1

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

2

IMAGE Spacecraft

500-m dipoles in spin plane

20-m dipole along z

<10 W radiated power3 kHz – 3 MHz300 Hz bandwidth

3

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

5

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

6

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

7

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

8

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

10

Empirical Magnetospheric Density Distribution

Average2000-2001

L = 7

6

5

June 20010800 LT

March 20011200 LT

11

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

20

Backup

21

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