Slide 1BackgroundBackground: GPSGlobal Positioning System (GPS): How it worksGlobal Positioning System (GPS): How it works cont’d.Global Positioning System (GPS): How it works cont’d.Global Positioning System (GPS): Sources of ErrorGlobal Positioning System (GPS): Error Correction: DGPSGlobal Positioning System (GPS): Error Correction: WAASGlobal Positioning System (GPS): ApplicationsGlobal Positioning System (GPS): NMEAGlobal Positioning System (GPS): NMEA Cont’d.Global Positioning System (GPS): NMEA Cont’d.Inertial Navigation SystemPhysics of Accelerometers/GyroscopesMechanical AccelerometersCapacitive AccelerometersPiezoelectric AccelerometersPhysics of Accelerometers/GyroscopesMechanical GyroscopesMicro Electromechanical GyroscopesOptical GyroscopesInertial Navigation SystemNavigation EquationsNavigation EquationsNavigation EquationsIntegration of GPS and INSGPS/INS IntegrationQuestions?Navigation Systems and Their ImplementationMichael BekkalaMichael BlairMichael CarpenterMatthew GuibordAbhinav ParvataneniDr. Shanker BalasubramaniamBackgroundAccessibilityPopularity of GPS and INS•Cell phonesApple iPhone, Blackberry, Android platform•Nintendo WiiWii Remote, MotionPlusBackground: GPSFirst put into practical use in the 90’s.) More commonly used in the 21st centuryGPS is for navigation, syncing computer networks time, missile guidance Some applications that make use of GPS are Garmin Car Navigation Systems, Google maps, mobile apps GPS satellites are maintained by the Air force and can be used by anybodyGlobal Positioning System (GPS): How it worksAt least 24 operational GPS satellites in orbit •12 hour orbit•11,000 miles above earth•Atomic clockMost accurate time and frequency standards known•Synchronized, send signals at same timehttp://en.wikipedia.org/wiki/GpsGlobal Positioning System (GPS): How it works cont’d.Satellites send data to earth which are picked up by a receiverSignals arrive at different times based on the distance from the satellite•L1 (1575.42 MHz)Receiver needs to determine distance to four satellites•Determines 3-dimensional position•Does not send out a signalBut how does the receiver determine its distance from each satellite?Global Positioning System (GPS): How it works cont’d.To calculate distance:•Distance = Speed • Time Speed ≈ Speed of LightHow to determine time?•Receiver’s clock becomes synchronized to Coordinated Universal Time (UTC) by tracking four or more satellites•Each satellite transmits a unique “pseudo random” code at extremely precise time intervals•Receiver knows each satellite’s pseudo random code and when they are sent•Receiver determines the time delay it takes to match the expected satellite pseudo random code with the received pseudo random code Time Delay = Time!Global Positioning System (GPS): Sources of ErrorAtmospheric Error•Speed of light is only a constant in a vacuumCharged Particles in the IonosphereWater Molecules in the TroposphereEphemeris Error•Error that effects the satellite’s orbit (ephemeris) •Caused by the gravitational pull of the sun, moon, and the pressure caused by solar radiation•Error monitored by the Department of Defense (DoD) and broadcasted to the GPS satellitesMultipath Error•Timing error from signals bouncing off of objects such as buildings or mountains•Can be reduced by signal rejection techniquesHow can we reduce errors caused by the atmosphere?Global Positioning System (GPS): Error Correction: DGPSDGPS = Differential GPSBasic Idea:•Use known locations as reference locationsExact Position is known, compare to the location determined by GPSDevelop error correction data by using the difference of the exact location and the GPS determined location•Broadcast error correction data to local GPS receivers (receivers within 200km of the reference station)•Error correction can remove errors caused by the atmosphere—makes GPS data more accurate!Global Positioning System (GPS): Error Correction: WAASWide Area Augmentation System (WAAS)•WAAS is an example of DGPS •Also referred to as a Satellite Based Augmentation System (SBAS)•Developed by the Federal Aviation Administration (FAA)•Uses a network of ground based stations in North America and Hawaii•Measures variations in satellite signalsRelays error to geostationary WAAS satellitesUsed to improve accuracy and integrity of data•Independent systems being developed in Europe (Galileo), Asia, and India.Global Positioning System (GPS):ApplicationsAerospaceAutomotiveMilitaryCivilian•Recreation•Augmented RealityThe list goes onGlobal Positioning System (GPS):NMEANational Marine Electronics Association 0183 (NMEA)•A standard which defines communication between marine electronic devices•Uses ASCII serial communicationCan be read by the microcontroller over UART and parsed appropriately•Defines message contenthttp://www.gpsinformation.org/dale/nmea.htmGlobal Positioning System (GPS):NMEA Cont’d.Requirements•Contain complete position, velocity, and time (PVT) data•Independent of other messages•Begin with a ‘$’, end with a ‘\n’•Content separated by commas •No longer than 80 charactershttp://www.gpsinformation.org/dale/nmea.htmGlobal Positioning System (GPS):NMEA Cont’d.$GPGGA,123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46.9,M,,*47GGA - essential fix data which provide 3D location and accuracy data•GGA Global Positioning System Fix Data•123519 Fix taken at 12:35:19 UTC•4807.038,N Latitude 48 deg 07.038' N•01131.000,E Longitude 11 deg 31.000' E•1 Fix quality: GPS fix (SPS)•08 Number of satellites being tracked•0.9 Horizontal dilution of position•545.4,M Altitude, Meters, above mean sea level•46.9,M Height of geoid (mean sea level) above WGS84 ellipsoid•(empty field) Time in seconds since last DGPS update•(empty field) DGPS station ID number•*47 Checksum data, always begins with *http://www.gpsinformation.org/dale/nmea.htmInertial Navigation SystemThe use of inertial measurements in navigationMeasurements come from inertial sensors such as:•Accelerometers•GyroscopesVery accurate over short termErrors integrate with timePhysics of Accelerometers/GyroscopesAccelerometers•Measure acceleration in x, y, z directions•Types:MechanicalMicro Electromechanical
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