Sin Ming Loo, Michael Owen, Josh Kiepert, Arlen 

Planting, Michael Pook, Derek Klein

National Air Transportation Center of Excellence for 

Research in the Intermodal Transport Environment

Boise State University

Boise, ID 83725

Byron Jones, Jeremy Beneke

National Air Transportation Center of Excellence for 

Research in the Intermodal Transport Environment

Kansas State University

Manhattan, KN 66506

Jean Watson

Office of Aerospace Medicine

Federal Aviation Administration

Washington, DC 20591

September 2009

Final Report

Modular, Portable, and Reconfigurable 

Wireless Sensing System for the Aircraft Cabin

DOT/FAA/AM-09/18

Office of Aerospace Medicine

Washington, DC 20591

OK-09-0434-JAH

Federal Aviation

Administration
NOTICE

This document is disseminated under the sponsorship 

of the U.S. Department of Transportation in the interest 

of information exchange. The United States Government 

assumes no liability for the contents thereof.

___________

This publication and all Office of Aerospace Medicine 

technical reports are available in full-text from the Civil 

Aerospace Medical Institute’s publications Web site: 

www.faa.gov/library/reports/medical/oamtechreports
i

Technical Report Documentation Page 

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. 

DOT/FAA/AM-09/18 

4. Title and Subtitle 5. Report Date 

Modular, Portable, and Reconfigurable Wireless Sensing System for the September 2009 

Aircraft Cabin 6. Performing Organization Code 7. Author(s) 

Loo SM,1 Owen M,1 Kiepert J,1 Planting CA,1 Pook M,1 Klein D,1 Jones B,2

Beneke J,2

 Watson J3

8. Performing Organization Report No. 

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 1

Boise State University 

Boise, ID 83725 

2

Kansas State University 

Manhattan, KS 66506 

 

3

FAA Office of Aerospace Medicine 

800 Independence Avenue, S.W. 

Washington, DC 20591 

11. Contract or Grant No. 

Co-Op Agrmt No. 07-C-RITEBSU 

 

12. Sponsoring Agency name and Address 

Office of Aerospace Medicine 

Federal Aviation Administration 

800 Independence Ave., S.W. 

Washington, DC 20591

13. Type of Report and Period Covered 

14. Sponsoring Agency Code 15. Supplemental Notes 

Work was accomplished under Public law 108-76 

16. Abstract 

This paper describes the design and prototyping of a modular, portable, reconfigurable, and wireless 

multipurpose sensor system for monitoring and recording environmental conditions in aircraft cabins. The 

objective of this small, portable embedded sensing system is to record aircraft cabin conditions on flights of 

convenience to generate a large database for determination of normal conditions. By designing a sensor system 

that can be easily reconfigured for different needs (with the ability and flexibility to accommodate different/extra 

sensors), the system can be used to measure parameters that meet multiple research requirements. Through 

design considerations such as generalized signal interface and hierarchical code structure that can be easily 

reworked for new sensors, this sensor system has been developed for optimum usefulness in aircraft 

environment research. In the current setup, the system is configured with temperature, humidity, sound level, 

carbon dioxide, and pressure sensors. The system has been calibrated in the laboratory without the use of a 

pressure chamber. Data from six flights are presented in this report. 

 

17. Key Words 18. Distribution Statement 

Sensor System, Wireless, Monitoring, Portable, Removable 

Storage, Modular, Reconfigurable

Document is available to the public through the 

Defense Technical Information Center, Ft. Belvoir, VA 

22060; and the National Technical Information 

Service, Springfield, VA 22161 

 

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price 

Unclassified Unclassified 18 

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized 

iii

ABBREVIATIONS

As used in this report, the following abbreviations/acronyms have the meanings indicated

Abbreviation Meaning

ACER ----------- Airliner Cabin Environment Research

ASCENT ------- Airliner Sensor Cabin Environment NeTwork

ASHRAE ------- American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

CFR ------------- Code of Federal Regulations

dBA-------------- Decibels with A-weight filtering 

EMC ------------ Electromagnetic Compatibility

EMI ------------- Electromagnetic Interference

FAA-------------- Federal Aviation Administration

I/O--------------- Input/Output

I2C -------------- Inter-Integrated Circuit

OSHA----------- Occupational Safety and Health Administration

ppm ------------- Parts Per Million 

SPI--------------- Serial Peripheral Interface

TSA-------------- Transportation Security Administration 

v

Contents

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Airliner Cabin Environment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Designing for Flexibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Hardware. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Software. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Calibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Validation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Data Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1

Modular, Portable, and Reconfigurable Wireless

Sensing System for the Aircraft Cabin

Introduction

For a sensor system to be useful in environmental 

sensing research, it needs to meet several criteria. First, 

a sensor system that can be easily reconfigured (e.g., to 

change sensing frequency or power input, or to add local 

data storage or wireless access) provides researchers the 

ability to tailor the system to the environment. Second, 

a flexible sensor system should permit extra sensors to be 

integrated into the existing system with limited development downtime. This means that, when a new sensor 

has been qualified for a sensing task, it can be integrated 

and deployed in a timely manner. Third, for convenience 

in research, portable systems are preferable. This allows 

the system to be easily transported and placed at desired 

sensing locations. Fourth, for some sensing applications 

(e.g. in commercial aircraft), the sensor system must be 

inconspicuous. The system must be able to perform its 

sensing task without disrupting the environment. Finally, 

in some environments such as the aircraft cabin, strict 

regulations for the use of electronics devices are another 

concern.1, 2

This document describes a portable sensor system 

design for airliner cabin environment research: The ASCENT 1000 is a portable (with exterior dimensions of 

6.0”x2.0”x3.5”), self-contained sensor node that can be 

retargeted to measure a number of different parameters 

for use in a wide range of environments. The objective of 

the current system setup is to determine “normal” cabin 

conditions using a preliminary set of sensors monitoring 

carbon dioxide (CO2

), temperature, humidity, pressure, 

and noise level. The measurements recorded from these 

sensors will be gathered by the research team during 

travel (flights of convenience) and stored in a database 

for detailed analysis. 

During the ASCENT 1000 sensing operations, the 

system periodically (at a user-adjustable sensing frequency) 

polls the on-board sensors to accumulate data on the 

surrounding environment. After collecting data from the 

sensors, the microcontroller writes this time-stamped data 

to a secure digital memory card (a commercial product). 

Analysis of data can be carried out by accessing the sensor 

data file on the memory card. 

Although single units are useful in some situations, 

many environments require more than one data collection 

point. This is due to the tendency of some cabin parameters 

being measured (e.g., temperature, CO2

 concentration, 

and noise) to exhibit spatial variation within the cabin. 

Thus, each ASCENT 1000 can be outfitted with an 

antenna to function as a sensor node in a wireless sensor 

network. This provides comprehensive collection of data 

on the environment in question. The units perform their 

operations independently (or cooperatively with the use 

of a common clock or real-time clock), needing only to 

be powered before establishing communications with the 

network and initiating the data collection. Even though 

ASCENT 1000 has wireless networking capability, it has 

been used primarily as a stand-alone sensing unit with 

one sensor node per aircraft. 

The remainder of this report addresses the overall aircraft cabin environment, design features of the ASCENT 

1000, calibration and validation, preliminary monitoring 

results, and conclusions. The next section discusses the 

conditions of and possible contaminants in the aircraft 

environment. The third section of this paper explains how 

the system has been designed for flexibility with regard 

to the hardware, software, and calibration processes. The 

fourth section describes independent validation of sensor 

accuracy. The fifth section presents some preliminary 

monitoring results from six commercial flights with the 

ASCENT 1000. Conclusions are presented in the sixth 

section.

Airliner Cabin Environment

Commercial airplane passengers and crews breathe a 

mixture of outside and re-circulated air similar to what is 

found in many homes and offices. However, the aircraft 

cabin environment is unique due to the proximity of the 

passengers, the need for cabin pressurization, and the low 

humidity. All of these aspects are complicated by the fact 

that the aircraft is a completely enclosed structure.3

 The 

objective of this project is the creation of a portable (easy 

to carry), modular (sensors can be swapped with minimum 

downtime), and reconfigurable (for new sensors) sensor 

node to determine the normal operating conditions in 

this environment.

The temperature, air pressure, and relative humidity 

in the environment external to the aircraft cabin vary 

widely during flight as the plane moves from taxiing 

and takeoff through cruising to descent and landing. 

At a typical cruising altitude of 36,000 ft (11,000 m), 

the atmospheric pressure is about one-fifth that at sea 

level. Airworthiness standards in the Code of Federal 
2

Regulations (CFR Title 14 part 25.841) require the pressurization system to “provide a cabin pressure at altitude 

of not more than 8000 ft (2,440 m)” under normal 

operating conditions.3

 This is approximately equal to 

an air pressure of 75 kiloPascals (kPa). At this pressure, 

the partial pressure of oxygen is 74% of that at sea level. 

Muhm et al. recently published a paper on the effect of 

aircraft cabin altitude on passenger discomfort.4

 In their 

work, the subjects were in a pressure chamber and were 

asked to complete comfort surveys at a set interval. For 

this project, we measure the environmental conditions 

during routine aircraft operation. These measurements 

can be used in conjunction with the research published 

by Muhm et al. to monitor and adjust cabin pressure to 

maintain a comfortable environment. 

The outside air is free of most pollutants at cruising 

altitudes and requires no additional cleaning.3

 However, 

the outside air near ground level could contain a wide 

variety of contaminants from industrial and urban sources. 

With a substantial number of contaminants that one 

may wish to measure, the need for flexibility becomes 

very apparent. Integrating enough sensors to measure 

all possible parameters in a single, compact unit would 

be impractical. Therefore, a good design solution must 

be able to accommodate the ability to integrate or swap 

several different types of sensors with minimum development time to alter the configuration. We have designed 

the ASCENT 1000 sensor node to meet this level of 

flexibility, both in hardware and software.

Designing for Flexibility

Hardware

To facilitate flexibility in the system, the ASCENT 1000 

has been implemented with a Microchip PIC18F8722 

(Microchip Technology Inc., Chandler, AZ). The 

PIC18F8722 is an 80-pin microcontroller with 128 

kilobytes of program memory and nearly four kilobytes 

of data memory. The microcontroller also has 70 digital 

I/O pins with the ability to implement a 12-channel, 

10-bit analog-to-digital converter (at 3.3V, the resolution 

is 3.22mV), I2

C, and SPI. I2C and SPI are particularly 

useful, as they are common serial data communication 

protocols. This memory size and pin variety allows 

the system to implement a large variety and number 

of sensors.5

 Microcontrollers such as Atmel AVR and 

other Microchip models were considered for the design, 

but further research determined that the PIC18F8722 

provided the most features with the least amount of 

development time for this application. 

The current design has five onboard sensors: temperature, humidity, pressure, CO2

, and sound level. In addition 

to the environmental sensors, the design provides some 

internal sensing capabilities for system status, including 

system battery voltage level sensing. To further enhance 

flexibility, the system employs two different methods 

for outputting data. Every ASCENT 1000 has a secure 

digital flash memory card (a commercial product) slot 

for storing sensor measurements and an interface for a 

Maxstream XBee wireless modem, which can broadcast 

the sensor measurements to a PC or other sensor nodes. 

Several images of the ASCENT 1000 sensor module are 

shown in Figures 1a through 1e. Figure 1b shows the 

sensor board (green printed-circuit board) and the main 

board (blue printed-circuit board).

Temperature and humidity are sensed by a Sensirion 

SHT15 sensor module that measures relative humidity 

from 0 to 100 %RH (± 1 %RH) and temperature from -40 

to 257 °F (± 0.9 °F). The pressure sensing is accomplished 

with a VTI Technologies SCP1000 sensor module that 

measures absolute pressure ranging from 30 to 120 kPa 

(± 200 Pa). Carbon dioxide levels are determined with a 

Telaire 6004 sensor that measures CO2

 concentration in 

a range of 0 to 2000 ppm (± 40 ppm + 3% of reading). 

Though this sensor will not be able to measure the limit 

set by the Federal Aviation Administration (5000ppm), 

the narrower range provides better accuracy for the lower 

levels more likely to be encountered. Because pressure has 

some effect on any gas reading, the actual concentration 

of carbon dioxide is calculated from air pressure and CO2

measurements. This calculation is done in real-time by 

the sensor once it receives the pressure measurement. 

Therefore, no post-processing is necessary. Finally, sound 

level is measured with a custom-built sensor module. The 

sound sensor is capable of measuring a range from 48 to 

110 dBA (± 2 dBA for readings above 60 dBA; ± 5 dBA 

for readings below 60 dBA). 

The ASCENT 1000 can also meet a variety of power 

requirements for future sensors. The system has two 

MAX710 step-up/step-down, dc-dc converters to meet 

the most common requirements of 3.3V and 5V.6

 Efficiency is one of the primary benefits of this power 

supply configuration, as it provides voltage regulation at 

efficiencies above 80%. The ASCENT 1000 also implements a MAX642 step-up switching regulator that can 

be configured to provide a voltage from 1.5V to 16.5V.6

This is expected to cover most situations that may arise. 

Currently, the MAX642 is configured to provide 9V (common supply requirement for some sensors). In addition 

to these supply voltages, there is an additional on-board, 

dc-dc converter that can be populated and configured as 

needed. These dc-dc converters were selected based on a 

survey of the working voltage ranges and current needs 

of many common sensors. It should be noted that similar 

specification dc-dc converters are available from several 

different manufactures.
3

Figure 1a: Side View of ASCENT 1000

Figure 1b: Interior View of ASCENT 1000

Figure 1c: Front End View of ASCENT 1000 Figure 1d: Rear End View of ASCENT 1000 

Figure 1e: Bottom View of ASCENT 1000 
4

Long-term battery tests have been conducted on the 

ASCENT 1000 sensor node to characterize its power 

consumption. The system is designed to draw power 

from four AA alkaline batteries arranged in two sets each 

with two batteries connected in series (standard consumer 

product battery holder without battery pack; see Figure 

1e). This battery arrangement results in a capacity of 

4200 mAh at 3V with standard alkaline batteries. In the 

current sensor configuration with five sensors (CO2

, pressure, temperature, humidity, and sound level), testing has 

proven that the entire system could run for more than 15 

hours at 60-second sensing intervals. At about 15 to 20 

hours, high current or high voltage (above 9V) sensors 

such as the CO2

 would cease functioning because the 

batteries could not provide enough current to allow the 

dc-dc converters to boost the voltage. However, the rest 

of the sensors would continue to function for a total of 

20-24 hours. Even at the 15-hour mark, where the highpower sensors drop out, this provides more than enough 

time for even the longest flights. In actual flight tests, the 

power sub-system performed as expected. 

A large bank of I/O pins (including analog, I2

C, SPI, 

and strictly digital) is routed from the main board of the 

ASCENT 1000 by a header to a secondary board that can 

be reconfigured for the needs of the application. Thus, 

the main board can remain unchanged when different 

parameters need to be measured.

The ASCENT 1000 sensor node has the flexibility to 

accommodate many different sensors to match a variety 

of circumstances. Thus, the ASCENT 1000 sensor node 

can be deployed wherever environmental measurements 

are needed and can provide a standardized data collection system for all manner of research and commercial 

applications.

Software

The ASCENT 1000 sensor system also incorporates 

important software design considerations to allow for 

a flexible system that can be easily adjusted and reprogrammed using a computer. With a system that could 

have many different and unknown sensors, there could 

potentially be a large amount of code maintenance to 

ensure that each type of sensor is handled properly. Each 

sensor will require different interface and data conversion 

codes. This type of code maintenance is unavoidable. 

However, the program can be structured so that changes 

only need to be made on a low level, leaving the highlevel code unchanged.

The code is arranged in a hierarchical manner where 

the upper levels handle the general, unchanging operations of the sensor system. The lowest level of the system 

contains code specific to the individual sensors. In between is a code structure for translating general code into 

specific sensor calls. Figure 2 shows the basic structure 

for performing this translation. This structure defines 

the addresses of the functions that need to be called to 

communicate with an individual sensor.

An array of these structures defines the configuration 

of the system, and the high-level code need only iterate 

through this list, calling the general names of the functions that reference the addresses to run the specific code. 

When a new sensor is added to the system, a new instance 

of this software structure is needed, but no modification 

is required to the basic structure. The software sensor 

configuration is shown in Figure 3. This is simply a list 

of sensor structures that contain the function pointers 

for each sensor in the system. New sensors can be added 

by adding extra entries to this list. 

When writing to the secure digital memory card, all 

data from the sensors are converted into a form that can 

be explicitly understood by a generic graphing program 

on the data server. This specific data format enables the 

Figure 2: Structure for Holding Sensor 

Reference Data 

struct sensor { 

 char desc[20]; 

 void (*identify)(char buff[]); 

 unsigned int (*measure)(void); 

 float (*convert)(unsigned int); 

 float calibration; 

}; 

Figure 3: Array of Sensors with Function Addresses 

rom struct sensor sensors[] = { 

 { "Fibonacci", fib_identify, fib_measure, fib_convert, 0.0 }, 

 { "Temperature", SHT75_temp_identify, SHT75_temp_measure, SHT75_temp_convert, 1.0 }, 

 { "Humidity", SHT75_hum_identify, SHT75_hum_measure, SHT75_hum_convert, 2.0 }, 

 { "Pressure", pressure_identify, pressure_measure, pressure_convert, 4.0}, 

 { "CO2", CO2_identify, CO2_measure, CO2_convert, 5.0 }, 

 { "Sound_Level" , sound_identify, sound_measure, sound_convert, 4.0 }, 

 { "Altitude", altitude_identify, altitude_measure, altitude_convert, 3.0 }, 

 { "Case_Temperature", temperature_identify, temperature_measure, temperature_convert, 3.0}, 

 { "Battery_Level", BTest_identify, BTest_measure, BTest_convert, 6.0 }, 

}; 
5

post-processing server to graph the collected data, regardless of the type of sensor used. This format utilizes two 

types of data strings: identify strings and measurement 

strings. When the unit is switched on, identify strings are 

written that give pertinent information about the sensors 

such as name, model number, and measurement units for 

the sensors. It also assigns an identification number to 

each sensor. When measurements are taken, measurement 

strings are written that record the value measured by the 

sensor. These strings use the same sensor identification 

number assigned in the identify string. This allows the 

data to be associated with the correct sensor in postprocessing analysis. The format of these strings and how 

they are related can be seen in Figure 4.

Calibration

With the integration of multiple sensors of varying 

accuracy, the ability to individually calibrate each sensor is 

a necessity. Additionally, since this system is to be flexible, 

the calibration process should be as simple as possible. 

This section will describe how this is accomplished for 

each sensor on the ASCENT 1000. 

The temperature and humidity sensors are calibrated 

using a simple offset in the code. This offset is stored in a 

calibration file on the secure digital memory card and read 

from the card on startup. This makes it easier to adjust the 

calibration offsets. The offsets are calculated by placing 

the ASCENT 1000 in a closed container with a TSI QTrak indoor air quality monitor. The closed container is 

used to keep the moisture and temperature fluctuations 

to a minimum. This setup is monitored for several hours. 

The resulting measurements are then compared using a 

spreadsheet, and the average difference is calculated. The 

average difference is used as the calibration offset. 

Due to its reliability, it is not necessary to calibrate the 

pressure sensor, but it is tested for each unit. The procedure 

for testing the pressure sensor is similar to the process used 

for calibrating temperature and humidity sensors. The 

ASCENT 1000 is placed next to a TSI Q-Trak indoor 

air quality monitor, and the conditions are recorded for 

several hours. The outputs of both are compared to ensure 

that the pressure sensor is calibrated correctly. 

The CO2

 sensor is calibrated using the zero-calibration 

feature (using N2

 gas) on the Telaire 6004. Using this 

feature requires that a special command sequence be 

transmitted to the device by the microcontroller. Therefore, a special program was written for the PIC18F8722 

to perform this task, and this program is downloaded to 

the ASCENT 1000 to calibrate the CO2

 sensor. Then, 

the CO2

 sensor is hooked up to a continuous supply of 

pure N2

. The ASCENT 1000 is turned on for over 5 

minutes to ensure that the CO2

 sensor has had sufficient 

time to stabilize. After being turned off at the end of the 

calibration routine, the ASCENT 1000 is immediately 

reprogrammed with normal operational software to ensure that the CO2

 sensor is not inadvertently recalibrated 

with ambient air. 

After calibrating the aforementioned sensors, the 

entire system is tested inside the chamber with the TSI 

Q-Trak. Any sensor not within the tolerance indicated 

by its datasheet is recalibrated using the same process.

The method for calibrating the sound sensor is slightly 

more involved than the methods used for the other sensors. First, the ASCENT 1000 is placed in a closed container with a speaker and a Center 325 Mini sound-level 

meter. The amplitude of the 1kHz signal broadcasted by 

the speaker is adjusted until the Center 325 Mini reads 

the desired dBA level. Then, the analog reading on the 

Figure 4: Data Format and Relation Between Identify String (Top) and Measurement String 

(Bottom) 
6

sound sensor is recorded for 20 samples. The outliers 

are ignored, and the average is calculated to be used as 

the reference value for that dBA range. This process is 

repeated at dBA levels of 48 and multiples of 5 for 50 

through 110. The average analog readings for each dBA 

range are then hard coded into an array of values in the 

software. This array is used to calculate the actual dBA 

values in real-time, based on analog measurements from 

the sound sensor. The ASCENT 1000 is then tested again 

to confirm that the sound sensor values are within 2 dBA 

of the Center 325 Mini readings for values greater than 

60d BA (5 dBA for values less than 60 dBA).

Validation

Once calibrated, several units were independently 

tested against laboratory standards over their operating 

range to verify accuracy, repeatability, and reliability. 

These tests were conducted at the Institute for Environmental Research at Kansas State University. Tests were 

conducted for all variables except sound. The validation 

of pressure and carbon dioxide (CO2

) measurements are 

reviewed here.

A small altitude chamber was constructed from PVC 

plastic pipe fittings (see Figure 5). The chamber is connected to a vacuum pump through a water column 

vacuum regulator to provide a stable, precisely controlled 

vacuum. Conditions from ambient up to a cabin altitude 

of 10,000 ft can be simulated. The laboratory elevation 

is approximately 1000 ft, and ambient atmospheric 

pressure is measured with a precision barometer (see 

Figure 6) that is verified periodically against a precision 

mercury barometer. 

Pressure inside the altitude chamber is measured 

with a 10-foot water manometer. Thus, all pressure 

measurements are based on first principles or directly 

related to them in order to minimize the potential for 

errors in the reference pressure (altitude) used when 

testing a unit. An example of pressure test results is 

presented in Figures 7a and 7b. Overall, there is a 

slight, but acceptable, sensitivity shift. Repeatability 

is good. Each data point in Figure 7a averages at least 

15 minutes of data. The pressure sensor has an inherent fluctuation a little greater than +1 kPa (Figure 7b), 

which is acceptable for this application. 

Carbon dioxide measurements were validated using 

commercial, bottled calibration gasses that provide dry 

air with a known CO2

 concentration. Several different 

concentrations were used to span the range from 500 

ppm to 2000 ppm (the expected operating range of the 

sensor when used in an aircraft). The calibration gas 

was fed directly to the CO2

 sensing port on the unit at a 

low flow rate so as to avoid pressurization of the sensor. 

Test were conducted at ambient pressure and at reduced 

pressure, using the altitude chamber described above. 

The altitude chamber was fitted with connections so 

that the calibration gas bottle was outside the chamber 

during tests. This allowed different calibration gasses to 

be fed to the unit while it is held at reduced pressure. 

An example of CO2

 test results is shown in Figure 8.

In this test, there was an offset of approximately 

400 ppm. This offset was later corrected in other units 

using the zero-calibration described in the previous 

section. The accuracy (except for the offset) and repeatability are reasonable for this type of measurement. 

The reduced pressure tests shown in this figure were 

conducted at an equivalent cabin altitude of 7000 to 

8000 ft. There was no evidence of sensitivity to pressure over the range of 0 to 8000 ft cabin altitude for 

which the unit is intended.

Figure 5. Altitide Chamber With CO2 

Calibration Gas

Figure 5: Altitude Chamber With CO2 

Calibration Gas Figure 6: Precision Barometer for Measuring 

Ambient Pressure 
7

a. Results From 3 Pressure Tests 

b. Sensor Fluctuation 

Figure 7: Example of Data From Unit 16 Pressure Tests 
8

Data Results and Discussion

All of these design considerations contribute to a 

system with sufficient flexibility for meeting the various 

requirements of the aircraft environment. The ASCENT 

1000 has been EMI/EMC-tested and approved (by 

Boeing Electromagnetic Effect Laboratory according to 

Boeing D6-16050-4, Rev D, Electromagnetic Interference Control Requirements). It is now being used to 

collect preliminary data on commercial flights. As of 

this writing, data have been collected from more than 

70 flights in various aircraft models (e.g., CRJ 100, CRJ 

200, CRJ 900, B737, B757, B767, A319, A320, MD80, 

MD88, MD90, EMB 145, EMB 170, and EMB 190) 

since mid-August 2007. Figures 10 through 13 show a 

sampling of the data collected from these flights from 

March 30 to April 6, 2009. Since these flights were of 

varying duration, the time axis has been normalized to 

permit comparison. The system is turned on near 10,000 

ft (aircraft altitude) while ascending (when permission 

was given) and turned off on descent around 10,000 ft, 

as required during landing. In a few cases, we graphed 

only a portion of the six flights to ensure that the figures 

would be more readable.

The cabin pressure data measured from six commercial 

flights can be seen in Figure 9. Generally speaking, the 

cabin pressure drops on take-off, reaches a steady level at 

cruise altitude, then rises again as the aircraft descends for 

a landing. The conversion from cabin pressure to altitude 

can be computed using the equation:

E=-312.5P+31434.375 (2) 

The symbol E represents the elevation in ft and P is 

the cabin pressure in kilopascals. Equation (2) gives an 

approximation for the elevation in an open-air environment where the given cabin pressure would be measured.8 

The data from the six flights show that the cabin altitude 

stays below 8000 ft, taking into account the sensor accuracy (±200 ft). 

Figure 10 compares the cabin sound levels measured 

for six commercial flights. In this figure, the data were 

processed and graphed with a 6-point moving average. 

The measured sound level of the cabin was typically 

around 86 dBA (± 2 dBA). This is above the level at which 

permanent hearing damage can occur (>80 dBA) over 

long periods of time.9

 However, this is below OSHA’s 

limit of 90 dBA for an 8-hour duration.10

Figure 11 shows the CO2

 concentrations for nine different flights. The concentrations of carbon dioxide for 

these flights typically fell between 900 and 1700 ppm 

(on average ±79 ppm) with only occasional measurements 

registering higher values. The carbon dioxide concentrations never rose to the FAA regulation limit of 5000 ppm.

On most of the flights, the cabin temperature ranged 

from 68 to 77oF (± 0.9 °F), see Figure 12. On a few 

flights, such as the flight from March 31, the temperature 

measured consistently on the higher end of the range 

(approximately 77oF). The data recorded on April 6 

showed low temperatures for the duration of the flight 

(ranging between 68 and 72oC). Data would be needed 

from more flights to establish any correlations between 

Figure 8: Example Data From Unit 16 CO2 Tests 
9

75000

80000

85000

90000

95000

100000

105000

Time

Pressure (Pa)

31-March 01-April 03-April

06-April 27-March 30-March

a. Cabin Air Pressure. The sensor system records the cabin air pressure in Pascal units. Each 

line shows the cabin pressure at different stages of a flight (ground, ascent, cruising, descent, 

and ground). 

-1000

0

1000

2000

3000

4000

5000

6000

7000

8000

Time

Altitude (feet)

31-March 01-April 03-April

06-April 27-March 30-March

b. Equivalent Cabin Altitude. The altitude data were calculated from data in Figure 9a using 

Equation (1). 

Figure 9: Cabin Air Pressure and Equivalent Cabin Altitude Data From Six Commercial Flights 

(Time Normalized) 
10

Figure 10: Aircraft Cabin Sound Level Data From Six Commercial Flights. Six flights are 

presented (Time Normalized). The levels through all flights are below the limits set by National 

Institute on Deafness and Occupational Safety & Health Administration. 

500

700

900

1100

1300

1500

1700

1900

Time

CO2 (ppm)

31-March 01-April 03-April

06-April 27-March 30-March

Figure 11: Carbon Dioxide Data (Time Normalized) 
11

temperature and other factors. One suspicion is that the 

temperature sensor may be sensitive to cabin pressure 

(even though the sensor manufacturer indicates that 

this should not be the case); this will be verifi ed when 

the system is chamber-tested. 

For the most part, the humidity data (see Figure 

13) demonstrated a trend of starting at around 35-

50% (± 1% RH) relative humidity at the beginning 

of the fl ight and dropping to around 10-25% (± 1% 

RH) as the fl ight progresses. On the fl ight that logged 

lower temperatures, the humidity did not drop down 

quite as far, but all fl ights showed a trend of reduced 

humidity levels.

Figure 12: Temperature Data (Time Normalized) 

0

10

20

30

40

50

60

Time

Humidity (%)

31-March 01-April 03-April

06-April 27-March 30-March

Figure 13: Humidity Data (Time Normalized) 
12

Conclusions

We designed a portable sensing system (ASCENT 

1000) that can be part of carry-on luggage to ensure 

that sensing tasks can be easily performed without heavy 

and troublesome equipment. Use of the system can take 

advantage of flights of convenience, thus adding no 

extra cost to the project. The sensing system can also be 

reconfigured for different sensing tasks when a new sensor has been identified, requiring only minor modification to the software (add necessary code to control and 

collect data from a sensor) and adding the sensor to the 

sensor board (add sensor power and control wires). The 

microcontroller’s memory is flash-based, and it can be 

reconfigured in a matter of minutes. ASCENT 1000 is 

also modular. Different sensor boards can be connected 

to the main printed-circuit board. The wireless feature, 

which can be installed and removed as necessary, also 

adds to the unit’s modularity. 

Based on the data from the six domestic flights presented in this paper, we provide the following observations:

•	 Cabin altitude typically remains below the 8000 ft 

specified in Federal Aviation Regulation 25.841

•	 Carbon dioxide levels are normally below 1700 ppm, 

which is less than the FAA limit of 5000 ppm

•	 Cabin temperature and humidity are in the range of 

68°F to 77°F, and 10% to 50%, respectively

•	 Sound level averages 86 dBA: less than the 90 dBA 

level allowed by OSHA

The limited amount of data collected at this point 

should be noted. The question still exists as to whether 

this is representative of normal cabin conditions. When 

all the sensors have been pressure chamber-tested to characterize the effect of pressure on the sensors, additional 

data will be collected as part of an ongoing project. The 

ultimate goal is to collect data on thousands of flights. 

This small, portable sensor system was designed for 

use in recording conditions on flights of convenience. It 

cannot replace extensive systems, as described in ASHRAE 

research project 1262 Phase I.11 In Phase I, there were 

four flights in the study. Phase II is ongoing as of the 

writing of this manuscript with the number of flights 

in the double-digits. An arrangement has been made 

to take the sensor system described in this manuscript 

along in Phase II. With this cross-instrument correlation, 

we can verify the effectiveness of this portable system. 

This can provide us with a broader view of the normal 

aircraft conditions. 

One other potential development for this project is that 

the system can potentially be an open source platform 

for other research in this field. As the system currently 

exists, once a new sensor has been identified, it can be 

soldered into the printed-circuit board. Minor coding 

will be required for interfacing the sensor to the system 

electronics for controlling and data collection.

In addition to the sensor system described in this paper, an Internet infrastructure has been built to manage 

the data. A database server has been prototyped so that 

data can be uploaded and graphed. If desired, the data 

can be converted to a format that can be graphed using 

standard graphing software. 

References

1. Federal Aviation Administration. “91.21 Portable 

Electronic Devices.” Federal Code of Regulations 

Title 14: Aeronautics and Space, Part 91 – General 

Operating and Flight Rules. September 19, 2007.

2. Boeing. “Electromagnetic Interference Control Requirements.” D6-16050-4, Rev D. July 24, 2002.

3. “The Airliner Cabin Environment and the Health of 

Passengers and Crew.” Washington, DC: National 

Academy Press, 2002.

4. J. Michael Muhm, Paul B. Rock, Dianne L. McMullin, et al. “Effect of Aircraft-Cabin Altitude on 

Passenger Discomfort.” The New England Journal 

of Medicine, 2007;357:18-27.

5. “PIC18F8722 Family Data Sheet.” Microchip Technology Inc., April 2, 2009, www.microchip.com, 

2004.

6. “3.3V/5V or Adjustable, Step-Up/Down DC-DC 

Converters.” Maxim Integrated Products, April 

2009, www.maxim-ic.com, Rev. 0, July 1997.

7. Michael Owen. “Portable Wireless Multipurpose 

Sensor System for Environmental Monitoring.” 

Master’s Thesis, Electrical and Computer Engineering Department, Boise, ID: Boise State University, 

December 2007.

8. “Atmospheric Pressure at Site Elevation.” Universal Industrial Gases, Inc., 2004. Retrieved September 23, 

2007, from www.uigi.com/Atmos_pressure.html.