###### Abstract
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Technique** & **Observing Geometry** & **Temporal Resolution** & **Spatial Resolution** & **Accuracy** & **Conditions** \\ \hline Surface meteorology & ground & 1–60 min & few meters–tens of meters & few mm & affected by environment \\ \hline Lidar & ground, air, and space & low–high depending on observing geometry & low–high & few mm & cloud-free sky \\ \hline Microwave radiometers & ground, air, and space & 5–15 min & low–high & 1–5 mm & rain-free sky \\ \hline Sun photometer & ground & few times with high solar illumination intensity & High & few mm & direct sunlight and clear sky \\ \hline VLBI & ground & mins–days (depends on schedule) & very low (few sites) & few mm & none \\ \hline Polar satellites & space & 6–12 h & 1–10 km & few mm–1 cm & none \\ \hline Radio sondes & air & 1–4 times a day & low & 1–3 mm & none \\ \hline Imaging & & MODIS (1–2 days) & MODIS (250 m–few km), MERIS (300 m) & few mm–1 cm & cloud–free sky \\ \hline Remotely piloted & & depends on flight duration (few mins–few hrs.) & few meters–tens of meters & few mm & depends on weather \\ \hline In-SAR & space & 6–12 days & high & 1–2 mm & none \\ \hline GNSS satellites (radio & & & & \\ occultation) & space & 1–60 min & high & 1–5 mm & none \\ \hline GNSS satellites to & & & & & \\ standard GPS ground & ground & 30 s–few mins & tens–hundreds of km & 1–5 mm & none \\ receivers & & & & & \\ \hline Geostationary & & & & & \\ satellites & space & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of existing techniques that measure atmospheric water vapor. and successfully operated two low Earth orbit (LEO) satellites, which are SPATIUM-I (2U CubeSat) and SPATIUM-II (1U payload on a 6U CubeSat), for a technology demonstration of ionospheric electron density mapping [19; 28; 29]. Even though the SPATIUM mission has an objective of three-dimensional (3D) ionospheric mapping for _TEC_ using a constellation of small satellites, SPATIUM I and II missions demonstrated technologies related to a chip-scale atomic clock and onboard processing capabilities that can receive and a demodulate spread spectrum (SS) signal for time delay measurements between the ground station (GS) and satellite receiver. As a result, the two satellites have not yet investigated the influence of _TWVC_ on the time delay in measurements, 3D mapping of the atmospheric parameters, and employing an orbital constellation in addition to intersatellite communication. On a global scale, GNSS satellites have carried out modeling and mapping of the atmosphere's geophysical parameters, such as _TWVC_, _TEC_, temperature, and pressure [30]. The GNSS constellation utilizes either the carrier phase or the pseudo-range radio occultation techniques to estimate the signal delay as a result of the mentioned geophysical parameters. The measurements are deduced along the GNSS signal array paths [15; 31; 32]. Models such as numerical weather prediction (NWP) models, global ionospheric models (GIM) and global circulation models were developed to map and model the vertical and horizontal profiles of the atmospheric parameters from GNSS satellites [33; 34; 35]. However, the precision of _TWVC_ and _TEC_ from GNSS carrier waves is compromised because GNSS higher L-band frequencies are less affected by the atmospheric geophysical parameters. The GNSS satellites were not developed to monitor water vapor distribution or electron density but rather for locating or timing purposes. Therefore, their precision in determining _TWVC_ or _TEC_ is limited. Like GEO satellites, GNSS's high orbital altitudes have a large plasma component which influences measurement accuracy [19; 36]. Furthermore, when estimating water vapor time delay from GNSS measurements, the satellites' antenna phase center (APC) is prone to fluctuations and they require correct APC modelling. Incorrect APC modeling could yield false and additional trends in _TWVC_ profiles [37].