Meinberg GPS161 - Manual (Page 11)

11
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 300 m. If a cable with less attenuation is used its length may be increased
accordingly (e.g. 600 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 GPS161.
Powering up the system
If both, the antenna and the power supply have been connected, the system is ready to
operate. About 10 seconds (OCXO MQ: 1 minute, OCXO HQ: 3 minutes) after power-
up the receiver´s master oscillator has warmed up and 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 synchronization can be
achieved maximally one minute after power-up.
If the receiver position has changed by some hundred kilometers since last operation,
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 the new
almanac is complete and the system switches to Warm Boot mode scanning for other
satellites.