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> Tracking
     > Doppler location
     > GPS positioning
> Monitoring

How does Argos track mobile platforms?
           How does Argos monitoring work?

Transmitters can be tracked two separate ways:

• Conventional Argos tracking: The Argos centers calculate your transmitter locations by measuring the Doppler Shift on its transmit frequency.
• GPS positioning: if a GPS receiver is integrated into your Argos transmitter, a dedicated processing module at the Argos center can extract the GPS positions from your Argos messages, validate them, and output them as regular Argos positions.

The coordinates in both cases are in latitude and longitude. The reference system is the World Geodetic System (WGS 84).

Doppler Shift

Argos locations are calculated by measuring the Doppler shift on the transmitter signals. This is the change in frequency of a wave when a source of transmission and an observer are in motion relative to each other. 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.

Each time the satellite instrument receives a message from a transmitter, it measures the frequency and time-tags the arrival. The Argos processing centers compute the locus of possible positions for the transmitter, a cone defined by:
-a vertex at the position of the satellite when it received the message,
-the angle at the vertex a function of the difference between the frequency measured on board the satellite and the transmitter frequency.

The processing center calculates an initial estimate of the transmitter's position from the first and last messages collected during the pass and the most recent calculated frequency. 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 positions.

For each position, least-squares analysis is used on the equations to refine the estimate of the transmitter's position and transmit frequency. The position with the better frequency continuity is chosen, and its plausibility checked.

Four checks validate the location calculation:

• Minimum residual error,
• Transmission frequency continuity,
• Shortest distance covered since latest location,
• Plausibility of velocity between locations.

For the location to be validated at least two must test positive.

GPS positioning

GPS positions are recorded in the transmitter and sent via the Argos system. Using GPS in addition to Argos allows you to:

• Have two location systems in one,
• Acquire positions as often as you want,
• Obtain highest accuracy (10 meters) without influence from transmitter quality,
• Spread positions evenly throughout the day.