> For the complete documentation index, see [llms.txt](https://docs.theseus.us/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.theseus.us/cyclops/autopilot-integration/ardupilot/cyclops-v1.x.x/non-gps-ekf-initialization-in-ardupilot/optical-flow.md).

# Optical Flow

This page features information on how to set up your autopilot to fuse the optical flow and synthetic rangefinder that cyclops provides. 

{% hint style="info" %}
This document assumes that you are using both the optical flow and synthetic rangefinder together. Your device can be configured to use an actual, more accurate, rangefinder if preferred.
{% endhint %}

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{% step %}

### Ardupilot Parameters

Set these on the FC via Mission Planner / MAVProxy.

#### Optical flow

```
FLOW_TYPE        5      # MAVLink optical flow source
FLOW_FXSCALER    <computed below>
FLOW_FYSCALER    <computed below>
FLOW_ORIENT_YAW  0      # 0 if camera +X aligns with vehicle +X; 180 if reversed
FLOW_DELAY       30     # ms; bump to 50 if post-flight XKFR innovations are biased

EK3_FLOW_USE     1

## for the following params you may use a different source if wanted
EK3_SRC1_VELXY   5      # OPTFLOW
EK3_SRC1_POSXY   0      # none (flow gives velocity, not absolute position)
EK3_SRC1_POSZ    1      # baro
RC7_OPTION       90     # EKF_SOURCE_SET — flip SRC1↔SRC2 from the TX in flight
```

#### Synthetic Rangefinder

```
RNGFND1_TYPE     10     # MAVLink rangefinder
RNGFND1_ORIENT   25     # PITCH_270 (down) 
RNGFND1_ADDR     0      
RNGFND1_MIN_CM   5
RNGFND1_MAX_CM   <ceiling, e.g. 5000 for 50 m> 
## this value (MAX_CM) varies depending on you use case (see below)
```

{% hint style="warning" %}
When using VTOL drones, the MAX\_CM parameter should be \~30m higher than your transition altitude. For example, if transition from quad into forward flight at 50m, set the param to <8000> cm. However, for stol drones, this parameter should be dropped down to <3000 cm
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### Compute the flow scalers

`oflow` sends `OPTICAL_FLOW` at 25 Hz, so `T = 1/25 = 0.04 s`, `T² = 0.0016`.

Read `fx`, `fy` (pixels) from the rectified camera intrinsics found on vozilla.

<figure><img src="/files/B3RSZxBchErY85CshZld" alt=""><figcaption></figcaption></figure>

```
SCALER = (1 / (f × 0.0016) − 1) × 1000
```

Round to integer. `FLOW_FXSCALER` uses `fx`, `FLOW_FYSCALER` uses `fy`.

| `f` (px) | Scaler |
| -------- | ------ |
| 320      | +953   |
| 514.57   | +214   |
| 800      | −219   |

{% hint style="warning" %}
AP rejects scalers outside roughly ±800 (true scale 0.20×–4.00×). If the formula gives something extreme, recheck `fx` / `fy` — you may be reading the raw sensor matrix instead of the rectified one.
{% endhint %}
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### Bench Verification

Power up the FC and the companion. In Mission Planner, hit `Ctrl+F` → **MAVLink Inspector**.

Under **Vehicle 255** (the companion) you should see:

* `OPTICAL_FLOW` — `flow_x` / `flow_y` change when you wave a hand in front of the camera
* `DISTANCE_SENSOR` — `current_distance` roughly tracks `relative_alt` reported by the FC

Under **Vehicle 1** (the FC):

* `RANGEFINDER` reports a fresh distance
* Mission Planner → **Status** tab → `RNGFND1` and `OF` rows are live

If `OPTICAL_FLOW` or `DISTANCE_SENSOR` is missing under Vehicle 255: SSH to the NanoPi and run `journalctl -u oflow.service -f`. You should see `SEND OPTICAL_FLOW` and `SEND DISTANCE_SENSOR` lines. If `RECV GLOBAL_POSITION_INT` is also missing, MAVProxy isn't routing the FC stream to UDP `127.0.0.1:14555`.<br>
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### GPS Spoof Params

#### VTOL

No need to set GPS parameters for VTOL drone.

#### STOL / fixed-wing (TALON)

```
GPS2_TYPE        14      # MAVLink
GPS_PRIMARY      1
```

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{% step %}

### Flying Without GPS

The FC needs an EKF origin and home position before any GPS-denied mode (LOITER, FlowHold, etc.) will engage.

#### VTOL

Set the EKF origin and home position via Mission Planner. \
Optical flow + the synthetic rangefinder give EK3 enough to hold position during takeoff, and into forward flight.

#### STOL / fixed-wing (TALON)

Same as above, but EK3 also needs a few seconds of plausible position fix before it'll release for arming. Use the GPS spoof tool on vozilla:

1. Vozilla → Settings → Lab → GPS Spoof
2. Set Duration to 15
3. Click Spoof Home Position
4. Wait the full duration before arming
5. Arm, take off.

<figure><img src="/files/4DHQJlyNWSyOVYnrxJPX" alt=""><figcaption></figcaption></figure>

{% hint style="info" %}
To use GPS Spoof you must ensure you set the params appropriately. You need to GPS spoof to be able to use optical flow on STOL bird.
{% endhint %}
{% endstep %}
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