IONOSPHERIC PROPAGATION

Properties of Atmosphere and Ionosphere

1 0 k m

5 0 k m

1 0 0 k m

T ro p o sp h e re

S t ra to sp h e re

M e so sp h e re

T h e rm o sp h e re

E x o sp h e re

3 0 0 k m

ION

OS

PH

ER

E

3 0 0 6 0 0 9 0 0 1 2 0 0 1 5 0 0

T e m p e ra tu re (K )

E

F

E le c t ro n d e n s ity (c m -3 )

1 0 4 1 0 5 1 0 6

The typical electron distribution in the ionosphere

E

F

F1

D E

F2

N (1/m3)

The ionosphere can be modeled as a lossy dielectric whose relative permittivity varies with height (electron density) and with the frequency of wave

2rf

N811

N is a function of heightWhen a wave penetrates into ionosphere, it is refracted continuously and follows a curved path, finally it will be returned to earth from a level where refractive index

ir sinn

i

For a given frequency f, the wave will return back if

i2sin

f

N811 or

81

cosfN i

22

d, skip distance

h´, virtual height: apparent height of reflection

i81

cosfN i

22

at this point

If the maximum electron density in ionosphere is Nmax, then with vertical incidence (i=0), the frequency that will be reflected back is

frequencycritical,N9f0f

N8110 maxc2

c

maxr

The frequencies above fc will not be returned back to earth with vertical incidence.

Critical Frequency (fc) and Maximum Usable Frequency (MUF)

h

N

Nmax

f=fc f<fc f>fc

escapesf>MUF

f=MUFf<MUF

i

Maximum Usable Frequency (MUF)

i2

2i2 cos

N81fsin

f

N811

N=Nmax f=MUF

ici

max secfcos

N9MUF

Note that there is an upper limit to i due to curvature of earth

i

max,i

h´ h´

ae

ae=4a/38497km

d1

ds,max

Radiation leaving the antenna in horizontal direction (at grazing angle)

ha

asin

e

emax,i

For the F2 layer, h´=300 km 9659.03008497

8497sin max,i

i,max75c

max,i

c f6.3cos

fMUF

max,i

Maximum skip distance,

ds,max=2d1,

d1=ae, in radians, =-/2- i,maxFor the F2 layer, h´=300 km,

=90- 75= 15 0.262 rad

ds,max=2 (8497)

0.262=4449km

ha22d emax,s

Following approximate formula can also be used:

km4515300849722d max,s

If the desired range is less than the maximum skip distance, the transmitter beam must be elevated above the horizon, resulting in a lower value for MUF.

h´

O

T

R

ds

Ionosphere F Layer

aeae

i

Local horizon

ae

ie

coteccoscosa

h1

Using the law of sines for the triangle TRO, it can be shown that

where =180--i 90 is used, e

s

a

2d

haa ee

i

sinsin

The law of sines for the triangle TRO

For a given skip distance what is or i?

Skip distance

First hop

Skip distanceSecond hop

Io n o sp h e re

E arth

h ' h '

Tx

A

B

If the desired range is greater than the maximum skip distance, a multi-hop link must be used.

Ionospheric MeasurementsA sample ionogramPlot of virtual height as a function of frequency

• Normal incidence

• the virtual heights increase steeply as the critical frequency is reached.

• there are double reflections from the F1 and F2 layers.

T x M

F req u en cy f1

Hei

ght

fob liq u e

r

(a ) (b )

M U F

Oblique incidence ionogram showing reflections from different heights

Oblique incidence sounding stations: transmitter & receiver are located at the end points of the pathDrawbacks: difficulty in syncronization and fixed locations

Oblique incidence backscatter sounding stations: transmitter & receiver are located at the same site

In order to establish an ionospheric propagation link between two stations on earth, one needs to know the MUF for that path.

Practically, daily MUF charts are prepared for different locations and propagation distances and these charts are used to determine the frequency of operation

T im e o f d ay

MU

F, M

Hz

0 2 4 6 8 1 0 1 2 1 4 1 6 1 8 2 0 2 2 2 4

6

1 2

1 8

2 4

3 0

3 6

4 2

4 8

Sun

rise

0 k m

5 0 0 k m

2 0 0 0 k m

1 5 0 0 k m

2 5 0 0 k m

3 0 0 0 k m

3 5 0 0 k m

Sun

set

1 0 0 0 k m

Typical MUF Chart for propagation paths of different heights

Critical frequency: the maximum frequency that can be reflected by a layer for vertical incidence

MUF: the maximum frequency that can be reflected by a layer for a given incidence angle.

MUF differs from the critical frequency by a factor of .

MUF may show variations about the monthly average of up to 15%

Optimum frequency: somewhere between about 50% and 85% of the predicted MUF.

There is a frequency below which radio communications between two stations will be lost due to reduced SNR.

Decrease in frequency multiple hops

• increase in the losses • increase in the losses

due to the D layer

The ITU-R Recommendation P.373-8 definitionsOperational MUF: The highest frequency that would permit acceptable

performance of a radio circuit by signal propagation via the ionosphere between given terminals at a given time under specified working conditions, (antennas, power, emission type, required SNR, and so forth).

Basic MUF: The highest frequency by which a radio wave can propagate between given terminals, on a specified occasion, by ionospheric refraction alone.

Optimum working frequency (OWF): The lower decile of the daily values of operational MUF at a given time over a specified period, usually a month. The frequency that is exceeded by the operational MUF during 90% of the specified period.

Highest probable frequency (HPF): the upper decile of the daily values of operational MUF at a given time over a specified period, usually a month. The frequency that is exceeded by the operational MUF during 10% of the specified period.

Lowest usable frequency (LUF): The lowest frequency that would permit acceptable performance of a radio circuit by signal propagation via the ionosphere between given terminals at a given time under specified working conditions.

Attenuation of Waves in Ionosphere

Monthly average of diurnal variations of critical frequency and virtual height of regular ionosphere layers for summer

foE foF1

foF2

2

4

0

8

1 0

6

1 2

Sun

rise

Sun

set

Cri

tica

l Fre

quen

cy, M

Hz

2 0 0

4 0 0

Vir

tual

Hei

ght,

km.

0

2 0 0

4 0 0

0

F F

F2

F1

E

Vir

tual

Hei

ght,

km.

foE

foF1

foF2

2

4

0

8

1 0

6

1 2

Cri

tica

l Fre

quen

cy, M

Hz

Sun

rise

Sun

set

F F

F2

F1

E

S u m m e r , su n sp o t m in im u m S u m m e r , su n sp o t m a x im u m

0L o c a l t im e , h r

4 8 1 2 1 6 1 8 2 40L o c a l t im e , h r

4 8 1 2 1 6 1 8 2 4

2

4

0

8

1 0

6

1 2

Cri

tica

l Fre

quen

cy, M

Hz

0L o c a l t im e , h r

4 8 1 2 1 6 1 8 2 4

f o E

f o F 2

Sun

rise

Sun

set

2

4

0

8

1 0

6

1 2

Cri

tica

l Fre

quen

cy, M

Hz

0 4 8 1 2 1 6 1 8 2 4L o c a l t im e , h r

f o E

f o F 1

f o F 2

Sun

rise

Sun

set

2 0 0

4 0 0

Vir

tual

Hei

ght,

km.

0 Vir

tual

Hei

ght,

km.

F FF 2

F 1

E

W in te r , su n sp o t m in im u m

2 0 0

4 0 0

0

F FF 2

E

W in te r , su n sp o t m a x im u m

Monthly average of diurnal variations of critical frequency and virtual height of regular ionosphere layers for winter