Investigating the Utility of Global Positioning 

System (GPS) Technology to Mitigate the Carbon 

Monoxide (CO) Hazard Associated with Portable 

Generators – Proof of Concept Demonstration

June 2013

Han Lim

Division of Combustion and Fire Sciences

Directorate for Engineering Sciences

U.S. CONSUMER PRODUCT SAFETY COMMISSION

4330 East West Highway, Bethesda, MD 20814

This report was prepared by CPSC staff and it has not been reviewed or approved by, and may not 

necessarily reflect the views of, the Commission.
EXECUTIVE SUMMARY

The CPSC Directorate for Engineering Sciences’ Division of Combustion and Fire 

Sciences (ESFS) staff has conducted a limited, exploratory exercise to demonstrate the 

feasibility of using Global Positioning System (GPS) technology to distinguish indoor 

and outdoor environments, with the intent of shutting down portable generator operation 

when a generator is used indoors. Through a series of experiments, ESFS staff 

demonstrated that a modified, off-the-shelf, commercially available GPS receiver unit, 

with a staff-developed electromechanical shutoff system installed on a portable generator, 

can distinguish indoor and outdoor environments and shut off the generator accordingly, 

with the exception of a dense tree cover scenario. 

If ultimately proven effective and reliable, this technology may not suffice as a sole 

carbon monoxide (CO) mitigation technology because it does not protect against all 

generator use scenarios that pose a CO poisoning risk. To protect against CO poisoning 

resulting from outdoor operation near an opening into a home, as well as to minimize CO 

accumulation from a generator operating indoors before the GPS receiver/shutdown 

system can shut it off, additional technology would be needed to reduce the generator’s 

engine CO emission rate. As noted in Reference [1], page 5, deaths have occurred when 

generators were operating outside the home, near open windows, doors, or vents. CO 

emission reduction is CPSC staff’s primary strategy to reduce the generator CO 

poisoning hazard; and staff has demonstrated that current emission control technology 

adapted onto a prototype generator is effective in greatly reducing portable generator CO 

emissions in generator indoor use scenarios that are commonly fatal. The high CO 

emission rate of current generator designs can rapidly create an unsafe exposure in the 

space where the generator is operating, as well as in adjoining spaces. Implementation of 

a shutoff strategy, without requiring a reduced CO emission rate, can create a hazardous 

CO exposure if the generator runs for even a brief period of time before indoor operation 

is detected. Therefore, CPSC staff views a GPS receiver shutoff strategy as a 

supplemental means of further reducing the risk of CO poisoning [1]. 

The issue of dense tree cover requires further exploration by GPS technology experts. 

ESFS staff believes that the challenge posed by the scenarios in this study that involved

dense trees can be met as GPS receiver technology continues to evolve and become more 

sophisticated. However, ESFS staff recommends that the appropriate experts in the 

GPS/radio frequency (RF) engineering community explore the issue of dense tree cover 

or any additional situational limitations, as they influence this unique engineering 

challenge. 
TABLE OF CONTENTS

BACKGROUND ................................................................................................................1

THE CONCEPT.................................................................................................................2

IMPLEMENTATION .......................................................................................................3

The Candidate GPS Receiver Unit ...................................................................................3 

Preliminary Considerations and Experiments ..................................................................5 

Generator Shutdown System Demonstration Package .....................................................6 

Plan for Evaluation for the Demonstration Package ........................................................8 

RESULTS AND DISCUSSION ........................................................................................8

Preliminary Test Data .......................................................................................................8 

Test Data from Seven Indoor and Outdoor Environments ...............................................9 

Verification of the Shutdown System Functionality ......................................................11 

Testing at NIST Test House ...........................................................................................12 

POSSIBLE FUTURE PROTOTYPE .............................................................................15

CONCLUSION ................................................................................................................16

ACKNOWLEDGEMENTS ............................................................................................16 

REFERENCES .................................................................................................................17
1 

BACKGROUND

In support of the CPSC Portable Generator Rulemaking activity, ESFS staff explored the 

application of GPS technology to differentiate between indoor and outdoor portable 

generator operation. The goal was to determine if existing GPS receiver-based 

technology could be used to sense when a generator is operating in an enclosed location, 

and then, in combination with an electromechanical shutoff system, shut off the generator 

to prevent continued unsafe operation and mitigate the carbon monoxide (CO) hazard. 

The scope of portable generators examined for this study is limited to a class of internal 

combustion engine-driven generators rated at 15 kW (20 hp) or less, 250 V or less, which 

contain receptacle outlets for alternating current (AC) output circuits. Marine generators, 

recreational vehicle generators, and stationary generators are excluded from the scope 

[4]. Portable generators generally are equipped with wheels for ease of transport. The 

stated class of portable generators also comes in smaller sizes, where those generators are 

equipped with handles for hand carrying. 

This investigation was conducted to perform the following: (1) determine if there is 

utility in pursuing this GPS receiver strategy, and if initial field tests with a modified GPS 

system proved hopeful, (2) develop a prototype shutoff system with the ability to sense 

that a generator is running indoors and to shut the generator down. This exploratory 

exercise was not intended to demonstrate every possibility/scenario; it was strictly limited 

to documenting the concept’s utility. CO emission tests were not conducted; nor was 

there an attempt to analyze the time needed to shut down the generator. 

If ultimately proven effective and reliable, this technology may not suffice as a sole CO 

mitigation technology because it does not protect against all generator use scenarios that 

pose a CO poisoning risk. To protect against CO poisoning resulting from outdoor 

operation near an opening into a home, as well as to minimize CO accumulation from a 

generator operating indoors before the GPS receiver/shutdown system can shut it off, 

additional technology would be needed to reduce the generator’s engine CO emission

rate. As noted in Reference [1], page 5, deaths have occurred when generators were 

operating outside the home, near open windows, doors, or vents. CO emission reduction 

is CPSC staff’s primary strategy to reduce the generator CO poisoning hazard; and staff

has demonstrated that current emission control technology adapted onto a prototype 

generator is effective in greatly reducing portable generator CO emissions in generator 

indoor use scenarios that are commonly fatal. The high CO emission rate of current 

generator designs can rapidly create an unsafe exposure in the space where the generator 

is operating, as well as in adjoining spaces. Implementation of a shutoff strategy, without 

requiring a reduced CO emission rate, can create a hazardous CO exposure if the 

generator runs for even a brief period of time before indoor operation is detected. 

Therefore, CPSC staff views a GPS receiver shutoff strategy as a supplemental means of 

further reducing the risk of CO poisoning [1].

 
2 

CPSC staff has explored another shutoff system alternative. CPSC staff contracted work 

to the University of Alabama (UA) to develop a shutoff system onto a prototype 

generator using sensors such as intake air temperature and manifold air pressure and an 

algorithm to detect and shutdown the generator, as described in Reference [1], Tab G. 

The UA approach differs substantially from the GPS receiver approach. The GPS 

receiver approach, as discussed in this paper, presents another supplemental alternative to 

the UA shutoff strategy. 

THE CONCEPT

GPS receiver technology is ubiquitous, portable, and relatively inexpensive. Generally, 

GPS receiver units do not function properly indoors because the indoor structures 

attenuate the satellite signals the unit uses to identify its location. When this happens, 

some GPS receiver units activate a visual “Poor Satellite Reception” display to alert the 

user. ESFS staff hypothesized that it may be possible to exploit this feature and 

evaluated a commercially available GPS receiver unit in a variety of indoor and outdoor 

settings to investigate the capability to discriminate between indoor and outdoor 

operation. The degree of satellite signal attenuation will vary from receiver unit to 

receiver unit. 

ESFS staff anticipates that certain use scenarios will present challenges and that the GPSreceiver based concept will have limitations. Notwithstanding the above, ESFS staff 

believes that the GPS receiver technology may prove useful as an adjunct to CO emission 

reduction technologies to extend protection into enclosed spaces. Therefore, the use of 

GPS receiver technology merits investigation. 

The graphic below shows how GPS receiver technology can be integrated into a portable 

generator to detect indoor operation and shut down the generator. The concept is rather 

simple: once the GPS receiver cannot track the required number of satellites (from which 

one can infer that the generator may be operating indoors), an actuation circuit will 

disable the engine from running. Otherwise, the indoor-outdoor detection system 

concludes that the generator is running outdoors and the generator will run as intended. 
3 

Figure 1 - Concept of GPS-Based Generator Shutdown System

IMPLEMENTATION

The Candidate GPS Receiver Unit

The worldwide GPS uses a system of 24 satellites that orbit the earth twice a day sending 

data to earth. A GPS signal contains three components of information–a pseudorandom 

code, time data, and date data. These components of information identify the satellite, its 

current status, date, and time, as well as pinpoint where it is located at any given time. 

ESFS staff initially evaluated two GPS receiver units that were commercially available. 

ESFS staff compared the relative strengths of a 2005 GPS receiver and a 1998 receiver 

selected by ESFS staff for the exploratory exercise. The 2005 GPS unit, without an 

external antenna, was readily able to receive clear reception in every room of a detached, 

single family home. ESFS staff acquired a 1998 model GPS receiver equipped with an 

external antenna that could be modified to obtain the optimal “balance” to discriminate 

between indoor and outdoor environments. 

The 1998 GPS receiver used in this study must continuously receive signals from at least 

three satellites to calculate the GPS receiver’s location and track the receiver’s movement 

[2]. At times, additional satellites may be needed to determine its location. The GPS 

receiver can only process radio signals from satellites that are above the horizon and 
4 

provide adequate radio signal strength to the GPS receiver’s antenna. Once the GPS 

receiver is initialized to a location, it will compute a position in a few minutes. Shortly 

after the 1998 GPS receiver is turned on, a flashing triangle with a question mark first 

appears, indicating that it is locating the required satellites to determine its location. 

After several minutes, the GPS receiver will either display its location, complete with 

longitudinal, latitudinal, and elevation coordinates, or display an error message, 

depending on whether the GPS receiver unit has adequate reception. 

The GPS receiver requires an unobstructed view of the sky for optimal performance 

because GPS satellite signals are generally weak and cannot travel through rock 

materials, metal, or heavy tree cover. If the GPS receiver does not detect the required 

number of satellite signals, it will display the error message, “Poor Satellite Reception” 

[2].

To improve the receiver’s ability to discriminate between indoor and outdoor locations, it 

was necessary to make the receiver less sensitive to satellite reception. ESFS staff chose 

to use the receiver intact as a “black box,” rather than attempt to modify the proprietary 

internal components of the 1998 GPS receiver unit. Using the above approach greatly 

simplified concept evaluation. For example, if a new method or adjustment was made to 

attenuate the GPS receiver signal by partially shielding the antenna, the only output for 

evaluation was either the presence or absence of an error signal on the GPS receiver’s 

screen displaying “Poor Satellite Reception.” 

Various signal attenuation approaches using metal shields on the receiver’s antenna were

evaluated: mesh steel wire cage, aluminum foil, and copper tubing. The above three 

materials are known for attenuating radio frequency signals. A fixed length of copper 

tubing gave promising results during the initial screening evaluations and was chosen as a 

shield. 

As mentioned in the background section, there may be instances where the location, 

nearby structures, and terrain, for example, may impede the reception of satellite radio 

frequency signals. To understand some of these effects, ESFS staff evaluated the 1998 

GPS receiver unit in various homes and surrounding spaces, such as the front yard and 

back yard. 

According to the owner’s manual [2], the 1998 GPS receiver unit determines in 

approximately 5 minutes whether the required number of satellites is not detected. If the 

appropriate number of satellites is not detected, the unit displays a “Poor Satellite 

Reception” message. The decision time of 5 minutes is particular to this 1998 receiver,

and a shutoff system could be designed to make the decision more quickly. However, it 

would be at the risk of shutting the generator down while it is properly being operated 

outdoors. Deaths have occurred where generators were operated outdoors, near the 

home, near open windows, doors, or vents [1]. This tradeoff would have to be carefully 

considered because the utility of a generator and, possibly, acceptance of the GPS 

receiver controlled shutoff system by the consumer could be negatively impacted by a 

system that shuts the generator off when used properly outdoors. 
5 

A section of copper tube was used to reduce the sensitivity of the 1998 GPS receiver 

antenna strength to obtain the optimum balance of “strong enough to detect outdoors” 

and “weak enough not to detect indoors.” While the copper tube was a somewhat better 

alternative to aluminum foil or mesh steel wire cage, the copper tubing had limitations. 

ESFS staff tried several lengths of tubing to fine-tune the amount of attenuation. Various 

evaluations were done using 17 mm and 15 mm long copper tubes. As noted previously, 

the only output for evaluation was either the presence or absence of an error signal on the 

GPS receiver’s screen displaying “Poor Satellite Reception.” After conducting many 

trials in various indoor and outdoor environments, such as an ESFS staff’s house and 

parks with heavy tree cover, it appeared that the 15 mm copper tube did not provide 

sufficient shielding. Ultimately, ESFS used the 17 mm copper tube. Further fine-tuning 

was attempted by positioning the 17 mm copper tube on different locations of the antenna 

(held in place with a set screw). After several trials (approximately five to six different 

positions on the antenna), the optimum position was obtained. Figure 2 shows the GPS 

receiver with the 17 mm copper tube in its optimum position. The GPS receiver shows 

an error message indicating “Poor Satellite Reception”, as illustrated in the right-side 

photograph of Figure 2 when the GPS receiver cannot track the required number of 

satellites. The change in luminosity (from light to dark) in the area inside the “optical 

sensor target” box (illustrated in Figure 2) is what is detected by the optical sensor to 

trigger the shutdown function. 

Figure 2 - GPS Receiver with External Antenna and Copper Tube for Signal Attenuation; 

(Left): LCD Screen Does Not Show Error Message in this view 

(Right): LCD Screen Shows Error Message

Preliminary Considerations and Experiments 

Stray electromagnetic interference (EMI) can disrupt the proper operation of the GPSbased shutoff circuit. Current GPS designs effectively shield interference from many 

sources, such as radio and television antennae, garage door operators, and more. One 

source that may pose a particular challenge for a GPS receiver unit mounted onto a 

generator is the EMI that is radiated from the alternator in the generator. A few limited 

evaluations were conducted to gauge if such sources could have an adverse effect 

(interference) on the GPS receiver’s ability to sense indoor versus outdoor environments. 
6 

ESFS staff deemed that interference did not exist if the “Poor Satellite Reception” error 

message did not display on the LCD screen when positioned near the alternator of an 

operating engine. Evaluating for interference was done by tests with the GPS receiver 

inches away from a parked car alternator and an unloaded1 portable generator alternator. 

There did not appear to be interference in these two trials. However, it is unknown 

whether trials with a generator supplying an electrical load (a configuration that was not 

tested in this study) may give different results. A third trial consisted of placing a 

television antenna with a booster amplifier a few inches from the 1998 GPS receiver; 

again, no interference was observed. Although adverse EMI effects were not observed 

with this limited testing, further investigation may be needed to determine whether these 

effects will ever pose a problem. 

Generator Shutdown System Demonstration Package

The demonstration system consists of three main components: the GPS receiver, the 

photo sensor activation circuit, and a portable generator. The generator, which was used 

in emissions testing for the CPSC staff study in reference [1], is hereafter referred to as 

“SO1” (short for shutoff generator 1). While the GPS receiver determined indoor or 

outdoor status, an actuation system was developed by Engineering Sciences, Division of 

Electrical Engineering (ESEE) staff that would interface the generator to the GPS 

receiver. A circuit that would recognize a change in state from normal GPS receiver 

operation (with satellite reception) to poor satellite reception was developed. This circuit 

was not integrated into the actual GPS receiver unit; rather a separate external circuit box 

was constructed. An error message that appears on the LCD screen was the trigger 

source. A relatively simple circuit was employed to sense the “Poor Satellite Reception” 

error signal on the GPS receiver’s screen and deactivate the generator as shown 

schematically in Figure 3. Figure 4 shows the fully assembled physical circuit layout. 

Figure 3 - Schematic Diagram for Photo Sensor Activation Circuit for Shutdown 

System

 

1

 An electrical receptacle panel was not connected to the generator, so the generator was not electrically 

loaded, although the alternator was being energized.
7 

The SO1 prototype generator already had a remote engine shutoff switch installed, and it 

was activated by a remote controller. This feature was used quite often for the testing 

conducted at the National Institute for Standards and Technology (NIST) [1] to shut off 

the generator safely using the remote control feature. ESEE staff decided to make use of 

this existing generator shutoff feature. Figure 5 shows the components that comprise the 

shutdown system: modified GPS receiver, actuation circuit box, and remote controller.

The photo resistor, Rp, senses the change in the brightness of the LCD panel when the 

“Poor Satellite Reception” message is displayed. This signal is sent to the operation 

amplifier (triangular shaped symbol in Figure 3) turning on transistor Q1, which, in turn, 

activates relay K1. The relay contacts of K1 were connected to the “lock” button on the 

remote control, and when activated, the remote control sends a signal to shut off the 

generator’s engine. Shutting off the generator is achieved by grounding the engine’s 

spark plug. 

Figure 4 - Top View of Photo Sensor Activation Circuit

Figure 5 - Components of the Shutdown System, from 

Left to Right: Remote Controller, Photo Sensor Activation 

Circuit Box, and GPS Receiver
8 

Plan for Evaluation of the Demonstration Package

As mentioned in the background section, the GPS receiver is not immune to external 

influences. Varying tree cover is a potential issue. ESFS staff investigated to what 

degree trees may affect the GPS reception of the particular 1998 GPS receiver used in 

this exploratory exercise. Originally, ESFS planned to test the demonstration system

(generator plus shutdown system) in many different indoor and outdoor locations, such as

homes, buildings, front yards, and back yards. However, since generator shutdown 

system depends upon the generation of the “Poor Satellite Reception” message on the 

GPS receiver’s LCD screen, only the GPS receiver was used for these evaluations. 

As presented in the results and discussion section, a variety of home types, with and 

without garages, and with and without basements, were evaluated. 

RESULTS AND DISCUSSION

The data and discussions are presented in the chronological order that the evaluations 

were conducted. The 1998 modified GPS receiver is hereafter referred to as GPS 

receiver.

Preliminary Test Data

To test the feasibility of using the GPS receiver unit, ESFS staff initially evaluated the 

effectiveness of the GPS’s ability to differentiate indoor and outdoor environments. 

During these initial steps, the exact locations of the GPS receiver unit (e.g., middle of the 

room, near a window) were not noted. While these exact locations are important, ESFS 

staff conducted these limited, estimated experiments to determine if the GPS’s ability to 

differentiate indoor and outdoor environments would work. Table 1 below shows the 

results.

The evaluations, as indicated in Table 1, led to further investigation of the feasibility of 

using GPS receiver technology for differentiating indoor and outdoor environments. 

The data shown in Table 1 did not specify particular locations in the various rooms, but 

rather, they were merely collected to determine if further study should commence.

Therefore, a detailed evaluation was conducted on more homes, and the GPS receiver

was placed in a variety of specific locations (see Tables 2 and 3). The office building 

indicated in Table 1 is CPSC’s National Product Testing and Evaluation Center 

(NPTEC), 5 Research Place, Rockville, Maryland 20850. 
9 

Table 1 – Preliminary Data from Two Homes and a CPSC Office Building

Location Room 

Does the GPS Receiver Get 

Satellite Reception Using the 

Modified Antenna?

Single Family 

Home with 

2-Car Garage

office room NO

2-car garage, near garage door NO

2-car garage, near back wall NO

Basement NO

back yard deck YES

front yard YES

Single Family 

Home with 

1-Car Garage

living room NO

1-car garage NO

2nd floor bedroom NO

sunroom NO

back yard deck YES

front yard YES

CPSC Office

Building 

(NPTEC)

cubicle NO

parking lot YES

Test Data from Seven Indoor and Outdoor Environments 

Seven homes of CPSC Engineering Sciences and Laboratory Sciences personnel were 

evaluated to help characterize the reception qualities of the selected GPS receiver. There 

were a variety of home types, with all but one having garages of various types and one 

home was a townhome. Homes with garages were of particular interest, as some 

fatalities occurred when the generators were run in the garage and CO infiltrated into the 

homes [1]. It should be noted that this list of homes is not intended to represent any 

particular cross-section of homes in the United States; rather, it is a limited list of homes 

that were available at the time of this study.
10 

Table 2 – Detailed Evaluation of the Modified 1998 GPS Receiver in Seven Homes

Home Indoor 

Detection

Front Yard 

(Outdoor) 

Detection

Back Yard 

(Outdoor) 

Detection

Single family detached home with 2-car 

garage (HOME A) YES YES YES

Townhouse with 1-car garage (HOME B) YES YES YES

Single family detached home with 4-car 

garage (HOME C) YES NO* YES

Single family detached home with 1-car 

garage (HOME D) YES YES YES

Single family, one level detached home 

with detached garage (2-car garage) 

(HOME E)

YES YES YES

Single family detached home with 1-car 

garage (HOME F) YES YES NO*

Single family, detached one-level home, no 

garage (HOME G) YES NO* NO*

* GPS receiver failed to properly detect outdoor status, possibly due to dense tree canopies. 

However, the following observations were made: 

HOME C detected outdoor status in the back yard 18 ft from the sunroom 

HOME F detected outdoor status 9 ft from the front yard stoop 

HOME G detected outdoor status 27 ft from front yard stoop [side walk], which is not 

considered part of homeowner’s property 

Additional notes: 

One-level homes were indicated as such. Other homes listed either had a full second floor or 

split-level foyer type of second level.

Table 3 – Specific Areas of the Seven Homes That Were Evaluated

Indoor Rooms Tested: garage, basement, bedrooms, living room, 

kitchen, sunroom/enclosed porch, dining 

room – all near windows to maximize the 

opportunity for the GPS receiver to obtain

satellite signals

Outdoor Areas Tested: decks, open grassy fields, varying distances 

from the front yard stoop

For indoor detection, a “YES” indicates that the GPS receiver generated a “Poor Satellite 

Reception” error message, which means that the GPS receiver correctly sensed indoor 

status. For outdoor detection, a “YES” indicates that the GPS receiver was able to obtain 

its location coordinates and did not generate an error signal, which means the GPS 

receiver correctly detected outdoor status. For outdoor detection, a “NO” indicates that 

the “Poor Satellite Reception” error signal appeared which means the GPS receiver 

incorrectly detected indoor status. As the data tables show, the GPS receiver unit did not

have any problems detecting indoor status. However, the 1998 modified GPS receiver 

had difficulty detecting outdoor areas in three homes with dense tree cover. See Figure 6. 
11 

In contrast, homes with fewer trees did not exhibit problems obtaining a clear satellite 

signal outdoors. See Figure 7. 

 Figure 6 – Three homes that had difficulty receiving a satellite signal outdoors; left 

(Single family, detached one-level home, no garage); middle (4-car garage home–only 

had difficulty in the front yard); and right (1-car garage home–only had difficulty in the 

back yard). 

 Figure 7 – Three homes that did not have difficulty receiving a satellite signal outdoors;

left (townhouse); middle (1-car garage home); and right (2-car garage home). 

ESFS staff observed that if the GPS receiver was located where a roof-type structure was 

over it, even a clear, tempered glass sunroom (of Home C) provided sufficient shielding 

to detect that the GPS receiver was located indoors. One caveat to this observation is that 

it is not clear whether the indoor detection was due solely to the glass ceiling, or if 

surrounding trees or other obstructions played a role.

Verification of the Shutdown System Functionality

ESFS and ESEE staff verified the physical functionality of the shutdown system after 

installing the GPS receiver-photo sensor activation shutdown system onto the SO1 

generator. This verification was independent of the indoor and outdoor GPS receiver 

evaluations. Using the GPS receiver-based shutdown circuit, ESFS and ESEE staff 

demonstrated successful shutdown of a generator within 2 minutes after having started 

the engine. This demonstration was conducted in the all-terrain vehicle (ATV) laboratory 

at NPTEC. All safety checks were conducted prior to running the verification, such as 

securing the generator to the floor and connecting the engine exhaust pipe to the exhaust 

snorkel. Figure 8 shows the generator fitted with the shutdown system. The successful 

shutdown system operation was captured on video. 
12 

Figure 8 - Shutdown System Fitted onto SO1 Generator at NPTEC

Testing at NIST Test Home

While evaluations of the seven houses provide a rough sketch of how the GPS receiver 

performed in those scenarios, ESFS staff also evaluated the GPS receiver at the National 

Institute for Standards and Technology (NIST) test home (Building 423 - NIST Indoor 

Air Quality House, see Figures 9 and 10). This building was designed for testing indoor 

air quality of a single story home. This test home is furnished like a residential home 

complete with bedrooms, bathrooms, fireplace, window curtains, kitchen appliances, such 

as a range, and the building has been used for a multitude of indoor air quality related 

projects, including recent emission testing programs for the Prototype Generator testing

[1]. 
13 

Figure 9 - Floor Plan of NIST Indoor Air Quality Building 423 

Figure 10 – Aerial View of NIST Building 423 
14 

Table 4 – GPS Receiver Evaluations Conducted at NIST Indoor Air Quality House 

Building 423, July 23, 2012 

INDOORS

Room Approximate Times (MM:SS) for “Poor 

Satellite Reception” Error Message to 

Display

Location K* – Middle of the Garage 5:13

Location J** – Window of the Garage 1:48 // 1:52 // 1:48 // 5:05 // 1:56

Family Room 1:42

Kitchen 1:09

Dining Room 1:34

Living Room 1:55

Master Bedroom 1:54

Bedroom 2 1:30

Bedroom 3 5:30

OUTDOORS

Location Approximate Times (MM:SS) for 

Indicating GPS Acquired All Necessary 

Satellites 

Location A* – Grass field 15 ft from window 1:58

Location B* – Grass field 15 ft from window 3:09

Location L* – Grass field 15 ft from window 3:12

Location M* – Grass field 15 ft from 

window

1:42

NOTES:

* Refer to the specific lettered locations in Figure 9

** Since the garage area was of interest and with time permitting at the NIST facility, five trials were 

conducted at Location J

The results in Table 4 show that the GPS receiver unit was capable of distinguishing 

indoor from outdoor environments. The “indoor” times in Table 4 represent the time 

from the moment the GPS receiver unit was powered on, to when the “Poor Satellite 

Reception” error message appeared. The “outdoor” times represent the time from the 

moment the GPS unit is powered on, to indicating that the GPS receiver has acquired the 

minimum number of satellites for proper operation. The GPS receiver then displays its 

exact location, complete with longitude, latitude, and elevation coordinates. When the 

GPS receiver unit is powered on, a triangle with a question mark flashes continuously 

until either a location is determined or an error occurs (“Poor Satellite Reception” 

message appears). One caveat is that Building 423 had only a few surrounding trees. 

The perimeter of the building has grassy fields and open spaces. 

ESFS staff observed that, on occasion, the GPS required somewhat longer times to detect 

the indoor environment status.2 This observation regarding longer times is believed to be 

 

2 The times were approximate, as the staff’s reaction times to the start and stop events were performed with 

an electronic stopwatch. 
15 

due to the complicated attenuation of the GPS radio signal from the satellites because of

the interaction of varying terrain, tree cover, building materials, building structure, and 

location within the building, among other variables. The above-mentioned factors make 

GPS shutdown time challenging to predict. 

It is necessary to keep in mind that the GPS receiver was treated like a “black box,” and 

staff was only monitoring the output (error “Poor Satellite Reception” signal or no error 

signal). Also, the selected GPS receiver unit was manufactured in the 1998 timeframe, 

and there have been many improvements and refinements in GPS circuitry and software 

since then. Additionally, the shutdown times are somewhat irrelevant because GPS 

technology is evolving. These times only reflect how this particular GPS receiver 

performed, and they are not necessarily indicative of how a future GPS receiver, designed 

specifically for the generator shutdown feature, could function. However, it was more 

important to discover whether the GPS receiver was capable of correctly identifying 

whether it was operating indoors or outdoors. 

The demonstration supplements the work performed on the reduced-CO generator by 

University of Alabama (UA), and it is not in place of the UA work. ESFS staff 

recommends that GPS technology experts and generator manufacturers creatively explore 

the feasibility of possible solutions (on the indoor-outdoor detection aspect). 

POSSIBLE FUTURE PROTOTYPE

ESFS staff envisions an efficient “black box” that uses GPS receiver technology with a 

specifically designed antenna (internal or external) to detect indoor or outdoor status 

only. This simple detection scenario means that there is no need for populating maps, 

generating complex algorithms to log routes, or any of the other auxiliary computationdemanding tasks found in common consumer GPS receivers, thereby, minimizing 

demands on the chip processing power, circuit complexity, and costs. These design 

parameters would enable the ease of a GPS receiver-based shutdown solution for 

integration into the generator. 

A true production model would have a fully integrated design. GPS receiver circuitry 

could be housed safely in a weather-resistant enclosure, with only the antenna exposed; 

or, the manufacturer could opt to design an internal antenna. It should also be designed 

to be tamper-resistant so that a consumer is unlikely to defeat this safety feature. This 

safety feature could be used in conjunction with mandatory labeling/packaging to the 

effect: “This product will not operate indoors.” This feature may actually help consumers 

heed the warning: “do not use indoors.” Depending on how the power configuration is 

designed, the GPS receiver unit could be powered by a battery or super capacitor that is 

internal to, and charged by, the alternator. The options are limitless, but quite achievable, 

given the ubiquitous nature of GPS receiver technology found in everyday electronics, 

such as cellular phones and tablet computers. 

Looking ahead, CPSC staff envisions a robust test method that will ensure the shutoff 

system performance has the following characteristics: (a) produce repeatable 
16 

performance in the variety of indoor environments that generators have been reported to 

be operated in; (b) tamperproof (the generator will not start if the shut off system has 

been removed or bypassed); (c) durable after prolonged exposure to a generator’s typical 

storage and operating environments; and (d) features a fail-safe mechanism (prevents the 

generator from starting) when the shutoff system is not functioning for some reason or its 

battery/supercapacitor is discharged or does not sustain charge. 

CONCLUSION

ESFS staff has conducted a limited, exploratory exercise to demonstrate the feasibility of 

using GPS receiver technology to distinguish indoor and outdoor environments, with the 

intent of shutting down portable generator operation when used indoors. ESFS staff 

demonstrated that a modified off-the-shelf, commercially available GPS receiver unit can 

distinguish indoor and outdoor environments, with some exceptions where there were 

dense tree cover scenarios. The issue of dense tree cover requires further exploration by 

GPS technology experts. The above scenarios can be addressed as the GPS receiver

technology continues to evolve and become more sophisticated. 

ACKNOWLEDGEMENTS

Don Switzer (ESFS) provided oversight and his breadth of knowledge to help facilitate 

this exploratory exercise. Mark Gill (ESEE) developed the photo sensor activation 

circuit system and was instrumental in demonstrating the concept of this exploratory 

exercise.

The author thanks the numerous staff who have contributed and provided support: 

CPSC Staff:

Patricia Adair, Director for Division of Combustion and Fire Sciences (ESFS) 

Janet Buyer, ESFS, Project Manager for Portable Generators 

Susan Bathalon, Directorate for Hazard Identification and Reduction (EXHR)

Brian Baker, Laboratory Sciences staff for his assistance in the ATV laboratory

Stephen Patterson, Laboratory Sciences staff for fabricating the GPS receiver antenna 

modification components 

Andrew Trotta, Director for Division of Electrical Engineering (ESEE)

CPSC staff who have volunteered their homes for the various GPS experiments 

NIST Staff: 

Steve Emmerich (Energy and Environment Division, Engineering Laboratory) 

Dan Greb (Energy and Environment Division, Engineering Laboratory) 
17 

REFERENCES

1. Buyer, J et al. “Technology demonstration of a Prototype Low Carbon Monoxide 

Emission Portable Generator,” August 2012. Source: 

http://www.cpsc.gov/library/foia/foia12/os/portgen.pdf

2. [brand name removed] Corporation, “GPS Receiver Unit Owner’s Manual and 

Reference Guide,” May 1998.

3. Marks, L, “Standard Handbook for Mechanical Engineers Tenth Edition,” 1996 

McGraw-Hill Companies. 

4. Underwriters Laboratories Inc. “UL 2201 – Standard for Safety – Portable EngineGenerator Assemblies,” March 4, 2009.