1. Long RAnge Navigation version C
LORAN C
Long RAnge Navigation version C
Originally a marine navigation system Became feasible for aircraft navigation with the introduction of microprocessors
Frequency of Operation: 100kHz (all stations)

2. LORAN C A HYPERBOLIC SYSTEM i.e. lines of position are hyperbolas
This results from the fact that the lines of position are determined by measuring the DIFFERENCE in distance from two points.

3. LORAN C
One station is referred to as the Master and the others as Slaves

4. LORAN C
At least two lines of position are required for a position fix thus more than one slave is required

5. LORAN C
A useful property of the hyperbola is that its tangent at any point bisects the angle subtended by the line joining the two foci
Exercise: Use this property to determine where the best geometry occurs (LOP at 90º)

6. LORAN C How do we determine the time difference?
Each station, starting with the Master, transmits a series of pulses with the following shape:
This pulse has a bandwidth of about 20kHz

7. LORAN C Each station transmits a series of eight of these pulses
Pulse separation is 1000μs (1ms)
Note: In most chains the master transmits a ninth pulse after 2000μs. This can be used to indicate the status or integrity of the chain’s signals

8. How do we identify the pulses from each station?
LORAN C
How do we identify the pulses from each station?
The stations transmit their signals in sequence. The delay between signals from each station is such that the signal from the previous transmission is out of the coverage area before the next is sent.
Thus they always appear in the same order

9. LORAN C Chains
A group consisting of a Master and up to four slaves is called a chain
Each chain is identified by a Group Repetition Rate (GRI) which is the time between transmissions from the master.

10. LORAN C Chains
Each slave transmits its pulse train at a specified interval after the master has transmitted.
This is called the emission delay (ED) and is made up of the master-slave time (MS) and a coding delay (CD)

11. LORAN C Transmitters
Due to the long distances covered by each LORAN C chain, the power transmitted must be high (0.5 to 4 MW)
Propagation is by ground wave and thus has to be vertically polarized
Antenna therefore is a vertical mast (ideally a quarter wavelength long (3km) (10,000 ft.)
Not very practical!!

12. LORAN C Antennas Antennas are typically about 400m high
To improve the current flow, many are “top loaded”
They are still not very efficient (~10%)

13. LORAN C Antennas
“Top loaded” antenna with ground plane

14. LORAN C Receivers Receivers require a data base which provides
the location (Lat/Lon) of the Master and Slave stations
the GRI of the chains to be used
the Time Delays for the individual stations
The LORAN C signal travels both by ground wave and sky wave
ground wave gives stable, reliable timing
sky wave does not due to the variable nature of the ionosphere
ground wave is attenuated more and hence is weaker and can be contaminated by the sky wave

15. LORAN C Receivers
Since sky wave is always delayed by a minimum of 30μs, the positive-going zero crossover of the third cycle of the ground wave is used for timing

16. LORAN C Receivers Problems to be solved by receiver
Signals strength may vary by 120dB
Large dynamic range required
Noise at LF can be very high due to long range propagation of interference (e.g. lightning in tropics)
Signal to noise ratio can be – 20 dB

17. LORAN C Receivers Receiver Operation:
Searches for Master pulses using known GRI
PLL locks on to carrier to generate master clock
Locks on to slave pulses
Measures Master/slave time interval and subtracts the Emission Delay (ED)
Calculates the distances and position

18. Phase Locked Loops (PLLs)

19. LORAN C Accuracy Error Sources
Variation in propagation speed (land vs water, type of terrain)
Changes in signal strength
Absolute Accuracy depends on geometry
0.1 to 0.25NM
Repeatability
20 to 100m

20. LORAN C
Integrity
Monitors are installed throughout the LORAN C coverage area
These monitors adjust the transmitter timing to compensate for changing propagation conditions
If excessive errors are detected, the master transmitter is commanded to “blink” the ninth pulse off and on to indicate which station is unreliable
For airborne use, this can be done within 10 seconds of detection

21. LORAN C Coverage