VIA Technologies EPIA SN-Series Specifications download pdf (Page 93)

FASTER Engineering Model

6.5.2 Crossbow NAV420 Acquisition Block

The same structure used in the previous section has been adopted for the NAV420

acquisition block shown in Figure 69. While in the GPS acquisition block no commands

can be sent to receiver , in this case configuration data can be transmitted using the bi-

directional RS232 connection to set one of the three Data Packet Mode: Scaled Sensor

Packet, Angle Packet and NAV Packet.

As stated in Section 6.1.1, due to several problems with the external active antenna

the unit has been used in Angle mode. The unit outputs the stabilized pitch, roll and yaw

angles along with the angular rate, acceleration and magnetic field information. Being

GPS information not available the Kalman filter operates using an AHRS type filter

tracking the rate sensor bias and calculating stabilized roll, pitch and yaw angles. Angular

rate sensors are used to integrate over the aircraft rotational motion finding the actual

pitch, roll and yaw angles.

Accelerometers correct for rate sensor drift in the pitch and roll angles while

magnetometers correct for rate sensor drift in the yaw angle. Accelerometers are also used

to provide long term stability keeping the rate gyro drift continuously within acceptable

boundaries. Data is streamed at 100 Hz and to convert the digital data to angle the

following relation is used:

 = 

180

where angle is the actual angle in degrees and data is the signed integer data output in

the data packet. Pitch angle is defined as 90° <  < +90°. Other relations are used to

convert accelerations, angular rates, magnetic field components and temperature:

= 

=  

630

= 

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VIA Technologies EPIA SN-Series Specifications Page 93