Campbell Scientific TX312 High Data Rate GOES Transmitter - Manual (Page 42)

TX312 Transmitter
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received power will be half of a properly aimed antenna. Beyond 25 degrees,
the received power drops off very quickly.
6.3 GPS Antenna
6.3.1 How the GPS Signal is Acquired and Used
The GPS receiver will acquire a complete GPS fix at power up and once a day.
The TX312 transmitter will continue to operate normally for 28 days without a
GPS fix.
The GPS signal is used for two functions. The obvious use is to keep track of
time. The GPS receiver requires 3 satellites to acquire the time. For best
accuracy, four satellites are required. The second use of the GPS signal is to
correct the oscillator frequency. The GPS receiver will output a very accurate
1-second pulse. The 1-second pulse is used to correct oscillator drift caused by
changes in temperature and crystal aging.
The GPS is required for proper operation. After the transmitter is reset, or first
powered up, it can’t schedule a transmission until a GPS fix has been
established or the internal clock has been manually set. After the first fix, the
TX312 will acquire a GPS fix once a day. Each time the GPS system acquires
a fix, the entire GPS almanac is downloaded, which requires about 15 minutes.
6.3.2 GPS Antenna Location
The ideal location for the GPS antenna is above everything, with the shortest
cable possible. The GPS antenna will not receive the GPS signal through a
steel roof or steel walls. Concrete will probably act like steel. Heavy foliage,
snow, and ice will attenuate the GPS signal. The more of the sky the antenna
has a clear unobstructed view of, the better the GPS performance. Better GPS
performance will show up as less or no missed transmissions. Poor GPS
antenna placement will increase the number of missed transmissions, or
possibly stop all transmission