Understanding Smart Sensors - Nomads.usp

186 Understanding Smart Sensors(10,898 nautical miles). A single satellite orbits the earth twice for each earthrotation, tracing exactly the same path twice each day but passing four minutesearlier than it did the day before. The design of the system ensures that at leastfour satellites are in view at any one time, on or above the surface of the earthunder all weather conditions.GPS satellites transmit at two frequencies: L1, centered at 1,575.42MHz, and L2, centered at 1,227.6 MHz. Each satellite broadcasts a navigationmessage that includes a description of the satellite’s position as a function oftime, an almanac, and clock correction terms. Each message comprises 25frames, each 30 seconds long. Commercial GPSs are capable of locating towithin 100m.GPS technology is an integral part of the automotive ITS (discussed inSection 8.3.6). GPS can be used to track field personnel and for locating positionrelative to location on a CD-ROM map. GPS products have been developedfor consumer use. In addition to the RF components necessary to convertthe signals from satellites, a 32-bit microprocessor is also required to computethe position algorithm quickly and provide frequent updates to the user. Thecomplex sensor (receiver) is contained within a board that is 50.8 mm by 82.6mm by 16.3 mm [17]. Increased acceptance and application in high-volumesystems such as the ITS will allow further component reduction throughincreased integration.8.3.4 Remote Emissions SensingThe remote measurement of vehicle exhaust emissions is one way that remotesensing will start to affect a broad range of the population. Identifying vehiclesthat emit high levels of carbon monoxide and hydrocarbon is possibly the nextstep toward reduced emissions from vehicles with combustion engines.Figure 8.7 illustrates a remote sensing system [18]. The main parts of the systeminclude an infrared (IR) detector and source; a video camera to record thelicense plate number; a modified police radar gun; and a personal computerwith specially developed software.The remote sensing device (RSD) system operates by continuously monitoringthe intensity of an IR source. The presence of a vehicle is indicated whenthe beam is broken, resulting in the reference voltage dropping to zero and aspan voltage measurement being made. The value prior to beam interruption isalso stored. As the vehicle exits the beam, samples are taken at 125 Hz for over1 second. The carbon dioxide spectral region is filtered at 4.3 mm and thecarbon monoxide region at 4.6 mm to isolate those values. Hydrocarbon valuescan be detected as well. The system incorporates three noncontact remote