### Proposed Technique Analysis  
Detecting atmospheric water vapor using low-frequency radio signals from a constellation of small LEO satellites is proposed as an alternative method. Radio signals through space are primarily influenced by atmospheric water vapor content and total electron density. Approximately 99% of _TWVC_ is in the troposphere, whereas _TEC_ plasma density is dominant in the ionosphere [38]. The troposphere is a layer in the atmosphere's neutrosphere region. _TWVC_ and _TEC_ profiles could be acquired using a satellite constellation with high spatiotemporal resolutions and better accuracy than the GNSS and other conventional technologies. As a result, small satellites could be considered due to their short development span, affordable costs, and effectiveness in completing space scientific missions. Low Earth orbit contains a lower percentage of plasma than the GNSS orbits. Unlike SPATIUM-I and SPATIUM-II missions, the influence of both _TWVC_ and _TEC_ are taken into consideration and _TWVC_ is separated from _TEC_. Spread spectrum (SS) ranging signals which allow multiple signals to share the same frequency are transmitted from a network of satellites using intersatellite link (ISL) communication [19]. Software-defined radio (SDR) technology has been adopted to transmit and receive the SS signals. SDR technology is considerably effective, in that communication digital signal processing (DSP) hardware can be implemented as software; it supports a wide range of radio frequencies, and its architecture is reconfigurable, extensible, reprogrammable, and upgradable in real time [39]. The SS signals are modulated with binary phase shift keying (SS-BPSK) modulation because BPSK-modulated data travels long distances, and the original message is easily detected at the receiver due to the efficient properties of BPSK technology [40]. ISL technology has been employed for various satellite communication and formation flight applications, enabling ranging, commanding, timing control, and data transfer in a distributed network system [41]. In this proposal, the intersatellite ranging from satellites in different LEO orbital planes is performed in all directions to acquire three-dimensional mapping of atmospheric water vapor and ionospheric density along each line of transmission. Each satellite in the ISL network has a mission payload for _TWVC_ derivation. The mission payload is designed to fit small satellites with limited power, mass, and size. To derive the water vapor and electron density in three-dimensional distributions, we solve the inverse problems [19]. For a given set of water vapor and electron density distributions, we integrate the two quantities along each measurement path and compare _TWVC_ and _TEC_ with the ones derived from the measurement. We then find the 3D water vapor and electron density distribution that best matches with the measurement. Figure 1 depicts the main notion of water vapor and electron density dispersion measured from a constellation of satellites. The _TWVC_ and _TEC_ are detected based on signal propagation time delay, and the delay of SS signals along the intersatellite ranging path is calculated onboard each satellite. Since the total electron density varies inversely to the square of the radio signal frequencies, whereas _TWVC_ does not, solving the inverse problem and manipulating multiple radio frequencies (RF) help to distinguish and separate _TWVC_ from _TEC_[42]. Two remotely tuned algorithms that use extensible markup language remote procedure call (XML-RPC) and transport control protocol/internet protocol (TCP/IP) were coded to remotely reconfigure the SDR transmitter and receiver frequencies in real time [43,44]. The algorithms were built using GNU Radio digital processing blocks and the Python programming language. The algorithms achieve frequency translation within a second of reconfiguration. This is the shortest possible time to retrieve the mission data with high accuracy based on this method.