PROJECT BY:PAUL DIEMERTedition 6: December 3,2011LONG RANGE NAVIGATION (LORAN)

LORAN is an acronym for LOng RAnge Navigation.

Loran is a terrestrial, high power, hyperbolic navigation system operating in the 90 to 110 kHz frequency band. The Loran system became operational in 1958 and has been in service since then primarily serving the maritime community.
Loran is suited for a backup role for GPS. It’s a terrestrial low frequency navigation system whose signal pulses are high powered and not dependent on line of sight. While it’s able to provide similar capabilities in both navigation and timing/frequency services as GPS it does have drawbacks. One is that LORAN has never been certified for approaches. Attempts in the late 1980’s failed to certify the system due to deficiencies. New technology and upgrades to the system has eliminated all of the noted deficiencies.

HISTORY:

During the second world war, Loran was used as a bombing aid. The first system, LORAN A, used an oscilloscope type of display. The operator would have to count the pulses on the display and interpret the information using special maps. It wasn’t until the 1970s that Loran became popular.


apn4.jpg


Figure 1: APN-4 photo courtesy of Signals Collection '40-'45


LORAN: Theory of operation:

  • Loran operates in the VLF frequency range of 90Khz to 110Khz, centered at 100Khz,
  • Loran c transmitters consist of a 3,4,or 5 stations in a chain Figure 2-C ,
  • Secondary stations radiate pluses in bursts of eight. The master signal has a ninth pulse burst for identification purposes (Figure 2-b)
  • A loran pulse is 27 cycles in duration building up to maximum on the eighth pulse then diminishing in the following 19 cycles. (Figure 2-a)
  • Maximum power is achieved on the eighth pulse but the 3rd pulse is the principal timing point. The frequency has stabilized at 100 KHz at this time and there is no skywave contamination yet,
  • Each Loran pulse is 270 micro/sec in duration,
  • The interval between pulses in a pulse group is 1000 micro sec, except for the last two pulses which have a 2000 micro sec interval.




lorc_pulse_system.jpg
Figure 2: Loran-C pulse format and sequencing.
Courtesy of jproc.ca/hyperbolic/loran_c




Group Repetition Number:

Every LORAN chain in the world uses a unique Group Repetition Interval, the number of which, when multiplied by ten, gives how many microseconds pass between pulses from a given station in the chain. LORAN chains are often referred to by this designation (Figure 3)



220px-LORAN_Station_Malone-Outside-Large.jpg
Figure 3 LORAN Station Malone, Florida Great Lakes chain (GRI 8970)/Southeast U.S. chain (GRI 7980

Figure 3: LORAN Station Malone, Florida Great Lakes chain (GRI 8970)/Southeast U.S. chain (GRI 7980


There is a master station (M) and two or more secondary stations (W,X,Y,Z) as summarized in Figure 4,


CONFIGURATION
DESIGNATORS
EXAMPLE



Master with 5 secondary’s
M V, W, X, Y, Z
SouthCentral U.S.9610
Master with 4 secondary’s
M W, X, Y, Z
Southeast U.S.7980
Master with 3 secondary’s
M X, Y, Z
Canadian West Coast 5990
Master with 2 secondary’s
M W, X
Calcutta5943
Master with 2 secondary’s
M X, Y
East China8390

Figure 4: Loran-C transmitters Courtesy of jproc.ca/hyperbolic/loran_c



  • It consists of an array of a fixed station and secondary stations that transmit synchronized PULSE (figure 4) signals to mobile receivers.
  • The sequence of signal transmissions consist of a pulse group from the master station followed at precise time intervals by pulse groups from the secondary stations. The Receiver measures the differences in the times of the arrival of the pulsed signals from the various stations (Figure 4),


  • The fixed difference in the time of arrival of these signals from any two stations will define a HYPERBOLIC arc on which the receiver must lie (Figure 5),

Hyperbola.jpg


Figure 5: Time Difference Measurements (loran-c) Courtesy of jproc.ca/hyperbolic/loran_c


  • Two or more secondary stations are required to remove ambiguities in the position of the receiver
  • The time difference between the master and the first secondary station identifies one curve.
  • The time difference between the master and the second secondary station identifies the next curve.
  • The intersection of these curves will determine a point in relation to the position of the three stations.


Future of Loran:

LORAN-C use has been in steep decline, with the satellite based GNSS systems being the primary replacement.In January 2010, both the Canadian Coast Guard (CCG) and the United States Coast Guard (USCG) announced the termination of the Loran-C signal. But, with the perceived vulnerability of GNSS systems, and their own propagation and reception limitations, renewed interest in LORAN has appeared, as E-LORAN.
E-LORAN’s advancement in receiver design and transmission characteristics increases the accuracy and usefulness of traditional LORAN and with reported accuracies of plus/minus 0.8 meters E-LORAN becomes an alternative where GPS is unavailable or degraded.

Confusion abounds: Is Loran history? There are many different reports and papers about this question and it is a confusing path as can be seen in the following:

Quote from U.S. Coast Guard Navigation Center on the United States of America's intention to terminate the loran signal (or not terminate the signal) is as follows:
"The Coast Guard published a Federal Register notice on Jan. 7, 2010, regarding its intention to terminate transmission of the LORAN-C signal Feb. 8, 2010. A LORAN Programmatic Environmental Impact Statement Record of Decision stating that the environmentally preferred alternative is to decommission the LORAN-C Program and terminate the North American LORAN-C signal was published in the Federal Register on Jan. 7, 2010."

The entire Specification may be downloaded also.

LORAN may be gone but as the following states there is still E-LORAN .

E-LORAN:

Despite the closure of the the Loran-C chain in the USA and Canada, E-LORAN is alive and well in other parts of the world.
Because GPS is vulnerable to intentional, unintentional, and natural interference and Loran is virtually unjammable because of its high power there is still a need for LORAN as a save back-up or complement global navigation systems(GNSS).
Loran uses ground-based transmitters, low frequency, and are very high-powered, so they penetrate cities, buildings and densely foliaged areas where low level GPS signals are often blocked.



Terms to remember:

Loran pulse
A pulse 27 cycles in duration, known as a burst transmission, building up to its maximum on the eighth pulse and then diminishing in the following 19 cycles.

Pulse group
Groups of Loran-C pulses.

Group repetition interval (GRI)
Specific timing interval of the group of pulses starting and finishing with the master pulses.

Hyperbolic line of position
The intersection of two pulses.

Hyperbolic Navigation
Creating map intersections from known earth-based sources.

Baseline
A line from the master station to the slave station.

TD Lines
The difference between the time of reception of synchronized signals of two radio stations is constant along each hyperbolic line, when demarcated on a map; such lines are known as Time of arrival.


Credits:
1) Alan Cordwell <alancordwell(at)blueyonder.co.uk>
2) SAIT polytechnic, Ms. Lisa Soderquist Weatherby
3) Wikipedia
4) From the ground up, 28th edition,2000
5) Signals Collection '40-'45 web page: www.qsl.net/pe1ngz
6) Hyperbolic Radio navigation Systems, compiled by Jerry Proc, VE3FAB
7) High Speed Loran-C Data Channel Communications, James M. Boyer,United States Coast Guard
8) Canadian Coast Guard,www.ccg-gcc.gc.ca/
9)Loran-C Future Developments, jproc.ca/hyperbolic/loran_c_future.html
10)U.S. Coast Guard Navigation Center - NAVCEN MS 7310, 7323 Telegraph Road, Alexandria, VA 20598 - 7310 | (703) 313-5900