SX BLUE GPS iSXblue II+ GNSSTM, Руководство пользователя

Страница: 99 / 107

SXblue Series User Manual 98
Note - The satellite elevation angle lowers with increasing distance away
from the equator and from the satellite’s longitude. Although a good amount
of signal coverage is shown in Northern latitudes for both WAAS and
EGNOS, it may not be usable due to its low elevation angle and the potential
for it to be obstructed. Ideally, testing of the system in the area of its use is
recommended to ensure that the signal is sufficiently available.
Note - The SBAS signal coverage may be present in some areas without
either sufficient ionospheric map coverage or satellites with valid orbit and
clock correctors. In such a case, differential positioning with SBAS may not
be desirable or possible, as four or greater satellites (with correctors) must
be available to compute a DGPS position. The next section provides further
information on the ionospheric map features of SBAS and the SXblue
receiver.
SBAS Ionospheric Map Extrapolation
To improve upon the ionospheric map provided by SBAS, the SXblue receiver
extrapolates a larger ionospheric map from the broadcast coverage map,
extending its effective coverage. This allows the SXblue receiver to be used
successfully in regions where competitive products may not. For example, the
extrapolation provides enhanced coverage through the northern part of South
America for WAAS, through the northern part of Africa for EGNOS, etc.
Please note that the process of estimating ionospheric corrections beyond the
SBAS broadcast map would not be as good as having an extended SBAS map in
the first place. This difference may lead to minor accuracy degradation. As an
example, Figures B-5 depicts the broadcast WAAS and EGNOS ionospheric map
extent and the extrapolated version. As can be seen, the extrapolated grid extends
further in all directions than the broadcast one, enhancing usable coverage.
Figure B-5 Broadcast and Extrapolated Ionospheric Correction Map