Use of frequency measurement on satellite signals for computing differential doppler and solar flare detection

Journalof Atmosphericmd Printed in Great Britain.

FwreFfriolPhysics.

Vol. 4.

No. 2. pp.195-19% 1982.

0021-9169/82/M019SMS.~/O

Pcrgamon Press Ltd.

Short Paper USE OF FREQUENCY MEASUREMENT ON SATELLITE SIGNALS FOR COMPUTING DIFFERENTIAL DOPPLER AND SOLAR FLARE DETECTION

Istituto di Ricerca sulle Onde Elettromagnetiche de1 C.N.R. Florence, Italy P.P. Tiezzi University of Florence, Italy +

Deceased June 1980

(Receivedin final form 15 October1981)

Abstract _-During 1975-76 ionospheric measurements were made at the 1,R.O.E. station located in Tuscany. Using the ATS-6 radio beacon, total electron content measurements and solar flares detection were carried out, by means of a new kind of Doppler differential method. Some results are presented. 1. Total Electron Content Data --lll_One of the most widely used methods for studying the temporal and spatial behaviour of ionospheric total electron content 1T.E.C.) is to compare on the ground the relative phase of tt~o radio waves of different frequency radiated from a satellite. This relative phase is related to the T.E.C. along the propagation path by the dispersive properties of the ionosphere. The satellite transmits signals at frequency fl and f2 = mfl where m is a constant rational number. This implies that these signals are in fixed phase relation and in fact they must be derived from a common oscillator. The ground receiving equipment performs the mixing between the signal at the lower frequency and that at the upper frequency divided by m. Due to dispersion and variability in the propagation path length and/or in the T.E.C., a beat signal occurs. By recording such beats and by counting the cycles, the differential phase n?(t)

can be derived. This technique is well known as the dif-

ferential Doppler technique (Ross, 1960; de Mendonca, 1962) although phase data are actually used. The equipment required is quite sophisticated, involving the use

Of

a double channel phase locked loop system and is in general designed for a specific satellite with no compatibility with other satellites using a diffe195

196

Short Paper

rent frequency ratio m. The equipment we used in the ATS-6 campaign (Checcacci et al., 1967) supplies accurate frequency data from the 40-41-140 MHz signals. The frequency is measured by a digital frequency meter on a signal of about 100 Hz obtained by down conversions, Triple conversion is used and all of the local oscillator signals are derived from a common frequency standard having an accuracy of 10-10. We used the frequency data for obtaining the differential phase +t,

by successive integration on a computer. Apart from the resulting accu-

racy which will be discussed later, this new method offers some other advantages namely: the equipment is simpler and easily compatible with a beacon having a different frequency ratio m and further it offers the possibility of using the most suitable data handling procedure. We have used the ATS-6 signals at 40.016 MHz and 140.05'6MHz for computing the function

n y(t)

=

Algl,t, +by, =

140 - mf40 )dt +n T o

(1)

which is related to the T.E.C. along the propagation path by

“J

(t)

=

&_A

(2)

1

%40

where 9 a I

in radians 2 253 m3secV2 = Nds is the total electron content I is 'the propagation path

r N

is the electron density along path in el/m3

c

is the velocity of light Ims-1)

m

is the frequency ratio

f4oy fllrO are the lower and upper frequencies in Hz in expression (1) must be computed, as is done iso in the other methods, by c.ombining simultaneous and Faraday rotation data. 9 Because the difference of frequencies being very small, in order to improve

The integration constantA

the accuracy when evaluating n, l(t) from expression cl), we first performed s the integration then the subtraction. Further, since the frequency data are sampled at fixed time intervals, then ten as

A T,tt)

31

- fl4Ofi)) S =fi(fllo(i+l> z

function at the time t can be writ-

[l(f40(i+l)

T - f40(i)) ;

where T

is the time interval (set) between two measurements

(3)

197

Short Paper

N

is the number of measurements

f40 is the frequency value at 40 MHz (scaled at 140 MHz) fl4C is the frequency value at 140 MHz An example of the b?l ft) behaviour evaluated from expression (1) is

shown in Figs. 1 and 2. The wave-like oscillation shown in Fig. 2 is not

related to the ionosphere, but is an instrumental effect, which, at nresent, has not been fully explained. Fig:. 3 shows an example of T.E.C. data for May 2, 1976 measured respectively with the Faraday rotation and Doppler differential method. The calibration of Doppler data was obtained by the method De Mendonca et alii (1962). The standard deviation of fi(pi.e. G

can be obtained from the expresAS sion (3), knowing the standard deviation of the frequency measurements &f. In fact it is reasonable to suppose that the Wf

on the measurement on the

two frequencies is the same, since both are taken using the same frequency meter, the S/N ratio being comparable and the local oscillator errors negligible with respect to those of the frequency meter. Consequently we can write c+(t)

= Tcf

(4)

Since N )'>3

where A = G'"f\fi. Therefore G increases with ifi, the difference of phase hy= With N and t^hge relative error increases with l!im.

AfNT increases

The T.E.C. variations we can measure will depend on the time of integration: obviously it is possible to detect smaller variation increasing this time, For example, integrating over one minute, the minimum detectable T.E.C. variation is 8.3*1012 el m'2sec'l; integrating over ten minutes, the minimum detectable T.E.C. variation is 3.3.101' el m-2sec- 1. This time range (one to ten minutes) is still sufficient for observing not only the diurnal behaviour of T.E.C. but also small perturbations.

198

Short Paper

ATS-6

3May

1976

50 t

I

I

:

E

c

L.

ii. .

40-

2

so-

:

.

.

.

N.:*..

i 20-

,

.

!” :/:

0

. 1

06

I

I

12

18

24

VT

3 - T.E.C. curve)

Fig.

2. Solar

Flares

Detection

Measurements the detection disturbances high

of solar

atmosphere

tical

flares.

constituents the flares

frequencies

In fact,

radiation

a large

source,

energy

-

ses an immediate ionospheric T.E.C.

the flare 5% with

made

1967;

et alii,

from

Davies

rotation

have been

method

of change

1966)

over

mY2s-l.

on op-

becomes

of heights.

the

et alii,

important

1967;

many

1967). Garriot

effect

enhancement

cauThe

by monitoring

However

a

of particles

(Donnelly,

and large T.E.C.

1 - 4*1014

and

methods

enhancement

1967; Garriot

authors

flares

based

flares

a range

show that the most

is a sudden

of about

solar

ionospheric

indirect being

and a source

investigated

(Donnelly,

1967 to 1974 by various et alii,

on the ionization

a rate

flares

used

The X-UV radiation

of the ionization

of solar

widely

The sun during

also permit

during

other methods

in X and UV bands,

(1 MeV - 1 GeV).

enhancement

effects

by the Faraday

Experiments

mainly

data

of sudden

solar emission

is one of the most

themselves,

range

for T.E.C.

the observation

between

or radioemissionobservations.

strong with

on the signal

due to the interaction

for revealing

(continue

measurements obtained with the Faraday method and the Doppler differential method (dots).

of

of some

cases

are

Short Paper

observed vies

in which

et alii,

1976).

are monitored re to flare sions. rates

and often

In Fig.

ting shown

that

cycle

rate

of change

the solar

was

flare

at a minimum

with

shift

stable

variations

in general

the

sufficient

a frequency

(Da-

the flares

considerably

from

simultaneous

H,

emis-

for flare detection. of about

receiver

fla-

The

0.3 - 1.5 Hz on a

it is possible

to de-

shift measurements.

record periods

to detect

5. A frequency

in Fig.

nt with

during

T.E.C.

changes

correlated

is not

a suitably

4 a frequency

of hertz

appreciable

from the fact that

of frequency

it is possible

to a T.E.C.

cause

cause

but X emission

they are not

Thus with

by means

tenths

derives method

of T.E.C.

of 40 MHz.

by some

do not

Hti observation

of change

flares

This

by the H,

Therefore

signal tect

the flares

shift

of about

of December and no other

is given greater

sudden

showing

T.E.C.

of about 3.5*1014

variations.

mT2sa1.

of such

Such an event

We have during

varying

thus demonstra-

1.2 Hz is observed,

5, 1975. However phenomena

the frequency

than one hour

corresponding

related 1975-76

intensity

is

this the

eve-

solar

occurred.

Fig.5

Fig.4

References

Ross,

W.J.

1960

J. Geophys.

Res. E,

No.9.

Res. 67, No.6,

1962

J. Geophys.

Checcacci, P.F., E. Capannini, P. Spalla

1977

COSPAR

Donnelly,

1967

J. Geophys.

Res. 72, No.21.

1967

J. Geophys.

Res. 2,

Davies, K., R.F. Donnelly

1966

J. Geophys.

Res. 71, N0.11,

Davies, K., R.F. Donnelly

1976

in: Proc. COSPAR Symposium Geophysical Satellites Boston, USA, 345-359.

De Mendonca,

F.

R.F.

Garriot, O.K., A.V. Da Rosa

Space

Research

XVII,

2315-2337. 81-85.

No.23.

et al. 2843-2845. Beacon