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Argos Location Processing New features

Jean-Pierre MalardИ ­ Christian Ortega


Argos Location Processing
Location processing : principles Location processing : how does it work?







Argos + GPS positions
Data provided Improving Location Conclusion: getting better locations








Doppler effect
f < f0 f0

f > f0

Apparent frequency shift observed as the receiver is moving
Relative motion between transmitter and receiver

TX f0


Doppler effect


Doppler effect : Each Message Received Tells us the Tag is somewhere on a CТne
ity Satellite veloc

A

s xi ta pt ellit te sa


Some geometry : intersection between a plan and a conic surface


Intersection with Altitude Sphere: Two Solutions!!


Position quality varies with Satellite Pass Geometry versus the Transmitter

subsat track

PTT

PTT

PTT

Subsat track

Mean distance

Long distance (grazing angle )

IsoDoppler lines: one line correspond to one message collected ­ ie one frecuency measured Accuracy varies with PTT Distance to Track (CTA)


Doppler model algorithm: Geometrical Effect
ge om e tric e ffe ct
1200

1000

T=120s

HDO P (m/Hz )

800

600 400

T=60s T=30s At 8°, eP = 200*0.3 = 60 m At 2°, eP = 600*0.3 = 180 m
0 5 10 CTA (de g.) 15 20 25

200

0


Analogy with GPS

Idem for 4 satellites at grazing angle

To maximize the volume


Argos location processing : principle

Doppler

Localisation

Doppler = 0.3 Hz

estimated Parameters (Position)

1 3 5

Model

S10

7 9

11

S7

13

15

S4

17

3 Unknowns Position: Latitude, Longitude Transmitter exact frequency at the time of Satellite pass Altitude is assumed to be known

Each Argos Message provides one Doppler -> 1 Equation
4 Equations/ 4 Messages or more are needed to estimate Position Accuracy

19

S1


Complete Localisation Processing (4 messages received at least)

1. Geometric Intialization

2. Least meansquare Calculation

3. Selection of best Solution

4. Choice Validation / Plausibility tests

5. Accuracy Estimation Location class


Step 1: Geometric Initialization


The transmitter frequency is supposed to be known Two initial locations are calculated from the first and last Doppler measurements of the satellite pass


Step 2 - Least-squares calculation
Minimize the distance between the Measured Doppler data and the Theoretical Doppler curve for each initial solution

The iterative processing stops when the residual error does not change significantly from an iteration to the next one

F Fm r
Fc

F

T

Doppler curv e

Result of the calculation (for each initial location) : - Longitude, latitude, Tx Frequency and residual error (IC = internal consistency


Step 3: Selection of best solution


Step 3 ­ Choice Validation -Plausibility Tests ­ 2 tests OK required for Loc distribution

Minimum residual error (=<4 msgs) Solution chosen has the minimum residual error

Best Tx Frequency continuity Delta Frequency between previous and current locations is minimum Distance Test Shortest distance from previous Loc

Speed Test Speed is below Max Speed


Step 4 ­ Accuracy Estimation, Modeling the Error

Doppler

Localisation

Doppler = 0.3 Hz

estimated Parameters (Position)

1 3 5

Model

S10

7 9

11

S7

13

15

S4

17

^ GDOP

z

only wit h a gaussian noise
Er r or Ell i pse without model er r or

esti mated Posi tion ( bi as and noi se) tr ue Posi tion

19

S1


Step 4 - Location classes are based on estimated Circles of error
North

Limit of 1-sigma radius error for class 1 Circular error Elliptic error

Possible real location! East

Estimated location



Circle surface = Ellipse surface


GPS Location Decoding


GPS decoding

If manufacturer format is provided, CLS processes & displays the GPS positions
· Speed test are applied to remove bad GPS locations · GPS Positions marqued as class « G »

·All positions available on same supports (ArgosWeb, data files, Archives...)
· Accessible on Google Earth


Data provided: Location Classes
CLASS G Type GPS Nbr of Msgs 1 Accuracy (m) 10 to 100

3
2 1 0 A B

Doppler
Doppler Doppler Doppler Doppler Doppler

4 or more
4 or more 4 or more 4 or more 3 2

<250
<500 <1500 >1500 Unknown Unknown


For More Details: Ellipse of Error
For

class 0, 1,2,3 locations: Estimated error radius (m), The parameters of the ellipse of error direction (deg/East), semi-major and semi-minor axes (m) GDOP (*) (unit : m/Hz)
class A and B: The direction of the ellipse GDOP
nor i mi Se m xi s a

North

Orientation

For

xi s jor a i ma Se m

Ea st

* Geometric Dilution of Precision (GDOP) is a term used to characterize the geometric strength of satellite configuration on location accuracy. Argos location accuracy depends on the quality of the transmitter (frequency stability) as well as the GDOP. As a general rule, a smaller GDOP value indicates a more accurate position.


Multi-pass location (manual, on request)
One satellite pass over the transmitter with only 2 messages No or poor location !

With Multipas/Multisat

Selected PTT Location

(Better) Location available !


Example


How to get better results?








Carefully select your tag (Argos only, GPS.....) Apply max possible Transmitter power (Tag performance varies depending on the area on earth) Tune Tag duty cycle: performance is related to the mission (duty cycle, Transmission power, frequency...) CLS can help you with manufacturers Take benefit of all CLS tools: ArgosWeb, multisatellite position, ellipses of error ....


Thank You for your attention

Photo : Sandra Ferraroli



Who said I would be easy to Track?

photo : Jean-Yves Georges (CEPE, Strasbourg)