ARGOS LOCATION CALCULATION

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## ARGOS LOCATION CALCULATION

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**Roland Liaubet (CLS)**liaubet@cls.fr Jean-Pierre Malardé (CLS) malarde@cls.fr ARGOS LOCATIONCALCULATION**Argos location principles**Standard location processing description Others types of location Argos location class definition, interpretation and limits Argos location : main causes of error How to improve your number of locations and their accuracy Argos location software improvements foreseen Summary**Location based on Doppler shift measured on each message**Argos location principles Apparent frequency shift observed when the receiver and the transmitter are in motion relative to each other**TCA Time of Closest Approach**CTA Cross Track Angle Doppler curve characteristicsTCA/CTA definition Distance too short Doppler shift Sub-satellite track Cross Track Angle CTA TX Time of closest approach TCA Time Distance too long CTA also called distance from the ground sat track**3 Hypothesis :**Transmission frequency is stable during the satellite pass The platform is motionless during the satellite pass ( in principle) the altitude is known Argos location principles**The location process follows 7 steps:**A priori checks Geometric initialization Newton linearization Least-squares method Removing ambiguity Plausibility checks Location class estimation Standard location processing**Standard location is attempted if at least 4 messages are**received during a satellite pass 3 parameters are calculated : latitude, longitude, transmission frequency Standard location processing 1 : A priori checks**TX frequency calculated from the previous satellite pass**2 messages -> 2 Doppler shifts -> 2 cones Intersection with ellipsoid = 2 positions Which is the true position? Which is the mirror position? Standard location processing 2 - Geometric initialization V1 V2**Standard location processing**4 : Least-squares method Minimize the quantity : IC = SQRT (|| AX-F||2) = SQRT [ Σni=1( Fri – Fci)2/ (n-3)] The iterative processing stops when the residual error does not change significantly from an iteration to the next one Fr IC = internal consistency or residual error**Convergence**towards the true position Convergence towards the mirror position Standard location processing 5 : Removing ambiguity Satellite ground track**Where :**K : is the coefficient corresponding to the probability that the actual location of the transmitter is inside the circle with radius R. K = 1,414…corresponds to a probability of approximately 63% (*) HDOP : is the horizontal dilution of precision (*) Q : is the frequency noise estimator ( residual error) B : is the orbit error (B=100 m) Radius error calculation The radius R of the circle of error is given by: (*) Argos location is assumed to be a bi-normal distribution (*) HDOP can be interpreted as the geometrical factor of observation error propagation**Good HDOP**T=120s T=60s T=30s HDOP effect on location accuracy At 8°, error = 1.414*200*0.25 ~ 70 m At 2°, error = 1.414*600*0.25 ~212 m TX at the horizon (2500km) TX closed to sat track**Effect of the PTT position relative to satellite ground**track Curves intersection is not precise Poor HDOP Curves intersection is not precise Poor HDOP Curves intersection is optimal Good HDOP**Residual error translates random errors**Modeling errors or bias errors ( except for orbit error) are not taken into account in the ARGOS location and underestimated when calculating the radius of the circle of error Classification limits**Only latitude and longitude are calculated. We assume the**transmission frequency has not changed since the last location ( CLASS B) A priori checks Geometric initialization Removing ambiguity 1 criterion :minimum distance traveled from last location Plausibility checks 2 criteria: Solution selected matches minimum distance traveled from the previous location Distance traveled from the previous location is compatible with the maximum velocity of the platform Two-message location processing**Location accuracy depends chiefly on the difference between**the transmission frequency used in the geometric initialization and the actual PTT transmission frequency Two-message location**Latitude, longitude and transmission frequency are**calculated. We assume the transmission frequency noise is negligible ( CLASS A) A priori checks Geometric initialization Newton linearization Resolution of a precisely determined linear system Removing ambiguity Frequency continuity with respect to the last calculated frequency Minimum distance traveled from last location Plausibility checks Transmitter frequency of the chosen solution is significantly closer to the previous calculated frequency than than the one of the solution candidate Minimum distance traveled from last location Distance traveled from the previous location is compatible with the maximum velocity of the platform Three message location**The platform is assumed to be moving from its previous**location with a mean velocity in latitude and longitude ( and during the current satellite pass) Standard location assuming a moving PTT Standard locations only 0.5 h < delta T < 3.5 h Pnew Plast The new location is kept if the residual error (IC) is smaller than the one obtained for a stationary platform**Argos location : main sources of error**- timestamp - orbit error Hardware - ionosphere - troposphere TX power - relativistic effect Hardware - Speed - Altitude - CTA**Error caused by frequency drift( due to temperature**variation ) Doppler shift without freq. drift Measured Doppler shift Real position Computed position Drift**Error due to platform speed**Error due to frequency drift 400 m 7 km Elat(m) = 200*Vlat(km/h) Elon(m) = 100*Vlon(km/h)**Error of altitude**Error due topropagation in ionosphere 2.0 km 0.5 km**Impact of output power**PTT 18781 - CLASSES 1,2,3 - KL Nesdis data streams 100 90 2W 80 70 60 % 50 40 0,25 W 30 20 10 0 0 250 500 750 1000 1250 1500 1750 2000 Distance from the true position (m)**How to improve your locations**• How to increase the number of messages received per satellite pass • How to improve the location accuracy**Number of locations :Today, not enough messages received per**satellite pass : it is the biggest cause of location problems Several explanations can be put forward : repetition rate too low, hardware quality and antenna efficiency, TX signal power too weak, TX environment (surrounding noise), data loss due to system occupancy ( TX concentration and transmission at the same frequency). More location & better Precision**How to increase the number of messages received and improve**location accuracy Use multi-satellite service, Select good quality USO ( CLASS A recommended) Tuning TX parameters such as : output power, repetition rate, transmission frequency ( outside the Argos 1 band) Declare platform (average) altitude Declare correct maximum velocity of the platform More locations & better Precision**Current status**PTT altitude is assumed to be known An error in PTT altitude is translated into an error varying between ½ and 4 times on longitude Command MOD is not much used Land platforms represent 20 % DEM : digital Elevation Model Using a DEM in Argos location**USGS Model**30’’ arc resolution / 100 m accuracy ( 1000 m)**Statistic in the European zone**Altitude declared at the User Office : 0 m in 93 % of cases Altitude error greater than 100 m in 40%**Experimentation**Validation Dh = 1000m Dh = 0m Using a DEM**Multi-pass location**• Current status : • Only single pass location • Seven satellites in operation • Waiting time between two successive satellite overpasses at 43 ° latitude : • 5 % : less than 5 minutes • 25 % : less than 10 minutes • 57 % : less than 15 minutes**Selected**PTT Location Multi-pass location**Multi-pass location**Advantages • Increase the number of standard locations • Decrease the risk of selecting the wrong solution Disadvantages • When the TX is drifting too much • When the platform is moving with a high speed