Loran-C P-Static
Presented to:
Loran Integrity Protection Panel Meeting
July 24-25, 2002, Stanford University
David W. Diggle, Frank van Graas
Ohio University • Avionics Engineering Center
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OVERVIEW
•
•
•
•
•
Precipitation Static (P-Static)
Two main mechanisms affecting Loran-C
Loop versus wire antennas
Loop antenna design/feasibility
Conclusions & Recommendations
Ohio University • Avionics Engineering Center
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Precipitation Static (P-Static)
• Charge build-up may be caused by:
» Charged rain droplets
» Dry air or snow
» Wing-tips in different potentials (clouds,
lightning)
• Potentials can easily reach 100 kV.
• Charge build-up causes pulsed discharges at a rate
of 1 to 1000 discharges per second.
• Most aircraft have static dischargers installed on
trailing edges to slowly discharge the aircraft.
Ohio University • Avionics Engineering Center
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P-Static and Loran-C Receivers
• Two main mechanisms
» Charging of antenna relative to the fuselage
• Wire (E-Field) antenna has a high impedance
and cannot be terminated to absorb charges
(continuous termination would also terminate
the Loran signal)
• Loop (H-Field) antenna has a very low
impedance (also a low profile)
» “Popping” discharge of aircraft to surrounding air
• Affects both wire and loop antennas
• Older wire antenna pre-amps could saturate
due to “ringing” in the pre-amp circuitry
Ohio University • Avionics Engineering Center
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Loran-C Wire versus Loop Antennas
Wire
(E-Field)
• Large effective height
 Little voltage
amplification needed
• High impedance (MW)
 Charge build-up (cannot
be terminated)
• Antenna phase pattern is
omnidirectional
• Whip or wire antenna
Loop
(H-Field)
• Small effective height
 Large voltage
amplification needed
(low noise pre-amp)
• Low impedance (10W)
 No charge build-up
(antenna is grounded)
• One loop creates 0 and
180 degrees.
• Conformal antenna
Ohio University • Avionics Engineering Center
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Wire P-Static Problem
Antenna is short
relative to the Loran-C
wavelength (3000 m):
- High impedance (MW)
- Pre-amp must have a
high input impedance
Voltage too high due
to charge build-up:
Neon bulb conduct
and shorts-out the
Loran signal
Ohio University • Avionics Engineering Center
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Loop Antennas for Loran-C
• Main Advantage for Aviation: Low sensitivity to PStatic due to antenna charge build-up (which
causes degraded navigation or loss-of-navigation)
• Perceived Issues:
» Phase Pattern is not Omnidirectional
• Combine two loops in phase-quadrature and
correct for the antenna phase using the
known, relative angle of arrival; or
• Process signals from two loops separately
» Signal-to-Noise Ratio
• Small effective antenna height necessitates an
ultra low-noise pre-amplifier (or a large
antenna)
Ohio University • Avionics Engineering Center
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Loop Antenna
Block of
Ferrite
Loop 2
Loop 1
Loop 1
Loop 2
90o
Ohio University • Avionics Engineering Center
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Phase-Quadrature
• Loops 1 and 2 are combined in phase-quadrature to
obtain an omnidirectional antenna pattern
• Received phase
is a function of
0o
Loop 2
the direction of
signal arrival
• Useful for
270o
90o
reception of
Loran Comm.
signals (phase
Loop 1
offset does not
180o
affect decoding)
Horizontal Antenna Pattern
Ohio University • Avionics Engineering Center
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Phase Correction
Master
X Secondary

Approximate User
Location
360 c
Time Difference M-X: Correction =
 
Ohio University • Avionics Engineering Center
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Loop Antenna System Design
• Effective Height:
1.5 mm (300 mm for E-Field)
» Need 200 times more voltage amplification
• SNR (in dB) = Loran (dB/mV/m) - Noise (dB/mV/m)
» Loran Signal at 300 nmi:
60 dB/mV/m
» Atmospheric Noise (avg.):
45 dB/mV/m
» At Ohio Univ., Seneca (M) SNR
15 dB
• Input Noise Voltage
» Should be below Atmospheric Noise at the
antenna output:  0.25 mV
Ohio University • Avionics Engineering Center
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Ultra Low-Noise Amplifier
• Good Amplifier (MAX 410) input noise
et 
e n  R p  R n  i n  4 kT R p  R n 
2 2
2
where : e n  1 . 9 nV /
Hz
i n  1 . 3 pA /
Hz
R p  0 W , R n  1200 W
Thus, et  0.5 mV
• This is the amplifier noise at the output of the
impedance transformer 1:25 (Voltage 1:5).
• Therefore, amplifier noise referred to the input of
the transformer is 0.1 mV
» Provides 8 dB of margin relative to 0.25 mV
Ohio University • Avionics Engineering Center
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Amplifier/Quadrature Combiner
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Antennas
Wire (E-Field)
II Morrow A-16
Loop (H-Field)
King Radio KA42A
Ohio University • Avionics Engineering Center
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Conclusions & Recommendations
• Conclusions
» Loop antenna reduces the Loran-C P-Static
problem
» Conformal Loran-C antenna exists on all aircraft
(use TSO’d ADF antenna)
» Ultra Low-Noise pre-amplifier is feasible for
Loran-C loop antenna.
• Recommendations for further work
» Detailed investigation into P-Static environment
(RF Data Collection)
» Develop circuitry to enable dual-use ADF/Loran
of a single ADF loop antenna
Ohio University • Avionics Engineering Center
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