Radionavigation
Navigation is the process of determining own position, orientation and velocity relative to a known reference point. It is pivotal for traveling from one point to another. For centuries sailors relied on stars or known landmarks to navigate the seas, however, at the beginning of 20th century, with the development of electronic equipment for producing, transmitting and receiving radio waves, paved the way for a more sophisticated way for navigation to be used onboard maritime, air and land platforms. This new method is called radionavigation that takes its name from the radio waves it utilizes.
The U.S. Federal Radionavigation Plan (FRP), which is the official source of positioning, navigation, and timing (PNT) policy and planning for the U.S., defines radionavigation as;
“The determination of position, or the obtaining of information relating to position, for the purposes of navigation by means of the propagation properties of radio waves.”
So, the basic components of radionavigation are i) radio waves, ii) radio wave transmitters located at known positions, iii) radio wave receivers on board the navigating platform and iv) a clever method to determine position.
One of the first applications of radio for navigation in ships was to fit the vessels with a radio beacon, and upon the shore, a series of radio direction finding stations. Radio compasses, also called radio direction finders, had been around since the beginning of 1900s and working principle was based on rotating a loop antenna and measuring and comparing the strength of the signal relative to the rotation as it points in different directions. This allowed for bearings to be based on the transmitting source. Using that information, two stations could relay their findings to a ship, from where the vessel's crew could determine their location. The basis of this system, which is finding two lines from two known positions in order to determine your own, is the foundation of most radionavigation systems.
The loop-antenna radio direction finder was then developed into a device in which a motor turned the loop and electronic circuitry identified the direction of the source of the signals. This instrument, originally called a radio compass, could guide the user toward any detectable transmitter. It was often linked to a compass so as to display not merely the direction of the radio station compared to the heading of the craft but the actual direction as plotted on a chart.
How does one find the position?
Since the speed that radio waves travel at is known the idea is simple; knowing the exact location of the transmitting station and the time that the signal was transmitted then recording the time when the transmitted signal is received allows to find the time that the signal took to reach the platform. Assuming that the radio signals travel at the speed of light, the distance between the station and platform is found by simply multiplying the calculated time difference by the speed of light. Then it is the matter of repeating the procedure for at least three transmitting stations to calculate the platform’s position. As explained in “What is GNSS?” blog and seen in Fig. 1, there could be only one intersection point where the signals from three transmitters meet and that is the location of the receiver.
Figure 1 : Determining position using radio signals (c: speed of light).
Some History
As mentioned above, the first of the major systems to be adopted for navigating aircraft was the four-course radio range (radio compass). This system, put in place in the late 1920's, offered aircraft just four beams from the station to navigate by. This allowed aircraft to fly towards the station along one of the beams, and then to fly away from the station on another beam. Prior to this, pilots navigated mostly by following the roads on the ground below. Of course, there are no roads to follow in the ocean, making navigation mostly dependent on memory and buoys and landmarks if there were any. On July 6, 1920, US Navy seaplane pilots used a radio compass to locate and navigate their way to a ship 100 miles offshore, marking the first use of radio navigation by an aircraft.
A similar system present in the 1900's, and still in use in some places (such as Alaska) today is called the Nondirectional Beacon/Automatic Direction Finder system (NDB/ADF). Very similar to the systems used in maritime navigation of the 1920's, these two systems work together to allow an aircraft to determine a bearing from their aircraft, to a beacon.
A combination of Very High Frequency Omnidirectional Range and Distance Measuring Equipment systems (VOR/DMEs), were to follow NDB/ADF as a more modern method of aircraft radionavigation. It functions very similarly to the original four-course range, but instead of offering only merely four 'ranges', it offers 360 (one for each degree), through the use of a clever transmission technique. These allow pilots to navigate with far more freedom in regards to course selection. The additional Distance Measuring Equipment - allows a pilot to determine their position with only a single reading from a VOR. The position of the aircraft can be determined by combining the distance indicated by the DME with the direction from a VOR beacon at the same site as the DME beacon.
Loran (long-range navigation) was a large engineered system, developed in the 1940s and successfully deployed World War II. By 1946, loran was used by thousands of navigators over three-tenths of the surface of the earth. Loran is a hyperbolic system of navigation based on pulse-modulated synchronized signals and is still in use. Loran in its original form (now called Loran-A) operated at frequencies near 2 megahertz, but interference with and by other services and unreliable performance at night and over land led to its replacement by Loran-C. Loran-C transmitters operate at frequencies of 90 to 110 kilohertz, and the signals are useful at distances of 1,800 nautical miles or more.
Before World War II, radio navigation could only provide a course or a bearing to a station. The invention of timekeeping technologies, such as the crystal oscillator, led to a new era of systems that could fix position accurately and were easier to use. See “What is GNSS?”
Taxonomy
A very comprehensive classification of radio navigation systems is given in an article which is repeated here to provide the big picture and to illustrate that radio navigation has penetrated to all aspects of life.
