Meinberg GPS169PCI - Manual (Page 13)

13
Mounting the antenna
The GPS satellites are not stationary but circle round the globe in a period of about 12
hours. They can only be received if no building is in the line-of-sight from the antenna to
the satellite, so the antenna/converter unit must be installed in a location from which as
much of the sky as possible can be seen. The best reception is given when the antenna
has a free view of 8° angular elevation above horizon. If this is not possible the antenna
should be installed with a mostly free view to the equator because of the satellite courses
which are located between latitudes of 55° North and 55° South.
If even this is not possible problems occure especially when at least four sattelites for
positioning have to be found.
The unit can be mounted using a pole with a diameter up to 60 mm. A standard coaxial
cable with 50
Ω impedance (e.g. RG58C) should be used to connect the antenna/
converter unit to the receiver. Cable thinner than RG58 should be avoided due to its
higher DC resistance and RF attenuation. When using the optional antenna diplexer the
total length of one antenna line between antenna, diplexer and receiver must not be
longer than 250 m. If a cable with less attenuation is used its length may be increased
accordingly (e.g. 500 m with RG213).
If the antenna cable was canned by the user: before powering up
the system, make sure that there is no short-circuit between the
inner and outer conductor of the antenna cable, because this
could cause a fault of GPS169PCI.
Powering up the system
After the board has been mounted and the antenna has been connected, the system is
ready to operate. About 10 seconds after power-up the receiver´s TCXO operates with
the required accuracy. If the receiver finds valid almanac and ephemeris data in its battery
buffered memory and the receiver´s position has not changed significantly since its last
operation the receiver can find out which satellites are in view now. Only a single
satellite needs to be received to synchronize and generate output pulses, so synchroniza-
tion can be achieved at least one minute after power-up. After 20 minutes of operation
the TCXO has achieved its final accuracy and the generated frequencies are within the
spezified tolerances.
If the receiver position has changed by some hundred kilometers since last operati-
on, the satellites´ real elevation and doppler might not match those values expected
by the receiver thus forcing the receiver to start scanning for satellites. This mode is
called Warm Boot because the receiver can obtain ID numbers of existing satellites
from the valid almanac. When the receiver has found four satellites in view it can
update its new position and switch to normal operation. If the almanac has been lost
because the battery had been disconnected the receiver has to scan for a satellite and
read in the current almanacs. This mode is called Cold Boot. It takes 12 minutes until