chapter 2: determining position all methods rely on intersections of lines or surfaces on which the...
TRANSCRIPT
Chapter 2: Determining position
All methods rely on intersections of lines or surfaces on which the sought position has to be.
1) Angle (bearing) to 2 objects with know position, gives 2 straight lines of possible locations
LANDN
θ1
θ2
θ1
θ2
Use compass, radio direction finder, or radar with angle reading.
Radio positioning either from ship to 2 beacons, of from 2 land stations to ship (emergency location)
LANDN
θ2
θ2
Use compass, radio direction finder, or radar with angle reading for direction.Use radar or height of objects for distance
2) Angle (bearing) and distance to 1 object with know position, gives 1 straight line and 1 circle of possible locations
10km
3) Astronomical navigation:
Measure maximum height above horizon of sun, moon, star (meridian transit) geographic latitude
Accurate timing of this event (needs good clocks) geographic longitude
3) Hyperbola methods: Two transmitters send out synchronized signals and receivers determines theDIFFERENCE in arrival time, i.e. the DIFFERENCE in distance.
Examples, still existing in coastal regions, are the DECCA, LORAN, OMEGA systems.
DECCA LORAN OMEGA
Range 300-400nm 1000nm 6000nm
Accuracy 0.5nm 1-5nm 1-2nm
24
86
10
12
AB2
4 1
86
10
20 18 16 14
A-B=10
A-B=6
Time difference positioning (hyperbolas)
2 pairs of transmitters gives 2 hyperbolas of possible positions, intersection is sought location
• Distance to satellites is measured via traveltime. If accurate time synchronization, position is intersection of 3 spheres.
• The receiver calculates the position for each satellite from the received orbit data, and then calculates its distance (pseudorange) from each satellite based on the time delay between when the transmission was sent and when it was received.(This assumes accurate time is known)
• Once the distances from the satellites are known, the reciever’s position is determined by determining the intersection point of four imaginary spheres, centered at the satellites, with the radius of each sphere equal to the pseudorange of the satellite.
Three spheres would be enough if time was known, i.e. with 3 unknowns x,y,z (Trilateration). With 4 unknowns at receiver (x,y,z,t) need 4 distances and equations (“spheres”)
GPS: Global Positionining System (see notes for various web sites)
Satellite navigation system run by the U.S. Department of Defense.
The satellites provide coded signals that can be processed by a GPS receiver to compute position, velocity and time
The system consists of a – Space Segment– Control Segment – User Segment
GPS: Global Positionining System
Space segment
Satellites orbit the earth in 12 hours.
5-8 satellites are visible to the user from any point on earth (4 satellites are needed to compute location).
Control segment
• Monitor stations at the master control facility (Schriever Air Force Base, Colorado) incorporate satellite signals into orbital models.
• The models compute orbital data and clock corrections for the satellites and upload them back to the satellites.
• The satellites send time-coded subsets of the orbital data to the GPS receivers via radio signals.
Accuracy of GPS:
1) Standard positioning service (SPS), signal made worse by selective availability (SA): ±100m horizontal
± 150m vertical± 350ns time
2) Precise positioning service (PPS), coded with P-codeaccuracy several meters
3) Differential GPS: measure errors and coding effect at a fixed reference stationand correct actual position. In simplest case, assume nearby position errorsare the same and correct (subtract) wander/scatter. For better accuracy recordexact waveform of all signals (phases etc) and post-process with referencesignal:accuracy: meters to centimeter, in best case subcentimeter
Why are differential GPS data more accurate than handheld GPS data? • GPS satellites send out two different signals: L1 and L2. Differential GPS receivers measure both L1 and L2, whereas handheld GPS receivers measure only L1. • Differential GPS uses antennas specially constructed to reduce multipath error.
D.Chadwell uses 1 dual-frequency receiver on ship and 3 reference ones on land. Data are post-processes with free (but complicated) NASA software (GIPSY/OASIS from JPL). Dual-frequency GPS costs $10,000-20,000.
Commercial products: They can provide sub-meter to decimeter absolute positioning with a few second latency – takes GPS and a communication link to the processing center to receive the corrections. These use a commercial version of the JPL software
http://www.cnavgnss.com/about http://www.gdgps.net/
Practical aspects:
• data are output on serial line (RS232) in NMEA format
• some receivers deliver time pulse on BNC output
• systems with antenna ARRAY can also deliver orientation and inclination (attitude), e.g. ASHTEC ADU (important since better than gyrocompass, e.g. for ship ADCP)
NMEA sentencesthat can be outputby GPS receivers
Example of most common NMEA sentence (GGA)
Principal Developers: Germany, France, Italy and The United Kingdom Principal Developers: Germany, France, Italy and The United Kingdom
Intended primarily for civilian use, unlike the U.S. system, which is run by and Intended primarily for civilian use, unlike the U.S. system, which is run by and primarily for the U.S. military.primarily for the U.S. military.
GALILEO Positioning SystemGALILEO Positioning System
• 30 spacecraft 30 spacecraft
• ORBITAL ALTITUDE:ORBITAL ALTITUDE: 23222 km23222 km
• 3 orbital planes, 56° 3 orbital planes, 56° inclination (9 operational inclination (9 operational satellites and one active satellites and one active spare per orbital plane) spare per orbital plane)
• SATELLITE LIFETIME:SATELLITE LIFETIME: >12 years >12 years
www.esa.int/esaNA/galileo.html
Galileo ServicesGalileo Services Open Service (OS):Open Service (OS): The OS signals will be broadcast in two bands, at 1164–1214 MHz The OS signals will be broadcast in two bands, at 1164–1214 MHz
and at 1563–1591 MHz. Receivers will achieve an accuracy of <4 m horizontally and <8 m and at 1563–1591 MHz. Receivers will achieve an accuracy of <4 m horizontally and <8 m vertically if they use both OS bands. Receivers that use only a single band will still achieve vertically if they use both OS bands. Receivers that use only a single band will still achieve <15 m horizontally and <35 m vertically.<15 m horizontally and <35 m vertically.
Commercial Service (CS).Commercial Service (CS). Accuracy >~ 1 m. The CS can also be complemented by Accuracy >~ 1 m. The CS can also be complemented by ground stations to bring the accuracy down to less than 10 cm. This signal will be ground stations to bring the accuracy down to less than 10 cm. This signal will be broadcast in three frequency bands, the two used for the OS signals, as well as at 1260–broadcast in three frequency bands, the two used for the OS signals, as well as at 1260–1300 MHz.1300 MHz.
Safety of Life (SoL) & Public Regulated Service (PRS). Safety of Life (SoL) & Public Regulated Service (PRS). Accuracy comparable to the Accuracy comparable to the Open Service. Their main aim is robustness against jamming and the reliable detection of Open Service. Their main aim is robustness against jamming and the reliable detection of problems within 10 seconds. They will be targeted at security authorities (police, military, problems within 10 seconds. They will be targeted at security authorities (police, military, etc.) and safety-critical transport applications (air-traffic control, automated aircraft landing, etc.) and safety-critical transport applications (air-traffic control, automated aircraft landing, etc.), respectively.etc.), respectively.
Search & Rescue (SAR). Search & Rescue (SAR). Detect and report signals from COSPAS-SARSAT search-and-Detect and report signals from COSPAS-SARSAT search-and-rescue beacons in the 406.0–406.1 MHz band, which makes them a part of the Global rescue beacons in the 406.0–406.1 MHz band, which makes them a part of the Global Maritime Distress Safety System.Maritime Distress Safety System.
http://ec.europa.eu/transport/galileo/index_en.htm
Open Service (OS)Open Service (OS)
– – Interoperable with other GNSSInteroperable with other GNSS– – Provided on 3 frequenciesProvided on 3 frequencies– – Free of chargeFree of charge– – World-wide coverageWorld-wide coverage– – Provides position and timing performances competitive with other Provides position and timing performances competitive with other GNSS systemsGNSS systems– – Service for the mass market: Possible applications: in-car navigation, Service for the mass market: Possible applications: in-car navigation, hybridization with mobiles…hybridization with mobiles…
Commercial Service (CS)Commercial Service (CS)
– – Same basic performance as open serviceSame basic performance as open service– – Added value by additional information in the datastream (i.e. Added value by additional information in the datastream (i.e. integrity, integrity, differential corrections) with guarantee of servicedifferential corrections) with guarantee of service– – Allows for a higher data rate throughput and enables users to Allows for a higher data rate throughput and enables users to improve improve accuracy, indoor navigation, integration with wireless accuracy, indoor navigation, integration with wireless communication communication networksnetworks– – Access is restricted by service provider via encryption for commercial Access is restricted by service provider via encryption for commercial
exploitation of the service by service providerexploitation of the service by service provider
Galileo ServicesGalileo Services
Galileo ServicesGalileo Services
Safety of Life (SoL)Safety of Life (SoL)
– – Same basic performance as the open serviceSame basic performance as the open service– – Additional provision of integrity information (providing timely Additional provision of integrity information (providing timely warnings warnings to the user when it fails to meet certain margins of accuracy)to the user when it fails to meet certain margins of accuracy)– – Free of direct user chargesFree of direct user charges– – Unencrypted with signal authenticationUnencrypted with signal authentication– – Envisaged that a service guarantee will be provided for this serviceEnvisaged that a service guarantee will be provided for this service– – Service for maritime, aviation, rail…Service for maritime, aviation, rail…
Public Regulated Service (PRS)Public Regulated Service (PRS)
– – Guaranteeing the continuity of public applications for European Guaranteeing the continuity of public applications for European and/or and/or national security (interference mitigation techniques: high national security (interference mitigation techniques: high continuity continuity of service, more robust signal)of service, more robust signal)– – Controlled by EU and Member State governmentsControlled by EU and Member State governments– – Wideband signalWideband signal– – Encrypted ranging codes and navigation massageEncrypted ranging codes and navigation massage– – Access restricted to users authorized by EU Member StateAccess restricted to users authorized by EU Member State
Galileo ServicesGalileo Services
Search & Rescue (SAR)
– Global broadcast of alert messages received from distress emitting beacons (reception and localization)– Enhances the performances of international
Search and Rescue systems such as COSPAS-SARSAT (average waiting time is currently one hour):• 5 GEOs: real-time reception• 4 LEOs: reception and position (with Doppler effect)
GLONASS (Russian ГЛОНАСС; ГЛОбальная GLONASS (Russian ГЛОНАСС; ГЛОбальная НАвигационная Спутниковая Система; НАвигационная Спутниковая Система;
Global'naya Navigatsionnaya Sputnikovaya Global'naya Navigatsionnaya Sputnikovaya Sistema. Global Navigation Satellite System)Sistema. Global Navigation Satellite System)
•24 spacecrafts 24 spacecrafts •ORBITAL ALTITUDE:ORBITAL ALTITUDE: 19100 km 19100 km•3 orbital planes, 64.8° inclination (7 operational satellites and one active 3 orbital planes, 64.8° inclination (7 operational satellites and one active spare per orbital plane). Location within the plane: 1-8, 9-16, 17-24. “Spares” spare per orbital plane). Location within the plane: 1-8, 9-16, 17-24. “Spares” separation 120°, satellites equally spaced within the same orbital plane, 45° separation 120°, satellites equally spaced within the same orbital plane, 45° apart. apart. •ORBIT:ORBIT: Roughly circular, approximately 11 hours, 15 minutes. The spacing of Roughly circular, approximately 11 hours, 15 minutes. The spacing of the satellites in orbits is arranged so that a minimum of 5 satellites are in view the satellites in orbits is arranged so that a minimum of 5 satellites are in view at any given time. at any given time. The satellite orbits repeat after 8 days. As each orbit plane contains 8 satellites, there is a non-identical repeat (i.e., another satellite will occupy the same place in the sky) after one sidereal day. •SIGNALS:SIGNALS: Standard Precision (SP) and high precision (HP). SP signal L1 have a Standard Precision (SP) and high precision (HP). SP signal L1 have a frequency division multiple access in L-band: L1= 1602 MHz + frequency division multiple access in L-band: L1= 1602 MHz + nn0.5625 MHz, 0.5625 MHz, where where nn is frequency channel number ( is frequency channel number (nn=0,1,2...).=0,1,2...).•HORIZONTAL POSITIONING ACCURACY :HORIZONTAL POSITIONING ACCURACY : ~57-70 meters ~57-70 meters•VERTICAL POSITIONING ACCURACY :VERTICAL POSITIONING ACCURACY : ~70 meters ~70 meters •VELOCITY VECTOR MEASURING:VELOCITY VECTOR MEASURING: ~15 cm/s ~15 cm/s•TIMING :TIMING : ~1 µs ~1 µs
The first three test satellites were placed in orbit in October 1982 with the first operational satellites The first three test satellites were placed in orbit in October 1982 with the first operational satellites entering service in December 1983. The system was intended to be operational in 1991, it was entering service in December 1983. The system was intended to be operational in 1991, it was announced to be operational on September 24, 1993 but the constellation was not completed until announced to be operational on September 24, 1993 but the constellation was not completed until December 1995.December 1995.
GLONASSGLONASSThe Russian Aerospace Agency has the approval of the Russian government to continue a long-The Russian Aerospace Agency has the approval of the Russian government to continue a long-term plan for the period 2002-2011, during which time it plans to reconstitute a GLONASS term plan for the period 2002-2011, during which time it plans to reconstitute a GLONASS constellation of 24 satellites. Russia plans to have 18 operational satellites by the end of 2007 constellation of 24 satellites. Russia plans to have 18 operational satellites by the end of 2007 and 24 operational satellites by the end of 2010. and 24 operational satellites by the end of 2010.
The GLONASS-M satellites have two civil signals and have an expected life of 7 years. In The GLONASS-M satellites have two civil signals and have an expected life of 7 years. In addition to the one currently in orbit and the one scheduled for launch on 26 December, 7 more addition to the one currently in orbit and the one scheduled for launch on 26 December, 7 more have been ordered for production. have been ordered for production.
GLONASS-K will be the future generation GLONASS satellite and will have a third civil signal. It GLONASS-K will be the future generation GLONASS satellite and will have a third civil signal. It will also weigh much less than the GLONASS-M (800 kg versus 1,400 kg). GLONASS-K will will also weigh much less than the GLONASS-M (800 kg versus 1,400 kg). GLONASS-K will transmit integrated information and will support search and rescue operations. transmit integrated information and will support search and rescue operations.
www.glonass-ianc.rsa.ru
Galileo vs. Galileo vs. GLONASS PhasesGLONASS Phases
Now by 2013
GALILEO + GPS INTEROPERABILITYGALILEO + GPS INTEROPERABILITY
The following key interoperability aspects have been identified so far:The following key interoperability aspects have been identified so far:- signal structure and frequency selection,- signal structure and frequency selection,- geodetic and time reference frames,- geodetic and time reference frames,- constellation configuration,- constellation configuration,-system policies and services guarantees.system policies and services guarantees.
The GPS-Galileo time offset The GPS-Galileo time offset (GGTO) will represent an (GGTO) will represent an important issue for GPS-Galileo important issue for GPS-Galileo interoperability, since it will cause interoperability, since it will cause a bias between measurements in a bias between measurements in combined GPS/Galileo receiverscombined GPS/Galileo receivers(Moudrak et al., 2004)(Moudrak et al., 2004)
Acoustic navigation
d1 d2
h1 h2
s1 s2
Measure slant ranges s1, s2 acoustically, know depths d1, d2 independently,then do only horizontal position problem for h1, h2 (2 intersecting circles).With 3 transponders, can measure 3 slant ranges and do 3-D problem,i.e. solve for x,y,z from 3 intersecting spheres.
Long baseline navigation
Terms:
• Transducer• Beacon/pinger• Transponder• Responder
(x1,y1)
(x,y)
(x2,y2)
(x-x1)2 + (y-y1)2 = h12
(x-x2)2 + (y-y2)2 = h22
• Typical accuracy 1m (i.e. approx. 1 ms)• Have to use harmonic mean sound speed to convert traveltime to distance
Determining transponder position:Perform acoustic survey to collect many slant ranges s i
and corresponding GPS positions xi, yi. Each position duringthe survey lies on a sphere around transponder atX,Y,Z, i.e. many equations (X-xi)2 + (Y-yi)2 + (Z-0)2 = si
2
With 3 unknowns (do this for each transponder).
Errors to correct: offset between GPS antenna and transducer, direction-dependent. Also movement of ship during traveltime needs to be corrected.
Precision transponders allow timing with microsecond accuracy. Originally developed at SIO (see MPL report by Spiess et al), mainly by adding a fixed-time delay circuit (always know to within some microseconds how much time signals spends in transponder).Measures round-trip traveltime to +/-5microseconds.Now also commercially available from Linkquest:
http://www.link-quest.com/html/intro3.htm
Additional uncertainty comes from uncertain ray path and multi-paths, a lot of averaging helps here, and also being in the center of the transponder array (less sensitive to depth/height errors then).
Short baseline navigation
Super/Ultra short baseline navigation
Geodetic measurements (GPS, acoustic, pressure)from ships and moorings
