Another title in the series What you should know Marine electronics about Loran-C receivers

by Daniel A. Panshin much more of the ocean off the U.S. than What is Loran? Extension Oceanographer Loran-A, with no gaps, and it will also Loran stands for long range include the Great Lakes and much of navigation. It is an electronic system that Oregon State University the continental . Loran-C is helps vessels to determine their position accurate, dependable, and long-range. It at sea by using shore-based radio will replace the present Loran-A system, transmitters and onboard receivers. Loran Are you ready for Loran-C? What kind of which will be turned off after two years works in any kind of weather, 24 hours Loran-C receiver will you use? What of overlapping Loran-C service. (For a day. should you know about Loran-C information on the schedule and on the receivers before deciding? Can you extent of Loran-C coverage in your area, convert your Loran-A receiver? Selection check with your local Sea Grant advisory What are the basic differences of a Loran-C receiver best suited to your agent or your Coast Guard District office, between Loran-A and Loran-C? navigational needs is an important or write to Loran-C Information, U.S. Loran-A was developed during World decision. It is time to start thinking about Coast Guard, Washington, B.C. 20590.) War 11. Loran-C is an improved version that decision. Thit bulletin assumes you are already dating back to the late 1950's. Loran-C Loran-C covers part of the U.S. waters familiar with Loran-A. If you are not, uses a lower frequency, 100 kHz (kHz = now, and by 1980 it will be available all you may want to do some background kiloHertz = kilocycles/second = around the United States except reading. See "For more information," thousand cycles per second), compared northern . Loran-C will cover page 8, for suggestions. with 1850 to 1950 kHz for Loran-A. The lower frequency gives Loran-C a longer range and allows for greater accuracy. Loran-C uses advanced electronic technology, including the more accurate cycle-matching method, rather than the envelope-matching Loran-A method. (To help you understand technical terms, those terms that are italicized in the text are defined in the glossary on page 8. In addition, most of the terms used here are explained in following sections.) Other technical improvements of Loran-C include multiple-pulse transmissions, phase coding, freedom from sky-wave contamination, and the ability to use more sophisticated receivers. As a result, Loran-C can tell you where you are within one-tenth to one-half nautical mile (geodetic, or absolute, accuracy) and help you return to the same spot, time after time (repeatable accuracy), within 15 to 90 meters (50 to 300 feet), depending on your location within the Loran coverage By 1980, Loran-C will area. cover coastal waters all around the United States (except the North Slope of Alaska), the Great Lakes, and most of the contiguous 48 states as well. Loran-C will serve a much larger area than Loran-A, with higher accuracy and no gaps in marine coverage. Loran-C is also available in the Hawaiian Islands Oregon State University and in portions of the western Extension Marine Advisory Program Pacific, the North Atlantic, and the A Land Grant / Sea Grant Cooperative Mediterranean. SG 50 May 1978 In some ways, Loran-A and Loran-C receivers are similar; in others, they are quite different. Envelopes and cycles. Both Loran-A and Loran-C receivers measure time differences between master and secondary signals, but they work in different ways; Loran-C signals are different from Loran-A signals. If you could look at both a Loran-A and a Loran-C signal on the same oscilloscope, they would look like this:

What does a Loran receiver do? To fix your position, more information A Loran receiver measures the is needed, such as a visual bearing, In this case, the oscilloscope presents the difference in time of arrival of two range, depth sounding, or second Loran pulse envelopes, which are an outline of different radio signals, one from a master reading. In many locations, two good the peaks of the actual radio-frequency transmitting station, the other from a Loran-A readings are available; in other cycles that have been transmitted. By secondary transmitting station. The time locations, as on the West Coast, for contrast, the cycles that make up the difference is very small and is read out in example, it is common to fix your position pulse envelopes look like those shown microseconds (millionths of a second), with one Loran-A reading plus a below. or "mikes" in common marine language. sounding. With Loran-C, however, two A Loran-A reading contains four digits, good Loran readings will be available in such as 1526, whereas a Loran-C reading all parts of the coverage area. So assume contains five digits and usually includes that you take a second Loran reading, tenths as well, such as 13370.6. this time from master station M and Because Loran signals travel at a secondary station S2, obtaining a time known speed, a time-difference reading difference of B. You have thus located allows you to plot your position as a line yourself at the intersection of lines A and of position on a chart. If, for instance, B (as illustrated above). you tune your receiver to master signal M and secondary signal SI and obtain a time-difference reading of A, you know that you are somewhere on line A, which is printed on the chart (as suggested by the drawing above). Ab^iut '/'Z./t

L-ora.n Uor^n - o ^Sy o L e- With Loran-A, only the envelopes are used for making time-difference measurements. This technique is called envelope matching. In the case of Loran-C, however, the signal is broadcast so that both envelopes and cycles are used to make time-difference measurements. To obtain the full accuracy of Loran-C, the receiver makes a coarse match on the envelopes and a fine match on the cycles. This technique is called cycle matching. Loran-A signals have cycles, too, as shown on the bottom of page 2, but they are too closely A - £.nv

Multiple-pulse transmissions. Another difference between Loran-A and Loran-C is the number of pulses each transmits. When operating with Loran-A, you select K a rate that covers your area, such as 1H5. This selection tunes in one master transmitter and one secondary _A transmitter, each of which broadcasts just one pulse. With your receiver controls set on Function 1, Loran-A signals look like this on the oscilloscope: L.or&.n-A; FunctLon 1 When operating with Loran-C, you also select a rate that covers your area, such as 9940. This selects one master transmitter and from a minimum of two to as many as five secondary transmitters. The master transmitter broadcasts a series of nine pulses, and each secondary transmitter broadcasts a series of eight vIUIIIII pulses. If we once again set our receiver MaL-ste-r ■ to Function 1 and look at 9940 (the U.S. £c-£?nd a,r L£<5 West Coast chain), for instance, there are three secondaries, and its appearance on an oscilloscope might look like the drawing at the right : Lora^n - C- . Function 1

Phase coding. Each of the Loran-C pulses is also phase-coded. A pulse can have positive phase or negative phase, which indicates whether the cycles start positively (up) or negatively (down), as shown in the drawings at the lower right. The series of eight (secondary) or nine (master) pulses is transmitted in a sequence of positive and negative pulses, such as +- + -+ +—. The sequence or code for master-station transmissions is different from that of secondary-station transmissions. Phase coding allows automatic Loran-C receivers to distinguish between master and secondary transmissions, and it helps the receivers to operate when the Loran signals are weak compared to the Pha.se Cod e Phgtse. Code. background noise. By contrast, Loran-A signals are not phase-coded. Other differences. There are a number of other differences among Loran-C receivers, whether manual or automatic. SKy Wa.ve,'^2Pr^'^ &\rour\A Wa-V^ Some of them are: 1. the number of chains the recevoer can process (some receivers are intended for regional use and can only process one chain; if you wish to operate in another area with this kind of receiver, you must have the factory or a dealer change your receiver from one chain to another); 2. the number of secondary stations simultaneously tracked (one, two, or Sky waves. Yet another difference Manual acquisition and automatic more); between Loran-A and Loran-C is the acquisition. Manual-acquisition Loran-C 3. the number of time differences effect of sky waves—that is, signals that receivers are like most Loran-A receivers: displayed (if the receiver tracks only bounce off the ionosphere instead of You must match pulse envelopes visually one secondary, it will only display one traveling directly from transmitter to as your first step. To be able to match time difference; if it tracks two or receiver. Sky waves are especially strong envelopes on an oscilloscope, you must more, however, it will have either one at night (above). be able to see the envelopes, or signals, display that shows two time differences Sky waves always arrive later than above the background radio noise, or alternately, or two separate displays); ground waves—that is, signals that have "grass." The signal levels need to be and traveled a direct path from transmitter to somewhat higher than the noise level 4. the type of signal matching (cycle or receiver. The sky-wave signal can be before you can match them. envelope). much stronger than the ground-wave If, for instance, a signal is exactly signal, a situation that can make twice as high as the noise, this situation envelope time-difference measurements is described as having a signal-to-noise confusing or difficult. ratio, or SNR, of 2:1. Once you have With Loran-A, sky waves are a acquired the signals manually, you may problem, just as they can be with Loran-C then track them manually or manual receivers. Since Loran-C sky automatically, depending on the type of waves can arrive before the peak of the receiver you have. A point to note, by ground wave, the Loran-C ground-wave the way, is that there is appreciably more envelope will be distorted under this background noise at 100 kHz, the condition, and the apparent position of Loran-C frequency, than there is at the peak of the envelope will shift. The Loran-A frequencies. result is that under this condition Loran-C Automatic-acquisition Loran-C envelope matching on a manual receiver receivers match signals electronically. will be erroneous. With Loran-C By fully using the information available automatic receivers, sky waves pose no through cycle matching, multiple-pulse problem as long as the ground-wave transmissions, and phase coding, signal is strong enough to use: The automatic receivers can match signals standard point for matching Loran-C that would be invisible to the eye on an signals with automatic receivers is 30 oscilloscope because they are not strong microseconds after the beginning of the enough to be seen above the noise. pulse, and sky waves do not arrive before Full Loran-C coverage depends on the this point. ability of receivers to acquire and track signals under conditions where the noise is as much as three times higher than the signal, that is, where the SNR is 1:3. (Of course, in much of the Loran-C coverage area, the signal is stronger than this level.) All receivers that can acquire signals automatically also track signals automatically. What are some Dynamic range indicates the ability of time difference, and thus determine your basic characteristics a receiver to process both weak and position from Loran. common to all receivers? strong signals. As stated above, If the receiver tracks two secondaries sensitivity provides a measure of how simultaneously, you have enough First, a note of caution. As with all far from Loran transmitters a receiver information to determine your position marine equipment, let the buyer beware. can function under low-noise conditions; from Loran anytime without delay. A Performance data, or receiver dynamic range, in conjunction with receiver's ability to track more than two specifications, that are presently available sensitivity, provides a measure of how secondaries provides flexibility represent information that manufacturers close to Loran transmitters a receiver immediately to display time differences provide. Because there are no standard can function. from another secondary of the same testing procedures, no common basis Differential gain indicates the chain, a feature which may be desirable exists for making numerical comparisons. on a long transit. For this reason, this bulletin does not difference in strength between a master generally present numbers. signal and a secondary signal that the Cycle-step switch. Since an automatic receiver can handle. A high differential receiver can match the wrong cycle You should be aware of these basic gain is desirable and means that the under certain conditions, time-difference characteristics as you make your receiver can successfully compare a very readings in such a case will be in error by receiver decision: strong signal with a very weak signal, an a multiple of 10 microseconds (1 Loran-C especially important capability if you cycle is 10 microseconds long). A cycle- Receiver band width. At the heart of will be operating close to a Loran-C step switch allows the operator manually a Loran set is a special-purpose radio transmitting station. to increase or decrease the time-difference readings by 10-microsecond intervals, receiver. Since the Loran-C signal falls Power consumption is an important thereby helping the receiver to correct an within the frequency band of 90 to 110 consideration, especially for small boats error. kHz, peaking at 100 kHz, the receiver that have limited electrical power. must cover this frequency range. Receivers vary widely in their power A memory button allows you to freeze Because of different electronic approaches requirements, drawing from 10 to 100 an important reading until you have a to processing the Loran-C signal, some watts. Most receivers can operate on chance to write it down. When you receivers are relatively broadband (that either AC or DC, although AC engage the memory button, the receiver is, they receive signals well outside 90 to capability is often an optional feature. continues to track internally, and the 110 kHz), whereas others are quite time-difference readings then update as Display. Most Loran-C sets show time- narrow in the range of frequencies they soon as you release the memory button. receive. difference readings on a lighted display. If the receiver tracks two secondaries, the Alarm indicators on automatic There are trade-offs with both time differences can be shown receivers inform you of various conditions approaches. If the receiver is broadband, alternately on one display, or they can be when the time-difference readings are it may also receive unwanted signals; shown simultaneously on two separate not usable: you will then need to remove them (by displays. • Blink alarm indicates that the master using notch filters, which are discussed Readout resolution—that is, whether or one of the secondary stations is in the next paragraph). If the receiver the display shows time differences in not transmitting the correct signal. is too narrow-band, the Loran-C signal is whole microseconds, tenths, or • Lost-signal alarm indicates that the distorted, resulting in possible errors in hundredths—is important. Regardless of signal is too weak for the receiver to time-difference readout. the features of your receiver, you need a function properly. display that shows time increments at • Cycle alarm indicates the receiver least as small as tenths of a microsecond Notch filters is the name given is tracking the incorrect cycle on one if you are to obtain full Loran-C accuracy. electronic filters in the receiver that or more signals. The cycle alarm There are other display features you reduce the effects of interfering signals. may also come on when the will probably want to check into. How Interfering signals are human in origin; receiver is in fact tracking the frequently does it update the time they can be from radio broadcast stations correct cycle but has not passed all differences? Is the display easy to read? or from special stations such as Decca. internal tests necessary to assure that Is it legible in bright sunlight? Can you (The Coast Guard has assembled a list it is tracking the correct cycle. of known interfering signals by region; dim the display at night? to obtain a copy, write to Loran-C You should also consider design Can you convert your Loran-A Information, U.S. Coast Guard, features such as dimensions, weight, receiver to Loran-C? Washington, DC 20590, specifying the convenience of controls, mounting, and It is possible to convert some Loran-A region in which you are interested, and moisture resistance as you select your receivers to Loran-C. There are also ask for a copy of the "Loran-C Loran-C set. Loran-A/C combination receivers that Interference List.") To obtain optimum can operate on both Loran-A and Loran-C performance, it is important Are there other features Loran-C signals. Convertible and that your receiver have the proper you should consider? combination receivers are basically number of notch filters, usually two to Definitely, yes! Some of the other Loran-A receivers. Whether or not one of four, and that they be set properly. features that may be available are these receivers will work for you is important to have; others are nice to something only you can decide. Sensitivity is a measure of the have: In part, the answer depends on your minimum signal level at which a Number of secondaries tracked. All needs for accuracy and the strengths of receiver can function. The more sensitive Loran receivers track and display time- the Loran-C signals in your area. the receiver, the greater the distance it difference readings between the master Convertible and combination receivers will work from Loran transmitters, so and one or two secondaries. If the require very strong Loran-C signals from long as the background noise is not receiver tracks or matches just one three transmitting stations (if you want excessive (that is, so long as the signal- secondary, then it is necessary to go two readings), and they do not offer the to-noise limit for the receiver is not through the acquisition and matching accuracy of a fully automatic Loran-C exceeded). process again to obtain a second Loran receiver. Is the installation of motors, fluorescent lights, , 3. If at all possible, try out the set you your receiver important? television sets, and other electronic are considering on a vessel of the Proper installation of your Loran-C equipment. Interference suppression may same type as yours, in your general receiver is critical to good performance, require installation of filters, shields, area, before you buy. At a minimum, much more so than with Loran-A. Proper grounds, and capacitors plus considerable operate the set in a dealer's showroom installation requires time and skill, and a ingenuity and persistence. Make sure to on a live antenna using real Loran-C competent, factory-trained dealer should accomplish the interference suppression signals, not on a test signal. Try out do it. Antenna location, grounding, with your engine running—along with the set during nighttime as well as interference suppression, and receiver all the equipment you would normally daytime. How easy is the set for you placement are all important: operate at sea. to operate? What is its reputation in the fleet? How long does it take the Antenna. Make sure you use a correct Receiver placement. Protect your set to acquire Loran-C signals, to lock antenna and antenna coupler for your receiver from excessive heat, dampness, receiver. Mount the antenna as high as on to the correct cycles, and start salt spray, and vibration. Do not mount it tracking? possible and as far away as possible (at in direct sunlight or within 1 meter least 3 meters or 10 feet) from all metal (3 feet) of your magnetic compass. objects, stays, and other antennas. Do not Provide adequate ventilation. 4. Buy from a reputable dealer. Make connect any other equipment to your sure that the dealer is technically Loran-C antenna. competent. Make sure that the dealer A half-dozen practical tips Grounding. Connect both the antenna will install and service your set. Make coupler and the receiver to a good 1. Buy the type of Loran-C set that sure that the dealer has adequate ground. At lower operating frequencies, matches your needs. Do not buy more factory support. such as Loran-C at 100 kHz, the ground than you need; do not buy less than becomes more critical. you need. Buy carefully. It is an important decision. Consult table 1, 5. Check the warranty carefully. A good Interference suppression. Electrical summarizing types of receivers, warranty is important; Loran-C repairs and electronic interference, or noise, can and the consumer checklist, page 7. can be expensive. Watch out for come from many sources, both aboard the unusually short warranty periods. vessel as well as from the surrounding 2. How automatic is automatic? All you environment. Onboard noise comes from have to do with some sets that are everything that generates or uses called automatic is turn them on. With 6. Do not depend exclusively on electricity; it is a much more severe others, you have to perform one or Loran-C, regardless of how fine a problem at the 100-kHz Loran-C more of these additional functions: receiver you buy. The prudent frequency than at higher frequencies, Select chain, set notch filters, select navigator uses all the navigational and you must suppress it if you are to secondary stations. Understand what information that is available. Loran-C, have good Loran-C results. you are getting, what the set will do just like all other marine electronics Special culprits include alternators and for you, and what operations you have equipment, can break down and give generators, ignition system, electric to do. erroneous readings.

Table 1.—Comparison of types of Loran-C receivers

Manual acquisition Automatic acquisition Manual Automatic Automatic tracking tracking tracking Sophisticated Operator Turn on Turn on Turn on Turn on actions Select chain Select chain Select chain (unless Select chain required Set notch filters Set notch filters preset) Set notch filters Acquire master Acquire master Set notch filters Select secondaries Select secondary Select secondary Select secondaries (unless Match envelopes Match envelopes preset) Match cycles Match cycles (if available) Set gain controls Set gain controls

Advantages Lowest cost Moderate cost High accuracy High accuracy Can convert from A or use Continuous tracking (one Long range Long range A/C combination secondary) Full Loran-C coverage Full Loran-C coverage (some models) High accuracy Simple operation Simple operation Can convert from A or Extra features (latitude- use A/C combination longitude conversion, (some models) etc.)

Disad- Lowest accuracy Needs high SNR for Expensive Very expensive vantages Shortest range (needs acquisition Complex electronics Complex electronics high SNR) Tracks one secondary at (Some sets more Possible unrecognized Not usable in some areas a time automatic than others; errors in computations Tracks one secondary at a Medium degree of features vary) associated with extra time operator involvement features High degree of operator involvement CONSUMER CHECKLIST—LORAN-C RECEIVERS

Brand/ Model Store/Salesperson My needs Retail price Includes antenna and coupler? (Yes/No) Includes installation? (Yes/No) Design Dimensions (HXWXD) Weight Mounting: Convenience, Space required (HXWXD) Controls: Convenience Power required AC, DC (volts?), watts Basic features Matching (envelope, cycle) Acquisition (manual, auto) Tracking (manual, auto) Number of pulses tracked? Number of notch filters? Chain selection (preset, selectable) Other features Number of secondaries tracked? Cycle step switch? (Yes/No) Memory button? (Yes/No) Alarms: Blink? (Yes/No) Lost signal? (Yes/No) Cycle? (Yes/No) Reputation Manufacturer Model Dealer Reputation Install? Service? Factory support Warranty Length Coverage Personal Impression Have you operated set? (Yes/No) Other comments SG 50, What you should know about Loran-C receivers, Oregon State University Extension Service, Sea Grant Marine Advisory Program. Corvallis (5-78) Accessories Glossary of terms For more information There is a wide range of equipment Absolute accuracy: See geodetic accuracy. On the principles of Loran: Consult available other than the basic Loran-C Cycle matching: A technique that any good basic text on navigation. Two matches the radio-frequency cycles of the best are American Practical Navigator ("Bowditch"; Washington: A remote readout is a separate display that make up the pulse envelopes. U.S. Government Printing Office, 1962 or unit that presents the same time- Cycle matching is used to obtain full difference information as the receiver at Loran-C ranges and accuracies. later edition) and G. D. Dunlap and H. H. Shufeldt, Button's Navigation and a different location on your vessel. Envelope matching: A technique that Remote readouts are valuable if you need matches the pulse envelopes that are Piloting (Annapolis: U.S. Naval Institute, 1969or later edition). Loran readings in a second location, or if an outline of the radio-frequency you wish to locate your receiver in a cycles. Envelope matching is used in On Loran-C: Loran-C User Handbook, more protected part of your vessel and Loran-A receivers and for coarse U.S. Coast Guard Publication CG-462 use a remote readout in your navigation matching in Loran-C receivers. (Washington: U.S. Government Printing area. Geodetic accuracy: A measure of your Office, 1974). Track plotters take their signals from ability to determine true geographic On Loran-C receivers: Wild Goose a Loran-C receiver and provide an inked position—namely, latitude and Association Radionavigation Journal trace on a chart of the vessel's location. longitude—from a navigation system 1977 (Acton, Mass.: Wild Goose Some track plotters work from a relative such as Loran; it is also known as Association, 1977). reference system, where one set of absolute accuracy. On the economics of Loran-C: F. J. Loran-C lines is vertical on the plotter Ground wave: A signal that travels Smith, Financial Considerations in and the other is horizontal. Others work directly from transmitter to receiver. Switching from Loran-A to Loran-C and from a conventional reference system, Notch filters: Filters in the receiver that Loran-C Receivers: Making the Decision, where north-south is vertical, east-west is may be tuned so as to reduce the Oregon State University Extension horizontal, and you can select any effects of interfering signals. Service, Sea Grant Marine Advisory Program Publications (in press). chart scale you need. Track plotters Phase coding: The predetermined pattern present a continuous record of your of positive and negative phases for On how to use your Loran-C receiver: vessel's movements, a more complete and each pulse in master and secondary R. F. Dugan and D. A. Panshin, How to convenient recording of the same kind of Loran-C signals. Phase coding reduces Get the Most out of Loran-C, Oregon information that many mariners record sky-wave interference and helps State University Extension Service, Sea in log books. Loran-C receivers to distinguish Grant Marine Advisory Publication (in Track plotters take particular between master and secondary signals. press). advantage of Loran-C's repeatable accuracy, and can be especially valuable Repeatable accuracy: A measure of your to commercial operators such as ability, through using a navigation crabbers, shrimpers, lobstermen, and system such as Loran, to return to a draggers, who need to return as position where you have been before. accurately as possible to a previous Signal-to-noise ratio, or SNR; The ratio of location. Satisfactory track-plotter the Loran-C signal level to the level of operation depends on accurate location background noise. Loran-C coverage of your initial position. charts show the area within which the Some receivers use microprocessors— SNR is 1:3 or stronger. special-purpose minicomputers—to aid Sky wave: a signal that reflects off the in processing Loran-C signals. ionosphere, rather than traveling a Microprocessors can also allow direct path from transmitter to computation of latitude and longitude through the use of coordinate converters. While such information may be desirable, you may get less rather than more: Microprocessor computation of latitude and longitude at this time does not take into account Loran-C transmission anomalies and may include other minor computational errors as well. Unless you determine and apply a local correction, the resultant latitudes and longitudes will be less accurate than if you plotted your Loran-C position on a chart and read off latitude and longitude. Microprocessors can also compute Loran-C signal strength, receiver Extension Service, Oregon State University, Corvallis, Henry tracking status, speed, distance, range, A. Wadsworth, director. This publication was produced and dis- and courses to steer. (Depending on how tributed in furtherance of the Acts of Congress of May 8 and June the calculations are made, the above 30, 1914. Extension work is a cooperative program of Oregon State University, the U.S. Department of Agriculture, and Oregon cautions also apply to microprocessor counties. Extension's Marine Advisory Program is supported in computations of speed, distance, range, OREGON STATE UNIVERSITY part by the Sea Grant Program, National Oceanic and and course.) A development to watch for Atmospheric Administration, U.S. Department of Commerce. in the near future will be the introduction EXTENSION Extension invites participation in its activities and offers them of accessories that use microprocessor- a SERVICE equally to all people, without discrimination. computed steering information to drive autopilots. 5-78/20M