SERVICES & FACILITIES

THE ARCOS SYSTEM: MONITORING THE WORLD'S ENVIRONMENT

By Katherine R. Swenson and A. E. Shaw III

THE ARGOS SYSTEM is a satellite- through asynchronization of the transmis- based Data Collection and Location Sys- sion and by use of different repetition peri- tem (DCLS) used to collect in situ environ- ods. PTT location is determined by measur- mental data from fixed and mobile plat- ing the Doppler shift of the carrier fre- forms. Coupled with an accurate position quency transmitted by each PIT. The proba- determination, these data are time-tagged bility of collecting data from a PTT during \ and disseminated world-wide. The year a given spacecraft pass is 99%, providing 1990 marks twelve years of continuous the messages are identical during the whole operation of the Argos system, primarily in overpass (approximately 10 minutes). the service of oceanography. The Satellites Argos is the result of a cooperative GrounraS The Argos onboard package is carried venture between the Centre National by two spacecraft (currently NOAA- 10 and d'Etudes Spatiales, (CNES--the French NOAA- 11) simultaneously in circular, near- space agency), the National Aeronautic and Fig. 1: Argos swath overlap polar, sun-synchronous orbits to provide Space Administration (NASA), and the the Argos system with complete global National Oceanic and Atmospheric Ad- coverage. Sun-synchronous means that each ministration (NOAA). A Memorandum of sponsible for ensuring that the system is satellite orbit transects the equatorial plane Understanding (MOU) was signed in 1974 managed according to the MOU. at fixed local solar times. Orbit revolution defining each partner's responsibilities. The The Argos DCLS comprises four essen- period is approximately 101 minutes. system became operational in October of tial components: platforms equipped with The instantaneous field of view of each 1978 with the launch of the protoflight Platform Transmitter Terminals, satellites, satellite is a circular footprint approximately satellite, TIROS-N. An updated bilateral receiving stations, and the Global Process- 5000 km in diameter. The satellites receive agreement, signed in March 1986, insures ing Centers. messages from the PTTs within the foot- continued joint cooperation through the Platform Transmitter Terminals print, time-codes them, measures the fre- rest of the TIROS series (into the late- An Argos-certified Platform Transmit- quency, and demodulates the PTT ID 1990s). CNES is planning future joint space ter Terminal (PTT) is a data acquisition number and sensor data. Four messages can cooperation with NOAA involving pos- station designed for transmission via the be received and processed simultaneously. sible additions to the NOAA/TIROS-N Argos system. Argos PTTs are simple to With the launch of NOAA-K in 1993, and series and also succeeding-generation po- operate, light (< 1 kg), inexpensive, and use subsequent spacecraft, an additional four lar orbiting space platforms. little power (200 mW on average). PITs Data Record Units will be added, increas- The Argos system is reserved for pro- can be mounted on many types of terrestrial ing the capacity of the Argos system four- grams involved with the collection of envi- and maritime platforms; such as drifting fold. ronmental data; defined as measurements buoys, balloons, animals, ships, trucks or The swath created by the satellite's of physical, chemical or biological proper- land-based stations. footprint as it sweeps the globe is displaced ties of solid earth, earth's waters (rivers, Since Argos is a random access system, 25 ° in longitude to the west, at the equator, lakes and oceans), and the atmosphere the PTTs uplink (transmit) their messages every satellite revolution, due to rotation of (including space). Programs not directly at preset intervals without satellite interro- the earth. There is an element of swath involved with environmental monitoring gation. Each message may contain up to overlap for each successive orbit (Fig. 1). may be approved by the Operations Com- 256 bits of sensor data. A message uplink Global coverage was guaranteed by selec- mittee on a short-term basis. A bilateral takes between 360 and 920 ms, as a func- tion of this type of orbital geometry. The France-USA Operations Committee is re- tion of message length. The uplink time- number of passes over a given location out, or repetition period, is normally set each day is a function of latitude. Satellite Katherine R. Swenson and A. E. Shaw III, Service between 50 and 200 seconds, depending on coverage ranges from six passes per day at Argos. Inc., 1801 McCormick Drive. Suite 10. Lan- the user's application. All PTTs transmit at the equator to twenty-eight passes per day dover, MD 20785, 401.65 MHz. Separation in time is achieved at the poles.

60 OCEANOGRAPHY,APRIL,1990 Argos data are "played back" from the for each program and PTT. • Class 3--Quality Location: (Accu- satellites in three modes: Each of the processing centers is able to racy: 68% of the results within 150 m of the • The S-band which transmits the re- take on the entire global processing load true position.) Uses the same criteria as that corded data on 1698.0, 1702.5, or 1707.0 should one of the GPCs fail. for standard location, plus favorable geo- MHz at 2.66 mbps. Location Calculation and Quality metric conditions of the PTT relative to the • The real-time S-band on 1698.0 or PTT location is determined by measur- space-craft ground track and oscillator sta- 1707.0 MHz at 665.4 kbps. ing the Doppler shift of the carrier fre- bility providing good internal consistency. • The real-time VHF beacon on 136.77 quency (401.65 MHz) of messages received Data Distribution or 137.77 MHz at 8.32 kbps. by the spacecraft. Since the transmission Processed data are available generally Telemetry Receiving Stations frequency is fixed for all PTTs, the satel- within three hours after the receipt of data Argos currently utilizes three major te- lite-received frequency at any instant can from the satellite(s) in computer files ac- lemetry receiving stations (Wallops Island, be used to define the field of possible posi- cessible by the user through telephone, VA, and Fairbanks, AK, in the United States, tions for a PTT. The field is in the form of telex or telecommunication networks (i.e., and Lannion in France) where all the re- a half-cone, with the satellite at its apex, the TYMNET or TRANSPAC). These net- corded data are downlinked when a satellite satellite velocity vector as the axis of sym- works may be inter-connected through in- is within station visibility. metry, and the apex half-angle (A) such ternational transit nodes. All Argos data received from the U.S. that: There are three types of on-line files that stations are routed through NOAA's Na- contain processed data: tional Environmental Satellite, Data and • The AJOUR file containing the most Information Service facility in Suitland, cos(A)_(~-fe), c recent location and sensor message for each Maryland, for decommutation. These data v PTT. are then transmitted to the two Global • The TELEX file containing in chrono- Processing Centers (GPCs). Where: logical order the most significant sensor Until mid-87, Service Argos received c = Speed of light message for each PTT and the correspond- and processed only the recorded data stored V = Satellite speed relative to PTT ing location for each satellite pass received onboard the satellite and played back ap- f = Transmission frequency (401.65 during the past 100 hours. proximately ten times per day per satellite. MHz) • The DISPOSE file containing all loca- With the satellite orbital period being 101 f = Receive frequency tion data and all sensor messages for each minutes, processing delays from the time of satellite pass and each PTT received during PTT data transmission to on-line data availa- In April 1987, three different levels of the past 100 hours in chronological order. bility were usually about two hours for the location accuracy were implemented (Class A Direct Data Distribution service is recorded data. However, since July 1987, 1,2 and 3). Since January 1988, a new class also available as an alternative to dial-up Argos has received the processed DCLS (Class 0) has been defined to accommodate for users who require their data immedi- data (imbedded in NOAA's High-Resolu- the needs of certain users. The quality index ately following processing or at set inter- tion Pixel Transmission data stream) in depends on location quality control and vals during the day (not to exceed four near real-time. Today, ten real-time passes geometric conditions. The user may choose times per day). It is possible to request that are acquired by Service Argos. between these four classes of location accu- only real-time data, only recorded data, or Service Argos is currently investigating racy described below: both real-time and recorded data be sent to the possibility of receiving data through • Class 0--Special Location: Service your site. The Direct Data Distribution another receiving station in the western US based on a minimum of two messages re- service is also available to users who have to reduce the data throughput times and ceived by the satellite. Both Doppler loca- access to computer networks such as SPAN, provide real-time coverage to all of North tion solutions are given; therefore, the since the interconnection protocol is the America. quality of the result is left to the user's same whether the receiving system is a Global Processing Centers (GPCs) determination. computer or a network. The Argos system is administered by • Class 1--Non-Guaranteed Location: Data are also available on computer Service Argos, Inc., in North America, and (Accuracy: 68% of the results are within 1 compatible tapes, floppy disks or printouts by Collecte Localisation Satellite (CLS)/ km of the true position.) Based on at least on a fortnightly or monthly basis. Data are Service Argos, S.A., in Toulouse, France. four messages being received by the satel- archived for three months. Appropriately Redundant, identical processing centers are lite, with an interval of at least 240 seconds formatted data may be distributed over the in place at each of these facilities, where the between the first and last message received. Global Telecommunication System. following operations are performed: The location is not necessarily of low qual- Applications for Argos • Decoding of the received PTT mes- ity, since it may be a location for which it Argos PTTs typically transmit oceano- sages and conversion of data into physical was not possible to conduct all of the qual- graphic and atmospheric parameters: at- units, as specified by the end user, ity control checks. mospheric pressure, air temperature, sea- • Very accurate computation of the • Class 2--Standard Location: (Accu- surface temperature and temperature at satellite(s) orbits, racy: 68% of the results within 350 m of the various ocean depths, wind speed and di- • PTF location computation using or- true position.) At least five messages re- rection, and currents. Argos applications bital data and the Doppler frequency shifts, ceived over a duration of at least 420 sec- include weather monitoring from various • Storage of all these results in computer onds. Requires good short-term oscillator types of vessels, guidance of fishing fleets, files for on-line user access, stability. There is no ambiguity in the solu- iceberg drift prediction, hydrological moni- • Generation of the archived data base tion. toring, and tracking of animals. []

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