Challenges and Solutions for GPS Receiver Test Presenter: Mirin Lew January 28, 2010 Agenda GPS technology concepts GPS and GNSS overview Assisted GPS (A-GPS) Basic tests required for GPS receiver verification Test solutions Signal creation for GPS receiver test A-GPS test systems GNSS = Global Navigation Satellite Systems
GPS and GNSS Overview GPS: Global Positioning System System owned and operated by the U.S. government Civilian service freely available to users worldwide Military service available to selected agencies only GNSS: Global Navigation Satellite System General term for any satellite-based navigation system Includes multiple systems worldwide Global Navigation Satellite Systems (GNSS) Galileo Joint effort of European Community and European Space Agency 2 test satellites in orbit, contracts awarded for first 14 satellites, up to 32 satellites operational by 2014 Interoperability agreement signed with GPS 4 services (open service, paid commercial service, safety of life service, public regulated service) as compared to 2 GPS services (public and private) Global Orbiting Navigation Satellite System (GLONASS) Russian system first launched by Soviet Union in 1982 Became non-functional for most applications in the 1990 s Currently being restored, 22 satellites in orbit as of Dec. 2009 Particularly good coverage over upper latitudes (Northern Europe) Compass (Beidou-2) Chinese system 3 satellites are up, 12 satellites by 2012 to provide regional service Eventually 30 satellites
GNSS Related Systems Satellite Based Augmentation Systems (SBAS) Geostationary satellites transmit correction and integrity data for GNSS system over the GNSS frequency. Provides increased positioning accuracy. North America: Wide Area Augmentation System (WAAS) India: GPS and GEO Augmented Navigation (GAGAN) India: Indian Regional Navigation Satellite System (INRSS) - 2012 Europe: European Geostationary Overlay Service (EGNOS) Japan: Multifunctional Satellite Based Augmentation Satellite System (MSAS) Japan: Quazi Zenith Satellite System (QZSS) - 2013 Regional Navigation Satellite Systems (RNSS) Intended for improved coverage over limited areas GPS Technology Overview Constellation of 24 active satellites in orbit (up to 32 satellites total) Each satellite transmits its current location and time Each satellite transmission is synchronized to the rest by atomic clock Minimum of 4 satellites required for 3D location calculation Major segments of the system Space: Satellites or Space Vehicles (SV) orbiting the Earth twice a day at 20,200 km Control: Ground stations provide navigation information update and SV control User: GPS receiver
How Does GPS Work in the Real World? Space Segment Uplink data: Satellite orbital information Position constants Clock correction factors Atmospheric data Monitor Stations GPS Data Master Control Station User Control segment GPS Transmitted Signal Satellite GPS signal has 3 components: Carrier wave: 1575.42 MHz (L1) & 1227.60 MHz (L2) Ranging (pseudo-random) codes: Coarse acquisition (C/A) code and precise (P) code Navigation message: 50 bit/s contains ephemeris data (detailed orbital information for the transmitting satellite) and almanac data (more general orbital information for all satellites) L1 Carrier 1575.42 MHz C/A Code 1.023 MHz Navigation Data 50 Hz + 90 Σ + L1 Signal (Civilian use) modulo 2 adder P Code 10.23 MHz + modulator L2 Carrier 1227.6 MHz L2 Signal (Military use)
Navigation Message Navigation message 25 pages/frames 37,500 bits 12.5 minutes 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Sub-frame 2 300 bits Sub-frame 1 6 seconds Sub-frame 3 Sub-frame 4 Sub-frame 5 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 Frame (page) 1500 bits 30 seconds Satellite health and clock correction data Ephemeris Ephemeris Partial almanac & other data Telemetry word (TLM) 30 bits 0.6 seconds 8 bits preamble 16 bits reserved 6 bits parity Handover word (HOW) 30 bits 0.6 seconds 17 bits Time of week TOW 7 bits ID 6 bits parity Worst Case: 30 seconds to receive full ephemeris data 12.5 minutes to receive full almanac data and Ephemeris Files File Contains data on the health and general orbital information for every satellite in the constellation. Updated weekly. http://www.navcen.uscg.gov/gps/almanacs.htm Ephemeris File Contains detailed information on the orbit of an individual satellite. Updated every 2 hours. http://cddis.gsfc.nasa.gov/gnss_datasum.html#brdc Data from the almanac can be used to create a scenario file that contains the satellite information for a specific date, time, and location. Ephemeris data files can be used to create a GPS signal that more accurately represents the actual signals broadcast at that date, time, and location.
Agenda GPS technology concepts GPS and GNSS overview Assisted GPS (A-GPS) Basic tests required for GPS receiver verification Test solutions Signal creation for GPS receiver test A-GPS test systems GNSS = Global Navigation Satellite Systems Assisted GPS (A-GPS) Technique for cellular network to assist mobile phone s GPS receiver to lock to satellites and achieve location fix more quickly Fulfills U.S. FCC s E911 directive which mandated fast and accurate location of mobile phones by emergency services Needed due to low GPS signal levels that may be seen by mobile phones when indoors or in areas without direct view of sufficient satellites Allows mobile phone s GPS receiver to acquire location fix much more quickly Base station provides assistance data to mobile phones. Data includes: Navigation: precise satellite orbital information : coarse orbital information Time of Week: GPS time Ionosphere: single frequency (L1) correction factors Reference location: initial estimate of location Acquisition assistance: data to aid in locating or tracking satellites Real-time integrity: list of bad satellites UTC model: leap second time correction for GPS time
How Does A-GPS Work in the Real World? Space Segment Uplink data: Satellite ephemeris Position constants Clock correction factors Atmospheric data Monitor Stations GPS Assistance Server Master Control Station Network Downlink: Coarse Time Ephemeris Data Coarse Location (100m accuracy) Control segment Cellular Network User A-GPS Operation Assistance Data Transportation Control plane: Uses dedicated messaging on network control channels User plane: Uses existing standard Internet protocol (IP) based data connections; also called Secure User Plane Location (SUPL) A-GPS Modes Mobile station/user equipment (MS/UE) Assisted (older method) MS/UE supplies GPS measurements to network Network combines with assistance server data, calculates and transmits location back to mobile Typically used with control plane MS/UE Based (newer method) MS/UE uses assistance data to calculate location Transmits location back to BS Used with user plane (less network dependent)