SATELLITE COMMUNICATIONS ENGINEERING 'LOOK-ANGLES' LABORATORY INTRODUCTION The purpose of this laboratory is to investigate the primary parameters affecting satellite receiving systems. This includes, for the purposes of this experiment, the following: The field of view from the receiving antenna platform The ‘look-angles’ for each satellite viewed from that platform The expected spectrum for the received signal Downlink frequency, polarisation, modulation format and sub-carrier separation are investigated. In addition to the above, the noise level of the receiving system and the expected ratio of carrier power to noise power spectral density. The signals are measured as broadcast transmissions. SCHEDULE A) In the Laboratory (i) Using the data provided calculate the satellite look angles for the required satellites. (ii) Ensure that the required satellites fall within the field of view of the receiving equipment (iii) Draw a graph showing the elevation and azimuth of all satellites within this field of view B) On the Laboratory roof (i) Using the look-angle values calculated; align the dish antenna to listed satellites, making allowance for magnetic variation. (ii) Using the remotely controlled dish, ‘lock’ onto other satellites to confirm previously calculated values. (iii) Using the method already described, measure the ratio of carrier power to noise power spectral density. Access and telecommunications will be provided by a member of staff who will accompany students onto the roof. A C Stamp October 2002

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1

Concepts in Navigation NORTH

NORTH

0O 60O

LATITUDE

N

LONGITUDE

W

30O

E

0O

S

30O 60O

PARALLELS

SOUTH

GREENWICH MERIDIAN 0O N-S

N

NE

In the ‘Azimuthal’ Plane direction is conventionally measured ‘clockwise’ with o reference to True North (000 ). Consequently, East is 090o, o o. South 180 , and West 270

E

W

SW

MERIDIANS

SOUTH

THE EQUATOR IS 0O

NW

30O E

30O W

SE R EA D

BE A

R G IN

S

S

RE HE

When a magnetic compass is used, an allowance for the Magnetic Variation must be applied.

W E N

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ANTENNA LOOK ANGLES The orbital height of the satellite in geosynchronous orbit is 35,786 km, and the actual (slant) range to the satellite is d (metres). Given; The Latitude of the Earth station is represented by; The Longitude of the Earth station is represented by; ΦΕ , The Longitude of the satellite is represented by; The equatorial radius of Earth (6378.14 km) is represented by; The radius of Earth at the location of the Earth station is represented by; The longitudinal difference, B, (ie B = ΦΕ − ΦS) Note that by convention

λΕ, ΦS ,

Re R

North & East are Positive South and West are Negative

In figure 2, the triangle ‘on’ the surface of the globe is spherical, whilst the other is plane. Napiers rules of spherical geometry show that:

A = tan-1(

− tan B sin λE

)

Find c given that

cos c = cos λΕcosB,

Find R

R = Re(1 - (sin2 λE)/298.257)

d=

Hence find

(R

2

)

+ (RE + h) − 2R(RE + h)COSC 2

θ, the elevation angle, is given by; θ = cos-1(sin.c (Re+h)/d) Az the azimuth angle, depends on the value of ’A’ found above, and relative LatitudeS of the Earth station, and the Sub-satellite point and the magnitude of the difference in Longitudes. The azimuth angle is represented by ‘Az’

(a) (b) (c) (d)

λE0

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B< 0

then Az = A

B> 0

then Az = 360o - A

B< 0

then Az = 180o + A

B> 0

then Az = 180o - A

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Spherical Geometry for Satellite ‘Look-Angles’ 90 80 70

60 50 40 30 20 W

-50

R

-40

-30

10

-20 -10 ΦE

0 -10

RE

Φs

E SS

30 10 20

40

50

-20 -30

ES

h

d

Diagram for Azimuth angle A SAT

B

From the figure above a b

Plane triangle obtained from the above figure.

C

A

a = 90o b = 90 o

Note range ‘d’ and elevation angle ‘Elo’

c

c R

λE 90o + ELo RE + h

d

S

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Calculation of Azimuth angle based on the relationship between the latitude of The Earth Station, The Sub-Satellite Point; and Longitudinal difference. N

N

SS SS

Az

ES Az

ES a. λE0: AZ = 180o – A

Earth Station in Northern hemi-sphere and West of Sub-Satellite point

Earth Station in Northern hemi-sphere and East of Sub-Satellite point

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VISIBILITY LIMITS The angle subtended by the satellite is represented by S o, and assuming a reasonable horizon limit of 5o, such that the angle in figure 2 (shown as 90o + elevation) becomes 95o. So using the sine rule: sin S =

R sin 95o RE + h

Angle c in the diagram is therefore, 180 o -95 o - S o = 85o - S o The Easternmost and Westernmost limits of visibility are given by the positive and negative values of B, where B = cos-1 (cosc/cosλΕ) Use B = ΦΕ − ΦS to find ΦS

5 5 52 5 5 49 48 47 R EA D

BE

AR

IN

S

64

65

66

E

63

67

68

W

62

RE HE

61

G

4

N

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Measurement of received signals C of Broadcast Signals No C is quite straightforward, but necessitates some detailed No knowledge of the measurement instrument. Most broadcast systems still use FDMA and this forms the basis of the analogue measurement made here.

The measurement of

Recall that C C 1 = x N N0 Bandwidth So in dBs C C = N NO

(working in dB’s)

−

Bandwidth

C C = +B NO N

Assuming that the base-band bandwidth of a TV signal is 5.5MHz, and allowing for C is given by the measurement of FM, the modulated bandwidth is 27MHz. The NO C and making allowance for the following: N (i)

The bandwidth of the modulated signal (assume 27 MHz)

(ii)

The shortfall in analyser resolution (i.e. the resolution bandwidth is 1 MHz, compared with a signal basebandwidth of 5.5 MHz)

(iii)

The noise floor lift of the Low Noise Converter

(iv)

The analyser video bandwidth

(v)

The noise bandwidth (compared with signal bandwidth) of the analyser (called the analyser correction factor by HP & Techtronix)

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