iGage iG9 User Manual
71
DOP is a function of how many and where the satellites are in the sky. We prefer more satellites,
spread over a larger portion of the sky, with one or more satellites in every quadrant:
11:30 am
Great
8:50 pm
Bad
One pitfall of OPUS-RS is very short occupations may entirely fall into a very high-DOP period. As you
can see from the DOP plot above, high DOPs rarely last for more than an hour and longer OPUS-
Static occupations will usually have some periods of low DOP and excellent coverage.
The change in satellite constellation, which determines PDOP is why a receiver will work one day
and then not work in a nearby location at a different time.
#11 Be Procedure Smart: avoid Blunders
Assuming that your receiver is in a location that is suitable for GPS observations, at a suitable time,
there are several procedural blunders that you can do to force a bad result:
Mounting system is not level and receiver is not centered over the ground mark.
Antenna height (HI) is wrong.
Antenna is mis-rotated, doubling antenna compensation errors.
Wrong antenna type is selected.
No battery in head with external power
Use a Fixed Height Tripod, Get the HI Correct!
The #1 OPUS procedure failure is a blundered instrument height. The ONLY HI that OPUS will
accept is the vertical height above ground to the ARP (Antenna Reference Point) in meters.
If you use a tribrach, you are going to have to make a slant measurement and then reduce the
slant distance and SHMP (Slant Height Measurement Point) vertical offset to a metric vertical
height. The process is described
in the “
‘Slant Height’ to ‘Vertical Height’:
” section on page
83.
Slant reduction error is also very common source of blundered instrument height. The
iGx_Download tool makes this computation automatically for you; however, you must keep track
of Slant vs. Vertical and Feet vs. Meters.
Transposition of digits in random heights that occur with tribrachs on tripods is a common source of
error. Measurement to the wrong place on the antenna is a common source of error. Mixing slant
measurements in feet with metric SHMT and radius constants is a common source of error.
Confusing slant heights between multiple occupations is a common source of errors. Using ‘inch’
tapes instead of ‘tenths’ tapes is a common source of errors.
