The Fox Vehicle is a six-wheeled, light armored, NBC Reconnaissance Vehicle. On-board NBC detection capabilities include the MM-1 Mobile Mass Spectrometer, which is the primary detection device, the M43A1 Chemical Agent Detector, the M256 Series Chemical Agent Detector Kit, the AN/VDR2 radiation detector, and the ASG1 radiation detector. [6] The Fox does not provide any biological detection capability, but does protect the crew from biological hazards, and allows the crew to mark areas of potential hazard and safely take samples for laboratories to analyze for biological hazards. With these capabilities, the Fox vehicle was used according to the context of each military operation. Tactics associated with each type of operation often restricted the operation of the Fox vehicle and reduced its capability to detect chemical hazards. For instance, troops performing offensive operations need to move quickly to exploit the momentum of the assault and reduce troop and equipment losses from enemy fire.

There are also several considerations about the Fox vehicle that should be understood before drawing any conclusions about chemical detections or alarms reported during Operation Desert Storm. First, the Fox was designed as a reconnaissance system, with a primary function to detect, identify, and mark persistent ground contaminated areas. Although it can detect chemical warfare agent vapors, the basic Fox with its MM-1 mass spectrometer is not optimized for this purpose and is significantly less effective than existing chemical vapor detectors (such as the M43A1). For this reason the M43A1 was added to the Fox vehicle. Second, the Fox has a two-step alert and confirmation process.[7] It makes an initial quick scan for possible chemical presence to provide maximum warning to troops. This may cause false alarms. The second step is a more time-consuming analysis that can more precisely identify what chemicals are present. Third, the Fox cannot determine the specific concentration of a chemical agent. It has a mass spectrometer (the MM-1) that can identify what chemicals are present, but not how much is present. Fourth, recording many MM-1 actions and results (such as the outcome of a spectrum) on a Fox tape requires additional, time-consuming steps on the part of the operator.


Figure 2. Fox Reconnaissance Vehicle


The Fox can conduct NBC reconnaissance and chemical agent detection on the move. It can keep up with maneuver forces at a pace of 30-40 km/h using several methods of operation. This allows it to cover large areas. The Fox provides both "real time" alerting and detailed confirmation of chemical agents during offensive and defensive maneuver operations. The Fox crew is protected from outside contamination by pressurizing and sealing the vehicle. This allows the crew to conduct NBC reconnaissance, retrieve and retain samples, and mark contamination boundaries without leaving the vehicle or wearing chemical protective equipment. The on-board air conditioner increases crew comfort and keeps electronic equipment from overheating. The heart of the Fox vehicle detection system is the MM-1 Mobile Mass Spectrometer which can detect and identify chemical agents that have been preprogrammed into its library. The chemicals programmed into the library are chosen based on the suspected chemical threat.


The MM-1 Mobile Mass Spectrometer

The MM-1 Mobile Mass Spectrometer is a tool used to analyze chemical compounds. All chemical compounds are made up of small pieces called "molecules." A mass spectrometer excites each molecule, breaking it into smaller charged particles called "ions," and then counts each ion in a sample. These ions are sorted by their atomic weights, providing a unique signature for each chemical substance. The MM-1 graphically displays the relative intensities of selected ion patterns to the operator’s screen. A spectrum is a listing of the relative intensity of each ion the mass spectrometer counted for the molecules in the sample. Additionally, this information can be printed to a hard copy tape for later, more detailed analysis and a record of the detection. Since a spectrum for each chemical taken under the same conditions is unique, using a mass spectrum to identify a chemical substance is similar to identifying a person by using a fingerprint.

Because the MM-1 can detect only relative intensities and not concentrations or amounts, it requires a baseline spectrum of air taken in an uncontaminated area. This baseline spectrum, called a background, is taken upon starting up the equipment, whenever a change in methods occurs and periodically while in use. The minimum detectable amount for each ion mass is calculated from the background. [8] All subsequent readings the MM-1 makes are compared to that background.


Taking a Sample

When a substance contacts the sampling port, as shown in Figure 3, the sampling port heats it until the substance vaporizes. Because many different chemical compounds may be in the vaporized sample, it is important to separate them so they can be identified. As the vaporized sample travels through the sampling probe, it separates due to temperature and because lighter molecules travel faster than heavier molecules. The MM-1 can operate at two different temperatures: the Hi temperature of 180 C, and the Lo temperature of 120 C. When the probe is hot (Hi temperature), all the molecules travel fast and there is less separation. If the probe is less hot (Lo temperature), the molecules travel slower and there is more separation. After the vaporized sample molecules are separated traveling up the probe, they enter the MM-1 where they are broken into smaller charged pieces called ions, which the MM-1 uses to identify the substances in the sample and the relative-intensity of each substance. It is important to note that when the MM-1 takes a spectrum it analyzes only the substance with the highest relative intensity, even if several substances are present.[9]


Figure 3. Close-up view of the back of the Fox Vehicle


The MM-1 can operate in two modes, each with multiple methods, but US troops were only trained in the Air Monitor Mode of operation. [10] Consequently, this paper only addresses the Air Monitor surveillance mode. Three methods of detection were used to search for chemicals during Operation Desert Storm. These methods were Wheel/Hi, Air/Hi, and Surface/Lo. Table 1 shows the temperature and sampling probe position for each of the methods.


Table 1. Methods

Mode Method Probe Temperature Wheels Used Probe Position
Air Wheel/Hi Hi 180 C Yes 2-3 feet from ground
Monitor Air/Hi Hi 180 C No 2-3 feet from ground
  Surface/Lo Lo 120 C No 2-4 inches from ground


The Wheel/Hi Method

The Wheel/Hi method is designed to alert the crew to the possible presence of a liquid chemical warfare agent. The Wheel/Hi method uses two sampling wheels which trail behind the Fox to pick up liquid chemical samples from the ground. The wheels lift alternately to the probe’s sampling port where the liquid present on the wheels is vaporized by the heat of the sampling port. During Operation Desert Storm, the wheels did not lift automatically; a Fox crew member had to manually push a switch each time a wheel needed lifting to take a sample. If the wheels were not lifted, the probe still sampled the surrounding air, which was effectively the Air/Hi method.


The Air/Hi Method

The Air/Hi method does not use the sampling wheels but is otherwise similar to the Wheel/Hi method. The Air/Hi method can detect only chemical vapors in the surrounding area. According to the chemical engineers at the US Army’s Chemical and Biological Defense Command, Edgewood, Maryland, (experts in the system performance and capabilities), the Fox is not well suited for generalized vapor detection, because the air volume drawn through the sampling probe is approximately 300-400 times less than the air volume drawn through other detectors such as the M43A1. The result is that the MM-1 is approximately 500 times less sensitive to nerve agent vapors than the M43A1[11] . Consequently, when the Fox was modified for American use, the M43A1[12] was added as a vapor detector. Table 2 shows the vapor sensitivities of the MM-1 as compared to other vapor detectors.


Table 2. Vapor Chemical Agent Detector Characteristics[13]

Item Agents Sensitivity Response Time
M8A1 Alarm G, V Nerve 0.1-0.2 mg/m3 <=2 min
M256A1 Kit G







0.005 mg/m3

0.02 mg/m3

2 mg/m3

9 mg/m3

3 mg/m3

8 mg/m3

9 mg/m3

15 min

15 min

15 min

15 min

15 min

15 min

25 min

CAM GA, GB, VX, HD, HN <= 0.1 mg/m3 <=1 min
MM-1[14] GB[15]



62 mg/m3

46 mg/m3

115 mg/m3

<=45 sec


The Surface/Lo Method

The Surface/Lo method uses the lower operating temperature (120 C) allowing the maximum amount of separation among multiple chemical compounds. Surface/Lo is the recommended method to take a spectrum (but a spectrum could be taken from any method). After the MM-1 alerts for a chemical agent, the normal operating procedure is for the operator to stop the vehicle, change the method to Surface/Lo, and wait for the probe to cool from 180 to 120 C. The MM-1 requires at least three minutes to cool the probe temperature from Hi to Lo. The operator then lowers the sampling probe until it is approximately 2-4 inches from the suspected contamination, takes another sample, and performs a spectrum. All of this takes time, and in several operational scenarios such as the Marine breaching operations, the confirmation procedure (taking a spectrum) could not be performed without interfering with the accomplishment of the primary mission. Therefore, confirmation was not done.


The Initial Search For Chemical Agents

Regardless of the method being used, a quick response time is paramount to the safety of troops involved in military operations. In order to provide the response time necessary for military operations, the MM-1 continuously monitors against a target list[16] of approximately 10 selected chemical agents most likely to be present, based on intelligence reports and the suspected chemical threat. The 10 chemicals usually on the target list were:

  • TABUN (GA)
  • SARIN (GB)
  • SOMAN (GD)
  • VX (VX)
  • To speed the initial search, the MM-1 looks for only four ion peaks for each chemical and attempts to match the pattern and ratio of these peaks against the target list of chemicals. If an initial match is made with these four ion peaks, the MM-1 sounds an alarm. However, this first alarm does not confirm the presence of a chemical agent since there are many chemical "interferents" that have similar ion peaks and many combinations of chemicals that may yield ion patterns similar to those in the target list. Consequently the MM-1 can falsely indicate the presence of dangerous chemicals . For example, the four ion peaks used to initially alert for the nerve agent Sarin and the riot control agent CS are similar. Additionally, Sarin has an ion peak at 125.0 molecular weight (m.w.) and a relative intensity of 25.0%, while the riot agent CS has an ion peak at 126.0 m.w. with a relative intensity of 18.7%. Because this peak in particular is so similar, the MM-1 may initially alert for Sarin when the actual chemical is CS resulting in a "false positive" for Sarin.

    A "false positive" is an initial alert for a dangerous chemical that is not present. To positively determine what chemical is present, the MM-1 operator must run a spectrum to analyze all the ions present, not just the four used in the initial alert. The spectrum of the suspected chemical is compared to all the detection algorithms stored in the MM-1 chemical library. If a match is found, the MM-1 confirms the initial alert. If a match is not found, the MM-1 displays "unknown." For later analysis and a permanent record of the alarm, the complete ion spectrum by atomic weight can be printed on the Fox tape; however, this is a manual function that the operator must perform and is not an automatic feature of the system.


    Minimizing Alarm Errors

    Since not alerting to a chemical agent seriously jeopardizes the safety of unprotected troops, the Fox has been specifically designed to ensure an alert occurs if a substance is present at the expense of generating potential false alarms. This ensures maximum warning time and safety. However, so the Fox is not continuously alerting to a variety of substances, there are several design considerations to minimize the "false positives." The MM-1 uses mathematical algorithms to reduce "false positives" while assuring an alarm is generated if a chemical warfare agent is present. The algorithms depend on three compound-specific values to separate genuine alarms from alarm errors. These values are the Interference, Reliability, and Impossible Ion parameters, and they may be uniquely set for each compound in the library. An example of these values is shown in Table 3; a complete list is provided at Tab C.


    Table 3. Examples of Parameter Values


    Interference Parameter

    Reliability Parameter

    Impossible Ion

    by Molecular Weight












    109 m.w.




    97 m.w.










    In general, the function of the interference parameter is to suppress alarms when large amounts of interfering substances are present. Larger values mean a higher amount of an interfering compound is required to suppress an alarm. The scale is logarithmic so a chemical warfare agent with an interference parameter of 1.0 would require the interfering compound to have only ten times the amount before the alarm would be suppressed. The alarm for a chemical warfare agent with an interference parameter of 8.0 (like Sarin) would only be suppressed if the presence of the interfering agent were 100,000,000 (108.0) greater. By properly setting the interference parameter, one can assure that the MM-1 alarms for the presence of a small amount of chemical warfare agent in the presence of a large amount of other compounds. The value of the interference parameter is preprogrammed into the MM-1 and is determined by experience and testing.

    Table 4 provides examples of chemicals whose ion patterns in certain conditions are known to resemble those of chemical warfare agents. The Sarin-CS similarity was mentioned earlier. In subsequent Fox vehicle testing after the war,[17] it was determined that the silicone material in the Fox sampling wheels and silicone lubricants on the wheels would emit certain ions when raised to the heated sampling probe. These ions could confuse the MM-1, causing an initial alert for the chemical warfare agent Lewisite. A detailed spectrum analysis would indicate that the alarm is in fact a "false positive" by displaying "unknown" to the MM-1 operator. Although this was discovered after the Gulf War, it is relevant to the Fox vehicle configuration during the war and is a factor in explaining several of the alerts to the chemical agent Lewisite, which were never confirmed. Benzyl Bromide (a tear-producer and skin irritant) was not routinely monitored by the MM-1 but was in the Fox Chemical Library and could be identified by spectrum analysis. The ions used to identify Benzyl Bromide are also found in Toluene (a common solvent) and Cyclopentadiene (an insecticide).



    Table 4. Examples of Interfering Agents[18]

    Chemical Warfare Agent

    Interfering Agent




    Silicone Plasticizers[20]

    Benzyl Bromide[21]

    Toluene[22] (solvent) and Cyclopentadiene (insecticide)


    The reliability parameter allows a range of variation among the four initial ion intensities monitored and is predetermined to give greater latitude to detect a chemical warfare agent. Determination of this parameter is a tradeoff between detection of an actual agent and generating a "false positive". The higher the setting, the more likely a "false positive" could occur.[23] This parameter was programmed into the MM-1 and pre-set by technicians prior to Operation Desert Storm, based upon the suspected chemical threat.

    An impossible ion is an ion that is NOT present in a dangerous compound, but is present in another compound with similar peaks. If the MM-1 detects the impossible ion, it can determine the suspected chemical is not the dangerous compound being sought. For example the four peaks monitored for Phosgene are 65.0, 63.0, 98.0, and 109.0. The mass 109.0 is an impossible ion for Phosgene and is set with a relative intensity of 0.0%. If the MM-1 detects the mass 109.0 at any relative intensity other than 0.0%, the MM-1 would know the chemical could not be Phosgene, and it would not alert to Phosgene. [24]

    Given the manner in which the MM-1 initially alerts for chemical agents and the parameter settings used to prevent alarm errors, it is possible to understand how the MM-1 could initially alert for a dangerous chemical when only a less hazardous substance is present. In a multi-chemical environment[25] such as a battlefield, the MM-1 must compare the ions encountered with the ion patterns of the chemical warfare agents on the target list. Because the percentage of each ion in a sample may vary slightly, the MM-1 allows for variation (plus or minus) on either side of the ion relative intensities programmed for each chemical. In the case of chemicals with similar ion peaks well within the variation allowed by the reliability parameter, safety considerations dictate that the MM-1 choose the more dangerous chemical. The interference parameter also forces the MM-1 to choose the more dangerous chemical by requiring such an enormous amount of the less dangerous chemical be present. In other words, if there is any question about the identity of the suspected substance, safety considerations require the MM-1 to alert for the dangerous chemical.


    The Fox Tapes

    Every time the MM-1 performs a function, it can be recorded on a paper tape that looks similar to a grocery receipt. The printed tape records information such as calibration tests, alarms, warnings, method changes, and the results of spectrum analyses. If enabled, the autoprint function prints everything automatically; otherwise the MM-1 operator must press the print button to record.

    Following are four examples[26] of possible MM-1 tapes. Listed first on all four tapes is the word "background," which prints every time the MM-1 changes detection methods. Below "background" the detection method being used to monitor for chemical agents is printed (e.g. Wheel/Hi or Air/Hi). All four examples have Air Monitor printed on them. This is the mode of operation US troops were trained to use. The next item on the tapes is location information and is based on data provided by the crew at the start of a mission and updated throughout the mission. The compound initially detected and its relative intensity appear on the line below the location. The letter[27] preceding the relative intensity denotes the detection method being used.

    In the first example, the MM-1 initially alerts to compound A with a relative intensity of 6.3. The MM-1 operator switches the MM-1 to the Surface/Lo method. Surface/Lo is printed on the tape. The MM-1 again alerts to compound A with a relative intensity of 6.4. Because the relative intensity is above 4.0, the MM-1 operator runs a spectrum. The spectrum confirms a detection for compound A.


    Figure 4. Example 1 of a Fox Tape



    Figure 5. Example 2 of a Fox Tape


    In the second example, the MM-1 initially alerts for compound B with a relative intensity of 6.3. The MM-1 operator switches to Surface/Lo and the MM-1 again alerts for compound B with a relative intensity of 6.4. A spectrum is run, but this time the spectrum does not match any of the compounds in the chemical library. The computer classifies the substance as "unknown" which means the chemical is not in the MM-1’s library, and therefore is not compound B. If the substance is not in the library, the MM-1 has no basis of comparison, and thus can not determine if the substance is hazardous.

    In the third example, the MM-1 initially alerts for compound C with a relative intensity of 6.3. The MM-1 operator switches to Surface/Lo. Compound C is detected again with a relative intensity of 6.4. A spectrum identifies the substance as FATS/OILS/WAXES. The MM-1 has detected hydrocarbons, not a chemical warfare agent.


    Figure 6. Example 3 of a Fox Tape


    In the last example, the Air/Hi method is being used. The MM-1 initially alerts to compound D with a relative intensity of 1.9. The MM-1 operator switches to Surface/Lo and compound D is no longer being detected. The alarm is therefore not a confirmed detection.


    Figure 7. Example 4 of a Fox Tape



    Operational Employment During Operation Desert Storm

    There were three basic ways the Fox was used during Operation Desert Storm: as a reconnaissance tool, as a mobile detector, and as a point detector. Following a text-book approach for a reconnaissance mission, the Fox drove across an area where troops and equipment had to pass. The Fox operated using the Wheel/Hi method.[28] The MM-1 was programmed to send a warning[29] if a chemical agent was detected above a predetermined relative intensity and alarm at a second higher predetermined relative intensity. If the MM-1 alerted to a chemical warfare agent, the MM-1 operator changed to the Surface/Lo method. This required that the Fox vehicle stop, allow the probe temperature to cool 60 degrees, and back up to the contaminated area if the vehicle’s momentum carried it beyond the area to be tested. This process can take 5-10 minutes. The MM-1 operator runs a spectrum if the MM-1 continues to alert to a chemical warfare agent and the relative intensity is high enough.[30] Only a spectrum analysis can positively identify the chemical in question. If the spectrum analysis identifies a chemical warfare agent, the Fox moves back to the edge of contamination. The MM-1 operator then switches to the Air/Hi method and moves around the boundary, watching for low ion level readings on the MM-1 screen. When the readings become very low, the MM-1 operator switches to the Surface/Lo method and takes another reading. The switching between Air/Hi and Surface/Lo continues until the boundaries of contamination are identified and marked.

    If vapor agents were expected or if the operational considerations prevented the Fox from stopping, the Fox was used as a vapor detector. However, the Fox is not a very sensitive vapor detector and, therefore, not a good system for determining areas of vapor contamination. When operating as a mobile vapor detector, the procedures were similar to a reconnaissance mission except the Fox crews drove through areas using the Air/Hi method, sampling the airborne vapors. If a Fox initially alerted to a chemical warfare agent using the Air/Hi method, the MM-1 operator could switch to Surface/Lo and initiate a spectrum. However, operational considerations (such as exposure to enemy fire) often prevented the Fox crews from stopping and performing these important secondary functions. However, the initial alert from a Fox vehicle was enough cause for troops to don additional protective gear.

    The Fox is a capable point detector, and was used this way during Desert Storm when a small area was suspected to be contaminated. The MM-1 used the Surface/Lo method with the probe lowered until it touched the unknown substance. The MM-1 operator would run a spectrum to identify the substance. In this way, the MM-1 could analyze compounds on individual pieces of clothing or equipment.


    Observations from Desert Storm

    Commanders and Fox vehicle operators generally praised the operation of the Fox during Operation Desert Storm. "The Fox Reconnaissance vehicle proved valuable to commanders by rapidly confirming that agents were not present."[31] However, there were a couple of complaints, none of which hamper actual operations.

  • "The VOS [Vehicle Orientation System] is absolutely useless for extended off-road use with no opportunity to update location. On moves of 10 km or less, accuracy was usually within 300m. On moves of 50 km, location accuracy was often off by as much as 20 km."[32] The VOS provides location information based on the inertia of the vehicle. Simply stated the VOS calculates current locations by using the starting coordinates, the direction of travel, and the number of wheel rotations counted during the vehicles movement. The problem with the VOS occurs when a vehicle’s wheels rotate but the vehicle does not actually move. This situation can occur if the vehicle is trying to traverse areas where loose sand is prevalent.

  • "Sampling wheel arms need to be spring loaded to allow the use of sampling wheels in rough off road use. Currently large bumps and ridges will damage the sampling arms."[33]

  • "The Fox was ineffective in monitoring for agents through the breach because: it could not slow down to get good readings, it could not stop in the breach and take samples, and it was not allowed to go back and check breach areas that were thought contaminated."[34]

  • "Almost all maintenance jobs require ‘special tools’ which were not available to our mobile maintenance teams. ... The Filters and other parts required for regular services were difficult to obtain, and often unavailable ...."[35]

  • The smoke from oil well fires was a problem for all the US chemical detection equipment. Crude oil combustion forms many ions that are also present in various chemical agents. Other environmental effects that caused the Fox to initially alert to several chemical agents included diesel fumes, and fumes from explosives. When the first Fox vehicles arrived in the Gulf, the fine sand in this region desert caused a detection problem for the Fox, but it was corrected prior to the start of the ground war.



    The Fox vehicle is a powerful tool for detection of chemical warfare agents and was first introduced to US troops during Operation Desert Storm. In order to improve troop safety and assure alerting for chemical warfare agents, the US government accepted the possibility of increased frequency of "false positives" occurring. Critical design considerations allowed for initial false alerts that could not be confirmed in many situations, but comments from commanders and Fox operators were very favorable of the vehicle. Planned improvements, due to input from commanders and Fox crews include the installation of the Global Positioning System (GPS) and the addition of the M21 stand-off chemical detector.[36]


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