### Proposed Technique Analysis
Meteorological ultra-high frequency (UHF) bands were selected for communication since they are more vulnerable and influenced by the parameters of the atmosphere compared to GNSS L-band frequencies. The network of  
Figure 1: 3D mapping of atmospheric _TWVC_ and electron density. ground stations (GS to GS network) uses GS to satellite networks to operate missions, control the satellites, and acquire mission data from satellites in the ISL network. The following items summarize the primary goals of this study:  
1. To deduce the _TWVC_ measurement concept. 2. To design a satellite payload that measures atmospheric water vapor, determines the system requirements, selects the components and specifications, and conducts system interfaces and integration. 3. To implement a GNU radio-based SDR transceiver with both transmitting and receiving capabilities of SS ranging signals. 4. To perform ISL ranging. This is essential for 3D mapping when all LEO orbital planes will be considered. 5. To demonstrate dual frequency reconfiguration of SS ranging signals by remotely tuning SDR parameters during runtime onboard each satellite. This is required for mission measurement accuracy and to distinguish _TWVC_ and _TEC_. 6. To eliminate instruments' clock offsets and errors as much as possible. 7. To simulate how the signal time delay due to water vapor and electron density can be estimated. This is vital in deducing the final _TWVC_ measurement. In terms of mission success criteria, achieving the above primary goals is a huge milestone which guarantees the viability of conducting a _TWVC_ mission onboard the satellite based on this proposed method. Therefore, the purpose of this paper is to demonstrate the feasibility of the primary goals based on a ground test simulating an in-orbit demonstration mission. The ground test assumes satellites in the same orbital plane using two satellite payloads with the capability of both transmitting and receiving SS ranging signals. Therefore, this demonstrates the ISL network feasibility, and two frequencies are used for the frequency manipulation demonstration. The signals are time-stamped at both the transmitting and the receiving ends for time delay estimation. Unlike in the orbit scenario, for these ground tests there is no influence of atmospheric water vapor or _TEC_. For this reason, a simulation of signal time delay due to water vapor was implemented using a delay pulse generator. This functionality qualifies the ability of this system to determine the real water vapor content in the future. The most critical requirements of this study are listed below:  
* The frequency reconfiguration time and data processing time should be \(\leq\) 1 s.
* A water vapor column of approximately a few mm and a delay measurement accuracy \(\leq\)100 ns. * The size of the constellation should be more than 1000 small satellites [19]. * The temporal resolution should be between 5 min and 15 min, whereas spatial coverage should be between 15 km and 4600 km. * The payload should be able to fit within the limited constraints of power, size, and mass for a small satellite. The novelty of the present paper is the use of SDR technology to deduce atmospheric water vapor delay based on radio signals, frequency manipulation, and the ISL network from a constellation of small satellites in LEO. Also, time stamps were implemented with RF switches in order to deduce the signal propagation time delay. The SDR capabilities were adopted to implement an automated software transceiver that performs digital signal processing (DSP) and frequency manipulation remotely in the shortest possible time. Instead of each satellite being composed of several radio devices that operate at different frequencies, only a single SDR transceiver is mounted onboard each satellite to perform the communication mission requirements and this reduces the cost and strain on the limited resources of small satellites. In this way, the size of individual satellites becomes smaller and the number of satellites in the constellation can be increased.