TM 1-1520-238-10 3-64 Change 8 (EGI or HARS) may produce Hover Position Box drift that is random and unpredictable. In these modes of opera- tion, 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. System Description. 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 sen- sors 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 togeth- er; the faster the rotation, the greater the interference pat- tern. 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 EGI is approximately 4 minutes. The time is not significantly af- fected 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 re- ceiver 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 re- tain the key through power ON/OFF/ON cycles. Because of safeguards built into the EGI, it is not considered classi- fied when keyed. The antenna for the GPS receiver is lo- cated on the top of the vertical stabilizer. b. Operation. The operation of the GPS receiver is entirely automatic. The GPS receiver is powered when the EGI is powered. If the GPS is keyed it may take as long as 12 minutes to verify the key with the satellites. The GPS receiver will provide position and time. The Naviga- tion System automatically configures to handle the GPS and DNS velocities for velocity aiding of both the EGI and HARS. When the GPS has tracked the first satellite, it will provide the date and time to the Navigation System. The date and time (ZULU) is displayed at the top of the ADMIN page of the CDU. There are no specific operator actions that may be taken with the GPS. 3.16.3 Data Transfer Unit (DTU). The Data Transfer Unit (DTU) is located in the aft portion of the CPG right–hand console. The cartridge is pro- grammed using the Aviation Mission Planning System (AMPS). The AMPS writes the coordinate files for way- points, targets, present position and laser codes to the cartridge. In the helicopter, the FCC handles all reading and writing to the cartridge. Whenever the CPG makes changes to any of the data, the FCC will automatically up- date the cartridge with the changes. This will preserve any changes made by the CPG in the event of a power tran- sient. On power–up, either initial or following a transient, the FCC will check for copies of the files it has written out to the data cartridge. If it finds the saved files present, it will automatically load all of the saved files. It the saved files are not found, the FCC will load the AMPS files into the system. This has the effect of automatically loading the AMPS files when a “new” cartridge is used. The CPG has the capability of manually loading and saving files by using the pages on the CDU. FCC memory data may only be ”dumped” by powering down the FCC. 3.16.4 Required Navigation Data. The Integrated Navigation System uses two components of data for proper operation: map datum and altitude. Magnetic variation is automatically calculated by the Navi- gation System and requires no action by the flightcrew. Spheroid is not used in any portion of the Integrated Navi- gation System. a. Altitude. Altitude is used with all coordinate data to permit the accurate prepointing or cueing to