Page 38 - Satellite Communications, Fourth Edition

18 Chapter One

1.6 Argos System
The Argos data collection system (DCS) collects environmental data
radioed up from platform transmitter terminals (PTT) (Argos, 2005).
The characteristics of the PTT are shown in Table 1.8.
The transmitters can be installed on many kinds of platforms, includ-
ing fixed and drifting buoys, balloons, and animals. The physical size of
the transmitters depends on the application. These can weigh as little
as 17 g for transmitters fitted to birds, to track their migratory patterns.
The PTTs transmit automatically at preset intervals, and those within
the 6000 km swath are received by the satellite. As mentioned, the satel-
lite completes about 14 orbits daily, and all orbits cross over the poles. A
PTT located at the polar regions would therefore be able to deliver
approximately 14 messages daily. At least two satellites are operational
at any time, which doubles this number to 28. At the equator the situa-
tion is different. The equatorial radius of the earth is approximately
6378 km, which gives a circumference of about 40,074 km. Relative to
the orbital footprint, a given longitude at the equator will therefore
rotate with the earth a distance of 40074 × 102/1440 or about 2839 km.
This assumes a stationary orbital path, but as mentioned previously the
orbit is sun synchronous, which means that it rotates eastward almost
1° per day (see Sec. 2.8.1), that is in the same direction as the earth’s rota-
tion. The overall result is that an equatorial PTT starting at the west-
ern edge of the footprint swath will “see” between three and four passes
per day for one satellite. Hence the equatorial passes number between
six and seven per day for two satellites. During any one pass the PTT is
in contact with the satellite for 10 min on average. The messages received
at the satellite are retransmitted in “real time” to one of a number of
regional ground receiving stations whenever the satellite is within range.
The messages are also stored aboard the satellites on tape recorders, and
are “dumped” to one of three main ground receiving stations. These are
located at Wallops Island, VA, USA, Fairbanks, Alaska, USA, and
Lannion, France. The Doppler shift in the frequency received at the
satellite is used to determine the location of the PTT. This is discussed
further in connection with the Cospas-Sarsat search and rescue satellites.

TABLE 1.8 Platform Transmitter Terminals (PTT) Characteristics
Uplink frequency 401.75 MHz
Message length Up to 32 bytes
Repetition period 45–200 s
Messages/pass Varies depending on latitude and type of service
Transmission time 360–920 ms
Duty cycle Varies
Power Battery, solar, external
SOURCE: www.argosinc.com/documents/sysdesc.pdf