Campbell Scientific ST-21 Argos Satellite Transmitter - Manual (Page 7)

ST-21 Argos Satellite Transmitter
1. Overview
The ST-21 is a certified Argos PTT satellite transmitter for use with Campbell
Scientific CRBasic dataloggers. Service Argos data transceivers fly aboard
two NOAA polar orbiting satellites. With an orbit altitude close to 800
kilometers, the satellites are at a relatively low orbit. The low orbit allows for
low transmit power, low battery power requirements and the use of an omni-
directional antenna. The orbit period is approximately 1 hour and 45 minutes,
which allows hourly data at extreme northern and southern latitudes. Near the
equator there are about six satellite passes per day that are not evenly spaced.
During the average satellite pass, the satellite is in view for about 10 minutes.
FIGURE 1-1. ST-21 Argos Satellite Transmitter
1.1 Specifications
Power supply: Nominal 12 volts DC, 10 volts DC minimum,
16 volts DC maximum
Current drain
at 12 volts:
1.12 mA quiescent, 375 mA during transmit
Data buffers: 7 buffers, 32 byte each. Note: 28 bit ID numbers reduce
buffers to 31 bytes.
Operating
temperature:
-30° to 70°C
Dimensions: 7.5 L x 2.25 H x 1.25 W inches, 19 L x 6 H x 3 W cm.
Weight: 7.9 oz, 224 gm
1.2 Serial Port
The ST-21 is specifically designed to work with Campbell Scientific CRBasic
dataloggers. The ST-21 serial port connects to the CS I/O port of the
datalogger via an SC12 cable (see FIGURE 1-2). The datalogger provides 12
volt DC power and ground through pins 8 and 2 respectively. Communications
utilize the Campbell Scientific Synchronous Device for Communications
(SDC) protocol, which allows several devices to be connected to the CS I/O
port at the same time. Each device is addressed or pin enabled; the ST-21 is
address 4.
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