LORAN-A Loran-A operated in the 1850 to 1950 kHz band, uses pulse-time difference as its operating principle with a day/night range of about 800 to 1600 nm. Loran provided facilities whereby ships and aircraft derived their position at long distances. The system required at least three transmitting stations for each 'chain', and the observer used a special Loran receiver. A chain consisted of one master and two slave stations. Differences in the arrival time of pulses from a pair of stations was measured and displayed on the face of a cathode ray tube. Each fix required two observations and the operation normally took about five minutes. The readings were then transposed to a Loran lattice chart and position could be plotted. In some cases readings were referenced to special Loran tables. Because Loran-A signals were pulsed and not continuous transmissions, tremendous peak power levels could be achieved by a relatively small transmitter. The maximum reliable range for Loran-A was 700 miles by day and 1,400 miles at night. Each transmission pulse lasted about 40 microseconds and reoccurred at regular, accurately controlled intervals. This interval, called the Pulse Repetition Interval (P.R.I.) varied for each station and lasted between 29,000 and 40,000 µs. These pulses provided precise index marks for use in time measurements. The transmissions of corresponding master and slave pulses were separated by a fixed time interval which consisted of the time for a signal to travel from the master to the slave, plus one-half the P.R.I., plus an additional small time called the 'coding delay'. It should be noted that the observer is interested only in measuring the difference between the time of arrival of the two pulses, and not the actual time taken for each pulse to reach the receiver. There was no need, therefore, for an absolute synchronization of the receiver time base with the transmitter. At all points in the coverage area, the time interval between a master pulse and the next slave pulse was greater than the interval between a slave pulse and the next master pulse. That methodology provided a positive method of identifying the signals arriving from each station, even though their actual appearance was similar. In the measuring process, the time difference was always measured from the master pulse to the slave pulse, and the time delay of one half of the pulse recurrence interval was automatically removed. The lines of constant time difference for each pair of stations were pre-computed, taking into consideration the curvature and eccentricity of the Earth, the time for the master pulse to reach the slave station, and the coding delay. These "hyperbolic" lines were made available in the form of overprinted charts and tables. When a common master controlled two slaves, the master was called a 'double pulsed' station because it transmitted two entirely separate sets of pulses, one set paired with the pulses from each adjacent station. Pairs of Loran stations were situated up to 600 miles and more apart. If any trouble occurred at either the master or the slave station that might impair the accuracy of the pulse timing, the transmitters operated on a 2 sec ON then 2 second OFF mode. This appeared to the operator as a blinking signal. Blinking signals were not used for navigation. Reception of signals In order to properly display the pulses to be measured, the receiver's time base had to synchronized so the length of the trace on the C.R.T. matched the P.R.I. of the station. Failing to do so would cause the pulses to appear as if they were drifting to the left or to the right depending if the time base was too short or too long respectively. The face of the C.R.T. in the receiver displayed two time base lines because a pair of stations were always being compared. For convenience, the upper trace was called the "A" trace and the lower one the "B" trace. By convention, the master station was displayed on the upper trace and the slave on the lower one. The time difference measurement was the horizontal distance from the master pulse to the slave pulse. In an attempt to gain longer-range navigation, a variant of Loran-A was developed. It was known as SS (sky-wave-synchronized) Loran In the SS Loran system, the slave station of a pair was synchronized by a sky-wave pulse reflected from the 'E' layer, rather than by the ground wave as in standard Loran. This allowed the master and slave stations to be separated by as much as 1000 to 1200 miles. The Loran charts were calibrated in terms of sky waves, instead of ground waves, so that correction factors were unnecessary when sky waves were used. A disadvantage of the system was encountered when the indicator was located close to either or both stations, since erratic reception resulted when the angle of reflection of the sky wave from the E layer approached the critical angle. As the critical angle was approached, the radio waves exhibited increasing penetrating power and would go entirely or part way through the 'E' layer. Identification of LORAN-A pairs Loran-A stations did not transmit call signs. Instead, identification was made entirely by two distinguishing characteristics: a) radio frequency channel b) pulse repetition rate. A) By Channels Different groups of Loran stations operated on different frequencies Four fixed frequencies were available between 1,750 and 1,950 kc/s. The receiver was fitted with a channel selector switch for tuning to the desired frequency. They were assigned the following designations: Channel 1 - 1,950 kc/s Channel 2 - 1,850 kc/s Channel 3 - 1,900 kc/s Channel 4 - 1,750 kc/s B) By Pulse Repetition Rate In order to economize on frequency channels, a number of pairs of Loran stations were operated on the same frequency, but each pair operated at a different pulse repetition rate. That meant that signals from all stations on the same frequency within range appeared on the indicator, but they drifted across the scan at varying speeds. The operator selected a particular pair of stations by means of switches on the receiver which make the sweep repetition rate of the indicator the same as the pulse repetition rate of the desired pair. The desired signals would now be stationary, while the remainder still drifted across the scan and could be ignored. Two switches were provided. The first one adjusted for the basic pulse repetition rate, of which there were three in advanced Loran sets: High, Low and Slow. The second switch adjusted for a specific pulse repetition rate differing from the basic by a small amount. There were eight of these specific rates, numbered 0 to 7, for each basic pulse repetition rate. This system thus provided 96 separate station pairs using the four frequency channels available. Station identification symbols Each pair of Loran-A stations was given a three character identification symbol, of which the first character was the channel; the second was the basic pulse repetition rate, and the third for the specific pulse repetition rate. DISTANCE MEASURING EQUIPMENT Distance measuring equipment (DME) is a transponder-based radio navigation technology that measures distance by timing the propagation delay of VHF or UHF radio signals. The DME system is composed of a UHF transmitter/receiver (interrogator) in the aircraft and a UHF receiver/transmitter on the ground. Aircraft use DME to determine their distance from a land-based transponder by sending and receiving pulse pairs - two pulses of fixed duration and separation. A typical DME transponder can provide distance information to 100 aircraft at a time. Above this limit the transponder avoids overload by limiting the gain of the receiver. Replies to weaker more distant interrogations are ignored to lower the transponder load. The technical term for overload of a DME station caused by large numbers of aircraft is station saturation. Timing The aircraft interrogates the ground transponder with a series of pulse-pairs (interrogations) and, after a precise time delay (typically 50 microseconds), the ground station replies with an identical sequence of reply pulse-pairs. The DME receiver in the aircraft searches for pulse-pairs (X-mode= 12 microsecond spacing) with the correct time interval between them, which is determined by each individual aircraft's particular interrogation pattern. The aircraft interrogator locks on to the DME ground station once it understands that the particular pulse sequence is the interrogation sequence it sent out originally. Once the receiver is locked on, it has a narrower window in which to look for the echoes and can retain lock. Distance calculation A radio pulse takes around 12.36 microseconds to travel 1 nautical mile (1,852 m) to and from; this is also referred to as a radar-mile. The time difference between interrogation and reply 1 nautical mile (1,852 m) minus the 50 microsecond ground transponder delay is measured by the interrogator's timing circuitry and translated into a distance measurement (slant range), stated in nautical miles, and then displayed on the cockpit DME display. The distance formula, distance = rate * time, is used by the DME receiver to calculate its distance from the DME ground station. The rate in the calculation is the velocity of the radio pulse, which is the speed of light (roughly 300,000,000 m/s or 186,000 mi/s). The time in the calculation is (total time - 50µs)/2. Radio frequency and modulation data DME frequencies are paired to VHF omni directional range (VOR) frequencies and a DME interrogator is designed to automatically tune to the corresponding DME frequency when the associated VOR frequency is selected. An airplane’s DME interrogator uses frequencies from 1025 to 1150 MHz. DME transponders transmit on a channel in the 962 to 1150 MHz range and receive on a corresponding channel between 962 to 1213 MHz. The band is divided into 126 channels for interrogation and 126 channels for reply. The interrogation and reply frequencies always differ by 63 MHz. The spacing of all channels is 1 MHz with a signal spectrum width of 100 kHz.Technical references to X and Y channels relate only to the spacing of the individual pulses in the DME pulse pair, 12 microsecond spacing for X channels and 30 microsecond spacing for Y channels.DME facilities identify themselves with a 1350 Hz morse code three letter identity. If collocated with a VOR or ILS, it will have the same identity code as the parent facility. Additionally, the DME will identify itself between those of the parent facility. The DME identity is 1350 Hz to differentiate itself from the 1020 Hz tone of the VOR or the ILS localizer. Accuracy The accuracy of DME ground stations is 185 m (±0.1 nmi). DME provides the physical distance from the aircraft to the DME transponder. This distance is often referred to as 'slant range' and depends trigonometrically upon both the altitude above the transponder and the ground distance from it. A terminal DME is a DME that is designed to provide a 0 reading at the threshold point of the runway, regardless of the physical location of the equipment.
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