1 receiving ground-based vlf transmissions with rpi on image bodo w. reinisch environmental, earth,...

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Receiving Ground-based VLF Transmissionswith RPI on IMAGE

Bodo W. ReinischEnvironmental, Earth, and Atmospheric Sciences Department

Center for Atmospheric ResearchUniversity of Massachusetts Lowell

AFRL Hanscom AFB

19 December 2002

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IMAGE Spacecraft

500-m dipoles in spin plane

20-m dipole along z

<10 W radiated power3 kHz – 3 MHz300 Hz bandwidth

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Cavity

4

Overview of Radio Plasma Imager (RPI)

• RPI transmits coded EM waves and receives echoes at 3 kHz to 3 MHz

– Uses advanced digital processing techniques (pulse compression & spectral integration)

• RPI uses a tri-axial orthogonal antenna system– 500 meter tip-to-tip X and Y axis dipole antennas– 20 meters tip-to-tip Z axis dipole antenna– X axis antennas are used for transmissions– Echo reception on all three

• Basic RPI measurements of an echo at a selected frequency – Amplitude– Time delay (distance or range from target)– Direction of arrival– Wave polarization (ordinary or extra-ordinary)– Doppler Spectrum

• In situ density and resonances measurements

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Timing, Digitizer & Synth (DC1)

+Z De-ployer

CIDPMulticonductor Cable

RF Amp/Coupler

+Y Deployer

Oscillators, Exciters & Z Receiver (AN1)

RS-422

Secondary Power Distribution (SPD) (includes serial comm port)

RF Coax

Transmitter Power Distribution (TPD)

TMS320C30 DSP - based CPU (SC7)

Rcvr Preamp

+X Deployer

-Y Deployer

-X Deployer

VME Bus

X & Y Receivers (AN2)

Preamps

-Z De-ployer

Rcvr Preamp

RF Amp/Coupler

Rcvr PreampRcvr Preamp

RF Amp/Coupler RF Amp/Coupler

VME Bus

VME Bus

VME Bus

VME Bus

BeCu Wire

VME Chassis

10 mBoom

10 mBoom

250 mAntenna

250 mAntenna

250 mAntenna

250 mAntenna

RPI Block Diagram

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Transmission with Tuned Antennas

V a £ 1.5 kV rm s

T ra n sm itter C o u p ler

1 /2(R a + iX a)

A n ten n am o n o p o le

1 /2(R c - iX a)

6- 24 V

0 V

5 V

0 V

5 V

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RPI SpecificationsItem Specification Comments

Radiated power 10 W Tuned Antennas

Antennas 3 orthog. dipoles 2 x 500 m, 20 m

Frequencies 3 kHz - 3 MHz 10-1 - 105 e/cm-3

Bandwidth 300 Hz Matched

Pulse Width 3.2 ms Range resolution

Range Resolution 480 km

Range 20 RE SNR dependent

Angular Resolution 2o 500 km res. at 2 RE

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Local plasma resonances

X-mode echo trace

Sounding in the Polar Cap

S/CZ trace

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Field-Aligned Propagation Plasmagram

[Reinisch et al., GRL, 4521, 2001]

SX from southern HP

NX from northern HP

SX+NX double reflectionRecalculatedEcho traces

Plasma resonances

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Empirical Magnetospheric Density Distribution

Average2000-2001

L = 7

6

5

June 20010800 LT

March 20011200 LT

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12

13

05 Apr 2001

14

20

30

40

50

60

70

210 240 270 300 330

Maine, NAA 44.5 N, 291 ESeattle, NLK 48.0 N, 238 EAustralia, NSW 32.0 S, 116 E

year 2001

0424 UT

0600 UT

0438 UT

March 26

1600 UT

1544 UT

1400 UT

IMAGE B-field projection

April 4

0056 UT

0034 UT

2300 UT

February 14/15

NLK

NAA

GEO Longitude, deg

GE

O L

atit

ud

e,

de

g

15

Seattle, NLK 24.8

16

Seattle, NLK 24.8

17

Seattle, NLK 24.8

18

Maine, NAA 24.0

19

Australia NSW19.8

Maine NAA 24.0

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Backup

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Antenna Coordinates and Wave Polarizations

y

x

z

z'

e-wavei-wave

B0

y'x'

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Quadrature Samplesy'

x'

E Q = E R (w t=p/2 ) E I= E R (w t= 0 )

a0R

a

b

Reinisch et al., Radio Sci. 34, 1513-1524, 1999

Reinisch et al., Space Sci. Rev. 91, 319-359, 2000

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The Wave Normal

Reinisch et al., Radio Sci. 34, 1513-1524, 1999

Reinisch et al., Space Sci. Rev. 91, 319-359, 2000

I Q

I Q

x

x

Ε Ε

Ε Εn

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Wind – RPI

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Signal Transmitted byRPI

Signal Received byWBD

IMAGE to Cluster Radio Wave Transmission Experiment, 23 April 2002

Received Waveform with Fitted Pulse and Background Levels

04:44:37 04:44:39 04:44:41 04:44:43

E (

mic

rovo

lts

/ m

)

-1.0

-0.5

0.0

0.5

1.0

E (

mic

rovo

lts

/ m

)

-1.0

-0.5

0.0

0.5

1.0

-1.5

1.5

SC

2 S

als

aS

C3

Sam

ba

Spectrograms of Received Signal at Each Spacecraft

04:41:00 04:41:30 04:42:00 04:42:30 04:43:00 04:43:30 04:44:00 04:44:30

Ez

Fre

q [

kH

z]

502

504

506

508

510

Ez

Fre

q [

kH

z]

502

504

506

508

510

Ez

Fre

q [

kH

z]

502

504

506

508

510

Ey

Fre

q [

kH

z]

502

504

506

508

510

10-16

10-15

10-14

10-13

10-16

10-15

10-14

10-13

10-16

10-15

10-14

10-13

10-16

10-15

10-14

10-13

V2m-2Hz-1

SC

4 T

an

go

SC

3 S

amb

aS

C2

Sal

sa

SC

1 R

um

ba

Relative positions roughly to scale

University of Iowa

Funding for this experimentprovided by NASA

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System Parameter Nominal Limits RationaleRadiated Power 10 W @ 5% to

20% duty cycle10 W per antennaelement

Required for adequate SNR

Frequency Range 3 kHz – 3 MHz Covers expected range of plasmadensities.

Freq. Accuracy1x10

-5 Accurately measures observed plasmadensities

Freq. Steps 5% steps 100 Hz 5% in frequency gives 10% in plasmadensity resolution.

Measurement Duration 1 s to minutes 50 ms Different spatial and temporalrequirements along orbit

Maximum VirtualRange

120,000 km 300,000 km Extent of expected magnetosphericecho ranges

Minimum VirtualRange

980 km w. 3.2 msshort pulses

0 km for passivemodes

Pulse width + receiver recovery time is7 ms

Range increments 240 km 240 or 480 km Required sampling resolutionPulse Rep Rate 1 s-1 0.5 to 20 s-1 Sets unambiguous rangePulse Width 3.2 ms 3.2 ms to 1.9 s Provide 480 km range resolution

Receiver Bandwidth 312 Hz Consistent with 3.2ms pulse widthReceiver Sensitivity 25 nV/Hz (X&Y);

8nV/Hz (Z)Keeps receiver noise below cosmicnoise

Coherent IntegrationTime

8 s 125 ms to 64 sec Provides both processing gain &Doppler resolution

Doppler Resolution 125 mHz Determined by coherent integrationtime

Receiver saturationrecovery

6 ms Specially designed monostatic radarreceiver

Doppler range ±2 Hz 150 Hz To measure expected plasma velocitiesAmplitude resolution 3 dB 3/8 dB Data format allows 3/8 dB, but typical

display is 3 dBAngle-of-Arrivalresolution

2

1 when SNR is

40 dB or better

Identify echo direction with requiredaccuracy

Antenna Length 10 m & 250 m SNR required

Processing Gain 21 dB 0 to 33 dB To enhance weak echoes

Mass incl. Antennas 56 kg

Average Power 32 W

Table 1. RPI Specifications

RPI Specs