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About ARGOS

Argos services for wildlife tracking

 

Argos is a satellite-based system which collects, processes and disseminates environmental data from Platform Terminal Transmitters (PTTs) fitted on wildlife. Since 1978, when the Argos system went into service, worldwide thousands of animals have been tracked.

 

View from TUBSAT, source:
Earth and Moon viewer

 

 

 

 

 

 

 

I n t r oduction

Argos is a satellite-based system operating since 1978.  The Argos system collects data from Platform Terminal T ransmitters, PTTs, and delivers this telemetry data to the users desktop. It was established under an agreement between:

the National Oceanic and Atmospheric Administration (NOAA, USA),
the National Aeronautics and Space Administration (NASA , USA),
the French Space Agency (CNES).
Argos is operated and managed by:

Collecte, Localisation, Satellites (CLS), a CNES subsidiary near Toulouse, France,
Service Argos, Inc., a CLS subsidiary in Largo, near Washington, DC, USA.
 

The Argos intruments are housing on board of satellites of different organizations the NOAA, the Japanese space agency (NASDA) and the European Meterological Satellite organization (Eumetel). Two satellites are operational at any time.
The messge of the PTTs are received on bord of the satellites and relayed in real time to the ground. Additional the mesages are stored and transmitted every time they pass over one of the 3 ground stations, located at Wallops Island, USA; Fairbanks, USA and Lannion, France.
The data are sending directly onto the Global Telecommunication System (GTS).
Argos services offers the possibility your transmitters in the field, you'll be able to send messages to the PTT in the field.
 

PTT Charcteristics

      • each PTT has an individual ID number
      • uplink frequeny: 401.650 MHz
      • message length: up to 32 to 256 bits
      • transmission time: 360-920 ms, according to the no. of message bits
      • duty cycle: variable
      • transmitted are: ID number, sensor data, statement of message length, a preliminary synchronization sequence

 

Agros service

Describtion

Standard location and data collection

The basic service to geographically locate the animal and obtain data transmitted (heart rate, body temperature, flight altitude, etc.) , if any.

Location service plus
(known as Auxiliary Location Processing in North America)

Specially designed for wildlife applications, it provides more locations than the regular Argos service and crucial information on the location results and transmitter performance.

Multi-satellite service

Optional service, to further increase the number of locations of the animal by processing data from more than two satellites.

Other useful services

Automatic Data Distribution, offline products, etc. are described in the Argos catalog of Services and Products.

 

 

 

 

 

 

 

 

 

 

 



 

Wil dlif e loca tion via Arg os

The messages sent by PTT’s can be processed for: location or data collection or both.

Location
There are two separate ways of locating, i.e. tracking, your transmitter:
Conventional Argos location: the Argos centers calculate your transmitter locations by measuring the Doppler shift on its transmit frequency. Even if you do not send sensor or other data, the minimum message length is 32 bits.
GPS positioning: if a GPS receiver is interfaced or built into your Argos transmitter, a dedicated processing module at the Argos centers 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, in thousandths of degrees. The reference system is the World Geodetic System (WGS 84).

Data collection
You can also send sensor data — for example temperature readings or atmospheric pressure — or alphanumeric information entered through a keypad. Your message can contain up to 256 bits (32 bytes) in 32-bit blocks, as follows:

Argos estimated the following classes of location accuracy depending on the quality of received messages and other parameters like satttellite/transmitter geometry during satellite pass, number of messages received during the pass, transmitter frequency stability etc. .

 

Table 1: Location class

Class

Accuracy in latitude and longitude

0

> 1000 m

1

350 m < accuracy < 1000 m

2

150 m < accuracy < 350 m

3

< 150 m

A

no estimation of location accuracy

B

no estimation of location accuracy

Z

invalid locations

 

 

 

Location classes are based on:

    • satttellite/transmitter geometry during satellite pass,
    • number of messages received during the pass,
    • transmitter frequency stability.


Software
Argos telemetry data can automatically displayed with f.ex. the latest locations of the tracked animal by ELSA,  an cartographic application automatically polling the latest locations and sensor values from the data files.

Argos location
Locations are calculated from all messages received during a satellite pass over a transmitter. Various calculations are done, according to your request:

Standard locations are calculated on reception of four or more messages.
Users who have requested Location Service Plus (Auxiliary Location Processing in North America) also receive locations calculated from two or three messages. This service was developped to increase the number of locations for transmitters sending weak signals, or at irregular times, or in difficult environments such as steep-sided valleys. Typical users are animal trackers operating miniaturized transmitters.
Each location is assigned to a location class. The classes vary according to the estimated accuracy of the location, for standard locations, or the number of messages received, for Location Service Plus / Auxiliary Location Processing.

How the locations are calculated
Argos locations are calculated by measuring the Doppler shift on the transmitter signals. This is the change in frequency of a sound wave or electromagnetic 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 transmit 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 center computes 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 + 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.



GPS positions via Argos:

The advantages of sending GPS positions via the Argos system are that:

    • having two location systems is more reliable than just one
    • GPS positions can be generated as often as you want
    • accuracy is higher (within 100 meters) and does not depend on the transmitter quality
    • positions can be spread evenly through the day.
    • Providing the GPS positions in the Argos messages are in the format defined by CLS, they are output in latitude and longitude, and made available just like Argos locations.

Various Argos/GPS formats are available:

    • GPS positions only,
    • GPS positions interlaced with sensor data,
    • a single GPS position with the sensor data acquired at the same time.

 

For detail information about the Argos system please go to the Argos manual

 

 

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