Radio Range Stations
The following information is taken directly from Aeronautics Bulletin No.27 " Aeronautic Radio" (July 1, 1937); published by the U.S. Department of Commerce / Bureau of Air Commerce.
The Federal airway routes are marked by directional radio range stations which assist airmen to navigate in poor visibility, and serve as a verification of other methods of air navigation when weather is good.
Flying by radio guidance, the pilot listens for the Morse Code letters
A ( . _ ) and N ( _ . ). If the two letters are heard in equal volume, they blend
together into a continuous monotone, which is the signal to the pilot that he is on
course. If he moves to one side of the course he hears the letter "A"
predominantly, if to the other side, he hears the letter "N". Knowing the
orientation of the off-course signals, or determining it by reference to his air
navigation map, he changes direction to get back to the line of flight where "on
course" is heard.
(figure 1)Courses marked by a radio range beacon. Flying on course, the pilot hears a steady hum, broken off at intervals for a signal identifying the station. If he strays to one side of the course he hears the letter "A", on the other side the letter "N". When he is on course he hears both, but they interlock to form the steady hum which signifies "on course". After the airplane passes over the transmitter and proceeds along the opposite course the "off-course" signals are reversed. For example, if the pilot approaches the station on the lower course in the above diagram, he would hear "A" on the left side, "N" on the right. After passing over the transmitter he would hear "N" on the left and "A" on the right.
The manner in which radio courses are formed is illustrated by figure 1. As shown
there, the radio range station transmits the letter "N" into two quadrants of
circular area surrounding the transmitter, and sends "A" into the other two
quadrants. On course--the continuous monotone--is heard along the borders of the
quadrants. Thus, four courses are provided by each radio range station.
Ordinarily, two of the courses lie along the airway route, the other two serving as
guides for off-airway flying, or, in some cases, serving an intersecting airway, or to
assist in finding the local airport.
For station identification, the directional signals are punctuated at intervals of about 30 seconds by another Morse Code combination, distinctive for each radio range. The radio range station near Washington, for example, has the identifying signal WA ( . _ _ . _ ). It is transmitted twice, first into the "N" quadrants and then into the "A" quadrants. Flying on course, the pilot hears the two transmissions of the identifying signal with equal intensity. As he moves away from the course one of them begins to fade into the background until finally only one is heard. This affords another guide for quadrant identification. If the pilot moves into the "N" quadrant, say, the "N" signal predominates as the "A" fades out, and in addition the first of the two station identifying signals becomes more prominent--both indicating to the pilot that he is off course in an "N" quadrant.
(figure 2. Aerial view of a Bureau of Air Commerce radio range station, c. 1935)
Radio stations operated by the Bureau of Air Commerce are of several different types:
RA - radio range station with tower radiator antenna
RL - radio range station with loop antenna
MRA - medium-powered radio range station with tower radiator antenna
MRL - medium-powered radio range station with loop antenna
ML - directional marker, using low-powered transmitter with loop antenna
RA and RL class radio range stations may be received at distances up to 100 miles, and are located approximately 200 miles apart along the airways. Radio stations of these types are in operation on most of the airways.
MRA and MRL class radio range stations have a smaller range; they may be received at distances up to 50 miles. While these stations have to be installed at closer intervals to furnish continuous radio range guidance, requiring more stations, these medium-powered installations are less adversely affected by a difficulty known as "multiple courses" which occurs at some of the higher-powered stations in mountainous country, and they therefore have been installed on the Northern Transcontinental Airway and several other routes of the Federal Airways system. Medium-powered stations also are used on some other airways to supplement the service of the higher-powered stations.
Still another type of radio range station is the directional marker or miniature radio range station, with a range of about 15 miles. These stations are used to mark important intermediate landing fields and to serve as homing devices assisting pilots to reach these fields.
Most of the RA and RL stations and some of the MRA and MRL facilities operate in conjunction with radio broadcasting stations which announce weather reports on regular schedules. The communications and directional units may be housed in a single building, which is the newer type of installation, or they may be in separate buildings at a distance from each other. In either event, they are located at a distance from the airport so that the antennas will not constitute obstructions to landings and take-offs, and generally are operated by distant control from a Bureau of Air Commerce office on the airport or intermediate field.
A combined station providing both directional signals and weather broadcasts normally transmits both on the same frequency. This is necessary because of the limited number of channels available, and has the advantage that the pilot flying the radio range does not have to retune his radio receiver to get the weather information. However, the radio range signals have to be silenced while broadcasts are being made. These periods never exceed 3 minutes and are usually less, but they may occur at the very moment when a pilot flying on instruments is approaching the end of his run and wants the assistance of the radio range station in locating the landing area.
To avoid this difficulty, some of the radio stations are equipped for radiophone transmission on an alternate frequency of 236 or 278 kilocycles. At RA and MRA stations so equipped (the fifth tower in the vertical radiator antenna system is a part of the equipment for this purpose) the operator, upon request, will use the 236-kilocycle frequency for his weather broadcast. When he does so, he makes an announcement on the regular frequency that the weather broadcast will be given on the alternate frequency. The directive signals are silenced only for the few seconds needed for this brief announcement and then are resumed immediately. Thus, the pilot who needs continuous directional service gets it, and others listening to the station may retune their receivers for the weather, if they wish to do so.
Stations not equipped to broadcast on the alternate frequency will postpone broadcast for a few minutes to enable a pilot to work out his landing problem.
At a few stations on the Federal Airway system the 236-kilocycle frequency is used for all weather broadcasts, with continuous service at all times by the radio range stations.
