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4-4 Lassen-SK8 Embedded GPS Module
Operation and Performance
4.3.1 Elevation Mask
Satellites below a 5° elevation are not used in the position solution. Although low
elevation satellites can contribute to a lower/better PDOP, the signals from low elevation
satellites are poorer quality, since they suffer greater tropospheric and ionospheric
distortion than the signals from higher elevation satellites. These signals travel further
through the ionospheric and tropospheric layers.
In addition, low elevation satellites can contribute to frequent constellation switches, since
the signals from these satellites are more easily obscured by buildings and terrain.
Constellation switches can cause noticeable jumps in the position output. Since worldwide
GPS satellite coverage is generally excellent, it is not usually necessary to use satellites
below a 5° elevation to improve GPS coverage time. In some applications, like urban
environments, a higher mask may be warranted to minimize the frequency of constellation
switches and the impact of reflected signals.
4.3.2 SNR Mask
Although the Lassen-SK8 is capable of tracking signals with SNRs as low as 0, the default
SNR mask is set to 3 to eliminate poor quality signals from the fix computation and
minimize constellation switching. Low SNR values may result from:
• Low Elevation Satellites
• Partially Obscured Signals (e.g. Dense Foliage)
• Multi-Reflected Signals (Multi-Path)
The distortion of signals and the frequent constellation switches associated with low-
elevation satellites were discussed above. In mobile applications, the attenuation of signals
by foliage is typically a temporary condition. Since the Lassen-SK8 can maintain lock on
signals with SNRs as low as 0, it offers excellent performance when traveling through
heavy foliage.
Multi-reflected signals, also known as Multi-path, can degrade the position solution.
Multi-path is most commonly found in urban environments with many tall buildings and a
preponderance of mirrored glass, which is popular in modern architecture. Multi-reflected
signals tend to be weak (low SNR value), since each reflection attenuates the signal. By
setting the SNR mask to 2 or higher, the impact of multi-reflected signals is minimized.
4.3.3 PDOP Mask
Position Dilution of Precision (PDOP) is a measure of the error caused by the geometric
relationship of the satellites used in the position solution. Satellite sets which are tightly
clustered or aligned in the sky will have a high PDOP and will contribute to a lower
position accuracy. For most applications, a PDOP mask of 10 offers a satisfactory trade-
off between accuracy and GPS coverage time. With world-wide GPS coverage now
available, the PDOP mask can be lowered even further for many applications without
sacrificing coverage. For differential GPS applications, PDOP related error can be the
major contributor to position error. For differential GPS applications requiring the highest
level of accuracy, the PDOP mask should be set to 7 or below.