Technical Note: Argos System Background
The Argos PTT subsystems described in this document operate through the ARGOS system. Additional information on Argos is available in the Summer 1991 Telonics Quarterly. Argos is a French based organization with data processing centers in Toulouse, France, and Landover, Maryland, USA; and with user offices throughout the world. Arrangements can be made to use the ARGOS system by contacting ARGOS directly:
CLS America Inc.
4300 Forbes Boulevard, Suite 110
Lanham, MD 20706
Tel: (+1) 301-925-4411
Telex: 898 146
Fax: (+1) 301-925-8995
Australia and South Pacific:
Argos / Satellite IT Pty Ltd
PO Box 3108
Domain Road LPO
Tel: (+61) 418 368 917
Throughout the rest of the world, contact CLS corporate headquarters in Rammonville Cedex, France:
8 -10, rue Hermes, Parc Technologique du Canal
31520 Rammonville Saint-Agne
Tel: (+33) (0)561-394-700
Fax: (+33) (0)561-751-014
Argos maintains receivers onboard U.S. NOAA satellites. The satellites are in near sun-synchronous polar orbits, which means that unlike geostationary satellites, they are within "view" of any point on earth only a portion of the time and that the number of overpasses and total time in view is greatest at the poles and least at the equator.
Transmissions from PTTs are received at the satellite and then immediately downlinked for "real-time" data collection at Local User Terminals or LUTs (operated by Argos in some regions of the world or operated by the user community) while also being stored for delayed high-speed downlink to three ground stations. The real-time downlinks can be received by LUTs when the satellite's field-of-view, which is approximately 5000 km in diameter, encompasses both the PTT and the LUT. While some users recover data through LUTs, the majority depends on Argos for data collection and dissemination. Argos typically processes and makes data available to users within eight hours of transmission by the PTT.
It is important to note that Argos is a random access system with all PTTs transmitting on the same frequency (401.650 MHz). Individual PTTs are identified by an ID code, which comprises a portion of each transmission. Argos must be contacted for project approval, assignment of ID codes and their corresponding repetition periods, and to set up technical details regarding data collection, processing, and dissemination.
PTTs are constructed and certified in accordance with Argos specifications to assure compatibility of all users of the system. Each PTT transmission contains 160 milliseconds of unmodulated carrier followed by 48 bits (120 milliseconds) that includes a preamble, format synchronization, initialization, specification of the amount of sensor data to follow, and an ID number. Argos requires that each transmission also include a minimum of 32 data bits, thus resulting in a minimum transmission length of 360 milliseconds. The 32 data bits can be all zeros ("location-only" PTTs) or they can be used to transmit various sensor data collected by the PTT. Data streams up to 256 bits in length (total transmission length 920 milliseconds) are possible; however, data streams longer than 96 bits are utilized only when absolutely necessary in wildlife work due to the additional current required for longer transmissions and the limited power supply which can be carried by an animal.
In general, the more messages the satellite has received during a single pass-the more evenly distributed those messages are throughout the course of the overpass and the more stable the transmitter's reference oscillator-the more accurately the location of the transmitter can be calculated. Even under the best of conditions, as with all location systems, there is always some error in the determination of the location. Using the Doppler Shift of the Uplink Frequency and the time of signal arrival, Argos calculates the predicted location along with an index that relates to the magnitude of the error. The index is called a Location Class. When using the "Auxiliary Location Processing (ALP) recommended for wildlife tracking applications, location classes are assigned to provide a gross estimate of position accuracy to the end user. Argos Location Classes, in order from best (most accurate) to worst, are 3, 2, 1, 0, A, B, and Z. The majority of Class 3 locations are within plus or minus 150 meters in latitude and plus or minus 150 meters in longitude. Class 2, 1, and 0 locations show that at least four messages were received and that the accuracy of the majority of locations given are within plus or minus 150 to 350 meters latitude and longitude (Class 2), within plus or minus 350 to 1000 meters latitude and longitude (Class 1), and plus or minus 1000 meters or greater in latitude and longitude (Class 0) respectively of the true position. Class A and B locations show that three or two messages respectively were received from the overpass. These locations have been validated by the location software, but are not assigned accuracy. Class Z locations are "rejected" locations and are thus "invalid" ("User Manual, Argos", 1996, p. 27).
Location accuracy for many wildlife applications is in the order of 68 percent of locations plus or minus 1000 meters of the stated latitude and longitude. This statement of accuracy is provided as a general statement based upon previous experience and should be used as a reference only. The actual accuracy achieved in a study is also dependent on many other biological factors such as the amount of movement the study species may undertake during the course of a satellite overpass, and the behavioral patterns of the species under study. Behavioral patterns may significantly affect the ability of the transmitting unit to uplink information to the satellite during the overpass.