Introduction to Global Navigation Satellite Systems - Notes | ASEN 5090, Study notes of Aerospace Engineering

Download Introduction to Global Navigation Satellite Systems - Notes | ASEN 5090 and more Study notes Aerospace Engineering in PDF only on Docsity! GPS 101/Introduction to GPS Jargon ASEN 5090 Lecture 3 2 Jargon review  PRN (pseudorandom noise) tells you which code is being generated by a certain satellite. The codes get reused, so PRN15 in 2008 will be a different spacecraft than it was in 1989.  SVN (satellite vehicle number) tells you exactly which spacecraft it is. This number is never reused, as you need it to know the design specifications of the satellite.  Constellation status is maintained by the Coast Guard website, Notice Advisory to Navstat Users (NANU)  Why do they tell you whether a satellite is running a Rb or Cs clock? Because the noise properties of these clocks are pretty well known, so an advanced user could estimate the likely clock error for each satellite. A normal user won’t care. P Space segment 4 ‘isa Downlink data * Coded ranging signals Ca etieamuleniteltelt Uplink data Oe iestsiiilatsleliel * Satellite ephemeris position constants ¢ Almanac ¢ Clock-correction factors ee wy / iil) oe 6 Block II/IIA Block IIF • 24-satellite (nominal) constellation • Six orbital planes inclined at 55 deg, four satellites per plane • Not geosynchronous. • Semi-synchronous, nearly circular orbits (20,200 km altitude) • Redundant cesium and/or rubidium clocks on board each satellite • Antenna array pointed at the earth GPS Space Segment 7  Age Summary  All satellites have greatly exceeded original design lifetime  Currently 32 satellites on-orbit GPS Constellation Status Block II/IIA Built by Boeing Aerospace (Rockwell) Launched 1989 - 1997 Block IIR/IIR-M Built by Lockheed Martin Launched 1997 - 2007 IIR-M have C/A on L2 Block IIF Built by Boeing Launching 2009 ? L5 signal Block I 11 satellites (one launch failure) Built by Rockwell Launched 1978 - 1985 Block IIIA Built by Lockheed Martin Contract announced 5/2008 10 SPACE VEHICLE Broadcasts the SIS PRN codes, L-band carriers, and 50 Hz navigation message stored in memory SPACE-TO-USER INTERFACE CONTROL-SPACE INTERFACE MONITOR STATION  Sends raw observations to MCS MASTER CONTROL STATION  Checks for anomalies  Computes SIS portion of URE  Generates new orbit and clock predictions  Builds new upload and sends to GA GROUND ANTENNA  Sends new upload to SV Control Segment MCS = Schriever (previously Falcon) AFB in Colorado Springs 11 Critical Role of the Control Segment  Takes GPS ranging data. Assuming positions of tracking stations are known, computes orbital parameters.  In same process, estimates the bias and drift of each satellite’s clock relative to GPS time.  Uploads new orbits and clocks so that they can be broadcast on the GPS signal.  GPS orbits can be computed very simply. There are two forms used in this class.  The GPS almanac (1 km) is NOT for positioning. It is primarily used by the receiver to aid tracking (I.e. tells it which satellites should be visible).  The GPS broadcast ephemeris (1 meter) is for low-accuracy, real-time positioning.  The GPS precise ephemeris (2 cm) is not broadcast by the DoD. It is generated by geodesists, and will not be used in this class. 12 Orbits: Almanac  Almanac - YUMA format ******** Week 419 almanac for PRN-01 ******** ID: 01 Health: 000 Eccentricity: 0.6839275360E-002 Time of Applicability(s): 405504.0000 Orbital Inclination(rad): 0.9909240253 Rate of Right Ascen(r/s): -0.7794610391E-008 SQRT(A) (m 1/2): 5153.359863 Right Ascen at Week(rad): 0.4613413514E+000 Argument of Perigee(rad): -1.806921136 Mean Anom(rad): -0.1432259810E+001 Af0(s): 0.1506805420E-003 Af1(s/s): 0.3637978807E-011 week: 419 ******** Week 419 almanac for PRN-32 ******** ID: 32 Health: 063 0.99 radians = 56.72 deg Contrast with: 15 Control Segment – Monitor Stations  Original U.S. Air Force GPS Monitor Stations  Hawaii, Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs  New monitoring stations incorporated 2005  Cape Canaveral (USAF)  Washington, DC, UK, Argentina, Ecuador, Bahrain, and Australia (NGA) 16 GPS Signals  Codes  C/A Code – 1.023 MHz Clear Acquisition code, repeats @ 1 ms  P(Y) Code – 10.23 MHz, Encrypted  M Code – New Military Split spectrum, not relevant to this class.  L2C, L5C – New Civil codes  Carriers  L1 1575.42 MHz – C/A and P(Y)  L2 1227.6 MHz – P(Y) only, Block I, II, IIA, IIR  L5 1176.45 MHz – none currently.  Effect of encryption policy  Civilians only have single frequency (L1)  Military has both frequencies (L1 and L2).  The ionosphere is dispersive, and failure to correct for its effect can producing ranging errors up to 100 meters (and thus equivalent positioning errors).  Degradation (mostly of interest for historical reasons).  SA - selective availability  AS - anti-spoofing  Navigation Data  50 bps  Satellite ephemeredes and almanacs  Satellite clock parameters  Ionospheric model for single frequency users  Health and status 17 GPS User Segment  GPS receivers are specialized “radios” that track GPS signals and produce position and velocity solutions  Wide range of cost/sophistication depending on the application  Signals from 4 or more GPS satellites are required, but 8-10 are typically available at any time  Many civil (SPS) receivers track only the L1 C/A signal  Precision civil users track both L1 and L2, without the P(Y) codes.  PPS receivers have special keys that allow tracking of the military P(Y) code over both L1 and L2 courtesy Garmin courtesy General Dynamics Consumer Recreation (~$100) Military Spacecraft (~$1,000,000) Surveying/Science ($10,000) courtesy Trimble 20 Fundamentals of Satellite Navigation  Satellite based navigation is fundamentally based on:  The precise measurement of time (you have to agree on what you mean by time!)  The constancy of the speed of light  GPS and other systems use the concept of trilateration:  Satellite (transmitter) positions are known  Receiver position is unknown  Satellite-to-receiver range measurements are used to estimate position 21  The position solution involves an equation with four unknowns:  Receiver position (x, y, z) (in what reference frame)  Receiver clock correction (correction to what?)  Position accuracy of ~1 m implies knowledge of the receiver clock to within ~3 ns  GPS accuracy is based almost entirely on knowing satellite orbits and satellite clocks.  Requires simultaneous measurements from at least four satellites  The receiver makes a range measurement to the satellite by measuring the signal propagation delay  A data message modulated on the ranging signal provides the precise location of the satellite and corrections for the satellite clock. GPS accuracy is based almost entirely on knowing these two things. Position Solution 22 Since GPS accuracy is based almost entirely on knowing satellite orbits and satellite clocks, you have two choices: Provide inaccurate information about where the satellites are. Provide inaccurate information about the behavior of the clock. In GPS speak, this is called selective availability (SA). How Do You Degrade the Position Solution 25 Why did many people not care about SA? Longitude time series when SA was on. Most of this was clock “dither.” Longitude 100 T T T r : meters 0 10 20 30 40 50 60 time — minutes Longitude records for two receivers fairly close together. 26 27 Which means the longitude “difference” is much better known. You can get position for your site if you KNOW the position of the Other sites. This is the general principle used in “differential positioning” or “using a base station.” 30 Measurement Equation ! "r s = c tr # t s$ % & ' ( )  GPS receiver measures “Pseudorange” by measuring the transit time of the signal: time of transmission, encoded in signal by GPS satellite clock (better known) time of signal reception, (based on receiver clock, can be significantly in error) Again: you need to agree about what you mean by time. 31 Measurement Equation (cont)  true range  receiver clock error  satellite clock error (known)  ionosphere and troposphere delays (estimated or measured)  other errors (satellite ephemeris and clock mis-modeling, measurement errors, multipath, receiver noise)  Measured pseudorange to a satellite is comprised of: ! Rr s = "r s # c$s + c$r + "trop + "iono + "multi + "rel + % ! " r s (t r ) = r X s (t s )# r X r (t r ) 32 Solution Accuracy  Two primary factors affect the fundamental position and time accuracy possible from the system:  Ranging error – a function of the quality of the broadcast signal and data  Geometry – the distribution of satellites in the sky  The actual positioning accuracy achieved depends on many other factors:  The design of the receiver (receiver/antenna noise levels, modeling errors, etc.)  Environmental effects such as ionosphere and troposphere signal delays, field of view obstructions, multipath signals, and jamming/interference. Visibility *« LOS is a unit vector in the direction from the user location to the satellite (at time of transmission) (x* —X) ak a « How do you decide if a satellite is visible? x x — Upis the direction opposite the radius vector (approximately) or in ECEF: cos¢cos A. | [ve Jecee =| COSPsinA sing || — Elevation is n ; el = acos(E,p * Exo) — Azimuth is az = atan(LOS, /LOS,) 35  How we plot them Az/El 36  ir WY Gwe \\ Ss g Afi FEN Why | i YA \ 40 But at any given time, the constellation looks very different 41 Geometry – Dilution of Precision  Geometric Dilution of Precision (GDOP) is a quantitative measure of the quality of the receiver-to-GPS satellite range geometry  Related DOPs exist for position, horizontal, vertical, and time dilutions of precision  Used in conjunction with the URE (user ranging error) to forecast navigation and timing performance, weight measurements  For GPS, DOP can range from 1 to infinity, with values in the 2-3 range being typical Good GDOP Poor GDOP 42 The DOPs  We’ll skip the math for now.  HDOP - horizontal (σe + σn)  GDOP - geometric (XYZ)  VDOP - just vertical 45 Combining GPS and Galileo to increase # satellites, improve DOPs 24 satellites? 46  User Range Error (URE)  A measure of the accuracy of the pseudorange along the line- of-sight direction from a particular satellite to the user  Indication of signal quality  Composite of several factors  stability of particular satellite’s clock  predictability of the satellite’s orbit Ranging Error