## 7 Rationale for ka-band selection
### Technical aspects
#### Rain issues  
Propagation of Ka-band electromagnetic waves is known to be sensitive to atmospheric conditions which may lead to significant atmospheric attenuation. This attenuation includes contributions from gazes (with values varying from 0.4 to 2.1 dB when humidity varies from 7.5 to 50 g/m3), clouds (cumulus and cumulonimbus being generally the main contributors (0.2 to 0.7 dB) to the total loss; stratocumulus may also lead to an 0.05 to 0.2 dB attenuation) and rain. Attenuation by rain, and possibly snow, is computed from rain rate and rainfall thickness (see Table 2). Let us consider climatic zones as defined by UIT-R (Union Internationale des Telecommunications - Radiocommunications), and let us restrict our analysis to ocean and polar areas. Then, it comes that precipitation rates are less than 1.5 mm/hr during 95% of an year or even less than 1 mm/hr during 90% of the time, except in the Philippines area where figures should be doubled. (For information, if we assume a 99% figure to be the nominal specification in terms of altimeter measurement capability, then the maximum precipitation rate to be taken into account has to be increased to 12 mm/hr, or even 65 mm/hr for a 99.9% figure. This would be dramatic in terms of technical specifications of the instrument in the Ka-band). Assuming that the data return rate is respectively 90% and 95% level, the total attenuation (2 ways) is less than 2.4 dB and 3.6 dB respectively. Reaching the 90% or even 95% data return rates is thus fully achievable without any technical difficulty. \begin{table}
\begin{tabular}{|l|l|l|l|l|l|} \hline rate (mm/h) & 0.1 & 0.5 & 1 & 2 & 5 \\ \hline Attenuation due to rain (dB/km) & 0.02 & 0.1 & 0.2 & 0.4 & 1 \\ \hline Attenuation due to snow (dB/km) & 0.002 & 0.015 & 0.06 & 0.2 & 1.5 \\ \hline \end{tabular}
\begin{tabular}{|l|l|l|l|l|l|} \hline Latitude & 0\({}^{\circ}\) & 30\({}^{\circ}\) & 45\({}^{\circ}\) & 60\({}^{\circ}\) & 75\({}^{\circ}\) \\ \hline Maximum thickness 99\% of time (km) & 4.5 & 3.5 & 2 & 0.5 & 0.2 \\ \hline \end{tabular}
\end{table} 
Table 2: Rain rates (mm/hour) and thickness (km), as provided by UIT-R.  
Complementary studies can be performed in order to better analyze seasonal effects and optimize local time of ascending/descending nodes of the orbit if a sun-synchronous orbit is selected: indeed, besides improving the link budget -which should be a difficult task- local time of sun-synchronous orbits provides a degree of freedom to possibly enlarge the 90% and 95% figures that are reported above and then reach near full availability of measurements. (As a side comment, it may be considered that flying a sun-synchronous orbit opens the issue of tidal aliasing in terms of mission definition. The ocean tide topic has been widely addressed by oceanographers dealing with T/P or ERS data. As discussed in section 4, complementing T/P-like missions with sun-synchronous missions is now widely accepted as providing the needed altimeter data set to retrieve large scale and mesoscale ocean signals with the appropriate level of quality -if other parts of the systems are of course of the required quality-. Then, consideration of a sun-synchronous orbit for AltiKa missions complementing a T/P-follow on missions is fully acceptable on a mission point of view)  
In addition to the attenuation effect, rain cells also impact the shape of the waveforms: specific algorithms have to be used in order to retrieve geophysical parameters correctly in the presence of rain. Such algorithms are presently under development: for instance, simulations are being run to analyze the variation of the shape of the echoes in the presence of rain cells that are characterized by a given rain rate and a characteristic diameter. In the past, several studies investigating the effect of rain on dual frequency T/P altimeter data have proposed a rain flagging procedure based on the departure of measured Ku and C band data from theoretical Ku/C band backscatter coefficients relationships.