Source or Platform Collection Environment

Data were collected in the Antarctic sea ice zone. Buoys can be in the open ocean or within a sea ice pack. An ice flag, determined from Special Sensor Microwave/Imager (SSM/I) passive microwave data, is provided with all data and indicates the amount of sea ice present (0 = no ice, 1 = 1-30% concentration, 2 = 40-100% concentration).

Source or Platform Mission Objectives

Real time data from the IPAB drifting buoys are collected and sent to the GTS by the Collecte, Localisation, Satellites (CLS) Services Argos. Atmospheric pressure measurement and buoy position are the minimum data requirements. Other various surface and upper ocean parameters and sea ice properties would be valuable (WCRP 1993), but are generally not provided.

For global climate research applications, the WCRP suggests a network of drifting buoys with a spacing of 500 km to obtain surface pressure and sea ice drift data over the Antarctic sea ice zone. A network of 50 drifting buoys would provide adequate sampling of the ice covered zone of the Southern Ocean (WCRP 1993) but this goal has yet to be met.

The World Weather Watch (WWW) has also set processing requirements for data ingested by numeric weather prediction models. For basic surface variables (atmospheric pressure, air temperature, and wind vector), a horizontal resolution of 250 km is desired, with four observations per day. For ocean areas, the minimum target for horizontal resolution of surface data is 500 km. In the Antarctic region, the WWW requirements would be satisfied by a network of about 50 instrumented buoys, together with a corresponding network of meteorological stations on the Antarctic continent. (WCRP 1993).

All buoys in the basic network are to be equipped with transmitters to enable basic meteorological data to be transmitted in real time mode. Data are coded in a form suitable for extraction of basic meteorological parameters.

Source or Platform Program Management

The Argos system was developed to locate fixed and mobile buoy platforms and collect data from them. The system is a cooperative effort between the French Space Agency, the National Aeronautics and Space Administration (NASA), and the National Oceanic and Atmospheric Administration (NOAA). Its purpose is to provide an operational service for the duration of the Polar-orbiting Operational Environmental Satellites (POES) beyond the year 2000 (DBCP 1995).

Coverage Information

Satellite coverage is approximately 5200 km in diameter at any given moment, assuming that the line of sight to the NOAA Television Infrared Observation Satellite (TIROS)-N satellite is at least 5 degrees above the horizon. As the satellite orbits, the ground track of this circle produces a swath of 5200 km in width encompassing the earth. At each orbit, this swath covers both north and south poles. A satellite pass directly over a transmitting Platform Transmitter Terminal (PTT) will be within view of that antenna for about 13 minutes. The average satellite pass is approximately 10 minutes, independent of the PTT latitude. The number of satellite passes per day, however, is dependent on the PTT latitude (DBCP 1995).

Data Collection System

The Argos Data Collection and Location System (DCLS) receives messages transmitted by buoy platforms within satellite coverage. Messages are separated in time through the asynchronization of transmissions and the use of different repetition periods. Messages are separated by frequency according to Doppler shifts in carrier frequencies received from various PTTs. Up to eight simultaneous messages can be acquired by the DCLS. The DCLS records time of acquisition, measures the received carrier frequency, and demodulates and records the PDD identification code and sensor data. Data are formatted and transmitted to the TIROS Information Processor (TIP) (DBCP 1995).

Communication Links

All platforms used in conjunction with the Argos system carry an electronic transmitter called a PTT. A PTT includes an antenna, a Radio Frequency (RF) modulator and power amplifier, message generation logic, a sensor interface unit, an ultra-stable oscillator, and a power supply.

Since the NOAA-K satellite launch in 1996, all PTTs transmit on the same frequency band, from 401.620 to 401.680 MHz. Each PTT transmits at regular intervals. The period ranges from 90 to 120 seconds for platforms that utilize a Global Positioning System (GPS), and 200 seconds for platforms that collect data only. The duration of a single transmission burst depends on the data message length, but is usually less than one second (360 to 920 ms). The peak radiated power is less than 5 W (DBCP 1995).

Ground Segment Information

Data Acquisition and Processing

The NOAA TIROS-N satellites receive and record buoy data and transmit them to the NOAA National Environmental Satellite, Data, and Information Service (NESDIS) in Suitland, Maryland; or Météorologie National in Lannion, France; where they are separated from other satellite instrument data. They are then forwarded to the Argos Global Processing Centers (GPCs) in Toulouse, France, and Landover, Maryland, USA. The data are processed to calculate transmitter locations and interpret sensor outputs.

When data are sent to the GPCs, the following processing tasks are performed:

1. Decoding of the PTT messages and processing of the sensor data
2. Computation of PTT locations from Doppler shifts and orbital data
3. Storage of these processing outputs on computer files

Technical details of sensor data processing and platform location are provided in DBCP 1995.

Key Variables

See Parameters section.

Principles of Operation

Buoys collect and average sensor data and transmit them to two polar-orbiting satellites every 60 or 90 seconds. The buoy locations are determined from the Doppler shifts of the transmitter signals observed at different satellite positions. Data transmission and localization are conducted through the Collecte, Localisation, Satellites (CLS) Services Argos. Most buoys are equipped with GPS receivers for improved location accuracy. ARGOS maintains a monthly record of all platforms. Only the data with the largest number of identical transmissions within a satellite pass are saved. Approximately 25 independent transmissions are available for each platform per day. Data are transmitted in binary or hexadecimal format, and are converted to physical units based on the specifications of the sensors and on additional calibrations and buoy comparisons. All data are checked for permitted sensor tolerances, and a replacement value is assigned to erroneous data. Data are rearranged in chronological order. Outliers are found by objective methods and visual editing. Components of ice drift velocity are derived from the changes of positions. Using a smoothing cubic spline interpolation, regularly spaced time series are generated, which have a time resolution of three hours or one day. Long gaps are filled by linear interpolation (WCRP 1993).

Manufacturer of Sensor or Instrument

Buoys are built by several manufacturers who employ different mechanical and electronic designs. A chart summarizes buoy manufacturers used by each participating IPAB agency member. A complete list of buoy manufacturers is available from the DBCP web site. Further information on sensor specifications may be available from participating agencies:

• Alfred Wegener Institute, Germany
• Australian Antarctic Division, Australia
• Antarctic Cooperative Research Centre, Australia
• Finnish Institute of Marine Research and Dept. of Geophysics, University of Helsinki
• Geophysical Institute, University of Alaska, USA
• Lamont-Doherty Earth Observatory, Columbia University, USA
• Scott Polar Research Institute, United Kingdom