1. How it works #
The camera is not at the GPS antenna – it hangs below it and usually in front of it. TAG-E starts from the antenna position and adds the two sets of offsets, so every photo gets the position of the camera. The first set follows the aircraft, the second one follows the gimbal when it turns.
An offset error gives the same error in the geotag (1 cm wrong offset = about 1 cm wrong position), so measure carefully.
Figure 1 – A = GPS antenna, B = gimbal mount (yaw axis), C = camera. Red = X, green = Y, blue = Z.
Where is point C on the camera? C is the camera’s no-parallax point (perspective centre). It lies on the lens axis, usually inside the lens, where the aperture appears to be. To find it, look into the front of the lens from a slight angle: the bright image of the aperture (the entrance pupil) shows where C is (Figure 2).
Figure 2 – Point C: the no-parallax point of the lens, on the lens axis.
2. Polarity – which sign to use #
X points to the nose, Y to the right side, Z down. Note that Z is positive downwards: a camera hanging below the antenna has a positive Z.
Figure 3 – Positive and negative values. The arrow goes from the first point to the second one.
3. How to measure #
STEP 1 – Prepare
- Set the gimbal to look horizon with its yaw pointing forward.
- Prepare measurement tools
STEP 2 – Find the points
- A – centre of the GPS antenna.
- B – the vertical rotation axis of the gimbal (where it attaches to the aircraft).
- C – the camera’s no-parallax point (perspective centre) on the lens axis, usually inside the lens. Figure 2 shows where it is.
STEP 3 – Measure each axis separately
- OFF_ANT->MNT = from A to B. OFF_CAM->MNT = from B to C.
- X: forward is positive. Y: right is positive. Z: down is positive.
STEP 4 – Camera without a gimbal
- If the camera is fixed to the aircraft, measure the whole distance from the antenna to the camera into OFF_ANT->MNT and leave OFF_CAM->MNT at 0.
Figure 4 – Example. OFF_ANT->MNT = X +300, Y +60, Z +370 mm. OFF_CAM->MNT = X +45, Y +20, Z +110 mm.
4. GPS antenna position already set in ArduPilot / PX4 #
If the autopilot knows where the GPS antenna is (ArduPilot GPS_POS…, PX4 EKF2_GPS_POS_…, in metres), it moves basic GPS position from the antenna to the centre of the aircraft (CG). TAG-E then has to measure from that point, otherwise the distance is counted twice.
Figure 5 – With the autopilot antenna position set, measure X, Y and Z from the centre of the aircraft (flight controller).
| Autopilot setting | Measure OFF_ANT->MNT_X / Y / Z from |
|---|---|
| GPS antenna position not set (0) | the GPS antenna (A) |
| GPS antenna position set | the centre of the aircraft (flight controller) – all three axes |
TAG-E uses raw GPS (MAV_GPS_RAW = 1) or its own GNSS receiver | always use the GPS antenna (A) |
