Dead reckoning and radio horizon circles

This large passenger boat is actually
making some turns between the islands.

Today aprs.fi got a simple implementation of dead reckoning. Moving stations which have announced their position, speed and course within the past 30 minutes will show a blue line extending from the latest announced position. The line indicates where the station would be right now if it proceeded on that course at exactly the same speed.

To limit the performance impact of this feature the DR indicator will be shown for a single station at a time. Click on a station to enable it. It's also enabled automatically for a tracked station.

Naturally it'll show some rather bogus estimations for vehicles which have stopped moving but have not transmitted a new packet with a speed of 0. Most trackers installed in cars loose their power input before they have a chance of transmitting a packet at the parking spot.

A plane flying at 2648 feet, and the digipeaters
and igates which have received it.

Flying stations having a "flying" symbol such as airplane or balloon will have two circles drawn around them. A larger blue circle indicates the sea-level horizon from the current altitude of the station. A smaller green circle indicates the area where the station can be seen at an elevation larger than 5 degrees.

The circles are useful for estimating the distance at which the station can be heard, which can be of use for tracking teams of high-altitude balloons. It also gives pilots an idea of the large amount of digipeaters and igates in range.

The circles are drawn for stations having a suitable symbol when the altitude is over 100 meters. Other stations will also get the circles when their altitude reaches 1000 meters.