Thales MobileMapper Beacon, User Manual

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25
Spatial Decorrelation
Depending on the range from yourself to the beacon, the en-
vironmental conditions may differ between the base station
site and your location. This difference will show up as an ad-
ditional error in positioning, and is referred to as spatial
decorrelation.
The additional degradation will depend on the difference in
observed orbit and ionospheric errors between the reference
station and your GPS system. A general rule of thumb would
be a 1 m error for every 100 miles of distance to the beacon
station. This error is often seen as a bias in positioning, re-
sulting in a position offset. The scatter of the receiver is like-
ly to remain close to constant.
Coast Guard Beacon vs WAAS/EGNOS
Coast Guard beacon DGPS has certain strengths in compar-
ison to WAAS and vice versa, which are important to know
when deciding what correction source to use. Both services
are provided without user fees, so your choice of which sys-
tem to use must be made according to different criteria.
Benefits of Coast Guard Beacon
• Low Frequency - the low frequency of beacon is a signif-
icant strength when working in areas where a line of
sight is blocked to WAAS or EGNOS, such as under can-
opy or in urban canyon. The frequency of beacon is
located below AM radio stations. As you've likely expe-
rienced of AM radio stations, compared to higher fre-
quency FM stations, AM radio is more robust, and
diffracts well around obstacles, such as buildings, high-
way overpasses, etc. With often only two WAAS or
EGNOS satellites available, and sometimes only one, it's
possible that GPS can still be tracked effectively under
tree canopy. Being low frequency, the beacon signal is
able to penetrate trees very well, while a WAAS/EGNOS
lock cannot be attained reliably.
Note - Due to effects of
spatial decorrelation, we
recommend that you
consistently use a spe-
cific beacon station as
much as possible. If, for
some reason this station
is not available, you will
need to determine if a
secondary station is
available.