97 lines
3.8 KiB
Markdown
97 lines
3.8 KiB
Markdown
---
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title: Atmospheric propagation
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tags: troposphere, ionosphere, scintillation, attenuation, fade
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summary: What the air between a satellite and a receiver does to the signal, which is small in one band and decisive in another.
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---
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# Atmospheric propagation
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The signal path from a satellite to a receiver crosses atmosphere. Below
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about 10 GHz the atmosphere is close to transparent and the only loss is the
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geometry. Above it the same air becomes the largest single term in the
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budget, and above about 30 GHz it can be the reason a link does not close at
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all.
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## Definition
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**Attenuation** is the reduction in signal power along the path. **Scintillation**
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is the rapid fluctuation in amplitude and phase caused by refractive
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irregularities. **Fade** is the interval during which the received signal falls
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below what the link can use, whether because of attenuation, scintillation, or
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the geometry.
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The three are separate mechanisms with separate frequencies and separate
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mitigations, and a design that handles one does not handle the others.
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## Mathematics
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### Tropospheric attenuation
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Below about 10 GHz the clear-sky attenuation is the sum of contributions
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from oxygen and water vapour, each with its own mass frequency absorption
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curve. It is small, a fraction of a decibel at sea level, and nearly constant
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over a pass.
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Above about 10 GHz water vapour becomes significant, and above about 30 GHz
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it dominates. The specific attenuation for a given rain rate is
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$$
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\gamma = \frac{k R^{\alpha}}{\cos\theta}
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$$
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where $R$ is the rain rate, $\theta$ the zenith angle, and `k` and $\alpha$
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frequency-dependent. The $1 / \cos\theta$ is why a low-elevation pass is
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severe: at ten degrees from the horizon the path is about six times longer
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through the layer.
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### Rain fading
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The fade duration is a function of how fast the terminal moves relative to a
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rain cell. A high-elevation pass through a small cell gives a short, deep
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fade; a low-elevation pass gives a long, shallow one. Design mitigation
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therefore depends on the pass geometry, not only on the rain rate.
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### Ionospheric scintillation
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Above about 1 GHz the ionosphere becomes a refractive medium. The free
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electron content produces a phase shift proportional to the inverse square of
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frequency, and irregularities along the path produce rapid fluctuations. The
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scintillation index is the conventional measure.
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The effect is worst near the magnetic equator and near the plasma
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frequency peak around 10 to 20 UTC, and it is the reason a link that works
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at one station fails at another at the same moment.
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## Unit conventions
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- Attenuation and scintillation in decibels, index dimensionless.
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- Rain rate in millimetres per hour; the models are tabulated for that unit.
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- Free electron content in electrons per square metre, or in TEC units.
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- Frequency in gigahertz; the losses scale with its square or higher power.
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## Applicability
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The band decides everything. At 1 to 10 GHz, which covers a large part of
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the useful spectrum, the atmosphere is a detail in the budget. At Ku band it
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is a design figure. At Ka and above it is the design.
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The atmosphere also fails in a direction that a link budget does not show:
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scintillation is a fade that depends on the path and the time, not on the
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accumulated loss, so a link with adequate margin still degrades.
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This is why a modern constellation uses different bands and different
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mitigation for different shells, and why a handover that moves a user into
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a scintillation-active region is scheduled rather than opportunistic.
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## Standards
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- Specific attenuation: ITU-R P.838
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- Cross-polarisation and rain: ITU-R P.618
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- Scintillation: ITU-R P.1546
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- Local rain rate data: ITU-R P.837
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## See also
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- [[link-budget]] for the arithmetic these terms enter
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- [[access-and-availability]] for whether the geometry allows a pass at all
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- [[doppler-and-rate]] for what the measurement suffers |