Remote  Sensing:

       A Supplemental Tool for Vehicle Emission Control

\yhat is Remote Sensing?
Elemote sensing is a way to measure pollutant levels in a vehicle's exhaust while
the vehicle is traveling down the road. Unlike most equipment used to measure
vehicle emissions today, remote sensing devices (RSD) do not need to be physi-
cally connected to the vehicle.  The concept of an efficient tool to monitor the
vehicle fleet and identify excessive polluters has great appeal as a complement
to traditional mobile source emission control programs. A number of instrument
manufacturers are actively developing RSD systems.

What Pollutants are Measured by RSD ?
Current RSD systems can measure hydrocarbons and carbon monoxide in the
exhaust stream. Current systems cannot measure nitrogen oxides, another
exhaust pollutant and important contributor to smog. However, researchers are
actively developing RSD systems with this capability.

E'SD also cannot measure "evaporative" emissions  gasoline vapors that vent
into the air from hot engines and fuel systems. Fuel evaporation is a very sig-
nificant source of hydrocarbon pollution that can exceed tailpipe emissions on
hot days.

How does Remote Sensing Work?
Commercial RSD systems employ an infrared absorption principle to measure
hydrocarbon (HC) and carbon monoxide (CO) emissions.  These systems operate
by continuously projecting a beam of infrared radiation across a roadway.
                                 IR SOURCE
                                                    FACT SHEET OMS-15.
                                                       August, 1993

         Remote Sensing
 As a vehicle passes through the beam, the device measures the ratio of CO (and
 exhaust HC) to carbon dioxide in front of the vehicle and in the exhaust plume
 behind. The system uses the "before" measurement as a base and calculates the
 vehicle's CO emission rate by comparing the "behind" measurement to the
 expected ratio for ideal combustion.

 Exhaust HC is calculated in a somewhat similar manner by comparing the total
 carbon content of exhaust HC, CO, and carbon dioxide to the total carbon content
 of the gasoline the car burns.

 It is expected that RSD systems for nitrogen oxides (NOx) will use either a beam
 of ultraviolet light, or light from a tuneable diode laser projected across the road.
 The carbon dioxide to carbon monoxide ratio determined by current RSD sys-
 tems will still be needed to calculate NOx emissions.

 RSD systems employ a freeze-frame video camera and equipment to digitize an
 image of the license plate number so that it can be processed by a computer.
 This allows the computer to store emissions information for each monitored
 vehicle, based on the license plate number. Appropriate authorities can then
 identify and contact owners of vehicles with high RSD readings.

 Methods to measure a vehicle's speed and acceleration as it passes through the
 infrared beam are under development. This is important because the operating
 mode (e.g. acceleration, cruise, etc.) can significantly affect the instantaneous
 emission level from a vehicle. Some types of operation during an RSD test may
 be cause for invalidating a particular test.

 Computerized diagnostic technologies may also  play a role in future RSD sys-
 tems. Vehicle onboard diagnostic systems, capable of identifying certain mal-
 functions in a vehicle's emission control system, are required beginning with
 1994 models. The malfunctions could be reported to roadside RSD systems by a
 small electronic device on the vehicle called a radio frequency transponder.
 Similar transponder concepts have been used to time runners in marathons and
 experimental transponder systems are being used to assess toll road fees.

 Will Enhanced Inspection and Maintenance Programs
 Include RSD?

 Yes.  RSD and other "on-road" emission measurement methods will be an impor-
 tant part of EPA and state strategies to reduce emissions from motor vehicles.
 The 1990 Clean Air Act requires "enhanced" Inspection and Maintenance (I/M)
 programs in certain parts of the country where  air pollution exceeds national
 standards. Enhanced I/M programs must include on-road emission testing of a
 portion of the eligible vehicle  fleet.  RSD technology is expected to play a major
 role in these supplemental emission measurements:
  RSD will likely be used to identify vehicles with malfunctioning emission

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         Remote Sensing
  controls between scheduled I/M tests. Air quality benefits can result from
  early repair of vehicles that would otherwise not be identified or fixed
  until the next annual or biennial test.

 EPA studies have shown that properly repaired vehicles maintain low emis-
   sions for a long time. However, some individuals may tamper with their
   vehicle's emission control systems. The ability of the RSD to be easily moved
   from location to location provides a way to identify tampered vehicles between
   periodic I/M tests  and to enforce repair requirements on those found to be
   dirty. Other studies have found that RSD is more  effective in identifying
   tampered vehicles than the currently used random roadside pull-overs.

 RSD can detect unregistered or improperly registered vehicles. This will allow
  authorities to pick out drivers who cheat on registration or register out of the
  area to avoid participating in an I/M program.

 To take advantage of RSD's potential to identify dirty cars, EPA is requiring
  enhanced I/M programs to conduct supplemental emission measurements on
  at least 0.5% of vehicles subject to I/M testing each year. Vehicles that fail an
  RSD test would be required to be re-tested by the regular I/M test.  Repairs
  would be required for any vehicle failing this out-of-schedule I/M emissions

Can RSD Replace Enhanced Inspection and Maintenance

No. The Clean Air Act provides for use of RSD as a supplement to enhanced I/M
programs but not as a substitute for periodic emission testing. While RSD can
be extremely useful, it does have some limitations:

  RSD's capability to measure NOx has not been demonstrated and RSD meth-
   ods for measuring evaporative HC have not even been proposed. The Clean
   Air Act requires enhanced I/M programs to be capable of testing both NOx
   and evaporative HC.

  The Clean Air Act mandates that enhanced I/M programs, beginning in 1995,
   include an interrogation of the onboard diagnostic system to check for emis-
   sion control system malfunctions on 1994 and later model cars. Current RSD
   systems cannot interrogate the onboard diagnostic system.

  Studies by EPA and the California Air Resources Board have found that
   when RSD measurements are compared to emissions measurements made by
   EPA reference methods, the RSD incorrectly identifies clean cars as dirty.
   The reference methods typically involve measuring emissions over a driving
   cycle that includes a variety of driving modes such as acceleration, cruise,
   and braking.
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         Remote Sensing
    The RSD mis-identification rate has been as high as 20% for CO and as high
    as 60% for HC. More importantly, for clean air, EPA studies indicate that
    RSD does not identify 80% to 90% of the dirty cars that need repair. EPA
    believes that these results do not reflect on the instantaneous measurement
    accuracy of the RSD. Rather, EPA believes these results are indicative of
    changes in vehicle emission levels that typically occur when a vehicle is
    operated under driving conditions different than those observed by the RSD.
    A bibliography of studies is attached.

 Implementing RSD in Inspection and Maintenance Programs

 There are a number of administrative factors to consider in establishing I/M
 programs that include RSD. Some RSD advocates have suggested that RSD is
 capable of monitoring much more than 0.5% of the fleet. EPA agrees that RSD
 could be used by the I/M programs to measure emissions from many more cars,
 given adequate resolution of the following issues:

 41 Placement of Roadside Monitors:
    Current RSD technology can only measure emissions of vehicles driving in a
    single lane of traffic. It is not easy to find enough sites where appropriate
    single traffic lanes exist to monitor the majority of vehicles subject to I/M
    testing.  Restricting multiple lanes to a single lane for RSD measurement
    may not be practical in many cases, particularly during times of heavy traffic
    such as rush hour. Yet RSD testing during peak traffic periods would prob-
    ably be necessary to avoid missing high-emitting cars that could be parked
    during business hours.

    EPA has successfully used RSD monitors along multiple lane roadways in
    some studies without restricting traffic to a single lane. But pylons had to be
    placed between the lanes to protect some of the RSD equipment.  With such a
    set-up, drivers could potentially choose not to drive through the measuring

    Another issue involves limiting RSD placement to locations where represen-
    tative vehicle operation will be observed. It will be important for I/M pro-
    grams to avoid creating situations where a measurable portion of vehicles fail
    RSD monitoring at one location but pass at another location. For example,
    sites of high acceleration would likely be avoided because emissions tend to
    be higher during acceleration than during steady-speed driving.

  Appropriate Pass/Fail Levels:
    A difficult issue involves selecting an emission failure level (cutpoint) for the
    RSD that will identify most of the truly dirty vehicles and minimize false
    failures of clean vehicles. EPA studies indicate that the mis-identification of
    clean vehicles as dirty ones by  RSD is substantially reduced by measuring
    emissions from the same vehicle several times. However, multiple measure-
    ments also result in more dirty cars passing the test.
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          Remote Sensing
     Administrative systems need to be established so authorities can follow up
     with owners of vehicles that register high emissions during an RSD check.
     Whether vehicle owners are pulled over immediately at the time of the
     check or notified later by mail, oversight will be necessary to ensure that
     dirty vehicles undergo further testing and repair if necessary.

    Driver Behavior:
     To date, RSD emissions testing has occurred only in demonstration type
     projects with no consequences for drivers whose vehicles fail the test. In the
     future, RSD failures in enhanced I/M programs will result in mandatory
     re-testing and repair. These consequences may prompt drivers to change
     their driving route or regime (e.g., observing RSD testing on the way to
     work in the opposite direction, and choosing a different route on the way
     home), or otherwise alter their driving behavior to avoid passing an RSD
     monitor. The political implications of failing motorists, especially falsely,
     with this type of program are also yet to be realized.

   The prototype studies conducted to date do not provide the type of practical
   information I/M program managers need to effectively use RSD on a day-to-
   day basis.  However, EPA believes that most of these administrative issues will
   be resolved with experience, as states begin to integrate RSD into actual I/M
   programs. By starting out with a small fraction of the fleet (0.5%), I/M pro-
   gram offices can begin to develop administrative systems that will allow RSD
   to achieve its potential as a full player in the vehicle emission control program
   of the future

   A bibliography of RSD studies is attached to this fact sheet. These studies are
   separated into three groups: (1) those where RSD measurements were compared
   to measurements made by an EPA reference method; (2) those that measured
   on-road vehicles, but did not include any reference method measurements; and
   (3) analytical studies which did not include any measurements (RSD or refer-
   ence methods).
 For More Information:
 The Office of Mobile Sources is the national center for research and policy on air
 pollution from highway and off-highway motor vehicles and equipment. You
 can write to us at the EPA National Vehicle and Fuel Emissions Laboratory,
 2565 Plymouth Road, Ann Arbor, MI 48105. Our phone number is (313) 668-4333.

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