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The Impact of Weather

So far, the discussions in this chapter have been somewhat theoretical. One of the practical topics of most interest to designers and implementers of FSO systems is the weather.

Rain

Rain has a distance-reducing impact on FSO, although its impact is significantly less than that of fog. This is because the radius of raindrops (200–2000 μm) is significantly larger than the wavelength of typical FSO light sources.

Typical rain attenuation values are moderate in nature. For example, for a rainfall of 2.5 cm/hour, a signal attenuation of 6 dB/km can be observed. Therefore, commercially available FSO systems that operate with a 25 dB link margin can penetrate rain relatively unhindered. This is especially the case when systems are deployed in metropolitan areas where building distances are typically much less than 1 km. If, for example, the system is deployed over a distance of 500 m under the same rain conditions, the attenuation is only 3 dB/km. However, when the rain rate increases dramatically to beyond the cloudburst level (> 10 cm/hour), rain attenuation can become an issue in deployments beyond the distance scale of a typical metropolitan area. However, these kind of cloudbursts last for only a short period of time (minutes).

An interesting point to note is that RF wireless technologies that use frequencies above approximately 10 GHz are adversely impacted by rain and little impacted by fog. This is because of the closer match of RF wavelengths to the radius of raindrops, both being larger than the moisture droplets in fog. The lower unlicensed RF frequencies in the 2.4 GHz and 5.8 GHz ranges are relatively unaffected by rain or fog, but incur significant interference risks by nature of the lack of licensing in those frequencies.

Snow

Snowflakes are ice crystals that come in a variety of shapes and sizes. In general, however, snow tends to be larger than rain. Whiteout conditions might attenuate the beam, but scattering doesn't tend to be a big problem for FSO systems because the size of snowflakes is large when compared to the operating wavelength. The impact of light snow to blizzard and whiteout conditions falls approximately between light rain to moderate fog, with link attenuation potentials of approximately 3 dB/km to 30 dB/km.

Fog

Fog is the most detrimental weather phenomenon to FSO because it is composed of small water droplets with radii about the size of near infrared wavelengths. The particle size distribution varies for different degrees of fog. Weather conditions are typically referred to as fog when visibilities range between 0–2,000 meters. Because foggy conditions are somewhat difficult to describe by physical means, descriptive words such as "dense fog" or "thin fog" are sometimes used to characterize the appearance of fog. When the visibility is more than 2,000 meters, the condition is often referred to as hazy.

Table 3.1 relates visibility and different fog conditions. Scattering is the dominant loss mechanism for fog. Even modest fog conditions can highly attenuate infrared signals over shorter distances. The expected path attenuation in dB/km and its correlation to visibility is shown in the table. The table also clearly illustrates that rain has much less impact on FSO systems' path losses when compared to fog. For example, a medium rainfall results in less attenuation than a thin fog.

Table 3.1 International Visibility Codes for Weather Conditions and Precipitation

Weather Condition

Precipitation

Amount mm/hr

Visibility

dB Loss/km

Dense fog

 

 

 

0 m, 50 m

–271.65

Thick fog

 

 

 

200 m

–59.57

Moderate fog

snow

 

 

500 m

–20.99

Light Fog

snow

Cloudburst

100

770 m 1 km

–12.65

–9.26

Thin fog

snow

Heavy rain

25

1.9 km

2 km

–4.22

–3.96

Haze

snow

Medium rain

12.5

2.8 km

4 km

–2.58

–1.62

Light haze

snow

Light rain

2.5

5.9 km

10 km

–0.96

–0.44

Clear

snow

Drizzle

0.25

18.1 km

20 km

–0.24

–0.22

Very Clear

 

 

 

23 km

50 km

–0.19

–0.06


Fog is not well understood, and it is difficult to characterize physically. Although visibility is most commonly used to characterize foggy conditions, other methods such as particle size and density measurements have been undertaken to describe fog conditions in a more quantitative way. The FSO community mainly uses visibility data because these measurements have been taken at major airports over many decades. To some extent, these measurements allow you to characterize different regions and derive statistical availability figures for FSO systems. However, most of the data has been time averaged over years; in general, the temporal resolution of these data points is not very high.

Because microclimate environments such as ponds or rivers can induce foggy conditions, the data taken at airports sometimes is not reliable for nearby environments. However, it has been shown that the visibility at airports provides a good estimate for the minimum expectable availability figure. This is because airports typically are located outside metropolitan boundaries, and the microclimate inside a city typically generates less foggy conditions.

The density distribution of fog particles can also vary with height, which makes the modeling of fog even more complex. The limited amount of information regarding the local impact of fog on the availability of FSO systems is certainly one of the biggest challenges for the FSO industry.

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