Trimble Outdoors Part Number 45005-00-ENG, AcutimeTM2000 Synchronization Kit Signal Level Mask

Trimble Standard Interface Protocol A

Frequent constellation switching is undesirable because position jumps may be experienced when Selective Availability 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 default mask is set to 0.1745 radians (10°) and provides a reasonable trade-off between the benefits and drawbacks.

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. This mask should be lowered only with caution since it is also used to minimize the effects of jammers and reflected signals on the receiver. Users who require high accuracy can use a slightly higher mask of 6.0-8.0, since weaker measurements may be noisier and are often caused by reflected signals, which provide erroneous ranges.

Make sure that the elevation and SNR masks are not set too low. The satellite geometry is sometimes improved considerably by selecting low-elevation satellites. These satellites 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 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-80 Acutime 2000 Synchronization Kit User Guide