Aids to Navigation utilizing radio waves are referred to as radio aids to navigation． Radio aids to navigation are known as the radio navigation assistant system．
Vessels are able to find their positions without being obstructed by poor visibility due to rain or fogs．
Radio waves have the characteristics proceeding“straight forward”and of proceeding“at a fixed speed．”
Medium Wave Radio Beacon Stations and Microwave Radio Beacon Stations operate on the former characteristics of radio waves．
Vessels on the sea are able to recognize their positions by receiving the signals from the transmitting station．
Loran C and Decca stations based on the principle of hyperbolic navigation operate on the latter characteristics of radio waves．
By arranging a plural number of Loran C efficiently on the ground，it has been planned to secure as wide as possible area around the seas of Japan where vessels may obtain their positions （longitude and latitude）with high accuracy．
Today, by sending the transmitting radio wave from the earth satellite，it has become possible to determine the accurate position of vessels on a world-wide scale．
The system is called the Satellite Navigation System，and the Global Positioning System is in operation in the United States．
The Maritime Safety Agency has developed a Differential GPS of greater accuracy．
Around the seas of Japan it is possible to determine the position of a vessel with an error of less than 10 meters．

Radio Beacon Station

Medium Wave Radio Beacon

Medium wave radio beacons are for middle and near areas, using a low frequency round 300 KHz lower than the frequency range of 540 KHz to 1602 KHz used in the radios at our homes．
In order that vessels may find the direction of the transmitting station（radio beacon station）by a direction finder，non-directional radio waves are dispatched regularly．

Micro Wave Radio Beacon

●Ramark Beacon

The Ramark Beacon Stations emit radio signals of frequencies in the range of 9 GHz which may be detected by the radar on vessels．
When the antenna of the radar on the vessel turns toward the direction of the beacon station，a doted line appears between the vessel and the beacon station on the radar screen on the vessel．
Along the coastline or sand dunes where it is difficult to recognize the reflecting object，the Ramark Beacon will be a conspicuous mark so that the features of the area may be easily determined．

●Radar Beacon

Radar Beacons are quite similar to the Ramark Beacon，except that on the radar screen，a beacon signal with dots and dashes appears behind the location of the transmitting station．
The front edge of the dashed signal is the location of the transmitting station．

Loran C System・Decca System
（Radio aids depending on the time required for two or more radio waves to reach the vessel）

If from two radio stations，A and B,radio waves are transmitted at the same time，at the same speed，to a spot“a”in the ocean，as the distance from A is farther than B it will take a longer time for the radio waves to reach the spot“a”than the radio waves from station B．
Similarly at spot“b”and“c”,radio waves from station　A　will take a longer time than the waves from station B．
Connecting the　points，“a”，“b”and“c”where the difference of the time required is the same，a curve is obtained as shown in Fig．1．
The curve is known as the hyperbolic curve．
In transmitting radio waves from the so called “radio lighthouses”，as in the former case，it is possible to make a hyperbolic curve of a，b．c and d（Fig．1）．

If the vessel is receiving radio waves，the vessel will be in a position on the hyperbolic curve.
If it is possible to receive two pairs of radio waves，A and B and A and C，at the same time，the two hyperbolic curves in Fig．1 and Fig．2 may be ascertained．
The point where the two curves intersect will indicate the position of the vessel（Fig．3）．
This method of determining the position of the vessel is known as the hyperbolic navigation system．
Loran C and Decca Systems operate in the same principle, Today, facilities equipped on the vessel are capable of receiving radio waves，and the longitude and latitude may be calculated automatically by the computer．
There are four Loran C transmitting stations in Japan（Niijima in Tokyo，Gesashi in Okinawa，Minami Torishima in Tokyo and Tokachibuto in Hokkaido）covering the seas around Japan．
As Loran C is a world widely recognized radio navigation system，the North-West Pacific Chain, the Korean Chain and the Russian Chain are cooperating to provide the Loran C coverage in extended areas of the Far East Asian region．

Differential GPS

In the ocean，far from land，vessels depended on astronavigation．Watching the stars to identify their positions for a long time．
Today，vessels may ascertain their positions by radio waves from satellites． This is known as the satellite navigation system．
The GPS used in the car navigation system is well known．
According to the system，with several GPS satellites，measuring the distance between the receivers and the GPS satellites，the vessel may find its position from the intersection obtained．．
In the measured position using the GPS, due to the characteristics of radio waves，there is an error of about 100m． The GPS system has been developed to minimize the error．

In a location where the accurate position is already known（the reference point) the position is obtained by the GPS method，and the error from the reference point is calculated．
The error will be transmitted by radio beacons．
The receivers will be able to correct the GPS error to less than 10 meters．
The vessels must be equipped with a GPS receiver and a medium wave radio receiver．