## 4 Results and Discussions
### Discussions
This inversion produces more pronounced peaks in \(\left(\delta n/\delta z\right)^{2}\) compared to a sharp decrease in the vapor pressure. The inversion of vapor pressure can be due to cloud formation at heights of 2000 m and above. This explains the few occasions when very high levels of power were received from beams with elevation angles of 5 and above, mostly in the monsoon season. Variation in the vapor pressure was investigated, for the Ahmedabad as well as Guwahati  
Figure 4: Plot of \(\left(\delta n/\delta z\right)^{2}\) versus height (AMSL) on 5 June 2006 0530 IST for Ahmedabad link. when the maximum power was received from an elevated beam set. In all these cases, it was found that there was either a sharp fall or increase in the vapor pressure between certain heights falling in the common volume height range of the elevated beam set. Temperature during the night is generally low, and due to this, the water vapor content in the air becomes high, which being heavier than air, settles close to Earth's surface. This results in high vapor pressure at low heights, which decreases rapidly with increases in height. A sharp gradient of vapor pressure is thus observed from ground level only up to lower heights. This explains why maximum powers at 0530 IST were mostly received from beams with lower elevation angles, one to five, on the Ahmedabad link and from beams with elevation angles of one to four on the Guwahati link. However, during the daytime, the Earth's surface is heated up by the Sun's rays, which in turn, heats up the atmosphere at lower heights. Direct heating of the atmosphere from the Sun's rays also takes place. As a result, the water vapor content of air moves upwards, setting up a large gradient of vapor pressure even at higher heights. This explains the reason behind maximum powers being obtained from beams with almost all elevation angles (1 to 10) at 1730 IST on both the links. Heating of the atmosphere is more prominent in the summer season, especially in the evening hours. This results in a considerable upward shift in the vapor content and the development of large gradients of refractive index at all heights. Hence, the average received power is higher in the summer season, with more instances of maximum power being received from beam sets with higher elevation. The results indicate that for the Ahmedabad link, beam sets 1 to 5, corresponding to the height range of 249.3-2406.0 m, and for the Guwahati link, beam sets 1 to 4, corresponding to the height range of 323.8-1764.2 m, are more favorable for beam steering during the morning hours. Hence, beam steering should be restricted to the elevation angles of these beam sets. However, in the evening hours, beams with higher elevation angles are also favorable. Hence, beam steering should be carried out over the entire range of elevation angles 1 to 10. Beams with very high elevation angles like 11 rarely give high received powers. Hence, steering the beam to an elevation angle of 11 and higher will be a waste of resources and should be avoided. Thus, it can be seen that the beam pointing dynamics of a tropospheric link can be determined using a modified ray-based model with real-world meteorological data, and the knowledge obtained can be used for efficient beam steering.