## 6 Observation Requirements  
This ERA5 analysis confirms that the model reproduces migrating tides with a reduced amplitude at mid-latitude. Over the tropics, diurnal changes associated with steady non-migrating tides over continents were identified. Their characteristics need to be further investigated in order to be validated on a global scale with observations. However, operational global wind and temperature observations are missing in the middle atmosphere. In locations where steady waves appear, it is necessary to sample air masses several times per day to be able to retrieve tides. Ground-based observations are useful for complementing space observations. The European ARISE network [38], including the NDACC network [39], maintains remote sensing measurements of the middle atmosphere, allowing for continuous measurements during a full diurnal cycle [6; 8; 9; 21; 23; 40]. Some stations are located over the subtropics, allowing for local tidal retrieval. However, the network is not able to retrieve the largest non-migrating tides over the equator when no upper atmosphere observatories are deployed in these areas. In addition, while convective areas have a permanent roof made up of convective clouds, microwave technology [41] is preferred to lidar because the latter's abilities are highly insensitive to cloud cover. This is a critical issue because non-migrating tides are associated with atmospheric water vapor content, known to be challenging to accurately simulate in numerical weather/climate models.  
A recent study has been performed to envision how to provide temperature measurements unbiased by solar migrating tides [22]. This study suggests the use of a constellation of nano-satellites in different sun-synchronous orbits to increase the number of measurements per day over a given location for a reasonable cost. One potential proposed method is based on molecular scattering observed by limb viewing. This method provides the best vertical resolution. It was first used to view bright limbs from space using the Solar Mesosphere Explorer [42]. A similar method was used in space experiments like WINDII on UARS [43], and more recently it was successfully tested through the GOMOS experiment [44]. In the future, time continuity will be ensured by the ALTIUS (Atmospheric Limb Tracker for Investigation of the Upcoming Stratosphere) payload [45]. However, all these experiments that exhibit a similar observational geometry were not initially planned to provide temperatures and instead are research systems with a complex setup. However,  
Figure 11: See caption for Figure 10, here applied to tropical Africa (zone TA).  
these observations of the bright limb can be performed with a much simpler system that can employ CubeSat technology [22]. In 1999, a collaboration between California Polytechnic State University and Stanford University, California, developed a small educational platform called CubeSat, which was designed for space exploration and research for academic purposes [46]. CubeSats are now a common form of small spacecraft that can weigh only a few kilograms (up to 30 kg) and that correspond to a standard based on a form factor of a 10 cm square cube called unit (U). This principle allows easy deployment in space and use of COTS (commercial off-the-shelf) considerably reducing the price of each individual satellite. Retrieval of migrating tides require at least five payloads simultaneously in space to separate diurnal and semi-diurnal components. The presence of non-migrating tides yields much more information and requires increasing the number of space platforms to increase the sampling of these specific areas if we want to derive the local tidal characteristics directly from observations alone. This strategy has already been deployed to investigate several other issues, such as the radiative balance of Earth [47; 48].