GPS RECEIVER WITH SIMULATED DCF77 OUTPUT
1. General Information
The Global Positioning System (GPS) is a satellite system developed in the USA and based
on the NAVSTAR satellites made by Rockwell. With this system, it is possible to receive ex-
act time information anywhere in the world, anytime.
To meet these requirements, it was necessary to position a total of 24 satellites in 6 almost
circular orbits. The satellites’ orbits are inclined at 55° to the equator with an orbital period
of about twelve hours –corresponding to an orbital altitude of 20,183 km. These satellites
are now in operation. Each satellite has a mass of about 430 kg and contains a transmitter,
a receiver, an antenna, an atomic clock for maximum accuracy and a control system for the
satellite’s functions. The principal applications of the GPS system are for fixing positions and
as an aid to navigation.
2. Operating Principles
Once the GPS system is switched on, it first has to receive and evaluate valid data from the
satellites in its line of sight. This start-up phase may take as little as 2 –3 minutes if recep-
tion conditions are favourable. However, as long as half an hour may be needed if there is a
poor line of sight to the horizon. This start-up phase is also influenced by the configuration
the satellites are in at any particular time. It is impossible to fix a position or obtain time in-
formation with fewer than three satellites. The red LED “GPS IS WORKING...” flashes while
the satellite signals are being received. When the unit subsequently commences continuous
operation, this light-emitting diode only flashes when the number of satellites being re-
ceived falls below three.
It is simple to set up the system and the procedure does not require any particular special-
ist knowledge. The daylight saving time changes (start and finish) and the time zone can be
set using slide switches.
3. Installation and Start-up
3.1 Generals
The high frequency of the GPS signal (approx. 1.5 GHz) means an outdoor aerial is always
necessary for receiving GPS satellite data. The antenna must not have any directional char-
acteristics because it needs to receive data from all directions. Consequently, boosting the
reception characteristics of the antenna by shaping it mechanically as in the case of a para-
bolic antenna, for example, is not a viable option.
This means the signal magnitude at the antenna output is below the general background
noise level. An extremely low-noise pre-amplifier is therefore connected to the antenna in-
put to make it possible to evaluate the signals.
The antenna should have a clear “line of sight” to the entire horizon to guarantee that the
GPS reception will be as continuous as possible. The start-up phase may take a long time if
the reception arc is obstructed by objects or buildings, depending on the configuration of
the satellites. As a result, the antenna should be set up where the maximum possible clear
line of sight to the horizon is guaranteed. In general, the more of the sky you can see from
the installation site, the quicker the GPS clock will be radio-synchronized and the longer it
will remain so.
