Specifications

108
5.13 GPS position
With GPS (Global Positioning System) and HF communication the position of a drifting buoy,
be it on purpose or accidentally, can be tracked. GPS is standard on all buoys. Latitude and
longitude are updated once every half hour and transmitted to the user 8 times in one half hour.
The position accuracy is about 10 m (0.3”). Position integrity is monitored by the GPS receiver.
Only if the receiver flags the position as valid, will the position be updated and transmitted.
Otherwise a zero position will be retransmitted. Even though a GPS receiver consumes
considerable power, due to fast acquisition and a large duty cycle the net power consumption is
small.
5.13.1 Principle
The Global Positioning System consists of 32 satellites orbiting the earth and allowing GPS
receivers to calculate their position at any time and anywhere on the earth. To see how GPS
works let us look at its original name: NAVigation through Synchronized Timing And Ranging
(NAVSTAR). It says that GPS satellites and receivers all refer to a single time. Each satellite
transmits a unique repeating bit pattern (code signal) that is linked to the GPS timing. GPS
receivers receive these patterns. With the unique pattern the receiver can identify the satellite
source. The difference between the moment of reception and the moment of transmission of the
bit pattern, the travel time, multiplied by the speed of light determines the range between the
specific satellite and receiver. As the satellites also transmit orbit information, the so called
ephemeris, the GPS receiver can calculate the position of the satellite at the moment of
transmission. If the ranges to at least three satellites are known the GPS receiver can also
calculate its own three-dimensional position on earth. This would be the case if the GPS
receiver would exactly know what time it is. In practice, the GPS receiver time is offset from
the exact GPS time. To solve for the additional offset time, the receiver clock error, a minimum
of four visible GPS satellites is required for positioning.
5.13.2 GPS position antenna
The same GPS antenna in use for GPS wave measurement on DWR-G buoys, is used for GPS
positioning on WR-SG and DWR-MkIII buoys. On a DWR-G buoy the GPS wave antenna is
used for GPS positioning simultaneously.
Figure 5.13.1. DWR-G position antenna. Figure 5.13.2. Standard GPS position antenna