The Argos system calculates locations by measuring the Doppler Effect on transmission frequency. The Doppler Effect is the change in frequency of a sound wave or electromagnetic wave that occurs when the source of vibration and observer are moving relative to each other.
 

3.2.1 Physics and Measurement: The Doppler Effect

The classic case is when an observer notices a change in the sound when a train approaches and moves away. Similarly, when the satellite approaches a transmitter, the frequency of the transmitted signal measured by the onboard receiver is higher than the actual transmitted frequency, and lower when it moves away.

How the Doppler Effect influences transmitter frequency

Each time the satellite instrument receives a message from a transmitter, it measures the frequency and time-tags the arrival. A major feature of the Doppler location is the existence of two possible positions of the platform that give exactly the same frequency measurements on board the satellite: the nominal ("true") location and the mirror ("virtual") location. They are symmetrical about the sub-satellite track and, unfortunately, they are not a priori distinguishable.

Since 2011, users can choose between two location processing algorithms for Argos. Both techniques compute the Doppler frequency shift on the transmitters signal.

3.2.2 The positioning algorithm based on Least squares analysis (unchanged since 2007)

If four or more messages are received by the satellite, the location calculation process follows the following steps.

An initial estimate of the platform position is computed from the first and last messages collected during a single satellite pass and the last computed frequency of the transmitter. The intersection of the cones for these two messages with the terrestrial radius plus the height declared for the transmitter (altitude sphere) gives two possible locations.

tracking2.jpg

For each of the two possible locations, and by using all messages received during the satellite pass over the platform, a least-squares analysis is used to refine the estimates of the transmitter's position. If this analysis fails, the location calculation process cannot continue and no location is provided. The location with the minimal residual error is chosen, and its plausibility is tested.

Four plausibility tests are used to validate the location:

Two tests must be positive for the location to be validated. If the first location fails more than two tests, the second possible location is tested. If both locations fail more than two tests, then the location is not distributed to users, unless they subscribe to Service Plus/Auxiliary Location Processing (See Chapter 3.6 for more information). The location algorithm provides the two solutions where the first one is the most plausible and considered as the nominal location. An estimation of the location accuracy is calculated using the residual error and the satellite pass characteristics.

It is possible to calculate a position with two or three messages, but only partial information about the error will be available. These locations are distributed to users if they subscribe to Service Plus/Auxiliary Location Processing (See Chapter 3.6 for more information).

3.2.3 The positioning algorithm based on Kalman filtering (introduced in 2011)

In 2011, CLS introduced a location processing algorithm that takes into account platform dynamics and the use of a bank of Kalman filters to calculate positions. This method is extremely robust and positions can be calculated based on one message per satellite pass. In addition, the error estimate is an integral part of the algorithm and therefore systematically distributed to all users. Unlike the Least squares method, only the nominal location is calculated.

Kalman filtering is a 2-step process:

kalman_filter_en.gif

Three plausibility tests are used to validate the location:

All tests must be positive for the location to be validated. For all locations, an estimation of the accuracy is provided. Locations computed with less than 4 messages are distributed to users if they subscribe to Service Plus/Auxiliary Location Processing (See Chapter 3.6 for more information).

3.2.4 Error estimation

Due to the satellite’s polar orbit, the Argos position error is better represented by an ellipse rather than by a circle. For those users who wish to use it, CLS provides the following values corresponding to the ellipse of error for all locations:

ellipse.jpg

Users can employ this description of the location error for example to assimilate positions into an animal movement model.

Error ellipse is available for all locations with the Kalman filter and for locations computed with more than 4 messages with the Least squares method. For the Least squares method with 2 and 3 messages, CLS provides only:

*Geometric Dilution of Precision (GDOP) is a measure of the effect of the geometry of the satellite-beacon configuration on location accuracy. The actual Argos location accuracy depends on both the GDOP value and the quality of the transmitter (frequency stability). As a general rule, a smaller GDOP value yields a more accurate position.

 

How to access these parameters

These values are available as Diagnostic Data in ArgosWeb and distributed in tabular format ONLY via ArgosDirect.

Via ArgosWeb

Users can access the information via ArgosWeb in the Consultation/Data Table section (display Diagnostic Data by clicking on this icon sort.jpg , then selecting  diagnosticdata.jpg) or from the Data Download screen (display diagnostic data by clicking on diagnosticdata.jpg ). These parameters are available in clearly identified table columns (see below). For additional information, consult ArgosWeb’s online help.

Via ArgosDirect

These parameters are also available via ArgosDirect. Users must request them from their User Services. They are distributed in clearly identified table columns (see below).

Column title

Error radius

Semi-major axis

 Semi-minor axis

Ellipse orientation

GDOP

Units

Meters

Meters

Meters

Degrees (from North when heading East)

m/Hz

  

3.2.5 Altitude estimations

Location computations are extremely sensitive to altitude variations. A significant error in altitude can considerably reduce the accuracy of a location, especially if satellite visibility is unfavorable.

To improve location accuracy, a digital elevation model (DEM) is automatically included in all location computations for ground mobiles and birds. The DEM used is based on the USGS GTOPO30 model. It is broken down into squares with sides of 30 arc seconds of an arc and is used to estimate platform altitude.