IONOSPHERIC PROPAGATION
Jan 18, 2016
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