# Development Trend of Electronic Nose Technology in Closed Cabins Gas Detection: A Review
## 4 Application of Electronic Nose in Gas Detection of Closed Cabins
### Gas Detection in Space Station Cabin  
The utilization of an electronic nose in the space station needs to consider its microgravity environment, and to experiment with its feasibility, [PERSON] (2004) [30] designed and fabricated a miniaturized electronic nose array for the space shuttle using polymer-carbon composite sensing membranes to fabricate sensor arrays and developed an algorithm based on least squares (LM-NLS) that resulted in an accuracy of 85% for individual gas identification and 65% for mixed gas events. The JPL laboratory also brought this electronic nose into the space shuttle for testing and found it to be insensitive to the microgravity environment and suitable for environmental monitoring in actual chambers. Following this, [PERSON] (2007) [31] developed an electronic nose that continuously monitors the air quality inside the spacecraft using a quartz microbalance chemical sensor (QMB) and analyzed the data in real time using principal component analysis (PCA). The e-nose was tested on the ground for two years and brought into the spacecraft for practice. The experiments proved that the e-nose has good re-producibility in the space environment, proving that any sensor array not used can be utilized in the space station. Electrical fires are one of the main fires that occur in space stations, and as the wires heat up, the insulation releases chemical vapors, which can be detected by electronic noses far faster than smoke alarms, and the chemical vapors are emitted before the actual combustion, allowing for the warning. [PERSON] (2003) [29] listed three application scenarios of electronic noses in the space program (general-purpose air monitor for organic vapors in confined chambers, self-igniting fuel monitor with a pressure range of 3 to 15 psi, and pre-combustion alarms) and tested the recognition accuracy of the three electronic noses in various scenarios, in which the KAMINA electronic nose was able to detect 10 ppb of self-igniting fuel, and its recognition accuracy was tested to be as high as 90%. Prove the feasibility of electronic nose application in this field. To further improve recognition accuracy, [PERSON] (2008) [32] used the wavelet denoising method to pre-process the experimental data and used the information fusion theory Dempster-Shafer (DS) to fuse the results of multilayer perceptron (MLP), support vector machine (SVM), k-nearest neighbor (KNN) and probabilistic neural network classifier (PNN) classifiers. Even if one of the classifiers is less effective, this method can compensate for its shortcomings. After this, researchers started with sensor array composition, [PERSON] (2008) [33] evaluated 20 sensor arrays and experimentally demonstrated that a simplified array design (two metal oxide (MOX) and one electrical conductivity (EC) sensor) could identify the vapor and improve the accuracy of vapor identification to 100% using principal component analysis. This brings significant advantages to conventional design and data acquisition, as fewer sensors simplify the construction, cost, and weight of models and instruments, which is important in spacecraft and aircraft. Air pollution inside the space station can also seriously affect what health of the astronauts, and electronic noses can be used to detect air pollution inside the space station in real time. [PERSON] (2006) [34] described an improved algorithm for nonlinear least squares that was used to identify and quantify 12 target analytes in clean air. It was shown experimentally that the algorithm can effectively identify and quantify gas events if the concentration exceeds the electronic nose detection threshold. For a more comprehensive detection of constituent gases in the space station, [PERSON] (2010) [35] developed an electronic nose with an array of 32 conductivity sensors to monitor the air quality inside the spacecraft, and analysis of the detection data showed regular, periodic rises and falls in humidity and occasional releases of perfluoropropane, formaldehyde, methanol, and ethanol.