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 (M

W

)



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 (10

W

)



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 (M

W

) - 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 P Static 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

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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 1 Loop 1 Loop 2 90 o Ohio University • Avionics Engineering Center Loop 2

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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 the direction of signal arrival 0 o Loop 2 Useful for reception of Loran Comm.

signals (phase offset does not affect decoding) 270 o Loop 1 180 o 90 o Horizontal Antenna Pattern Ohio University • Avionics Engineering Center

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Master Phase Correction X Secondary



Approximate User Location Time Difference M-X: Correction = 360



c



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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/

m

V/m) - Noise (dB/

m

V/m)

» » »

Loran Signal at 300 nmi: Atmospheric Noise (avg.): At Ohio Univ., Seneca (M) SNR



60 dB/

m

V/m



45 dB/

m

V/m



15 dB

•

Input Noise Voltage

»

Should be below Atmospheric Noise at the antenna output:



0.25

m

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Ultra Low-Noise Amplifier

•

Good Amplifier (MAX 410) input noise e t



e 2 n

 

R p



R n



2 i 2 n



4 kT



R p



R n



where : e n



1 .

9 nV / Hz R p



0

W

, R n i n Thus, e t

 

1 .

3 pA / 0.5

m

V Hz



1200

W • •

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

m

V

»

Provides 8 dB of margin relative to 0.25

m

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Amplifier/Quadrature Combiner Ohio University • Avionics Engineering Center

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Wire (E-Field) II Morrow A-16

Antennas

Loop (H-Field) King Radio KA42A

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Conclusions & Recommendations

• •

Conclusions

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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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