Specifications
42
5.5.6 Selective availability
As mentioned the GPS system originally was and still is a military system, maintained by the
United States Department of Defence. As such a few features are incorporated to restrict the use
of precise GPS to selected users. This is known as Selective Availability (SA). When SA is
active a dither is added to the satellite GPS time thereby deteriorating the GPS position accuracy
from 10 m to 100 m. Furthermore the precision of the satellite orbit information may be
reduced. The information to correct for dithering is encrypted in the GPS signal. Unless one has
the encryption key one can not restore the intrinsic GPS accuracy.
On May 1, 2000 SA was officially discontinued, see
www.navcen.uscg.gov/gps/selective_availability.htm. If, in the unlikely event, SA would be
temporarily switched on for strategic reasons, the current GPS wave buoy will not work
anymore! However, it will continue measuring waves immediately after SA is switched off
again.
5.5.7 Filtering
Measured velocities are digitally filtered using an integrating high-pass filter with a cut-off at
0.01 Hz. Moreover, the intrinsic 2 Hz sampling is converted into 1.28 Hz north, west and
vertical motions to fit the long existing transmitter data format. Use is made of a so-called
decimation filter, which causes an extra delay.
5.5.8 Specifications
See Table 5.5.1.
Table 5.5.1. Specifications of DWR-G.
Parameter Value
Heave, north, west
Range −20 m-+20 m
Resolution 1 cm
Accuracy ±1 cm or 0.1% of value,
whichever is worse
Period time (frequency range) 1.6 s-100 s
(0.01 Hz-0.64 Hz)
Direction
Range 0º-360º
Resolution 1.5º
Accuracy 1.5º
Reference true north (WGS84)
Filter
Sampling frequency 2.0 Hz
Digital filtering type phase-linear, combined band-pass
and single-integrating FIR filter
Filter delay 256.0 s
Decimation filter delay 43.0 s
HF output buffer delay and
actual HF output
5.5 s (approximately, does not apply to logger files)
Data output rate 1.28 Hz
Band-pass characteristics 0.0154-0.59 Hz: 0.0013 dB
0.0132 0.009 dB
0.0115 0.09 dB
0.01 Hz 0.8 dB
low frequency side 52 dB/octave (< 0.01 Hz)