Trimble Outdoors SK8 - Manual (Page 82)

A-18 Lassen-SK8 Embedded GPS Module
Trimble Standard Interface Protocol
Frequent constellation switching is undesirable because position jumps may be
experienced when SA is present and DGPS is not available to remove these effects. The
benefit of a low elevation mask is that more satellites are available for use in a solution
and a better PDOP may be yielded. The current mask is set to five degrees and provides a
reasonable trade-off of the benefits and drawbacks. High accuracy users may prefer a
mask angle around ten degrees, where the ionosphere and troposphere begin to be more
predictable
A.15.4 Signal Level Mask
This mask defines the minimum signal strength for a satellite used in a solution. There is
some internal hysteresis on this threshold which allows brief excursions below the
threshold if lock is maintained and the signal was previously above the mask. The factory
default mask has been set to zero. High accuracy users may use a slightly higher mask of
6.0-8.0, since weaker measurements may be slightly noisier and are often caused by
reflected signals which provide erroneous ranges.
One should also resist the temptation to set the elevation and SNR masks too low. The
satellite geometry is sometimes improved considerably by selecting low elevation
satellites. They are, however, subject to significant signal degradation by the greater
ionospheric and tropospheric attenuation that occurs. They are also subject to more
obscuration by the passing scenery when the receiver is in a moving vehicle. The code
phase data from those satellites is therefore more difficult to decode and therefore has
more noise.
*
Note – A level of hysteresis in the signal level mask is allowed in the core operating
software. The hysteresis allows the receiver to continue using satellite signals which fall
slightly below the mask and prevents the receiver from incorporating a new signal until the
signal level slightly exceeds the mask. This feature minimizes constellation changes
caused by temporary fluctuations in signal levels.
A.15.5 DOP Mask and Switch
The DOP mask is the maximum DOP limit for any 2-D or 3-D position solution will be
made. The DOP switch is the level at which the receiver stops attempting a 3-D solution,
and tries for a 2-D solution when in automatic 2-D, 3-D mode. The switch level has no
effect in either manual mode. Raising the DOP mask will generally increase the fix
density during obscuration, but the fixes with the higher DOP will be less accurate
(especially with SA present). Lowering the mask will improve the average accuracy at the
risk of lowering the fix density.