TM 1-1520-238-MTF
Change 3
2-72
PROCEDURE
TROUBLESHOOTING
REFERENCE
HOVER CHECKS (CONT)
*8.
/
Doppler drift – Check.
NOTE
Rates as high as 21 feet per minute may oc-
cur with a properly functioning Doppler sys-
tem. Causes may be related to HARS, FCC,
or interface problems.
a. SYM SEL switch – Actuate to select
BOB-UP mode. Minimize velocity
vector and acceleration cue to the
extent possible.
b. Hover aircraft for one minute. Note
amount of Doppler drift from
original position. If drift is excessive
perform HBCM.
*9.
HAS/Hover Position Box Drift. The
Hover Position Box drift varies
according to whether the EGI or HARS
is in use as the inertial sensor, and if
using the EGI, whether or not the GPS
is operating in PPS (keyed) or SPS (not
keyed) mode or operating at all. Using
the EGI with the GPS keyed produces
optimal HAS/Hover Position Box drift
performance. The amount of drift may
be up to 6 feet the first minute, and as
much as 23 feet after 5 minutes. All
other modes of operation (EGI or
HARS) may produce Hover Position
Box drift that is random and
unpredictable. In these modes of
operation, the HAS/Hover Position Box
drift performance may be similar to that
of the non–integrated navigation system
(up to 21 feet per minute).
