## RDS (Vessel Meeting Point Calculation Along the Fairway)

The IENC performs the calculation of the meeting point with the selected target along the fairway.

```{important}
To avoid chart information overload, it is recommended to disable the RDS function when the vessel enters open waters.
```

The meeting point calculation is performed only for the selected target. If the navigator has selected multiple targets, the meeting point will be calculated for the last selected target.

The meeting point calculation is performed for both oncoming and following targets. The calculation is possible only when the oncoming or following target is on the same fairway graph as the vessel. If there are fairway gaps of more than 1 km in the ENC cell, or if the fairway is not encoded, the IENC will not perform the calculation.

If a meeting situation with the selected target occurs, the RDS button will be displayed in the target form.

### Purpose

Given the winding nature of the river channel, it is technically impossible to use ARPA tools for assessing passing conditions, even within the direct visibility range of the vessel and the target.

ARPA does not account for the winding nature of the river channel and other navigational-hydrographic features of the navigation area. Even if the target is behind a river bend, ARPA will calculate passing conditions as if the target were in direct or radar visibility.

The calculation is necessary for visual assessment of the passing point with an oncoming vessel. The meeting point may fall on fairway sections with restrictions: one-way traffic, reversible traffic, overtaking prohibited, speed limits.

**IENC STREZHEN** calculates 3 meeting point calculation scenarios:
 - oncoming target;
 - following target, overtaking — moving behind own ship but at a higher speed;
 - following target, being overtaken — moving in the same direction but own ship speed is higher than the target speed.

### Oncoming Target

![Oncoming target](images/RDS_1.png)

| No. | Element | Description |
|---|---------|----------|
| 1 |Own ship||
| 2 |Target| Any oncoming AIS target or radar target |
| 3 |Meeting point| Passing or overtaking point with the tracked target |
| 4 | --- | Distance from own ship to the meeting point in the set units of measurement |
| 5 | --- | Time and date of reaching the meeting point by own ship |
| 6 | --- | Transit time to the meeting point |
| 7 | --- | Geographic coordinates of the meeting point |
| 8 | --- | Go to meeting point button — when pressed, the chart in the main window is centered on the meeting point |
| 9 |![](images/btn_rds.png)| Go to meeting point form. Displayed when a vessel meeting is detected. If a meeting is impossible or the meeting point cannot be calculated, the button is not displayed |
| 10 | Back | Button to return to the target form |

The meeting point calculation is performed discretely, once every 5 seconds.

```{hint}
The meeting point position may change, as the current speed over ground of the vessel and the target is used in the meeting point calculation.
```

```{important}
When calculating the meeting point with an oncoming target, the point will be located between own ship and the target.
```

### Overtaking Target

![Overtaking target](images/RDS_2.png)

| No. | Element | Description |
|---|---------|----------|
| 1 |Own ship||
| 2 |Target| Any following AIS target or radar target |
| 3 |Meeting point| Passing or overtaking point with the tracked target |

```{important}
When calculating the meeting point with an overtaking target, the point will be located ahead of own ship.
```

### Overtaken Target

![Overtaken target](images/RDS_3.png)

| No. | Element | Description |
|---|---------|----------|
| 1 |Own ship||
| 2 |Target| Any following AIS target or radar target |
| 3 |Meeting point| Passing or overtaking point with the tracked target |

```{important}
When calculating the meeting point with an overtaken target, the point will be located ahead of the tracked target.
```

```{caution}
If the meeting point falls within a fairway branching area, the meeting point calculation will not be performed to avoid misleading the navigator.
```

```{caution}
The meeting point calculation function is not an ARPA element and does not calculate vessel passing conditions.

The meeting point position is always tied to the fairway axis. Meeting point coordinates are calculated periodically and with significant error.

The time to reach the meeting point is not related to the TCPA parameter.

The meeting point is advisory and informational in nature.
```

If the calculated meeting point falls on a fairway section with restrictions (overtaking prohibited, reversible traffic, one-way traffic), a notification will be displayed in the form indicating that the meeting point is in a restricted zone.

![Overtaken target](images/RDS_4.png)
