Argos DCS (Data Collection System)

Argos is a spaceborne data collection and location system (DCS) flown on NOAA (National Oceanic and Atmospheric Administration) spacecraft in LEO (Low Earth Orbit), an instrument package provided by NOAA (USA) and CNES (France), installed on all ATN (Advanced TIROS-N) family satellites since 1978 (also referred to as POES series); the first satellite equipped with Argos was TIROS-N.

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 Platform Transmitter Terminal (PTT) 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 Maryland USA, where it is processed, distributed to the user community, and stored on magnetic tape for archival purposes. ^{1) 2) 3) 4) 5) 6) 7)}

The processing and distribution service is provided commercially by CLS (Collecte Localisation Satellites - created in 1986), a CNES 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.

Argos is a joint program of CNES, NASA and NOAA, which 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, the ADEOS-II spacecraft of JAXA, and the EUMETSAT MetOp spacecraft in orbit.

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.

2) The space segment DCS payload of the service provider

3) The ground segment of the service provider.

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.

Figure 1: Illustration of a NOAA POES spacecraft with Argos/DCS instrumentation (image credit: NOAA)

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.

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

Communication Concept:

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.

Table 1: Characteristics of a PTT

Figure 2: Link access method of a PTT in the ground segment by a S/C (image credit: Service Argos Inc.)

Downlink: The data received by the Argos DCLS is multiplexed on-board by the TIP processor and transmitted to the ground via three paths:

Figure 3: The Argos system concept within the early NOAA POES/TIROS family

• Real-time: the TIP output (8.32 kbit/s, see Figure 3) 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.

Access Method:

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)}

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, 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: ¹⁰⁾

• 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.

Figure 4: Message handling scheme of the Argos-2 version (image credit: NOAA)

• Argos-2 offers greater sensitivity to low-power signals (receiver with sensitivity of -131 dB). The impacts of the higher sensitivity (from -128 to -131 dB) are broad in scope in that the general population of Argos transmitters will require less power.

• So-called PMTs (Platform Messaging Transceivers) are being introduced by the ground segment platforms (PTTs) able to receive and interpret messages sent by the satellite. The new service spectrum permits for example to calibrate platform sensors and to manage duty cycle by switching terminals on and off when needed.

The first NOAA satellite with the Argos-2 system package implemented was NOAA-15 (launch May 13, 1998) with Argos-2 system improvements continued on NOAA-16 (launch Sept. 21, 2000), NOAA-17 (launch June 24, 2002) and NOAA-18 (launch May 20, 2005).

Note: Initially it was planned to provide an enhanced service of a two-way messaging capability for Argos-2. However, this was only accomplished for Argos-Next.

The first Argos-Next (improved Argos-2) implementation has been flown on ADEOS-II (JAXA) with a launch Dec. 4, 2002; Argos-Next service provision tests started on Jan. 29, 2003. Note: end of ADEOS-II mission Oct. 25, 2003, due to spacecraft power failure).
ADEOS-II was the first satellite that carried an Argos two-way instrument package (Argos-Next) allowing users to send messages to their platforms equipped with an Argos receiver on the PTT, called PMT (Platform Messaging Transceiver), via a specific Argos downlink. Argos-Next also supports secure message transmissions. ^{11) 12)}

The Argos-Next implementation on ADEOS-II involved also a corresponding ground segment in Japan and integration into the existing CNES/CLS/NOAA Argos network. This new ground segment provides:

• Reception of data from ADEOS-II, extraction of Argos-Next data and transmission of these data to the CLS processing center in Tokyo, Japan (under JAXA responsibility)

• .Processing and distribution of Argos-2 (and Argos-Next) data and exchange of collected data with the processing center in Toulouse, France (under CNES and CLS responsibility).

Figure 5: The Argos-Next ground segment configuration (image credit: CLS)

Argos-3 instrument package:

The Argos-3 system design proposes three types of terminals to cover more applications. These terminals use different modulation waveforms: BPSK (Bi-Phase Shift Keying), QPSK (Quadra-Phase Shift Keying) and GMSK (Gaussian Minimum Shift Keying), channel coding, data volumes and bit rates with a good stability oscillator for the Doppler location. ^{13) 14)}

The major improvement of the new system is that it will now be able to send orders to its terminals whereas before they were only capable of receiving data (up to Argos-2 inclusive). There are some 10,000 Argos terminals throughout the world that send back data daily on the state of the oceans (currents, salinity, etc.), the habits and movement of some 3,000 animals, whether they be in the sea, on land, or in the air, as well as the localization and position of adventurers and navigators.

Figure 6: Message handling scheme of the Argos-3 version (image credit: NOAA)

Current Argos-2 buoys are sending data back continuously, even if no satellite is close enough to detect it, which uses up the buoys energy supply. With the new system, it will be possible to send messages to tell terminals to emit their data which will thus prolong their lifespan. The current terminals also send their data several times to assure that it is received; but the new system will be able to send messages letting the terminal know that the data has been collected. As a result, the terminals will not need to repeat themselves; hence, they will conserve their batteries. Engineers have also greatly enhanced the channels used for sending and receiving Argos-3 data; they now have broadband capabilities (4.8 kbit/s), which is ten times the amount of data transmission capability as was possible in the previous Argos generation.

Table 2: Main characteristics of the various Argos DCS generation implementations ^{15) 16)}

Table 3: Actual and projected launch dates of POES and MetOp series S/C with Argos instruments

• The MetOp-A spacecraft of EUMETSAT (launch Oct. 19, 2006) carries the first Argos-3 instrument demonstrator package (also referred to as ADCS (Advanced Data Collection System), equipped with a downlink capability of 400 bit/s (low) and the 4.8 kbit/s high data rate channel.

Argos-3 uplink:

• Uplink: support of 3 types of platforms @ 401 MHz

- Standard: 400 bit/s, basic Argos Service; STD (Standard Service): Argos-1, 2, and 3; Bi-phase-L, no coding

- High sensitivity: 400 bit/s, very low power; NG (High Sensitivity Service): additional 5 dB margin; Mix-QPSK, convolutional coding 7, 1/2

- High data rate (HD): 4.8 kbit/s, 5 W; HD: up to 50 kbit per pass; GMSK, convolutional coding 7, 3/4

• Improvement of system sensitivity for STD beacons (probability of correct processing > 99%)

- Argos 1: C/N0 (Carrier-to-Noise) ratio=43 dB Hz

- Argos 2: C/N0=40 dB Hz

- Argos 3 : C/N0=37 dB Hz (-134 dBm at instrument input)

• Improvement of system sensibility with new types of beacons (probability of correct processing > 99%)

- NG: C/N0=34 dB Hz (-137 dBm)

- HD: C/N0=48 dB Hz (-123 dBm)

Argos-3 downlink: transmits messages @ 466 MHz of variable length (~ 200 bit max) to the platforms fitted with appropriate receivers

• Beacon equipped with a receiver = PMT (466 MHz, 400 bit/s, half or full duplex)

• Main functionalities:

- Acknowledgement of uplink messages [from a designated PMT (Platform Messaging Transceiver)]

- Sending of PMT instructions (all kinds of information sent by the user to his PMT)

- Transmission of satellite ephemeris

- Time broadcasting

Figure 7: Message format layout for the various Argos-3 services

Figure 8: Overview of Argos-2/3 instrumentation (image credit: NOAA)

Figure 9: Comparison of Argos-2/3 system performance (image credit: NOAA)

Argos-3 onboard instrumentation:

The Argos-3 project began in 1997. Delivered in 2002, the instrument received new versions of the management and processing software in 2004 and was tested extensively, at satellite level by Astrium, and in conjunction with the operational ground segment by EUMETSAT. All integration activities and associated tests with the Argos-3 instrument are managed by the CNES Argos team.

Figure 10: Exploded view of RPU components (image credit: CNES/CLS)

The Argos-3 onboard instrument is composed of the following components:

• The RPU (Receiver Processor Unit) providing the following functions

- Processing of the received uplink signals

- Downlink management

- Interfaces with the receiver, the TxU and the satellite

• The TxU (Transmitter Unit) sending the emissions (messages) to the PTTs in the ground segment

• The harness for the RPU to TxU connection

The RPU (16 kg, 36 W) and TxU (8kg, 26 W) boxes have a cold internal redundancy that can be activated by TC level. In the same way, the USO (Ultra Stable Oscillator) has a cold redundancy.

RPU (Receiver Processor Unit). The RPU onboard a spacecraft processes received uplink signals @ 401.6 MHz, measures the incoming frequency, time-tags the message, creates and buffers mission telemetry, manages the downlink and acts as interface between the receiver, the TxU (Transmitter Unit) and the satellite. Featuring fully digital processing, the RPU stores messages and either relays them in real-time to the nearest regional antenna - or in deferred time to a global center (maintained by NOAA, Eumetsat). A backup RPU is included as part of the device. The RPU has dimensions of 195 mm x 280 mm x 365 mm.

Figure 11: Illustration of RPU (left) and TxU (right) devices, image credit: Thales Alenia Space

The TxU sends signals to platforms in the ground segment equipped with transceivers (PMTs) @ 466 MHz, including error-free message acknowledgement signals. The downlink allows users to send defined PMT instructions (TxU can selectively address one or more PMTs) and system operators to send global messages/commands such as satellite ephemeris or broadcasting time. Downlink software was specially designed for Argos-3. A backup TxU is included. The TxU has dimensions of 100 mm x 280 mm x 310 mm. The RPU and TxU instruments are being manufactured at Thales Alenia Space.

Figure 12: The MetOp-A spacecraft with the Argos-3/ADCS instrumentation (image credit: CNES/CLS)

International cooperation:

Plans are to embark also an Argos-3 instrument on an ISRO (Indian Space Research Organization) platform in the near future. As a result, the Argos system is an operational system exploited by a number of international programs. It is the main transmission channel and processing chain for data gathered by the following major international ocean observation programs:

• DBCP (Data Buoy Cooperation Panel), a network of drifters and moored buoys

• SOOP (Ship of Opportunity Program), XBT (Expendable Bathythermograph) lines

• Argo profiling float project.

In the future, Argos will be an important part of GEOSS (Global Earth Observation System of Systems) - an international initiative approved by over 60 governments and the European Commission (EC) and designed to improve our understanding of the Earth system.

2) http://www.argosinc.com/documents/sysdesc.pdf

3) D. Meldrum, D. Mercer , O. Peppe, "Developments in Satellite Communication Systems, Update Oct. 2001," URL: http://noaasis.noaa.gov/ARGOS/pdfiles/telecom-review-dec-2001.pdf

4) Argos Newsletters, http://www.cls.fr/html/argos/general/newsletter_tout_en.html

5) "Electronic Code of Federal Regulations (e-CFR)," May 2007, URL: http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=a871cad1eafae55f86452308ac668c56&rgn=div8&view=text&node=15:3.1.2.1.7.0.17.3&idno=15

6) B. Woodward, "The Argos Data Collection and Location System," 2004 Satellite Direct Readout Conference, Dec. 6-10, 2004, Miami, FLA, USA, URL: http://directreadout.noaa.gov/miami04/docs/tues/Bill_Woodward.pdf

7) http://noaasis.noaa.gov/ARGOS/

8) Note: the figure of 650 serviceable platforms in a footprint was provided by `CLS Argos' of Toulouse

9) "A Definition Study of an Advanced Data Collection and Location System (ADCLS)," prepared for GSFC by ECOSYSTEMS International Inc., January 1986

10) http://noaasis.noaa.gov/NOAASIS/ml/satservices.html

11) "Argos Joint Tariff Agreement, Twenty-third Meeting," Final Report, Angra dos Reis, Brazil, 27-29 October 2003, URL: http://www.jodc.go.jp/info/ioc_doc/JCOMM_Other/jta23_Final_Report.pdf

12) P. Schwab, C. Gal, "Argos DCS Ground Segment," 5th International Symposium on Space Mission Operation and Ground Data Systems (SpaceOps 98), June 1-5, 1998, Tokyo, Japan, URL: http://track.sfo.jaxa.jp/spaceops98/paper98/track2/2c003.pdf

13) E. Bouisson, J.-F. Dutrey, P. Guillemot, J. Delporte, "Post-Processing Solutions to Characterize Data Collection and Location Terminals for the Argos-3 System," PSIP'2003 - 3rd International Symposium on Physics in Signal and Image Processing., Jan. 29-31, 2003, Grenoble, France

14) Argos-3 - The New Generation, URL: https://www.argos-system.org/html/system/enhancements_en.html

15) "The Argos-3 (or A-DCS) instruments," 40th Argos Operations Committee meeting, CNES, May 24, 2005, URL: http://noaasis.noaa.gov/ARGOS/conf06/E-2_Argos3_Instrument_40.pdf

16) Information provided by Christopher O'Connors, NOAA, Suitland, MD, USA