Specifications

FASTER Engineering Model

parity bit) using the NMEA (National Marine Electronics Association) 0183 ASCII

interface specification (GPGGA, GPRMC, GPVTG and PGRMV sentences are currently

used).

Figure 53: Garmin GPS 18x 5Hz and its mechanical drawing [Ref. (33)]

The Crossbow NAV420 is a combined GPS navigation and GPS-Aided Attitude &

Heading Reference system (AHRS) that utilizes high stability MEMS-based inertial

sensors. Internally A 16 bit A/D converter acquires data from several sensors: a tri-axial

accelerometer, a tri-axial rate sensor, a tri-axial magnetometer and a temperature sensor;

the digital output and the GPS data is computed by an high speed sampling&DSP which

provides sensor compensation (factory calibration data, stored in an internal eeprom is

used by the DSP to remove temperature bias, misalignment, scale factors errors and non-

linearities from the sensor data) and applies a full-state Kalman filter algorithm resulting

in a state vector composed by GPS position (x,y,z); Velocity (x,y,z); Acceleration (x,y,z);

Roll, Pitch and Heading; 3-axis angular rate and the UTC time. The update rate is set to

100 Hz and under static conditions is possible to have fully stabilized data in about 60

seconds from power up [Ref (34)].

Due to some problems with the external GPS active antenna, for the purpose of this

study, the “navigation mode” provided by the I.M.U has not been used. Instead of the

“navigation mode”, the NAV420 has been utilized in “angle mode” acting as a complete

attitude and heading reference system giving as output the stabilized pitch, roll and yaw

angles together with the angular rates, accelerations along the 3 axis and the components

of the magnetic field. In the “angle mode” the onboard EKF tracks the rate sensor bias

and calculates the stabilized outputs. Accelerometers are used to correct for rate sensor

drift in pitch and roll, while magnetometers to correct for rate sensor drift in the yaw

angle.