ultra Rapid meant the recent past not the future So for LTO we see that the last 10 years have taken us from a situation of nothing available to the mobile device to today where these long term orbits have become codified in the 3rd Generation Partnership Project 3GPP and Secure User Plane Location SUPL wireless standards where they are known as ephemeris extension Imagine GPS is now reaching 100 percent penetration in smartphones and has a strong and growing presence in feature phones as well GPS is now in more than 300 million mobile phones at the very least credible estimates range above 500 million Now imagine every receiver ever made since GPS was created 30 years ago military and civilian smart bomb boat plane hiking survey precision farming GIS Bluetooth puck personal digital assistant and PND In the last three years we have put more GPS chips into mobile phones than the cumulative number of all other GPS receivers that have been built ever FRANK VAN DIGGELEN has worked on GPS GLONASS and A GPS for Navsys Ashtech Magellan Global Locate and now as a senior technical director and chief navigation officer of Broadcom Corporation He has a Ph D in electrical engineering from Cambridge University holds more than 45 issued U S patents on A GPS and is the author of the textbook A GPS Assisted GPS GNSS and SBAS GPS World December 2009 www gpsworld com 40