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directly from a sensor, there is no redundancy. If the
sensor providing the data has failed, then the affected
instrument or subsystem will not function. The distance--
to--go and steering information is always displayed
on the CDU and HMD symbology. The velocity
vector, acceleration cue, vertical speed indicator, and
heading tape are driven by the navigation system and
not by any single sensor. This allows redundancy in the
displays in the event of invalid data or failure of any
sensor. No one velocity sensor can cause the error. However,
because the Hover Augmentation System (HAS)
of the DASE receives velocity data directly from the
HARS, it is possible for a velocity error in the HARS to
affect HAS and not affect the Navigation System. In
the event that the HARS has been in a free inertial
condition for longer than 20 seconds the HARSVEL?
message is displayed in the CDU, NAV status page. If
the HARS has been in a free inertial condition for longer
than 20 seconds and the groundspeed of the helicopter
is less than 10 knots,the pilot HMD trim ball will
flash. The purpose of the flashing trim ball is to alert
the pilot that if the HAS is engaged theremay be an initial
drift of the helicopter.
With the integrated navigation system, 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 randomand unpredictable. In thesemodes 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).
3.16.1 Embedded GPS Inertial (EGI).
a. SystemDescription. The EGI is a velocity aided,
strapdown, ring laser gyro based inertial unit. The EGI
unit also houses the 5--channel GPS receiver. The ring
laser inertial unit and GPS receiver are treated as separate
sensors by the Navigation System and have separate
FD/LS indications. The ring laser gyro operates on
an optical principle called the Sagnac Effect which
deals with the properties of light beams traveling in opposite
directions around a closed loop. In operation, two
laser beams are directed around the ring in opposite
directions; clockwise and counter--clockwise. If the ring
rotates in the clockwise direction while the light beams
are in transit then the clockwise beam will seem to
travel a shorter distance than the counter--clockwise
beam. This is used as a measure of rotation by observing
the interference pattern created at the end of the
ring when the two laser beams mix together; the faster
the rotation, the greater the interference pattern.
b. Operation. The EGI begins alignment whenever
28 vdc power is available and aligns in the same manner
as the HARS. The vertical axis is found by aligning
the vertical ring laser gyro with earth’s gravity vector.
Inertial north is found by measuring the eastward rotation
of the earth about its axis. The time to align for the
TM 1-1520-238-10
EGI is approximately 4 minutes. The time is not significantly
affected by temperature. The heading tape
symbology will be displayed when the first platform,
EGI or HARS aligns. There is no effect on Navigation
System accuracy if the engines are started or the main
rotor is turning during alignment. There is no reason,
other than normal checklist items, to delay engine
start.
3.16.2 Global Positioning System (GPS).
a. System Description. The GPS receiver installed
in the EGI is a 5-channel receiver. The receiver is capable
of operating in either C/A code or encrypted P/Y
code. The Group User Variable (GUV) is the normal encryption
key used. The GUV key is loaded into the EGI
using a KYK-13 or equivalent device. When keyed the
GPS receiver will automatically use anti-spoof/jam capabilities
when they are in use. The EGI keying connector
is located on the aft portion of the right-hand FAB.
The EGI will retain the key through power ON/OFF/
ON cycles. Because of safeguards built into the EGI, it
is not considered classified when keyed. The antenna
for the GPS receiver is located on the top of the vertical
stabilizer.
b. Operation. The operation of the GPS receiver is
entirely automatic. The GPS receiver is powered when
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