Step 3. Hdop & Vdop Values Too High For Precision Requirements - ashtech Z-Xtreme System Manual

ity solution. Your field application software will also inform you which satellites are
being tracked by the base and which are being tracked by the rover and whether or
not these satellites are healthy. If you find that your solution will not fix, look to deter-
mine if the base and rover are indeed tracking at least 5 common healthy satellites.
Not tracking at least 5 satellites
1. Check satellite availability.
a. Use the Mission Planning software to check satellite availability for your
current location and time.
b. Look for the number of satellites higher than 15° above the horizon.
c. Ensure at least 5 healthy satellites are available.
d. If not, you will need to perform your survey at another time.
Go to step 3) below if the problem is not yet resolved.
2. Move base or rover if sites have satellite obstructions.
a. If your base or rover site has any obstructions higher than 15° above the
horizon, the obstructions may be blocking essential satellites. If
obstructions exist at the base or rover, more the system to an open area.
Tracking at least 5 satellites
Go to step 3 below.
Step 3. HDOP & VDOP Values Too High for Precision Requirements?
Dilution of Precision (DOP) values give a quality indication of the satellite geometry at
any given time. Satellite geometry is important to the precision of an RTK solution. In
fact, the DOP value is used as a multiplier in the computation of position precision.
For example, in the computation of horizontal RMS (HRMS), an estimated precision
value is multiplied by the HDOP at that given time to produce HRMS. The larger the
HDOP value, the larger the HRMS value. The same relationship holds for VDOP and
VRMS. Therefore, poor satellite geometry will result in poor solution precision. The
smaller the DOP value, the better the geometry and solution precision. Your field
application software supports the ability to view current DOP values. If your precision
estimates (HRMS, VRMS) do not meet expected values, use this feature to examine
the current DOP values.
DOP values not too high
Go to step 4) below.
DOP values too high
1. Look for a satellite window with more suitable DOP values to perform the
survey.
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