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66-415: The standard frequency of a modern EPIRB is 406 MHz. It is an internationally regulated mobile radiocommunication service that aids search-and-rescue operations to detect and locate distressed watercraft, aircraft , and people. The first form of these beacons was the 121.5 MHz ELT, which was designed as an automatic locator beacon for crashed military aircraft. These beacons were first used in

132-459: A mobile data terminal . A data terminal radio allows data communications to take place over the separate radio. In the same way that a facsimile machine has a separate phone line, this means data and voice communication can take place simultaneously over a separate radio. Early Federal Express (FedEx) radio systems used a single radio for data and voice. The radio had a request-to-speak button which, when acknowledged, allowed voice communication to

198-418: A 30-50 MHz link, (called crossband ). Early mobile radios used amplitude modulation (AM) to convey intelligence through the communications channel. In time, problems with sources of electrical noise showed that frequency modulation (FM) was superior for its ability to cope with vehicle ignition and power line noise. The frequency range used by most early radio systems, 25–50 MHz (vhf "low band")

264-498: A Doppler track is available. The COSPAS-SARSAT specifications say that a beacon location is not considered "resolved" unless at least two Doppler tracks match or a Doppler track confirms an encoded (GPS) track. One or more GPS tracks are not sufficient. An intermediate technology 406-MHz beacon (now mostly obsolete in favor of GPS-enabled units) has worldwide coverage, locates within 2 km (12.5 km search area), notifies kin and rescuers in 2 hours maximum (46 min average), and has

330-514: A beacon is about two hours. The first satellite constellation was launched in the early 1970s by the Soviet Union , Canada, France and the United States. Some geosynchronous satellites have beacon receivers. Since the end of 2003, there are four such geostationary satellites (GEOSAR) that cover more than 80% of the surface of the earth. As with all geosynchronous satellites, they are located above

396-536: A common trait of tube-type mobile radios: their heavy weight due to the iron -core transformers in the power supplies. These high voltage power supplies were inefficient, and the filaments of the vacuum tubes added to current demands, taxing vehicle electrical systems. Sometimes, a generator or alternator upgrade was needed to support the current required for a tube-type mobile radio. Examples of US 1950s-1960s tube-type mobile radios with no transistors : Equipment from different US manufacturers had similar traits. This

462-440: A cost savings, some systems employ vehicular chargers instead of a mobile radio. Each radio user is issued a walkie talkie . Each vehicle is equipped with a charger system console. The walkie talkie inserted into a vehicular charger or converter while the user is in the vehicle. The charger or converter (1) connects the walkie talkie to the vehicle's two-way radio antenna, (2) connects an amplified speaker, (3) connects

528-579: A few vehicles before making a commitment to a certain antenna location or type. U.S. Occupational Safety and Health Administration guidelines for non-ionizing radio energy generally say the radio antenna must be two feet from any vehicle occupants. This rule of thumb is intended to prevent passengers from being exposed to unsafe levels of radio frequency energy when the radio transmits. Dispatch-reliant services, such as tow cars or ambulances , may have several radios in each vehicle. For example, tow cars may have one radio for towing company communications and

594-425: A hex code. When the beacon is purchased, the hex code should be registered with the relevant national (or international) authority. After one of the mission control centers has detected the signal, this registration information is passed to the rescue coordination center, which then provides the appropriate search-and-rescue agency with crucial information, such as: Registration information allows SAR agencies to start

660-654: A mobile microphone, and (4) charges the walkie talkie's battery. The weak point of these systems has been connector technology which has been proven unreliable in some installations. Receiver performance is a problem in congested radio signal and urban areas. These installations are sometimes referred to as jerk-and-run systems. Mission Control Centre (Cospas-Sarsat) Within the Cospas-Sarsat programme, Mission control centres (MCCs) are responsible for receiving and distributing distress signal alerts from emergency position-indicating radiobeacon stations . MCCs are

726-470: A mobile radio installed in a locomotive would run off of 72 or 30 Volt DC power. A large ship with 117 V AC power might have a base station mounted on the ship's bridge. According to article 1.67 of the ITU , a mobile radio is "A station in the mobile service intended to be used while in motion or during halts at unspecified points." The distinction between radiotelephones and two-way radio

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792-507: A model of international cooperation, even during the Cold War . SARSAT means search-and-rescue satellite-aided tracking. COSPAS ( КОСПАС ) is an acronym for the Russian words " COsmicheskaya Sistema Poiska Avariynyh Sudov " (Космическая Система Поиска Аварийных Судов), which translates to "space system for the search of vessels in distress". A consortium of USSR, the U.S., Canada, and France formed

858-788: A national police force, or a coast guard). These international search-and-rescue points of contact receive SAR alerts from the USMCC. The U.S. NOAA operates the U.S. Mission Control Center (USMCC) in Suitland, Maryland. It distributes beacon signal reports to one or more of these RCCs: (RCC Boston) (RCC Norfolk) (RCC Miami) (RCC New Orleans) (RCC Cleveland) (RCC Alameda and Pacific SAR Coordinator) (RCC Seattle) (RCC Honolulu; operated as JRCC with DOD) (RCC Juneau) (sub-sector of RCC Miami) The US Coast Guard web page for EPIRBs states: "You may be fined for false activation of an unregistered EPIRB. The US Coast Guard routinely refers cases involving

924-499: A particular customer is a thing of the past. Modern mobile radio equipment is "feature rich". A mobile radio may have 100 or more channels, be microprocessor controlled and have built-in options such as unit ID . A computer and software is typically required to program the features and channels of the mobile radio. Menus of options may be several levels deep and offer a complicated array of possibilities. Some mobile radios have alphanumeric displays that translate channel numbers (F1, F2) to

990-1020: A phrase more meaningful to the user, such as "Providence Base", "Boston Base", etc. Radios are now designed with a myriad of features to preclude the need for custom design. For example, Hytera 's HM68X mobile radio, which was introduced in September 2022, offers a variety of features, including GPS location, emergency alarm, noise cancellation, and more. Examples of US microprocessor-controlled mobile radios: As use of mobile radio equipment has virtually exploded, channel spacing has had to be narrowed again to 12.5–15 kHz with modulation deviation dropped to ±2.5 kilohertz. In order to fit into smaller, more economical vehicles, today's radios are trending toward radically smaller sizes than their tube-type ancestors. The traditional analogue radio communications have been surpassed by digital radio voice communications capabilities that provide greater clarity of transmission, enable security features such as encryption and, within

1056-649: A precision of 100 m in the 70% of the world closest to the equator, and send a serial number so the responsible authority can look up phone numbers to notify the registrant (e.g., next-of-kin) in four minutes. The GPS system permits stationary, wide-view geosynchronous communications satellites to enhance the Doppler position received by low Earth orbit satellites. EPIRB beacons with built-in GPS are usually called GPIRBs, for GPS position-indicating radio beacon or global position-indicating radio beacon. However, rescue cannot begin until

1122-410: A radio transceiver, housed in a single box, and a microphone with a push-to-talk button. Each installation would also have a vehicle-mounted antenna connected to the transceiver by a coaxial cable. Some models may have an external, separate speaker which can be positioned and oriented facing the driver to overcome ambient road noise present when driving. The installer would have to locate this equipment in

1188-565: A rescue effort. Cospas Sarsat Monitoring include: A transmission is typically detected and processed in this manner: Once the satellite data is received, less than a minute is needed to forward them to any signatory nation. The primary means of detection and location is by the COSPAS-SARSAT satellites. However, additional means of location are frequently used. For example, the FAA requires that all pilots monitor 121.500 MHz whenever possible, and

1254-482: A rescue more quickly. For example, if a shipboard telephone number listed in the registration is unreachable, it could be assumed that a real distress event is occurring. Conversely, the information provides a quick and easy way for the SAR agencies to check and eliminate false alarms (potentially sparing the beacon's owner from significant false alert fines). An unregistered 406-MHz beacon still carries some information, such as

1320-464: A second for emergency road service communications. Ambulances may have a similar arrangement with one radio for government emergency medical services dispatch and one for company dispatch. US ambulances often have radios with dual controls and dual microphones allowing the radio to be used from the patient care area in the rear or from the vehicle's cab. Both tow cars and ambulances may have an additional radio which transmits and receives to support

1386-472: A serial number to look up phone numbers, etc. This can take up to two hours because it has to use moving weather satellites to locate the beacon. To help locate the beacon, the beacon's frequency is controlled to 2 parts per billion, and its power is five watts. Both of the above types of beacons usually include an auxiliary 25-milliwatt beacon at 121.5 MHz to guide rescue aircraft. The oldest, cheapest beacons are aircraft ELTs that send an anonymous warble on

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1452-456: A vehicle. Mobile radios are mounted to a motor vehicle usually with the microphone and control panel in reach of the driver. In the US, such a device is typically powered by the host vehicle's 12 Volt electrical system. Some mobile radios are mounted in aircraft (aeronautical mobile), shipboard (maritime mobile), on motorcycles, or railroad locomotives. Power may vary with each platform. For example,

1518-402: A way that does not interfere with the vehicle's sun roof, electronic engine management system, vehicle stability computer, or air bags. Mobile radios installed on motorcycles are subject to extreme vibration and weather. Professional equipment designed for use on motorcycles is weather and vibration resistant. Shock mounting systems are used to reduce the radio's exposure to vibration imparted by

1584-438: Is becoming blurred as the two technologies merge. The backbone or infrastructure supporting the system defines which category or taxonomy applies. A parallel to this concept is the convergence of computing and telephones. Radiotelephones are full-duplex (simultaneous talk and listen), circuit switched, and primarily communicate with telephones connected to the public switched telephone network . The connection sets up based on

1650-408: Is only accurate to 50 parts per million and the signals were broadcast using only 75–100 milliwatts of power. Coverage was partial because the satellite had to be in view of both the beacon and a ground station at the same time; the satellites did not store and forward the beacon's position. Coverage in polar and Southern Hemisphere areas was poor. False alarms were common, as the beacon transmitted on

1716-449: Is particularly susceptible to the problem of electrical noise. This plus the need for more channels led to the eventual expansion of two-way radio communications into the VHF "high band" (150–174 MHz) and UHF (450–470 MHz). The UHF band has since been expanded again. One of the major challenges in early mobile radio technology was that of converting the six or twelve volt power supply of

1782-523: The International Cospas-Sarsat Programme , the international satellite system for search and rescue (SAR). These beacons transmit a 406 MHz distress signal every 50 seconds, varying over a span of 2.5 seconds to avoid multiple beacons always transmitting at the same time. When manually activated, or automatically activated upon immersion or impact, such beacons send out a distress signal . The signals are monitored worldwide and

1848-478: The USCG has a network of direction finder sites along the coastlines. The National Oceanic and Atmospheric Administration maintains a near-real-time map that shows SARSAT U.S. Rescues. Several systems are in use, with beacons of varying expense, different types of satellites, and varying performance. Carrying even the oldest systems provides an immense improvement in safety over carrying none. The types of satellites in

1914-454: The 1950s by the U.S. military, and were mandated for use on many types of commercial and general-aviation aircraft beginning in the early 1970s. The frequency and signal format used by the ELT beacons was not designed for satellite detection, which resulted in a system with poor location detection abilities and long delays in detection of activated beacons. The satellite detection network was built after

1980-586: The 1980s, large system users with specialized requirements often had custom built radios designed for their unique systems. Systems with multiple- CTCSS tone encoders and more than two channels were unusual. Manufacturers of mobile radios built customized equipment for large radio fleets such as the California Department of Forestry and the California Highway Patrol . Examples of US hybrid partially solid state mobile radios: Custom design for

2046-455: The AM broadcast band) to improve safety. Instead of hanging out of a locomotive cab and grabbing train orders while rolling past a station, voice communications with rolling trains became possible. Radios linked the caboose with the locomotive cab. Early police radio systems were initially one way using MF frequencies above the AM broadcast band, (1.7 MHz ). Some early systems talked back to dispatch on

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2112-484: The ELT beacons were already in general use, with the first satellite not being launched until 1982, and even then, the satellites only provided detection, with location accuracy being roughly 20 km (12 mi). The technology was later expanded to cover use on vessels at sea (EPIRB), individual persons (PLB), and starting in 2016, maritime survivor locating devices (MSLD). All have migrated from using 121.500 MHz as their primary frequency to using 406 MHz, which

2178-449: The aviation band distress frequency at 121.5 MHz. The frequency is often routinely monitored by commercial aircraft, but has not been monitored by satellite since Feb. 1, 2009. These distress signals could be detected by satellite over only 60% of the earth, required up to 6 hours for notification, located within 20 km (12 mi) (search area of 1200 km), were anonymous, and could not be located well because their frequency

2244-401: The aviation emergency frequency, with interference from other electronic and electrical systems. To reduce false alarms, a beacon was confirmed by a second satellite pass , which could easily slow confirmation of a 'case' of distress to as much as 4 hours (although in rare circumstances, the satellites could be positioned such that immediate detection becomes possible.) The Cospas-Sarsat system

2310-503: The beacon and the orbit. If the beacon's frequency is more precise, it can be located more precisely, saving search time, so modern 406-MHz beacons are accurate to 2 parts per billion, giving a search area of only 2 km, compared to the older beacons accurate to 50 parts per million that had 200 km of search area. To increase the useful power, and handle multiple simultaneous beacons, modern 406-MHz beacons transmit in bursts, and remain silent for about 50 seconds. Russia developed

2376-411: The dispatch center. Each radio works over a single band of frequencies. If a tow car company had a frequency on the same band as its auto club, a single radio with scanning might be employed for both systems. Since a mobile radio typically works on a single frequency band, multiple radios may be required in cases where communications take place over systems on more than one frequency band. Intended as

2442-465: The end user. The standard "quarter wave" antenna in the 25-50 MHz range can be over nine feet long. A 900 MHz antenna may be three inches long for a quarter wavelength. A transit bus may have a ruggedized antenna, which looks like a white plastic blade or fin, on its roof. Some vehicles with concealed radio installations have antennas designed to look like the original AM/FM antenna, a rearview mirror, or may be installed inside windows, or hidden on

2508-471: The equator. The GEOSAR satellites do not cover the polar caps. Since they see the Earth as a whole, they see the beacon immediately, but have no motion, and thus no Doppler frequency shift to locate it. However, if the beacon transmits GPS data, the geosynchronous satellites give nearly instantaneous response. Emergency beacons operating on 406 MHz transmit a unique 15-, 22-, or 30-character serial number called

2574-801: The floor pan or underside of a vehicle. Aircraft antennas look like blades or fins, the size and shape being determined by frequencies used. Microwave antennas may look like flat panels on the aircraft's skin. Temporary installations may have antennas which clip on to vehicle parts or are attached to steel body parts by a strong magnet. Though initially relatively inexpensive mobile radio system components, frequently damaged antennas can be costly to replace since they are usually not included in maintenance contracts for mobile radio fleets. Some types of vehicles in 24-hour use, with stiff suspensions, tall heights, or rough diesel engine idle vibrations may damage antennas quickly. The location and type of antenna can affect system performance drastically. Large fleets usually test

2640-477: The location of the distress is detected by non- geostationary satellites using the Doppler effect for trilateration , and in more recent EPIRBs, also by GPS . Loosely related devices, including search and rescue transponders (SART), AIS-SART , avalanche transceivers , and RECCO do not operate on 406 MHz, thus are covered in separate articles. Cospas-Sarsat is an international organization that has been

2706-574: The manufacturer and serial number of the beacon, and in some cases, an MMSI or aircraft tail number / ICAO 24-bit address . Despite the clear benefits of registration, an unregistered 406-MHz beacon is very substantially better than a 121.5-MHz beacon, because the hex code received from a 406-MHz beacon confirms the authenticity of the signal as a real distress signal. Beacons operating on 121.5 MHz and 243.0 MHz only simply transmit an anonymous siren tone, thus carry no position or identity information to SAR agencies. Such beacons now rely solely on

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2772-492: The microphone and coiled cord were hidden inside the glove box. An operator tossed the mike into the glove box and shut it, causing the push-to-talk button to be depressed and locking the transmitter on. On taxi systems, a driver may be upset when a dispatcher assigns a call (s)he wanted to another driver and may deliberately hold the transmit button down (for which the owner can be fined by the FCC). Radios with time-out timers transmit for

2838-467: The motorcycle's modal, or resonant, shaking. Some mobile radios use noise-canceling microphones or headsets. At speeds over 100 MPH, the ambient road and wind noise can make radio communications difficult to understand. For example, California Highway Patrol mobile radios have noise-canceling microphones which reduce road and siren noise heard by the dispatcher . Most fire engines and radios in heavy equipment use noise-canceling headsets. These protect

2904-494: The network are: When one of the COSPAS-SARSAT satellites detects a beacon, the detection is passed to one of the program's roughly 30 Mission Control Centers , such as USMCC (in Suitland, Maryland), where the detected location and beacon details are used to determine to which rescue coordination centre (for example, the U.S. Coast Guard's PACAREA RCC, in Alameda, California) to pass the alert. The 406-MHz beacons with GPS track with

2970-545: The network, allow low band data transmissions to accommodate simple text or picture messaging as an example. (Examples: Project 25 (APCO-25), Terrestrial Trunked Radio ( TETRA ), DMR .) Commercial and professional mobile radios are often purchased from an equipment supplier or dealer whose staff will install the equipment into the user's vehicles. Large fleet users may buy radios directly from an equipment manufacturer and may even employ their own technical staff for installation and maintenance. A modern mobile radio consists of

3036-670: The nondistress activation of an EPIRB (e.g., as a hoax, through gross negligence, carelessness, or improper storage and handling) to the Federal Communications Commission. The FCC will prosecute cases based upon evidence provided by the Coast Guard, and will issue warning letters or notices of apparent liability for fines up to $ 10,000." Mobile service Mobile radio or mobiles refer to wireless communications systems and devices which are based on radio frequencies (using commonly UHF or VHF frequencies), and where

3102-430: The number of radio channels that could be accommodated in the available radio frequency spectrum were limited to a certain number, dictated by the bandwidth of the signal on each channel. Solid-state electronic equipment arrived in the 1960s, with more efficient circuitry and smaller size. Metal–oxide–semiconductor (MOS) large-scale integration (LSI) provided a practical and economic solution for radio technology, and

3168-418: The occupant's hearing and reduce background noise in the transmitted audio. Noise-canceling microphones require the operator speak directly into the front of the microphone. Hole arrays in the back of the microphone pick up ambient noise. This is applied, out-of-phase, to the back of the microphone, effectively reducing or canceling any sound which is present both in front and back of the microphone. Ideally, only

3234-435: The organization in 1982. Since then, 29 other countries have joined. The satellites used in the system include: Cospas-Sarsat defines standards for beacons, auxiliary equipment to be mounted on conforming weather and communication satellites, ground stations, and communications methods. The satellites communicate the beacon data to their ground stations, which forward it to main control centers of each nation that can initiate

3300-417: The original system, and its success drove the desire to develop the improved 406-MHz system. The original system was a brilliant adaptation to the low-quality beacons, originally designed to aid air searches. It used just a simple, lightweight transponder on the satellite, with no digital recorders or other complexities. Ground stations listened to each satellite as long as it was above the horizon. Doppler shift

3366-437: The path of communications is movable on either end. There are a variety of views about what constitutes mobile equipment. For US licensing purposes, mobiles may include hand-carried , (sometimes called portable ), equipment. An obsolete term is radiophone . A sales person or radio repair shop would understand the word mobile to mean vehicle-mounted : a transmitter-receiver (transceiver) used for radio communications from

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3432-418: The path of the satellite in space and the rotation of the earth. This triangulates the position of the beacon. A faster change in the Doppler indicates that the beacon is closer to the satellite's orbit . If the beacon is moving toward or away from the satellite track due to the Earth's rotation, it is on one side or other of the satellite's path. Doppler shift is zero at the closest point of approach between

3498-611: The preset amount of time, usually 30–60 seconds, after which the transmitter automatically turns off and a loud tone comes out of the radio speaker. The volume level of the tone on some radios is loud and cannot be adjusted. As soon as the push-to-talk button is released, the tone stops and the timer resets. Mobile radio equipment is manufactured to specifications developed by the Electronic Industries Association / Telecommunications Industry Association (EIA/TIA). These specifications have been developed to help assure

3564-419: The radio set had to be equipped with an indicator light, usually green or yellow, that showed power was applied and the radio was ready to transmit. Radios were also required to have a lamp (usually red) indicating when the transmitter was on. These traits continue in the design of modern radios. Early tube-type radios operated on 50 kHz channel spacing with ±15 kHz modulation deviation. This meant that

3630-510: The second satellite pass almost doubled the average time before notification of the rescuing authority. However, the notification time was much less than a day. Receivers are auxiliary systems mounted on several types of satellites. This substantially reduces the program's cost. The weather satellites that carry the SARSAT receivers are in "ball of yarn" orbits, inclined at 99 degrees. The longest period that all satellites can be out of line-of-sight of

3696-586: The terrestrial or aeronautical monitoring of the frequency. RCCs are responsible for a geographic area, known as a "search-and-rescue region of responsibility" (SRR). SRRs are designated by the International Maritime Organization and the International Civil Aviation Organization . RCCs are operated unilaterally by personnel of a single military service (e.g. an air force, or a navy) or a single civilian service (e.g.

3762-864: The user dialing. The connection is taken down when the end button is pressed. They run on telephony-based infrastructure such as AMPS or GSM . Two-way radio is primarily a dispatch tool intended to communicate in simplex or half-duplex modes using push-to-talk, and primarily intended to communicate with other radios rather than telephones. These systems run on push-to-talk-based infrastructure such as Nextel's iDEN , Specialized Mobile Radio (SMR), MPT-1327 , Enhanced Specialized Mobile Radio (ESMR) or conventional two-way systems. Certain modern two-way radio systems may have full-duplex telephone capability. Early users of mobile radio equipment included transportation and government. These systems used one-way broadcasting instead of two-way conversations. Railroads used medium frequency range ( MF ) communications (similar to

3828-444: The user that mobile radio equipment performs as expected and to prevent the sale and distribution of inferior equipment which could degrade communications. A mobile radio must have an associated antenna. The most common antennas are stainless steel wire or rod whips which protrude vertically from the vehicle. Physics defines the antenna length: length relates to frequency and cannot be arbitrarily lengthened or shortened (more likely) by

3894-471: The vacuum tubes. These circuits, called " inverters ", changed the 6 or 12 V direct current (DC) to alternating current (AC) which could be passed through a transformer to make high voltage. The power supply then rectified this high voltage to make the high voltage DC required for the vacuum tubes, (called valves in British English). The power supplies needed to power vacuum tube radios resulted in

3960-505: The vehicle to the high voltage needed to operate the vacuum tubes in the radio. Early tube-type radios used dynamotors - essentially a six or twelve volt motor that turned a generator to provide the high voltages required by the vacuum tubes. Some early mobile radios were the size of a suitcase or had separate boxes for the transmitter and receiver. As time went on, power supply technology evolved to use first electromechanical vibrators , then solid-state power supplies to provide high voltage for

4026-415: The voice present on the front side of the microphone goes out on the air. Many radios are equipped with transmitter time-out timers which limit the length of a transmission. A bane of push-to-talk systems is the stuck microphone: A radio locked on transmit, which disrupts communications on a two-way radio system. One example of this problem occurred in a car with a concealed two-way radio installation where

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4092-536: Was designed for satellite detection and location. Since the inception of Cospas-Sarsat in 1982, distress radio beacons have assisted in the rescue of over 50,000 people in more than 7,000 distress situations. In 2010 alone, the system provided information used to rescue 2,388 persons in 641 distress situations. The several types of emergency locator beacons are distinguished by the environment for which they were designed to be used: Distress alerts transmitted from ELTs, EPIRBs, SSAS, and PLBs are received and processed by

4158-414: Was made possible by Doppler processing. Local-user terminals (LUTs) detecting nongeostationary satellites interpret the Doppler frequency shift heard by LEOSAR and MEOSAR satellites as they pass over a beacon transmitting at a fixed frequency. The interpretation determines both bearing and range. The range and bearing are measured from the rate of change of the heard frequency, which varies both according to

4224-506: Was partly dictated by Federal Communications Commission (FCC) regulations. The requirement that unauthorized persons be prohibited from using the radio transmitter meant that many radios were wired so they could not transmit unless the vehicle ignition was on. Persons without a key to the vehicle could not transmit. Equipment had to be "type accepted", or technically approved, by the FCC before it could be offered for sale. In order to be type accepted,

4290-401: Was used in mobile radio systems by the early 1970s. Channel spacing narrowed to 20–30 kHz with modulation deviation dropping to ±5 kHz. This was done to allow more radio spectrum availability to accommodate the rapidly growing national group of two-way radio users. By the mid-1970s, tube-type transmitter power amplifiers had been replaced with high-power transistors . From the 1960s to

4356-401: Was used to locate the beacon(s). Multiple beacons were separated when a computer program analysed the signals with a fast Fourier transform . Also, two satellite passes per beacon were used. This eliminated false alarms by using two measurements to verify the beacon's location from two different bearings. This prevented false alarms from VHF channels that affected a single satellite. Regrettably,

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