Moving-Map Navigator: Instructions

A demonstrator in principle of a flight guidance system whose function is to steer an aircraft from an origin to a destination via a route, where a route is defined as a set of way points joined by 'great circle' arcs using an ellipsoidal model of the earth.

Message Panel

After the applet has finished loading and has started processing, the first task it does is load the images used for its panel backdrop. The progress or phase of the image loading process is indicated on the message panel along the bottom of the applet. If an error occurs during the loading process, a message specifying the type of error is displayed in red. In this case, the blank button at the bottom left of the applet becomes a "Retry" button. Press this to initiate a new attempt to load the images. If this fails, try pressing your browser's reload button.

After the images have loaded, the applet proceeds to load the list of available routes from the server. It then selects the first route in the list as the default route and loads the route data for this route. Again, the phases of the loading process are indicated by progress messages. If an error occurs, a red message is displayed indicating the nature of the error and again, the blank button at the bottom left becomes a "Retry" button, allowing you to re-attempt the loading of the route list or route data.

When all the loading processes have terminated successfully, you should see the applet complete with its clouds and sky backdrop photograph. At this stage it is a good idea to click on the link below the applet to duplicate the applet frame set in another tab in your browser so that you can read through all the text pages without terminating the applet. For example, you can switch back and forth between the tabs as you read this text so that you can see where things are on the applet.

Route Selector

At the top left of the applet is a drop-down choice menu. On this menu should be displayed the name of the first route in the list of available routes. Open this menu if you wish to select a different route to fly. When you select a new route, the waypoints and features data for that route will be downloaded as described above.

On the top right of the applet, next to the route selector, is a route search field. Here you can enter all or part of the name of a route. When you press the button marked "Find Route", this text will be searched for in the list of available routes. The first route name containing your text will then be selected automatically and its data loaded from the server.

Waypoint Selector

A route is made up of a sequence of waypoints. The route data also contains other route features such as cities, lakes and mountains to populate the map while you are flying the route. Beneath the map on the right of the applet is another drop-down choice menu. On this you will see the name of the starting point of the currently-selected route.

You can open this choice menu to reveal a full list of the waypoints in this route. Selecting any particular waypoint within this menu will move the aircraft instantly over that waypoint. Thus you can start a flight from any waypoint within a route.

Geoid Selector

At the bottom right of the applet panel is the Geoid Selector. This is a drop-down menu. It enables you to select which of the standard ellipsoidal models of the Earth's geoid you wish the Vincenty Method to use for computing distance and bearings.

The Earth, far from being a perfect sphere, isn't even a perfect ellipsoid. It has slight arbitrary bulges in various places. This means that different models give a more accurate fit in different regions of the planet. The most generally used geoidal model is the GRS80/WGS84(NAD83) ellipsoid. This is the one used by default by this navigation applet.

You can, however, select a different model from the drop-down menu that may better suit the region in which you are interested. Nevertheless, the gain in accuracy for navigation purposes is essentially ignorable.

The Selector Buttons

On the left of the applet beneath the flight data panel and above the Retry button and message panel are two banks of radio buttons. The bank on the left is to select the map refresh rate and the bank on the right is to select the map size (or scale).

The refresh rate selector allows you to select at which of the three available rates you wish the moving map to be refreshed. This determines how often all the navigational data for the map is recomputed and displayed. A higher rate uses more CPU time, so if in doubt, set it to refresh once every 1000ms, that is, once per second. However, the highest refresh rate should not use more that 13% of the CPU time of a modern PC.

The map size selector allows you to change the scale of the moving map to the dimensions shown adjacent to the radio button you select. You can change the map size at any time. For long legs of a flight such as transoceanic portions, you could set the map to cover the widest possible area. When approaching a destination, you could switch to a smaller coverage to show closer detail of your approach.

The Control Buttons

When a route is loaded, the aircraft is automatically set to the beginning of the route. To start the aircraft on its journey, press the "Start" button. Once the aircraft has started its journey, the "Start" button turns into a "Stop" button. Pressing this pauses the flight at the point it has reached.

The "Reset" button instantly returns the aircraft to the beginning of the route from wherever it may be. The "Next" and "Prev" buttons respectively step the aircraft to the next and previous waypoints in the route.


© January 2008 Robert John Morton