Installing the FC, GNSS and sensors

Why this matters

The flight controller knows the aircraft’s attitude and heading from its sensors. If they are mounted in the wrong orientation, in the wrong place or near interference, the drone flies wrongly even though every part is healthy. The classic symptom is a drone circling in a growing spiral in position hold (toilet-bowling) because the compass is disturbed. UAT 205 covered sensor principles and calibration; this module focuses on installation and configuration. The drone knowledge base’s unit on IMUs and magnetometers stresses bias, noise and vibration.

Compass and power wires

Current in a wire creates a magnetic field. A long straight wire carrying current $I$ produces $B={\mu }_{0}I/(2\pi r)$ at distance $r$ (OpenStax University Physics), with ${\mu }_0\approx 1.2566\times 10^{-6}$ N/A² (CODATA 2022). If the go and return wires lie close together at spacing $s$, their fields almost cancel, leaving about ${\mu }_{0}Is/(2\pi r^2)$ when $r$ is much larger than $s$. The compass measures the Earth’s horizontal field, which in Bangkok at the start of 2026 is about 41,168 nT (41.2 µT), with an inclination of 16.3° downwards and a declination of 0.68° west, according to WMM2025 through the British Geological Survey service.

import math

MU0 = 1.25663706127e-6                         # N/A² (CODATA 2022)
H_EARTH = 41_168e-9                            # T, horizontal, Bangkok 2026 (WMM2025)
I = 30.0                                       # A

def b_single(i, r):
    return MU0 * i / (2 * math.pi * r)

def b_pair(i, s, r):                           # go-and-return pair s apart, valid for r >> s
    return MU0 * i * s / (2 * math.pi * r ** 2)

print(f"single wire at 5 cm: {b_single(I, 0.05) * 1e6:.1f} uT ({b_single(I, 0.05) / H_EARTH:.1f} x Earth's horizontal field)")
for s in (0.005, 0.02):
    for r in (0.10, 0.20):
        b = b_pair(I, s, r)
        err = math.degrees(math.atan(b / H_EARTH))
        print(f"pair {s * 1000:.0f} mm apart at {r * 100:.0f} cm: {b * 1e6:5.2f} uT = {b / H_EARTH:5.1%} of H, heading error up to {err:.1f} deg")
    r_min = math.sqrt(MU0 * I * s / (2 * math.pi * 0.05 * H_EARTH))
    print(f"  distance for 5% of H with pair {s * 1000:.0f} mm apart: {r_min * 100:.0f} cm")

single wire at 5 cm: 120.0 uT (2.9 x Earth's horizontal field)
pair 5 mm apart at 10 cm:  3.00 uT =  7.3% of H, heading error up to 4.2 deg
pair 5 mm apart at 20 cm:  0.75 uT =  1.8% of H, heading error up to 1.0 deg
  distance for 5% of H with pair 5 mm apart: 12 cm
pair 20 mm apart at 10 cm: 12.00 uT = 29.1% of H, heading error up to 16.3 deg
pair 20 mm apart at 20 cm:  3.00 uT =  7.3% of H, heading error up to 4.2 deg
  distance for 5% of H with pair 20 mm apart: 24 cm

Mounting orientation

If the flight controller must be mounted rotated from the nose direction, the software must be told. ArduPilot uses AHRS_ORIENTATION, which rotates the IMU and internal compass readings by the mounting angle; the board must then be re-levelled and rebooted. An external compass is set separately with COMPASS_ORIENT. PX4 uses SENS_BOARD_ROT. An effective check is to tilt the drone by hand and confirm that the attitude on the GCS tilts the same way on every axis, then rotate the drone and confirm that the heading changes in the right direction.

GNSS antenna position

GNSS reports the position of the antenna, not of the IMU. If the antenna is offset from the IMU, the measured position moves when the drone tilts or turns even though the body has not moved. ArduPilot takes this offset in GPS1_POS_X/Y/Z in metres, with X forward, Y right and Z down, measured from the IMU (or from the CG if the IMU position is set). Newer PX4 versions use SENS_GPS0_OFFX/Y/Z instead of the older EKF2_GPS_POS_X/Y/Z. Always check parameter names against the documentation for the firmware version actually used.

import math

X, Z = 0.12, -0.10                             # m, GPS1_POS_X and GPS1_POS_Z
for roll in (10, 25):
    shift = abs(Z) * math.sin(math.radians(roll))
    print(f"roll {roll} deg: antenna moves sideways {shift * 100:.1f} cm while the IMU stays put")
yaw_rate = math.radians(60)                    # rad/s
print(f"yaw at 60 deg/s: antenna velocity {yaw_rate * X:.3f} m/s from rotation alone")
print(f"set GPS1_POS_X = {X}, GPS1_POS_Y = 0, GPS1_POS_Z = {Z}")

roll 10 deg: antenna moves sideways 1.7 cm while the IMU stays put
roll 25 deg: antenna moves sideways 4.2 cm while the IMU stays put
yaw at 60 deg/s: antenna velocity 0.126 m/s from rotation alone
set GPS1_POS_X = 0.12, GPS1_POS_Y = 0, GPS1_POS_Z = -0.1

Checking flight controller vibration

The ArduPilot vibration measurement page states that vibration below 30 m/s² is normally acceptable, 30–60 m/s² may cause problems, and above 60 m/s² nearly always causes position and altitude-hold problems. The clipping counter (accelerometer saturation at 16 g) should be zero or very small. The recommended mounting uses foam or gel at the corners to cut high-frequency vibration while still letting the board follow the frame’s slow motion.

Module lab

Common mistakes

Summary

• The field from power wires falls with distance and falls sharply when go and return wires are kept together; it should be a small fraction of the Earth’s horizontal field (about 41 µT in Bangkok)
• Set the mounting orientation with AHRS_ORIENTATION and COMPASS_ORIENT and check by tilting for real
• The GNSS antenna offset from the IMU goes in GPS1_POS_X/Y/Z and matters greatly with RTK
• Vibration below 30 m/s² and clipping near zero are the ArduPilot criteria

Check your understanding

1. What field does a single wire carrying 20 A produce at 10 cm?
2. An interference field of 4 µT perpendicular to a 41 µT horizontal field can shift the heading by about how many degrees?
3. What should you do if compassmot gives 45%?
4. The antenna is 0.1 m above the IMU. Is GPS1_POS_Z positive or negative?
5. Which ArduPilot range does a vibration level of 45 m/s² fall in?