Space Wave Propagation Questions and Answers

Q: How does rain attenuation depend on rain rate?

Specific attenuation follows a power law in rain rate with an exponent typically between one and one and a half, so heavy rain is disproportionately damaging. Since intense rain falls in small cells, long paths average over regions of different intensity and a path reduction factor is applied. Neither proportionality nor independence describes the relationship. A power law with an exponent above one means that the extremes of a distribution dominate the outage statistics.

Q: Why is a path reduction factor applied in rain attenuation calculations?

Heavy rain falls in cells a few kilometres across, so a twenty kilometre link is rarely immersed in the peak rate along its whole length and the effective path is shorter than the geometric one. The factor accounts for that statistically. The formula itself is correct for a uniform rate, rain does not systematically favour the middle of a path, and drop size is a meteorological matter. Correcting a point measurement for the spatial structure of the phenomenon is a general requirement in path averaged quantities.

Q: What availability figure is typical for a microwave link design?

Trunk links are commonly designed for 99.99 or 99.999 per cent availability, corresponding to about fifty minutes or five minutes of outage a year, and the fade margin follows from the rain statistics at that percentage. Lower figures are accepted for less critical services. One hundred per cent is unattainable, since any margin can be exceeded by a sufficiently rare event. Availability is a design parameter with a cost, and choosing it is a commercial decision as much as a technical one.

Q: What is a fade margin?

Fade margin is the number of decibels by which the received signal in clear conditions exceeds the level at which the link fails, and it is chosen so that fades exceed it only for the permitted outage time. It is not simply the difference between transmitter and receiver specifications, which ignores the path. Antenna gain and alignment enter the budget separately. A margin sized from the statistics of the impairment, rather than chosen arbitrarily, is what turns a prediction into a service level.

Q: Why does rain also cause depolarisation?

Large raindrops flatten as they fall and tend to align, presenting different attenuation and phase to horizontal and vertical polarisation, so a dual polarised system loses isolation. That matters most where frequency reuse depends on cross polar discrimination. Reflection, bulk birefringence and antenna wetting are not the mechanism, though a wet radome adds loss of its own. A scatterer whose shape and orientation are not random will always couple energy between polarisations.

Q: Which atmospheric gas absorbs strongly near sixty gigahertz?

Molecular oxygen has a complex of magnetic dipole transitions near sixty gigahertz that gives around fifteen decibels per kilometre at sea level, effectively blocking terrestrial paths but making the band attractive for short range systems that benefit from isolation. Water vapour has lines at twenty two and a hundred and eighty three gigahertz, carbon dioxide absorbs in the infrared, and nitrogen is transparent. A band that is useless for range and excellent for reuse is a good illustration of a property being an asset or a liability according to the application.

Q: Why is the sixty gigahertz band attractive for short range systems?

Absorption that kills a signal within a few hundred metres also prevents it from interfering with a system a kilometre away, so the same frequency can be reused very densely. Wide unlicensed allocations exist for that reason. Rain still attenuates the band severely, regulation exists but is light, and antennas are small rather than notably cheap. Turning a propagation liability into a system asset by choosing the right application is an instructive piece of spectrum engineering.

Q: What is an atmospheric window in the millimetre band?

Between the oxygen and water vapour lines lie ranges near thirty five, ninety four and a hundred and forty gigahertz where clear air attenuation is comparatively low, and imaging and radar systems are placed there. The windows are properties of the molecular spectrum rather than of time, regulation or geography. Choosing an operating frequency from the structure of the medium's absorption spectrum is the same reasoning that placed infrared astronomy at particular wavelengths.

Q: What is tropospheric scintillation on a satellite path?

Turbulent mixing produces small variations in refractive index that focus and defocus the wave, giving fluctuations of a fraction of a decibel to a few decibels at rates of a few hertz, worst at low elevation angles and in humid weather. Seasonal change, rain fading and ground reflection are separate effects with different timescales. Distinguishing impairments by the timescale on which they act is the quickest way to identify which one a record is showing.

Q: Why is scintillation worse at low elevation angles?

A path at five degrees elevation traverses many times more of the turbulent lower atmosphere than one directly overhead, so the accumulated phase perturbation and the resulting amplitude fluctuation are much greater. The same geometry increases gaseous absorption and sky noise temperature. Antenna gain is unchanged with pointing, rain is a separate impairment, and geostationary satellites do not move appreciably. Path length through an impairing medium is the governing variable for every low elevation problem.

21.

How does rain attenuation depend on rain rate?

A.It rises exactly in proportion to the rain rate in millimetres per hour
B.It is independent of rain rate once rain is falling at all
C.It falls as the rain rate rises because the drops coalesce
D.It rises faster than in proportion to the rain rate

Answer: D

Specific attenuation follows a power law in rain rate with an exponent typically between one and one and a half, so heavy rain is disproportionately damaging. Since intense rain falls in small cells, long paths average over regions of different intensity and a path reduction factor is applied. Neither proportionality nor independence describes the relationship. A power law with an exponent above one means that the extremes of a distribution dominate the outage statistics.

Discuss in Forum

22.

Why is a path reduction factor applied in rain attenuation calculations?

A.Intense rain occupies cells smaller than a long path
B.Rain falls less heavily at the ends of a path than in the middle
C.The attenuation is overestimated by the specific attenuation formula
D.Rain drops become smaller as they travel along the path

Answer: A

Heavy rain falls in cells a few kilometres across, so a twenty kilometre link is rarely immersed in the peak rate along its whole length and the effective path is shorter than the geometric one. The factor accounts for that statistically. The formula itself is correct for a uniform rate, rain does not systematically favour the middle of a path, and drop size is a meteorological matter. Correcting a point measurement for the spatial structure of the phenomenon is a general requirement in path averaged quantities.

Discuss in Forum

23.

What availability figure is typical for a microwave link design?

A.One hundred per cent
B.99.99 per cent of the year or better
C.Ninety per cent of the year, allowing outages on rainy days
D.Fifty per cent, since the median condition defines the design

Answer: B

Trunk links are commonly designed for 99.99 or 99.999 per cent availability, corresponding to about fifty minutes or five minutes of outage a year, and the fade margin follows from the rain statistics at that percentage. Lower figures are accepted for less critical services. One hundred per cent is unattainable, since any margin can be exceeded by a sufficiently rare event. Availability is a design parameter with a cost, and choosing it is a commercial decision as much as a technical one.

Discuss in Forum

24.

What is a fade margin?

A.The margin by which the antenna gain exceeds the design requirement
B.The tolerance permitted on the alignment of the antennas at each end
C.Extra received signal above the threshold to absorb propagation fades
D.The difference between the transmitter power and the receiver sensitivity

Answer: C

Fade margin is the number of decibels by which the received signal in clear conditions exceeds the level at which the link fails, and it is chosen so that fades exceed it only for the permitted outage time. It is not simply the difference between transmitter and receiver specifications, which ignores the path. Antenna gain and alignment enter the budget separately. A margin sized from the statistics of the impairment, rather than chosen arbitrarily, is what turns a prediction into a service level.

Discuss in Forum

25.

Why does rain also cause depolarisation?

A.Rain reflects part of the signal with a rotated polarisation
B.Water is birefringent and rotates the plane of polarisation
C.The receiving antenna is detuned by moisture on its surface
D.Falling drops are flattened and aligned, so the two polarisations differ

Answer: D

Large raindrops flatten as they fall and tend to align, presenting different attenuation and phase to horizontal and vertical polarisation, so a dual polarised system loses isolation. That matters most where frequency reuse depends on cross polar discrimination. Reflection, bulk birefringence and antenna wetting are not the mechanism, though a wet radome adds loss of its own. A scatterer whose shape and orientation are not random will always couple energy between polarisations.

Discuss in Forum

26.

Which atmospheric gas absorbs strongly near sixty gigahertz?

A.Oxygen
B.Water vapour, whose line dominates the whole millimetre band
C.Carbon dioxide, whose concentration has been rising steadily
D.Nitrogen

Answer: A

Molecular oxygen has a complex of magnetic dipole transitions near sixty gigahertz that gives around fifteen decibels per kilometre at sea level, effectively blocking terrestrial paths but making the band attractive for short range systems that benefit from isolation. Water vapour has lines at twenty two and a hundred and eighty three gigahertz, carbon dioxide absorbs in the infrared, and nitrogen is transparent. A band that is useless for range and excellent for reuse is a good illustration of a property being an asset or a liability according to the application.

Discuss in Forum

27.

Why is the sixty gigahertz band attractive for short range systems?

A.Antennas at that frequency are much cheaper than at lower frequencies
B.Oxygen absorption limits the range and so allows dense frequency reuse
C.The absorption makes the signal more resistant to interference from rain
D.The band is unregulated because no long range service can use it

Answer: B

Absorption that kills a signal within a few hundred metres also prevents it from interfering with a system a kilometre away, so the same frequency can be reused very densely. Wide unlicensed allocations exist for that reason. Rain still attenuates the band severely, regulation exists but is light, and antennas are small rather than notably cheap. Turning a propagation liability into a system asset by choosing the right application is an instructive piece of spectrum engineering.

Discuss in Forum

28.

What is an atmospheric window in the millimetre band?

A.A gap in the regulations allowing unlicensed transmission
B.A geographical region in which rain attenuation is negligible
C.A frequency range between absorption lines with lower attenuation
D.A period of the day during which the atmosphere is most transparent

Answer: C

Between the oxygen and water vapour lines lie ranges near thirty five, ninety four and a hundred and forty gigahertz where clear air attenuation is comparatively low, and imaging and radar systems are placed there. The windows are properties of the molecular spectrum rather than of time, regulation or geography. Choosing an operating frequency from the structure of the medium's absorption spectrum is the same reasoning that placed infrared astronomy at particular wavelengths.

Discuss in Forum

29.

What is tropospheric scintillation on a satellite path?

A.Slow variation of signal strength through the seasons of the year
B.Fading caused by rain cells drifting across the path
C.Interference between the direct signal and a ground reflection
D.Rapid fluctuation caused by refractive index turbulence

Answer: D

Turbulent mixing produces small variations in refractive index that focus and defocus the wave, giving fluctuations of a fraction of a decibel to a few decibels at rates of a few hertz, worst at low elevation angles and in humid weather. Seasonal change, rain fading and ground reflection are separate effects with different timescales. Distinguishing impairments by the timescale on which they act is the quickest way to identify which one a record is showing.

Discuss in Forum

30.

Why is scintillation worse at low elevation angles?

A.The path through the turbulent layer is much longer
B.The antenna gain falls when it is pointed near the horizon
C.Rain is more likely to be encountered at low elevation
D.The satellite moves more rapidly relative to the ground station

Answer: A

A path at five degrees elevation traverses many times more of the turbulent lower atmosphere than one directly overhead, so the accumulated phase perturbation and the resulting amplitude fluctuation are much greater. The same geometry increases gaseous absorption and sky noise temperature. Antenna gain is unchanged with pointing, rain is a separate impairment, and geostationary satellites do not move appreciably. Path length through an impairing medium is the governing variable for every low elevation problem.

Discuss in Forum

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