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                         ARGOS DCS GROUND SEGMENT
                                    Philippe Schwab1 and Claude Gal2
                                    Collecte Localisation Satellites
                          8-10 rue Hermes, 31526 Ramonville St Agne, France
                          Fax: +33-5-61-39-48-40, E-mail:
                                   Centre National d'Etudes Spatiales
                        18, avenue Edouard Belin, 31055 Toulouse Cedex, France
                          Fax: +33-5-61-28-18-82, E-mail:


Argos is a satellite-based location and data collection system oriented mainly towards studying and
protecting the environment. It has operated since 1978. Argos is accessible anytime, anywhere in the
world, thanks to a ground segment architecture designed to ensure high reliability and operational
performance, as well as good scalability. Scalability is key to the system’s continued success, as Argos is
something of a victim of its own popularity. Already in service for 20 years, Argos must now evolve if it
is to continue operating up to 2015 as planned. It is not always easy to improve system performance,
introduce new features and satisfy emerging needs while maintaining compatibility with existing systems.
Yet that is the task facing the French space agency CNES and its subsidiary CLS as they prepare to fly
new Argos instruments first on the Japanese space agency NASDA’s ADEOS-II satellite, and then on the
European organization EUMETSAT’s METOP satellites, not forgetting of course NOAA, which was the
first to fly Argos on its Tiros series of satellites.

                                            1. INTRODUCTION

Argos is a global satellite-based location and data collection service dedicated to the study and protection
of the environment. Established under an agreement between the French space agency (Centre National
d'Etudes Spatiales, CNES) and the US National Oceanic and Atmospheric Administration (NOAA), the
Argos system has been operating since 1978. It is operated and managed by Collecte Localisation
Satellites (CLS), a CNES subsidiary based in Toulouse, France.

Argos can locate any mobile carrying a suitable transmitter, anywhere in the world, to within around 150
meters. It also collects data from sensors connected to the transmitter. Argos receivers-processors are
flying on the NOAA Tiros-N series of polar-orbiting satellites, providing full global coverage.

Argos transmitters are used for a wide range of applications, chief among them oceanography and
meteorology, animal tracking, management of fish stocks and monitoring of hazardous materials. Today,
Argos processes data acquired round the clock by 6,500 platforms belonging to some 350 users in 63

The success of the Argos system is due largely to its ability to adapt to users’ evolving needs. In service
for 20 years, it will continue operating well after the turn of the century. CNES and its subsidiary CLS are
committed to maintaining the system’s adaptability and flexibility, with work under way to upgrade the
space and ground segments. Efforts are focusing on improving service, meeting new needs, ensuring

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compatibility with existing systems, and preserving the system’s key strengths (ease of use; worldwide
operation; lightweight, compact, low-power transmitters; operational availability, etc.) while serving new
markets and guaranteeing continuity of service for the next 15 years and beyond.

This paper describes some of the enhancements geared to take the Argos system on another 15 years. We
will concentrate on three major themes:
        - new international partners
        - reliability and operational features
        - system performance.

                               2. NEW INTERNATIONAL PARTNERS

As we stated in our introduction, Argos was established under an agreement between the French space
agency CNES and NOAA (USA). CNES supplies the Argos instruments which NOAA has committed to
fly on all satellites in its Tiros series, the last of these being NOAA N' scheduled for launch in 2004.

The rules governing the use of the Argos system and the partners’ mutual obligations are defined in a
Memorandum Of Understanding. An Operations Committee meets annually to discuss system operation
issues and to chart the system’s future development. The committee recently decided to seek new
partners for Argos, in Japan and Europe.

The agreement with Japan will see the launch of a second-generation Argos instrument on board the
Adeos-II satellite in July 2000, while the European organization EUMETSAT will fly a third-generation
instrument on its Metop satellites shortly afterwards.

                                 Figure 1 : Planning of the launchings

The extension of worldwide Argos cooperation, now comprising four major agencies—CNES, NOAA,
NASDA and EUMETSAT—reflects the size of the worldwide Argos user community and will bring

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many benefits. In particular, the increased number of satellites flying Argos will enhance reliability and
system performance. But above all, these organizations will prove a tremendous asset in stimulating
efforts to promote wider use of the Argos system.

As a result, CNES and its subsidiary CLS will be expected to operate and maintain an increasingly
complex ground segment processing a rich variety of data and supporting a large number of interfaces.
With more capabilities on offer, users will probably demand ever more reliability, availability and
operational performance.
The Argos system was designed to continue operating beyond 2010. This means that data processing and
services must evolve to match users’ changing requirements. Equally important, the level of performance
users accept today in terms of data collection capacity, processing throughput times, location accuracy,
data distribution and so forth will mostly be considered inadequate tomorrow, such is the pace of
technological change and the scale of users' expectations. Ten years ago, being able to retrieve data from
anywhere in the world was considered a great feat. Nowadays, users take it for granted that they can
consult or access their data from anywhere. It is therefore vital to keep abreast of new technologies and be
able to implement them.

The improvements to the Argos system made necessary by its expanding international reach therefore call
for a robust ground segment architecture able to stand the test of time while offering the flexibility to
adapt to new technologies and evolving needs. The people who operate this ground segment play an
essential role, as they must listen responsively to users’ needs, propose solutions and embrace
technological developments, as well as provide operational expertise. These qualities are the driving
factors behind the concepts described below.


Argos system users have come to appreciate continuous, year-round availability.

That kind of availability is a result of the ground segment organization established by CNES and CLS. As
we shall see, this organization is robust, designed to last, and geared to providing uninterrupted
operational service. It is also able to scale to users’ changing needs and support new partners.

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The current Argos ground segment architecture is shown below:

                          Fairbanks   Wallops   Lannion
                            USA        USA      France

                  Hawai                                     Réunion Island
        Halifax    USA                                         France Cap Town
        Canada                                                           South Africa
  Edmonton                                                                                Regional Processing Center
   Canada                         NOAA Satellites
                                                                                                  Tokyo / Japan


      Global Processing                Global Processing
           Center                           Center                                          Darwin       Perth
         Largo / USA                   Toulouse / France                     Melbourne     Australie    Australie     Wellington
                                                                              Australie                                  NL
                                                                                      NOAA Satellites

           USERS                      USERS

                                                          Regional Processing Center

                                                             Melbourne / Australia


Global centers
There are two Global Processing Centers (GPCs), one in Toulouse, France, and one in Largo, MD, USA.
These centers are the hub of the Argos ground segment architecture. Their main missions are to:
• process all Argos telemetry from all satellites and receiving stations;
• process and distribute data acquired from all Argos Platform Transmitter Terminals (PTTs), in
   identical fashion;
• monitor system technical parameters;
• perform vital system operation tasks (orbit determination, time-tagging, etc.).

The two GPCs operate concurrently but independently. So if one GPC goes down, the other will back it
Because both GPCs receive all the satellite telemetry simultaneously, they do not depend on each other.

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This is important in ensuring uninterrupted service in the event of an incident, assuming that the
probability of both GPCs being down at the same time is nil. However, we need to go further to achieve
total transparency.
In particular, the GPCs must run identical processing systems concurrently. Although this may seem
obvious, it is far from easy to implement, requiring effective configuration management, common
hardware and software policies at both centers, and strict application of the same corrective software
maintenance and revision procedures.
Each GPC must also support location of and data collection from all Argos PTTs, and they must perform
the task in the same way. This means that each time a GPC receives new information on PTT processing
requirements, it must relay that information immediately to the other GPC to maintain data consistency
and transparency for users.
Last, but not least, each GPC is operated by its own highly trained team. Neither Center is master or
slave, and redundancy is not provided by a “standby” center waiting to come on line if the nominal GPC
fails—both centers perform the same tasks independently using identical resources.

If one of the GPCs goes down, continuity of service is easily ensured by re-routing incoming calls to the
other. And as the two GPCs are linked by a leased line, users do not incur extra long-distance
communication costs.

Lastly, each GPC provides high availability and performance through hardware clustering, RAID 0+1
disk architectures and backed up communications links connected to dedicated routers.

Regional centers
Regional Processing Centers (RPCs) provide users with local access to Argos results. RPCs only process
data from PTTs operating within their region (e.g., the Japanese RPC only processes and distributes data
from Japanese users’ PTTs).
RPCs only process telemetry from the regional receiving stations to which they are directly connected.
For example, the Australian RPC does not process telemetry from the main ground stations in Fairbanks
and Wallops Island because it does not receive it directly. But it does process telemetry from Perth,
Darwin, Casey, Melbourne and Wellington. Telemetry not received directly by an RPC is relayed after
pre-processing from the GPC in Toulouse (or Largo if Toulouse is down). Since the GPC processes and
distributes data from all PTTs, it relays location and sensor data to each RPC for all PTTs within its
region. So in fact, an RPC does not have to be connected to a receiving station at all. This is the case at
the moment in Japan, although it may change very soon. More important, even if an RPC’s antenna fails
there are no delays in delivering results to users.

RPCs provide the same location accuracy as the GPCs. They can do this because they use the same
processing software, and because the GPCs send satellite ephemeris data daily to the RPCs. GPCs
perform all satellite orbit determination computations and other processing-intensive tasks.

RPCs are not backed up in the same way as the GPCs. In particular, they are not staffed round the clock.
RPCs are managed and monitored remotely from the Toulouse center, which backs them up in the event
of a failure.

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Main ground receiving stations
There are three main ground stations, in Fairbanks (USA), Wallops Island (USA) and Lannion (France).
The US stations are operated by NOAA. The French station belongs to the French weather bureau Météo-
France and is used only to cover blind orbits. These stations are called “main ground stations” as they
receive all messages recorded by the satellites during a complete orbital revolution. Argos thus provides
global coverage.
Ground stations are run and maintained by operational organizations to ensure high reliability and full
compatibility with the Argos system.

Regional receiving stations
Regional receiving stations worldwide receive data in real time acquired by the satellite as it passes over
transmitters within station visibility. This capability helps to make data available to users more quickly.
The ground stations at Fairbanks, Wallops Island and Lannion also act as regional receiving stations.

Not all regional receiving stations are owned by CNES/CLS but rather by national weather services
(Bureau of Meteorology in Australia, MetOffice in New Zealand, Météo-France in Réunion Island, etc.)
which use them mainly to acquire and process AVHRR satellite imagery. They do not guarantee absolute
operational reliability, but in practice performance has proved very satisfactory. To offset the possibility
that a station might become unavailable, we constantly strive to extend the network of regional receiving
stations. In this way, less than 100% reliability is counterbalanced by the large number of access points.

Communication networks
The wide variety of communications networks and equipment receiving telemetry for processing and
distribution of results to Argos users is a crucial element of the ground segment. These resources include
leased links, the Internet and public X.25 networks. The main protocols are DECNET, TCP/IP and X.25.

The two GPCs are interconnected by a dedicated 64-kbps link, itself backed up by an ISDN line.
Communications with the ground stations in Fairbanks and Wallops Island run over dedicated links
mostly belonging to NOAA.
Communications with regional receiving stations run over the Internet. It is common knowledge that the
Internet is risky and far from reliable. So why do we use it? In fact, it is the only available network
offering such easy access to all regional receiving station operators. Once again, likely problems can be
overcome thanks to the large number of receiving stations.
Today, most users communicate with Argos over the Internet using SMTP, Telnet, FTP and HTTP.
Direct PSTN links have been dropped in favor of SMTP messaging. Where users need enhanced
reliability, we suggest public X.25 networks which are widely available worldwide and offer outstanding
availability, but such services are not cheap. Fax and telex are also used for specific applications.


Many performance enhancements are planned for the Argos system, too numerous to mention here. We
shall therefore concentrate on what is undoubtedly the most spectacular—the Argos/Next project.
Underlying this project is the decision by CNES and NASDA to fly an Argos instrument on the Adeos-II
satellite. Adding a fourth satellite will immediately improve data throughput times, increase the amount
of location and sensor data distributed, and enhance system reliability.

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However, CNES has decided to go further in response to the main user requests which are not completely
satisfied with the only transmitter-to-satellite uplink and that its data collection capacity is insufficient for
certain kinds of users. The scope of the Argos/Next project has therefore been extended to include a new
satellite-to-transmitter downlink to give Argos two-way capability. This downlink will also increase data
collection capacity.

The downlink will enable sending of messages to PTTs fitted with suitable receivers. These messages,
called commands, will be interpreted by the PTT. The downlink will have no impact on the existing
uplink, and is therefore perfectly in line with the overall Argos system upgrading strategy. It will provide
three operating modes:

        - “Programmed” mode: for sending commands to a PTT or group of PTTs at given times over a
given period. This mode will generally be used with PTTs at known locations.

         - “Trigger” mode: for sending commands to a PTT on detection by the satellite, i.e., when the
satellite receives a message from the PTT on the uplink. This mode will be used mostly when the PTT’s
location is not accurately known or if it is operating intermittently.

        - “Uplink data acknowledgement mode” mode: the satellite will communicate with the PTT using
a special protocol designed to increase data transmission reliability. The satellite will automatically send
an acknowledgment to the PTT for each message successfully received, thus authorizing the PTT to
continue transmitting. This mode will increase the Argos system’s data collection capacity significantly,
with several kbps of useful data expected on each satellite pass.

The downlink will require the addition of two new elements to the ground segment:
         - a Downlink Message Management Center (DMMC) to centralize, validate, plan and transmit to
the satellite requests from users to downlink messages in the mode selected. This center will be located in
Toulouse, and closely linked to the existing processing center. A back-up center will also be installed at
the other global center in Largo (USA).
         - a network of four master transmitters at strategic locations around the globe, acting as the link
between satellites and the DMMC. These master transmitters will uplink all commands.

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The new Argos ground segment organization, including Argos/Next project elements, is shown below:

                                                                                                                    Wallops         Hatoyama
                                                                                                 Fairbanks           USA             Japan
                                                                                                               ADEOS-II Satellites

                                       Fairbanks   Wallops       Lannion
                                         USA        USA          France

                            Hawai                                          Réunion Island
                Halifax      USA                                              France Cap Town
                Canada                                                                  South Africa
          Edmonton                                                                                            Regional Processing Center
           Canada                              NOAA Satellites
                                                                                                                         Tokyo / Japan


              Global Processing                     Global Processing
                   Center                                Center                                               Darwin         Perth
                 Largo / USA                        Toulouse / France                       Melbourne        Australie      Australie   Wellington
                                                                                             Australie                                     NL
                                                                                                         NOAA Satellites

                   USERS                              USERS

                                                                                                             Regional Processing Center

             Dowlink Messaging                     Dowlink Messaging                                             Melbourne / Australia
             Management Center                     Management Center

                  Largo / USA                        Toulouse / France

                          Fairbanks   Kiruna Toulouse Hatoyama

The Argos/Next project will have a considerable impact on the existing ground segment. First, it will
need to interface with our partner NASDA’s ground segment, and second, the new downlink will mean
that it must support the master transmitters and the DMMC.

The Argos/Next project is also the precursor of the third-generation Argos instruments currently in
development and scheduled to fly on EUMETSAT’s METOP satellites, on NOAA N' and on ADEOS-II
follow-on, if NASDA confirms.

The third-generation Argos instrument will be compatible with existing systems while carrying a more
sensitive receiver to meet the needs of microtransmitter users. Data collection capacity will also be
increased by the planned 5 kbps link.
True to its underlying mission and development strategy, the Argos system continues to evolve by
building on its key strengths: compatibility, continuity, responsiveness and adaptability—strengths that
are the result of a ground segment organization providing high reliability and availability.


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