12.215 Modern NavigationReview of last classToday’s Class“Codeless” tracking“Codeless” tracking“Reduced” code trackingData messageReceiver start upReceiver start upSome guidelines for Receiver UseBasics on GPS OrbitsBasic ConstellationSatellite CoverageSpecifics of GPS signalSpecifics of GPSSpecifics of GPSBasic measurement typesBasic measurement typesBasic measurement typesPseudorange noiseToday’s class summary12.215 Modern NavigationThomas Herring11/20/2006 12.215 Modern Naviation L17 2Review of last class• Fundamentals of GPS• Method of encoding GPS signals (bi-phase, quadrature modulation)• Fundamentals of correlation methods used (Matlabcode)• Specifics of the GPS system– Frequencies– Chip rates – Data rates and message content11/20/2006 12.215 Modern Naviation L17 3Today’s Class• GPS measurements– Tracking methods used in GPS (“codeless”tracking)– Basic geometry of orbits (discuss more later)– Specific details of the GPS signal structure– Phase measurements (allow millimeter level position with GPS and cm in realtime)– Examine some GPS data.11/20/2006 12.215 Modern Naviation L17 4“Codeless” tracking• For security reasons (call anti-spoofing which stops GPS military receivers being faked out by false transmissions), the PRN code on the second GPS frequency (L2) is not fully known.• On the L1 GPS signal both C/A and P(Y)-codes are written, but at L2 on P(Y) code is written. How to track L2 since without code, signal will average to zero?• Three basic methods:– Codeless tracking: First method. Square the L2 signal. This removes the π phase changes but halves the wavelength. Used until the early 1990’s. Not used anymore. Requires Doppler shift to separate satellites and can have problems when Doppler shift is the same for two satellites11/20/2006 12.215 Modern Naviation L17 5“Codeless” tracking• Second method (still in use):– Cross-correlation of L1 (C/A removed) with L2. Since the C/A is known, it can be digitally removed from L1 signal leaving just the P(Y) code plus noise behind. Since the P(Y) code also is present in the L2 signal, cross correlation allows the time and phase differences to be determined. For L2 tracking, to correct ionospheric delay (discussed in future classes), thisdifference is all that is needed.– Since GPS signal is below noise level (spread spectrum), this is noisy method (but not as bad as squaring since L1 signal is stronger than L2)– Averaging times to get signal can be as long as 10 seconds (compared to 10-100 ms for L1)11/20/2006 12.215 Modern Naviation L17 6“Reduced” code tracking• The third class of methods: Reduced code tracking:– Several patented (actually codeless methods are patented) are used here and go under names like Z-tracking (Ashtech).– These all assume the P-code is known and that that the Y-code is a slowly modulated version of P-code (~20 KHz rates)– Basically pieces of the L2 signal are correlated using the P-code, but at the rate of Y-code additional modulation (called W-code), incoherent averages are formed because the code may have changed sign.– Because of low W-code rate, these tracking loops can be quite sensitive (averaging intervals of 100 ms)– Latest generation of receivers use these methods.11/20/2006 12.215 Modern Naviation L17 7Data message• GPS satellites need to transmit information about their location and clock errors. This done in a messages made of up “frames” and transmitted at 50 bits/second.• The 5 frames take 6 seconds each to broadcast so 30 seconds to get complete message (broadcast ephemeris for satellite plus almanac for one other satellite)• Complete almanac takes 12.5 minutes to receive.• Once one satellite is locked, the almanac is used to search for other satellites11/20/2006 12.215 Modern Naviation L17 8Receiver start up• The length of time a GPS receiver takes before it outputs good data and positions depends on starting condition– Hot start: Only recently turned off and back on again. Since all the satellite ephemerides are good and do not need to be re-read, receiver clock and position known, start can be a few seconds.– Warm start: Turned for a day or so. Clock probably OK, if position still OK (receiver can tell be how long it takes to acquire satellites), generally start-up takes a few minutes– Cold start: Turned off for some time: Can take tens of minutes to half an hour to start. May go into auto-locate mode where it assumes nothing about position11/20/2006 12.215 Modern Naviation L17 9Receiver start up• General procedure is to:– Acquire one satellite to get time and almanacs– Acquire two more satellites to get 2-D position (assumes height is last value or value given by receiver)– Acquire 4th satellite to get 3-D position (talk about how this is done in later classes)– Acquire any other satellites that should be visible• Most receivers use a type Kalman filter and so position and velocity states are estimated11/20/2006 12.215 Modern Naviation L17 10Some guidelines for Receiver Use• If you are going to use a GPS receiver in poor viewing conditions (e.g. downtown area or heavy foliation), best to lock on satellites before entering area –tracking will be much more complete in bad conditions• Some receivers make selection of satellites to get first position based on first seen ones. If these satellites become obstructed before full positioning is started, they will continue to wait for these satellites and not operate until they are reacquired. In these cases, best to turn receiver off and start again.11/20/2006 12.215 Modern Naviation L17 11Basics on GPS Orbits• Orbit characteristics are– Semimajor axis 26400 km (12 sidereal hour period)– Inclination 55.5 degrees– Eccentricity near 0 (largest 0.02)– 6 orbital planes with 4-5 satellites per plan• Design lifetime is 6 years, average lifetime 10 years• Generations: Block II/IIA 9729 kg, Block IIR 11000 kg11/20/2006 12.215 Modern Naviation L17 12Basic ConstellationOrbits shown in inertial space and size relative to Earth is correct11/20/2006 12.215 Modern Naviation L17 13Satellite Coverage• The GPS satellites point their transmission antenna to the center of the Earth.• Main beam at L1 is 21.3ohalf width, at L2 23.4o(due to lower frequency)13.9o21.3o L1, 23.4o L2EarthSatelliteBeam 1/2 power pointsL1 Beam extends to 4000 km above surface~4000 km11/20/2006 12.215 Modern Naviation L17