Hydrological Applications of Remote Sensing and Remote Data Transmission (Proceedings of the Hamburg Symposium, August 1983). IAHS Publ. no. 145.

Data collection and location by satellite: the Argos system

MICHEL TAILLADE Service Argos - Centre National d'Etudes Spatiales, 18 avenue Edouard Belin, 31055 Toulouse Cedex, France

ABSTRACT The French Argos system onboard meteorological NOAA satellites deals with the data collection and the platform location of environmental programmes (meteorology, oceanography, offshore, hydrology, biology, vulcanology, seismology). The Argos system is composed of the user platforms (buoys, balloons, fixed or offshore stations, animals, etc.), the onboard satellite equipment, the data processing centre and the data distribution system for Argos users. Platforms data are available in Toulouse (France) a few hours after their acquisition by satellite and are easily accessible, via usual telephone or telex networks, from anywhere in the world. After three years of operational use, it can be said this unique system has proved its efficiency and reliability and will continue to be used by dozens of countries during the next decade.

Collecte des données et localisation par satellites: le système Argos RESUME L'équipement français Argos est embarqué à bord des satellites de la NOAA; il permet la collecte de données et la localisation de plates-formes dans différents domaines (météorologie, océanographie, offshore, hydrologie, biologie, vulcanologie, seismologie). Le système Argos comprend dans son ensemble: les plates-formes des utilisateurs dont le type diffère selon le domaine d'activité (bouées, ballons, plates-formes fixes ou mobiles, etc.), l'équipement embarqué à bord des satellites, le Centre Informatique de Toulouse (France), le système de distribution des données. La mise à disposition des résultats a lieu au Centre de Calcul de Toulouse. Les données sont disponibles quelques heures après le passage du satellite dans une région donnée. Elles peuvent être prélevées par téléphone ou par télex suivant une procédure bien définie. Après trois ans de service opérationnel, on peut affirmer que le système Argos a fait ses preuves et qu'il sera utilisé dans la prochaine décennie par des dizaines de pays.

INTRODUCTION

The Argos system is primarily intended to locate fixed or mobile platforms and to collect environmental data from these platforms.

33

34 Michel Taillade

This system is the result of a cooperative programme between the Centre National d'Etudes Spatiales (CNES, France), the National Aeronautics and Space Administration (NASA, USA), and the National Oceanic and Atmospheric Administration (NOAA, USA).

CNES, NASA and NOAA are bound by a Memorandum of Agreement signed in December 1974. The main mission has been to provide an operational service for the entire duration of the TIROS N/NOAA programme, that is from 1979 until at least 1990.

The responsibilities of each agency (Fig.l) can be summarized as follows :

(a) NASA manages the design of TIROS-N (RCA is subcontractor) and the construction of TIROS-N and NOAA-A through NOAA-I spacecraft;

(b) NOAA develops mission requirements, operates the spacecrafts and the ground system;

(c) CNES supplies the Argos data collection system (DCS) for integration into the spacecrafts and the Argos data processing centre (DPC), and manages the Argos DPC and distributes the data.

Service Argos operates the system under the supervision of the Argos Operations Committee (NASA, NOAA, CNES). Its duties include attending to user's interests, coordinating system utilization and promotion, and supervising the system in general.

NASA

MANAGE CONSTRUCTION OF SATELLITES

NOAA

OPERATE SATELLITES AND GROUND SYSTEM „

/ OPERATE * THE ENTIRE

ARGOS SYSTEM

CNES

SUPPLY ON BOARD EQUIPMENTS ANO DATA PROCESSING CENTER

FIG.1 Responsibilities of each agency.

USER PLATFORMS

The electronics package to equip a platform of the user's choice is termed a Platform Transmitter Terminal or PTT. Argos PTTs may be mounted on fixed platforms (meteorological, hydrological stations, etc.) and moving platforms (buoys, balloons, icebergs, animals, etc.). However, all include as a minimum: ultrastable oscillator (severity or stability requirement depends on whether the platform's location is required), antenna, digital message generation logic, power supply, interfaces with sensors (Fig.2).

The transmitted standard message comprises an unmodulated part, for satellite receiver lock-on and a modulated one which includes synchronization signals, PTT identification code and sensor data.

ARGOS OPERATIONS COMMITTEE

/ SERVICE V I ARGOS J

The Argos system 35

\ i / \ | /

UHF TRANSMITTER

DIGITAL ELECTRONIC

STANDARD

INTERFACE FOR SENSORS

, i , i

USER'S SENSORS

FIG.2 The user platform.

The main characteristics of Argos PTTs can be summarized as follows :

(a) Transmission frequency is 401.650 MHz at regular intervals, i.e. 40-60 s in the case of platforms to be positioned, every 100-200 s for a data-collection-only-type platform. Message duration depends on the number of sensor output values to be transmitted, but it is always less than 1 s.

(b) Radiated power of 3 W with a low power consumption of just 200 mW which allows power sources such as dry cells, conventional batteries and even solar cells.

(c) Message capacity for sensor data is 32 to 256 bits; this length can be adapted to user requirements, in the case of standard processing (see Argos DPC description), by fields of 32 bits each. Each of 32 possible sensors can then be generated between 0 and 16 bits, inclusive.

(d) Ease of implementation, since all Argos PTTs operate on the same frequency and are of moderate price. The equipment for an individual up-link with the satellite can be purchased for US $2000.

THE ONBOARD EQUIPMENT

The input to the Argos onboard DCS "sees" a composite signal comprising a mixture of messages generated by a number of different PTTs within the satellite's coverage.

As each message is acquired, the DCS records the time and date, measures the carrier frequency and demodulates the platform identification number and sensor data (Fig.3). These data are then

36 Michel Taillade

RECEIVER PROCESSING

UNIT

TELEMETRY

ENCODER

SEARCH UNIT CONTROL UNIT TIP

FIG.3 The onboard DCS.

formatted and stored onboard magnetic tape recorders. Each time one of the two satellites passes over one of the three telemetry stations (Wallops Island, Virginia, USA; Gilmore Creek, Alaska, USA; Lannion, France), the data recorded on tape are read-out and transmitted to the ground stations. Once a satellite has completed telemetry data transmission for a particular pass, the received data are transmitted to NESDIS (National Environmental Satellite, Data and Information Service) Center at Suitland, Maryland, USA. Data concerning the Argos system are separated from those concerning other satellite experiments and transmitted to the Argos DPC located in the Toulouse Space Centre in France (Fig.4).

The instantaneous coverage on the ground corresponds to a circle with a radius of 2600 km for a minimum receiving angle of 5°. From one day to the next the "contact" passes for a given platform come nearly at the same time within a satellite's coverage (sunsynchronous satellites).

The indicated number of contact passes per day (Table 1) corresponds to the mean number of passes during which the potential contact time is at least 10 minutes.

The problem of how to select messages for acquisition is

FIG.4 The Argos data flow.

The Argos system 37

TABLE 1 Number of "contact" passes per day

Latitude Number of contacts: Minimum Mean Maximum

0 ±15

±30 ±45 ±55

±65 ±75 ±90

6 8 8

10

16 21 28 28

7 8 9

11

16 22 28 28

8 9

12 12

18

23 28

28

essentially the "random access" problem. The satellite must acquire the largest possible number of messages, yet the number of message processing channels is obviously limited. The key design concepts can be summarized as follows: messages from platforms within the satellite coverage appear at the input of the onboard receiver in a random fashion; message separation in time is obtained through the asynchronization of the transmissions and the use of different repetition periods; message separation in frequency is achieved as a result of the different Doppler shifts in the carrier frequency transmitted by the various platforms. Up to four simultaneous messages can be aquired providing they are separated in frequency. The onboard DCS performs within instrument specification, i.e. the probability of acquiring a message during one pass of a satellite, providing all messages transmitted during this pass are identical, is over 0.99.

Direct readout capabilities

PTT messages received by the Argos DCS are mixed onboard the satellite with data from other experiments. The combined data streams are handled in two ways: stored onboard tape recorders and transmitted to the ground in real time by a 136.77 or 137.77 MHz transmitter. Thus, the satellite acts like a relay that continuously transmits to the ground a signal containing data from PTTs within the satellite's coverage at that time. By using a suitable receiving station, the telemetry signal can be picked up (within 2000 km radius around the station) and the data extracted each time the satellites pass overhead (Fig.5).

These local user terminals (LUT) are designed mostly to handle data-collection-only platforms. The main advantages of a LUT are the real-time data availability and thus the alarm capabilities of this system. The drawbacks compared to a centralized system such as the Argos DPC include: the duplication of equipment and permanent manpower to ensure operational use; the satellite coverage limitations, and the non-availability of data at a single place (for example the decision centre).

38 Michel Taillade

Argos direct readout ground station

Jjft'Sfc/

tiiitGltite TUws N

station de reception dtmct readout station

FIG.5 The local user terminal.

THE DATA PROCESSING CENTRE

System configuration

The Argos data processing system was designed for a reliability of 99%. This is achieved by providing redundant facilities at all critical points. All telemetry data are transmitted from the telemetry receiving stations to the NOAA-NESDIS processing centre at Suitland, Maryland, where the Argos data are extracted for retransmission to the Argos DPC over a dedicated 7200 bps communications link. In Toulouse, the incoming Argos data are received and stored by a French-built Télémécanique T1600 acquisition computer. A second modem is on standby for the NOAA-CNES link. The computers, including a standby T1600, are linked to the main data processing facility via a high-bit-rate (4800 baud) link. The main facility is an Iris 80 bi-processor manufactured by Cii Honeywell-Bull.

Results are distributed in real time by a third T1600 computer which, through the intermediary of a microprocessor-based system, manages the dissemination of information to all system users. Backup is provided by the standby T1600 computer. All components of the system can be readily and quickly reconfigured so that operational services can be performed with minimum degradation under virtually all conditions (Fig.6).

Preprocessing

The real time preprocessing performed can be summarized as follows: management of telemetry data streams; elimination of redundant data and arrangement of telemetry data in chronological order; separation of users telemetry data and satellite housekeeping telemetry data;

The Argos system 39

OATA PROCESSING COMPUTER

IRIS 80 - Bl - PROCESSOR

FROM

NOAA-

ACQUISITION

COMPUTER

0 DATA SANK

ir

REAL TIME

DISSEMINATION

COMPUTER

0 IN REAL TIME

FIG.6 The Argos data processing centre flow chart.

processing of housekeeping telemetry data; processing of users telemetry data - message restitution and verification of content, and time coding of each message in universal time (UT); determination of incoming signal level (in decibels) and Doppler frequency (in hertz); and, finally, message classification by PTT (Fig.7).

Platform location and accuracy

The platform location is determined solely by measuring the Doppler effect on the carrier frequency of incoming messages. As the satellite's orbit is accurately known, five Doppler measurements during a given pass are sufficient to attempt a location calculation.

The number of location calculations per day as a function of

SENSORS

PROCESSING

FIG.7 Sof

TH

ACQUISITION

•

PRE PROCESSING

k

H i

MANAGEMENT

PLATFORM

LOCATION

' RESULTS

OUTPUT

/ / I tware f

A 7OPV

~\ DATATION AND

ORBIT DETERMINATION

\

chart.

40 Michel Taillade

LATITUDE m DEGREES

D 10 20 30 40 50 SO 70 SO 90

FIG.8 Number of location calculations per day.

latitude is shown in Fig.8. The location accuracy of the Argos system is affected by

numerous factors. In so far as the PTTs are concerned, the most important parameter remains the medium-term (20-minute) stability of the oscillator, irrespective of the cause of frequency drift.

Table 2 summarizes the results on location accuracy both at la and at 3a, given as a function of oscillator medium-term stability.

Two important conclusions can be drawn. First, even if the required accuracy is only very low, it is not advisable to use oscillators for which the medium-term stability is less than 10~7. At 2 x 10" the error is several tens of kilometres while the system does not operate at all for greater errors. Secondly, with very stable oscillators (10 ) the location accuracy may reach 150 m at la and 500 m at 3a. Since the system is apparently free from systematic errors, by averaging the positions obtained for fixed platforms over several days it is possible to reduce the error to less than 100 m.

TABLE 2 Location accuracy of the Argos system as a function of oscillator stability

Oscillator stability

2 5

2 5

X

X

X

X

10' 10' 10' 10' 10' 7.0"

- 9

- 9

- 8

"8

- 8

- 7

Accuracy At la 66°

(m). At 3a 99°

150 500

1100 2100 4000 5500

500 1300 2000 3600 6000 9000

The Argos system 41

Sensor data processing

Sensor data processing is also performed in real time. The data gathered by platform sensors can be processed in several different ways. Data from each platform sensor are processed independently so that a different processing option can be selected for each.

With respect to data code conversion, in the case of sensors that generate data in pure binary code, the code conversion program can supply data in octal, decimal or hexadecimal. For data received in BCD (binary coded decimal) form, the only available conversion is into decimal. For data encoded directly in ASCII, there is no code conversion, i.e. the output data will be in the same code.

Standard processing corresponds to the conversion of sensor data into engineering parameters. This is performed using a calibration curve, defined by a maximum of 20 points, provided by the user for each platform sensor. The points may be chosen so as to optimize the accuracy of the encoded data. Each sensor calibration curve must be independent of all others.

At the request of various users, a number of special sensor data processing options have been developed and integrated into the online processing system. The definition of such processing options is subject to negotiations between the user and Service Argos so that the complexity of the processing programs to be developed can be determined prior to their integration with the system software.

DATA DISTRIBUTION SYSTEM

It is important for a system of this type, which is primarily intended to provide user services, to offer data distribution options compatible with modern data transmission techniques. The data distribution system was designed to supply results the moment that they became available, that is, immediately after the corresponding data flow has been processed.

On-line distribution

On-line data distribution may be via: (a) The Global Telecommunications System (GTS): the Argos DPC is

linked to the main GTS hub in Paris; this means that results can be available in Canada or the USA in 20 minutes. The results are disseminated either as soon as the processing is completed or at synoptic hours, but in any case all data are converted into meteorological codes (special processing option), i.e. COLBA for position and scientific measurement from tropospheric balloons, DRIBU for the same type of data concerning drifting buoys, HYDRA for hydrological data, etc.

(b) Switched networks: telephone and telex lines are available at Service Argos to all system users. The dial-in and dialogue procedures are rather simple and the user requires a telephone and an acoustic coupler terminal or a standard telex.

(c) Dedicated links or networks: a permanent dedicated link has been developed for the Toulouse-Suitland connection. It is possible to gain direct access to the results of North American experiments

42 Michel Taillade

through a terminal concentrator at Suitland by dialling one of the two telephone numbers of this remote "Argos computer". A connection to the French TRANSPAC dedicated network is working allowing the possibility of access on other dedicated networks, such as EURONET and TIMENET, because of their mutual interconnection (Fig.9).

When accessing the computer, users can choose between three files :

(a) AJOUR: for all platforms belonging to a given experiment, this file contains the most recent platform location data and the last sensor message selected according to predetermined quality criteria.

(b) TELEX: for all platforms belonging to an experiment, this file contains, in chronological order, one sensor message and the corresponding position data for each of the satellites.

(c) DISPOSE: for all platforms belonging to a given experiment, this file contains, in chronological order, all sensor messages and all location data for all orbits.

In the last two files, data that are more than 12-h old are eliminated and replaced by more recent data.

COLBA. DR1BU. HYDR

EUROPEAN STANDARD

. TELEPHONE . ] Z-M SWITCHED

NETWORK

CALL

U S TERMINAL

TRANSPAC

• - DEDICATED LINES

RESULTS FILES ON LINE DISTRIBUTION

FIG.9 Argos data distribution,

Off-line distribution

Results for off-line distribution are obtained from the data bank which itself is supplied in real time. Data stored in the data bank are read out once a week for all current experiments. These data, in the form of computer printouts or magnetic tapes, are forwarded once a fortnight or once a month, as requested by the user, by the fastest postal means available. Magnetic tapes containing data from several experiments are also available.

Data availability

The critical parameter for an operational data distribution system is the overall response time. This corresponds to the interval

The Argos system 43

TABLE 3 Number of users of the Argos system

USA France Canada Norway United Kingdom Japan Australia Sweden Denmark New Zealand South Africa Portugal Finland Germany India New Guinea Brazil

17

1980: Number

30 28 17

6 5 5 3 2 1 2 1 1 1 1 1 1 1

106

%

29 27 16

6 5 5 3 2 1 2 1 1 1 1 1 1 1

1981: Number

38 33 22

8 7 5 3 2 2 2 1 1 1 1 1 1 1

129

%

29 25 17

6 5. 4 2 1. 1. 1. 1 1 1 1 1 1 1

5

5 5 5

1982: Number

45 35 19 11 12 11

4 3 2 1 2 1 0 3 2 0 1

152

%

30 23 12.5 7 8 7 3 2 1 0.7 1 0.7 0 2 1 0 0.7

between the time when a given platform message is acquired by the satellite and the time when the data are made available to the user following processing at the Argos DPC in Toulouse. The breakdown of this interval depends on the orbit interval (up to 100 m), the time for data transmission from the telemetry stations to NOAA-NESS facility, the transmission over the dedicated Suitland-Toulouse link and the processing time for data generated by a single orbit. The statistics corresponding to 31 months of observation are as follows:

56% of all data were made available in less than 2 h 30 min; 66% of all data in less than 3 h; 87.5% of all data in less than 6 h.

i i

* P T 7 1 x 2 4 4 \

5@ 2 1 6 - -2 0 2 - -188 174 1 S B S I Tl I l + H + H + m - H + l + H ! H + l + + m + I H + H + H I I I H-Hit-m-H

v-J

FIG.10 Number of platforms using Argos.

44 Michel Taillade

SYSTEM UTILIZATION

Platform number

Figure 10 indicates the evolution of the number of platforms in operation from the beginning of 1981.

Number of users

Table 3 indicates the evolution of the number of the users admitted to the system to the end of 1982. This corresponds to programs under preparation for implementation and those already operating in the system.