Campbell TGT1, Operator'S Manual

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1
TGT1 GOES Transmitter
1. Introduction
The TGT1 transmitter supports one-way communication, via satellite, from a
Campbell Scientific datalogger to a ground receiving station. Satellite
telemetry offers a convenient telecommunication alternative for field stations
where phone lines or RF systems are impractical. This transmitter features a
crystal oscillator that is digitally temperature-compensated to prevent the
frequency from drifting into adjacent channels. The TGT1 is manufactured for
CSI by Telonics Inc. and inter-faces directly to the datalogger's 9-pin I/O port.
2. GOES System
2.1 Orbit
The TGT1 transmitter sends data via Geosta-tionary Operational
Environmental Satellites (GOES). GOES satellites have orbits that coincide
with the Earth's rotation, allowing each satellite to remain above a specific
region. This allows a user to point the GOES antenna at a fixed position in the
sky.
There are two satellites, GOES East and GOES West. GOES East is located at
75° West longitude and GOES West is located 135° West longitude. Both
satellites are located over the equator. Within the United States, odd numbered
channels are assigned to GOES East. Only even numbered channels are
assigned to GOES West. Channels used outside the United States are assigned
to either spacecraft.
2.2 NESDIS and Transmit
−
Windows
GOES is managed by the National Environmental Satellite Data Information
Service (NESDIS). NESDIS assigns addresses, uplink channels, and self-
timed/random transmit time windows. Self-timed windows allow data
transmission only during a predetermined time frame (typically 1 minute every
3 or 4 hours). The self-timed data is erased from the transmitter's buffer after
each transmission. Random windows are for critical applications (e.g., flood
reporting) and allow transmission immediately after a threshold has been
exceeded. The transmission is then randomly repeated to ensure it is received.
A combination of self-timed and random windows can be executed by the
TGT-1.
2.3 Data Retrieval
Data retrieval via the TGT1 and the GOES system is illustrated in Figure 2-1.
The User Interface Manual, provided by NOAA/ NESDIS, describes the
process of retrieving the data from the NESDIS ground station. The data are in
the form of 3-byte ASCII (see Appendix B for a computer program that
converts the data to decimal). You can also retrieve data directly from the