Argos is a global spaceborne data collection and location system (DCS) dedicated to studying and protecting Earth's environment. The objective of the Argos DCS is to locate in the ground segment fixed and mobile platforms and collect environmental data from them. The system consists of in-situ data collection platforms in the ground segment equipped with sensors and transmitters and the Argos DCS instrument aboard polar orbiting weather satellites. What makes the DCS unique is the fact that a moving satellite platform allows for locating an in-situ platform using Doppler shift calculations. This positioning capability permits applications such as monitoring drifting ocean buoys and studying wildlife migration paths.
The system was developed under a cooperative program between CNES, the French Space Agency, NASA (National Aeronautics and Space Administration), USA, and NOAA (National Oceanic and Atmospheric Administration), USA. The purpose is to provide an operational service for the entire duration of the POES (Polar-orbiting Operational Environmental Satellites) program of NOAA (TIROS-N series), that is well beyond year 2000. 1)2)3)4)
The cooperative Argos program of CNES, NASA and NOAA started in 1974 for the purpose of long-term continued global satellite data collection services (in particular environmental data) from fixed and mobile platforms located anywhere in the world. The Argos system package has been flown on all TIROS-N family satellite of NOAA since 1978. The space segment comprises the NOAA POES satellites and more recently the ADEOS-II spacecraft of JAXA, and the EUMETSAT MetOp spacecraft in orbit.
The Argos/DCS supports NOAA in its overall environmental mission objectives, collecting (ground and space) truthing data. The concept uses many ground segment platforms (fixed and moving), i.e. buoys, free-floating balloons, wildlife, and remote weather stations, and equips them with a PTT (Platform Transmitter Terminal) package. These PTTs collect and process relevant environmental data and transmit them to the NOAA-POES satellites. The on-board Argos DCS receives the incoming signal and measures both the frequency and relative time of occurrence of each transmission.
The S/C retransmits these data via the CDA (Command and Data Acquisition) stations (one at Wallops Island VA, the other at Fairbanks, AK; there is in addition a downlink station at Svalbard Norway), to a central NOAA processing facility in Suitland, Maryland, USA. The DCS information is decommutated and sent to the Argos processing centers in Toulouse France and Largo (Landover) Maryland USA, where it is processed, distributed to the user community, and stored on magnetic tape for archival purposes. 5)6)
The processing and distribution service is provided commercially by CLS (Collecte Localisation Satellites - created in 1986), a CNES and IFREMER subsidiary in Toulouse, France, and by Service Argos Inc. of Largo, MD, USA (a CLS subsidiary). Service to the user community has been continuously provided since fall 1978.
The Argos system concept involves three interactive elements or subsystems:
1) PTTs (Platform Transmitter Terminals) of the various clients in the ground segment, equipped with sensors to measure environmental parameters. The PTTs are the Argos user platforms, fixed or mobile, deployed at sea, on land or in the air and transmitting independently.
2) The space segment DCS payload of the service provider. A desired complement of two operational NOAA spacecraft and two in backup in simultaneous orbit, with instrument packages that receive PTT messages on a random access basis, then separate, time-code, format and retransmit the data to ground stations,
3) The ground segment of the service provider. The ground stations and two Global Processing Centers (GPCs) in Toulouse, France and Landover, MD, USA, where data are retrieved, processed, and distributed to users. Each centre can take on the full operational workload if the other fails.
Flying the Argos DCS system aboard NOAA polar-orbiting satellites provides worldwide coverage. Additionally, incorporating the Argos instrument on a moving satellite allows for locating an in-situ platform using Doppler shift calculations. This positioning capability permits applications such as monitoring drifting ocean buoys and studying wildlife migration paths. Argos DCS can track platforms anywhere in the world, supplying positions to users around the globe. Platforms can be attached to practically any type of physical object, for example: an ocean buoy, a stream gauge, a bear, a bird, or a fishing vessel. Argos platforms are located by using the Doppler Effect, which gives an accuracy of up to 150 m. Doppler locations are good for compact, low-power transmitters and in difficult radio environments. The satellites receive the signals sent even in extreme conditions such as a platform transmitting from a dense rainforest or from transmitters on the polar ice caps.
Argos DCS is used with great success in the following applications:
- Studying oceans and atmospheric conditions (oceanography, meteorology)
- Preserving and monitoring wildlife
- Monitoring volcanoes
- Monitoring fishing fleets
- Monitoring shipments of dangerous goods
- Humanitarian applications
- Managing water resources
- Operation of automatic weather stations in remote areas such as in Antarctica
The majority of Argos DCS users are government/non-profit agencies and researchers. At the start of the 21st century, Argos DCS customers are engaged in over 1000 programs operating approximate ly 15,000 data collection platforms in 72 counties.
Argos space segment:
Each Argos payload is equipped with a DCLS (Data Collection and Location System), also referred to simply as DCS, which receives all transmissions from the platforms in view during a pass. Functionally a DCLS is comprised of the following subsystems:
• Housekeeping equipment, power supply and DCLS command interface
• Receive assembly (receiver and search unit, both with full redundancy)
• Signal processing assembly (four identical Data Recovery Units (DRUs). All data are tape recorded on board the spacecraft.
Argos ground segment:
A set of user platforms, fixed or mobile, deployed at sea, on land, or in the air. All platforms reporting to the Argos system must carry a certified PTT (Platform Transmitter Terminal) package for satellite uplink communication. Each PTT outputs a short message (of 0.36 to 0.92 seconds duration, or of 32 bits to 256 bits maximum length) modulating a carrier frequency. Message transmission intervals range from 90 - 300 s, depending on the application.
The ground segment of the service provider consists of two NOAA/NESDIS CDA (Command and Data Acquisition) stations, one at Wallops Island VA, the other at Fairbanks, AK. In addition there is a downlink station at CMS (Centre de Météorologie Spatiale) Lannion, France. All these stations also provide real-time data during the pass. Argos provides two GPCs (Global Processing Centers), one in Largo, MD, the other in Toulouse, France. Each GPC receives data from all platforms but processes only the data that belong to “its” users. Both centers will, however, immediately process all data in case of necessity, thereby ensuring full redundancy. 7)
As of 2002, the Argos ground segment provides five processing centers (the two global processing centers in Toulouse and Largo continue to process data sets from all receiving stations; the regional centers are: Melbourne, Tokyo, and Lima):
• Toulouse, France
• Largo, MD, USA
• Melbourne, Australia
• Tokyo, Japan
• Lima, Peru
Collection Uplink: Argos provides a total of four (eight in next series) parallel receiving channels for data collection, each at a rate of 400 bit/s. Each PTT in the ground segment transmits encoded messages at regular intervals (fixed platforms at 45 - 200 seconds, drifting or mobile platforms in the order of 90 - 150 seconds).
Note: the search unit is a spectrum analyzer that scans a 24 kHz band centered at 401.650 MHz. The next series of DCS will have 80 kHz of bandwidth (100 kHz allocated, two safeguard bands of 10 kHz at each end). Time tagging and frequency measurements are made by the DRUs and processed on the ground for location determination.
Uplink frequency (UHF band)
Up to 32 bytes
45 to 200 seconds
Varies depending on latitude and type of service
360 - 920 ms
Battery, solar, external
Table 1: Characteristics of a PTT
Downlink: The data received by the Argos DCLS is multiplexed on-board by the TIP processor and transmitted to the ground via three paths:
• Real-time: the TIP output (8.32 kbit/s, see Figure 2) directly modulates a VHF beacon which transmits continuously.
• Real-time: the TIP output is multiplexed on-board the satellite with HRPT data and transmitted in S-band
• Delayed Transfer: the TIP output is also recorded by a tape recorder, and each time the satellite passes over one of the ground stations, the recorded data is dumped via S-band telemetry.
The Argos communication capability is limited to the function of data collection from the PTTs. The concept does not offer a remote configuration control capability of the data collection platforms in the ground segment.
The on-board DCLS receiver picks up messages from the transmitting platforms in its area of visibility. The receiving system can discriminate between message arrival times and between frequency shift due to the Doppler effect. Up to four (eight in next series) messages may be processed simultaneously.
The Argos access scheme employs `pure (i.e. unslotted) ALOHA.' Messages from the PTTs are received on-board on a random access basis. The Argos Doppler system provides a position fix for drifter (or mobile) platforms. This setup requires between three and five successful transmissions, which must occur within one pass (footprint).
Within an average footprint of 10 minute duration, each platform in the ground segment usually has a number of attempts to make contact with the DCLS in the space segment.
• Fixed platforms: the number of transmission attempts of fixed platforms is three at a repetition rate of 200 seconds (average = 3).
• Drifting (mobile) platforms: the repetition rate is 90 - 150 seconds, hence the maximum number of transmission attempts possible within a footprint is 5 - 6 (average = 5). [About 80% of the possible position fixes are actually achieved by the system; 20% are rejected during ground processing for various reasons, mainly geometrical configuration: number of messages, pass duration, distance to the track, etc., according to CLS Argos].
The nature of random access very much degrades data collection performance by the space segment. The scheme of pure ALOHA permits under normalized offered channel traffic a maximum channel throughput rate of 18%. Any two signals overlapping in time and frequency may interfere, with the loss of both. The principal parameter that affects the performance of the Argos data relay system is “interference”: it occurs when the demand for service exceeds the system's capability. The result is loss of data from system `blockage'. The maximum number of platforms that a single Argos DCLS can actually service within a footprint is in the order of 650. In this number, there is a certain mix of fixed (collection service only) platforms and drifting (collection and location services) platforms, a further assumption is a certain message length.8)9)10)
The probability of good message reception is 67% with a traffic density of 2.6 Erlang, and 8.3 Erlang for the next improved DCLS series which is scheduled to be launched starting in 1998 with NOAA-K.
The total number of platforms registered as active in the Argos system globally is around 4000, out of which around 2300 are transmitting every day. The remaining platforms transmit once every two or three days, or less. This information was provided by CLS Argos (6/1993), the service provider of the system.
Argos-2 DCS, the next generation instrument package:
Argos-2 represents an enhanced instrument package over the old Argos system - in response to user-identified priorities. The new performance spectrum includes: 11)
• Implementation of eight DRUs (Data Recovery Units) instead of four on the previous Argos generation. Hence, the Argos-2 generation spacecraft are able to process eight messages simultaneously.
• The uplink bandwidth was increased from 24 to 80 kHz. This permitted a better distribution of PTT transmitter frequencies and a better discrimination of signal reception at the spacecraft. Hence, for a given platform population more messages can be received intact. The increased onboard capacity - wider receiver bandwidth, and more flexible management of transmitter repetition periods - offers more PPT sensor data transmissions.