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ARGOS LOCATION CALCULATION

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Roland Liaubet (CLS)

Jean-Pierre Malardé (CLS)

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

Location based on Doppler shift measured on each message

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

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

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 is attempted if at least 4 messages are received during a satellite pass

3 parameters are calculated :

latitude,

longitude,

transmission frequency

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?

2 - Geometric initialization

V1

V2

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

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)

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

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

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

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

Location accuracy depends chiefly on the difference between the transmission frequency used in the geometric initialization and the actual PTT transmission frequency

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

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 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

- timestamp

- orbit error

Hardware

- ionosphere

- troposphere

TX power

- relativistic effect

Hardware

- Speed

- Altitude

- CTA

Doppler shift without freq. drift

Measured Doppler shift

Real position

Computed position

Drift

Error due to platform speed

400 m

7 km

Elat(m) = 200*Vlat(km/h)

Elon(m) = 100*Vlon(km/h)

Error due topropagation in ionosphere

2.0 km

0.5 km

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 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).

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

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

30’’ arc resolution / 100 m accuracy ( 1000 m)

Altitude declared at the User Office : 0 m in 93 % of cases

Altitude error greater than 100 m in 40%

Experimentation

Dh = 1000m

Dh = 0m

Using a DEM

- 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

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