LoRaN

Experiments with old Loran-A and Loran-C equipment

Loran in general
The Long Range Navigation system is best known in the form of Loran-C being an important navigation system
for general aviation and shipping until the early 90' when the system gradualy due to cost considerations was shut down
and taken over by GPS based solutions.
Loran-C was one of the several hyperbolic navigation systems available and sometimes called the forerunner of GPS which basicly also is based on a more complicated form of hyperbolic navigation..
Hyperbolic navigation systems are discussed elsewhere on this website.

Also much information can be found at the website of Jerry Proc,
Which was of a great help for this and my other experiments regarding hyperbolic navigation equipment.
Jerry Proc
Some info i got from the US pattent buro.
US3736590A

BPRR, Basic Pulse Repetition Rate

H	33.333 pps
L	25 pps
S	20 pps
SH
SL	80 uSec
SS

SPRR, Specific Repetition Rate
0 - 7 * 100 uSec

ARI, Actual Repetition Interval
ARI = BPRR - SPRR

PPGRI, Pulse Group Repetition Interval
Same as ARI

Generating Loran signals
As Loran-C has been phased out there are nog signals available to play with the old deviced i have collected over the years.
To me the only way to overcome this problem is by generating the signals myselve using some form of programmable digital device.
Althoug i have experimented with a micro-controller to do this, using a FPGA or a CPLD seems to be more apropiate to me.
The VHDL code generates 8 pulses of 200uSec in a row and a extra pulse for the master station.
The extra pulse is used by the loran equipment to automaticy start the identification sequence.

Timings:

Single pulse on time	200µSec
Full pulse time	1000µSec
8 pulses	7000µSec
To master pulse	9000µSec
Incl. master pulse	9200µSec
Repetition time	89700µSec (for GRI:8970)

The FPGA based LORAN generator

The output from the LORAN generator shown on a osciloscope The extra master pulse clearly can be seen

A tiny piece of the VHDL code.

PortArena : LoranChain Port Map
(
    PulseOut   => PortArenaPulse,
    GRISet     => 3712, -- 2H3   29700 Port Arena
    NrOfSlaves => 2,
    Master     => True,
    Sync       => Ref_Out,
    PulseClk   => PulseClkA
);
   
Red_Out  <= Ref100kHz  And GreatLakesPulse;
Grn_Out  <= Ref1850kHz And PortArenaPulse;

Just an image while debugging

LORAN A Operation
While collecting old (useless) navigation equipment i was lucky to stumble on a LORAN A/C unit from the zeventies
branded Electronic Concepts ECI 105 AC, capable to display both Loran-A and C
It uses a CRT to display the signals that than had to be timed.
The such derived timings where used to find a hyperbolic line on a map such indicating the ships position.
Only much later when microchips became available this process could be done automaticly.
The unit i have rescued is verry heavy aproximate 40 kilo's even if it is all solid state.
After some fidling with the knobs (no manual, no clue how to operate the machine),
i managed to determine the slaves time difference equivalent as produced by the generator.
This time difference indicated which hyperbole on the special LORAN-A map was one of the position fixes.

Verry rare and vintage Loran A and C receiver

Detailed impression of the scope display

Loran-C Operation
In order to be able to bring the RF signal to the low impedance DC biased receiver input of most units,
i had to do some signal level conversion.

The signal coupler

Schematics

This is one of the more modern units i have been playing with.
It took some real effort to get all the timings exactly right.
It also turned the signal levels where far to high, causing ringing of
the receivers input filters nicelyforming the pulseshape but damaging their timing, as can be seen on the imge below.
After reducing signal strength by at least 50dB (duhhhh) the receiver started to recognise the stations.

A LORAN-C unit from the ninetees

Green: Signal from generator Purple: signal taken from the detector inside the unit

Aviation equipment