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84-491: The Sentinel-6 Michael Freilich ( S6MF ) or Sentinel-6A is a radar altimeter satellite developed in partnership between several European and American organizations. It is part of the Jason satellite series and is named after Michael Freilich . S6MF includes synthetic-aperture radar altimetry techniques to improve ocean topography measurements, in addition to rivers and lakes. The spacecraft entered service in mid 2021 and

168-416: A frequency mixer , and because the difference in the two signals is due to the delay reaching the ground and back, the resulting output frequency encodes the altitude. The output is typically on the order of hundreds of cycles per second, not megacycles, and can easily be displayed on analog instruments. This technique is known as Frequency Modulated Continuous-wave radar . Radar altimeters normally work in

252-403: A "humpy" pattern to them; for any given signal frequency, the problem would only be significant if the devices were located at specific points in the line. This led to the idea of sending a test signal into the line and then changing its frequency until significant echos were seen. This would reveal the approximate distance to the device, allowing it to be identified and fixed. Lloyd Espenschied

336-432: A 2.2 kHz audio tone produces a modulation index of 1.36. Suppose that we limit ourselves to only those sidebands that have a relative amplitude of at least 0.01. Then, examining the chart shows this modulation index will produce three sidebands. These three sidebands, when doubled, gives us (6 × 2.2 kHz) or a 13.2 kHz required bandwidth. A rule of thumb , Carson's rule states that nearly all (≈98 percent) of

420-724: A base frequency of 450 MHz, was among the highest frequency systems of its era which made it much more useful. In Canada, the National Research Council (NRC) began working on an airborne radar system using the Bell altimeter as its basis. This came as a great surprise to British researchers when they visited in October 1940 as part of the Tizard Mission , as the British believed at that time that they were

504-443: A baseband modulating signal may be approximated by a sinusoidal continuous wave signal with a frequency f m . This method is also named as single-tone modulation. The integral of such a signal x m ( t ) = c o s ( 2 π f m t ) {\displaystyle x_{m}(t)=cos(2\pi f_{m}t)} is: In this case, the expression for y(t) above simplifies to: where

588-435: A binary signal modulates the carrier, the modulation index is given by: where T s {\displaystyle T_{s}\,} is the symbol period, and f m = 1 2 T s {\displaystyle f_{m}={\frac {1}{2T_{s}}}\,} is used as the highest frequency of the modulating binary waveform by convention, even though it would be more accurate to say it

672-574: A few flight cancellations in the United States. Radar altimeters are also used in military aircraft to fly quite low over the land and the sea to avoid radar detection and targeting by anti-aircraft guns or surface-to-air missiles . A related use of radar altimeter technology is terrain-following radar , which allows fighter bombers to fly at very low altitudes. The F-111s of the Royal Australian Air Force and

756-481: A given time, T. In the first units, this was accomplished using an LC tank with a tuning capacitor driven by a small electric motor. The output is then mixed with the radio frequency carrier signal and sent out the transmission antenna. Since the signal takes some time to reach the ground and return, the frequency of the received signal is slightly delayed relative to the signal being sent out at that instant. The difference in these two frequencies can be extracted in

840-456: A lack of selectivity may cause one station to be overtaken by another on an adjacent channel . Frequency drift was a problem in early (or inexpensive) receivers; inadequate selectivity may affect any tuner. A wideband FM signal can also be used to carry a stereo signal; this is done with multiplexing and demultiplexing before and after the FM process. The FM modulation and demodulation process

924-489: A scientific and technical conversation in the Nizhny Novgorod Radio Laboratory , reported about his new method of telephony, based on a change in the period of oscillations. Demonstration of frequency modulation was carried out on the laboratory model. Frequency modulated systems are a widespread and commercially available assistive technology that make speech more understandable by improving

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1008-575: A series of experiments carried out in partnership with the BBC . After scheduled transmissions had ended for the day, a BBC transmitter in Bournemouth sent out a signal that slowly increased in frequency. This was picked up by Appleton's receiver in Oxford , where two signals appeared. One was the direct signal from the station, the groundwave, while the other was received later in time after it travelled to

1092-465: A target, its outgoing sounds return as echoes, which are Doppler-shifted upward in frequency. In certain species of bats, which produce constant frequency (CF) echolocation calls, the bats compensate for the Doppler shift by lowering their call frequency as they approach a target. This keeps the returning echo in the same frequency range of the normal echolocation call. This dynamic frequency modulation

1176-470: A tone-modulated FM wave, if the modulation frequency is held constant and the modulation index is increased, the (non-negligible) bandwidth of the FM signal increases but the spacing between spectra remains the same; some spectral components decrease in strength as others increase. If the frequency deviation is held constant and the modulation frequency increased, the spacing between spectra increases. Frequency modulation can be classified as narrowband if

1260-414: A wider signal bandwidth than amplitude modulation by an equivalent modulating signal; this also makes the signal more robust against noise and interference . Frequency modulation is also more robust against signal-amplitude-fading phenomena. As a result, FM was chosen as the modulation standard for high frequency, high fidelity radio transmission, hence the term " FM radio " (although for many years

1344-478: Is calculated based on the first signal return from each sampling period. It does not detect slant range until beyond about 40° of bank or pitch. This is not an issue for landing as pitch and roll do not normally exceed 20°. Radio altimeters used in civil aviation operate in the IEEE C-band between 4.2 and 4.4 GHz. In early 2022, potential interference from 5G cell phone towers caused some flight delays and

1428-512: Is called the Doppler Shift Compensation (DSC), and was discovered by Hans Schnitzler in 1968. FM is also used at intermediate frequencies by analog VCR systems (including VHS ) to record the luminance (black and white) portions of the video signal. Commonly, the chrominance component is recorded as a conventional AM signal, using the higher-frequency FM signal as bias . FM is the only feasible method of recording

1512-514: Is categorised as a safety-of-life service , must be protected for interferences , and is an essential part of navigation . Frequency modulated Frequency modulation ( FM ) is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. The technology is used in telecommunications , radio broadcasting , signal processing , and computing . In analog frequency modulation, such as radio broadcasting, of an audio signal representing voice or music,

1596-443: Is expected to operate for 5.5 years. The Sentinel-6 program includes two identical satellites, to be launched five years apart, Sentinel-6 Michael Freilich, which launched on 21 November 2020, and Sentinel-6B, which will launch in 2025. These satellites will measure sea level change from space, which have been measured without interruption since 1992. Formerly called Sentinel-6A and Jason-CS A ( Jason Continuity of Service-A ), it

1680-418: Is identical in stereo and monaural processes. FM is commonly used at VHF radio frequencies for high-fidelity broadcasts of music and speech . In broadcast services, where audio fidelity is important, wideband FM is generally used. Analog TV sound is also broadcast using FM. Narrowband FM is used for voice communications in commercial and amateur radio settings. In two-way radio , narrowband FM (NBFM)

1764-605: Is not permitted by regulations.) Older airliners from the 1960s (such as the British Aircraft Corporation BAC 1-11 ) and smaller airliners in the sub-50 seat class (such as the ATR 42 and BAe Jetstream series) are equipped with them. Radar altimeters are an essential part in ground proximity warning systems (GPWS), warning the pilot if the aircraft is flying too low or descending too quickly. However, radar altimeters cannot see terrain directly ahead of

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1848-487: Is reduced to an acceptable level. FM is also used at audio frequencies to synthesize sound. This technique, known as FM synthesis , was popularized by early digital synthesizers and became a standard feature in several generations of personal computer sound cards . Edwin Howard Armstrong (1890–1954) was an American electrical engineer who invented wideband frequency modulation (FM) radio. He patented

1932-626: Is that it has a larger signal-to-noise ratio and therefore rejects radio frequency interference better than an equal power amplitude modulation (AM) signal. For this reason, most music is broadcast over FM radio . However, under severe enough multipath conditions it performs much more poorly than AM, with distinct high frequency noise artifacts that are audible with lower volumes and less complex tones. With high enough volume and carrier deviation audio distortion starts to occur that otherwise wouldn't be present without multipath or with an AM signal. Frequency modulation and phase modulation are

2016-405: Is the frequency deviation , which represents the maximum shift away from f c in one direction, assuming x m ( t ) is limited to the range ±1. It is important to realize that this process of integrating the instantaneous frequency to create an instantaneous phase is quite different from what the term "frequency modulation" naively implies, namely directly adding the modulating signal to

2100-466: Is the Deviation ratio which is the ratio of frequency deviation to highest frequency of modulating non-sinusoidal signal. FM provides improved signal-to-noise ratio (SNR), as compared for example with AM . Compared with an optimum AM scheme, FM typically has poorer SNR below a certain signal level called the noise threshold, but above a higher level – the full improvement or full quieting threshold –

2184-486: Is the Modulation index which is the ratio of frequency deviation to highest frequency in the modulating signal and f m {\displaystyle f_{m}\,} is the highest frequency in the modulating signal. Condition for application of Carson's rule is only sinusoidal signals. For non-sinusoidal signals: where W is the highest frequency in the modulating signal but non-sinusoidal in nature and D

2268-468: Is the highest fundamental of the modulating binary waveform. In the case of digital modulation, the carrier f c {\displaystyle f_{c}\,} is never transmitted. Rather, one of two frequencies is transmitted, either f c + Δ f {\displaystyle f_{c}+\Delta f} or f c − Δ f {\displaystyle f_{c}-\Delta f} , depending on

2352-416: Is used for FM broadcasting , in which music and speech are transmitted with up to 75 kHz deviation from the center frequency and carry audio with up to a 20 kHz bandwidth and subcarriers up to 92 kHz. For the case of a carrier modulated by a single sine wave, the resulting frequency spectrum can be calculated using Bessel functions of the first kind, as a function of the sideband number and

2436-586: Is used to conserve bandwidth for land mobile, marine mobile and other radio services. A high-efficiency radio-frequency switching amplifier can be used to transmit FM signals (and other constant-amplitude signals ). For a given signal strength (measured at the receiver antenna), switching amplifiers use less battery power and typically cost less than a linear amplifier . This gives FM another advantage over other modulation methods requiring linear amplifiers, such as AM and QAM . There are reports that on October 5, 1924, Professor Mikhail A. Bonch-Bruevich , during

2520-536: Is widely used in computer modems such as fax modems , telephone caller ID systems, garage door openers, and other low-frequency transmissions. Radioteletype also uses FSK. Frequency modulation is widely used for FM radio broadcasting . It is also used in telemetry , radar , seismic prospecting, and monitoring newborns for seizures via EEG , two-way radio systems, sound synthesis , magnetic tape-recording systems and some video-transmission systems. In radio transmission, an advantage of frequency modulation

2604-586: The BBC called it "VHF radio" because commercial FM broadcasting uses part of the VHF band – the FM broadcast band ). FM receivers employ a special detector for FM signals and exhibit a phenomenon known as the capture effect , in which the tuner "captures" the stronger of two stations on the same frequency while rejecting the other (compare this with a similar situation on an AM receiver, where both stations can be heard simultaneously). Frequency drift or

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2688-427: The E band , K a band , or, for more advanced sea-level measurement, S band . Radar altimeters also provide a reliable and accurate method of measuring height above water, when flying long sea-tracks. These are critical for use when operating to and from oil rigs. The altitude specified by the device is not the indicated altitude of the standard barometric altimeter. A radar altimeter measures absolute altitude :

2772-829: The European Commission , the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), NASA , and the National Oceanic and Atmospheric Administration (NOAA), with funding support from the European Commission and technical support from France's National Centre for Space Studies ( CNES , Centre national d'études spatiales). The Sentinel-6 mission is part of the Copernicus programme initiative,

2856-405: The FM capture effect removes print-through and pre-echo . A continuous pilot-tone, if added to the signal – as was done on V2000 and many Hi-band formats – can keep mechanical jitter under control and assist timebase correction . These FM systems are unusual, in that they have a ratio of carrier to maximum modulation frequency of less than two; contrast this with FM audio broadcasting, where

2940-451: The U.S. Air Force have a forward-looking, terrain-following radar (TFR) system connected via digital computer to their automatic pilots . Beneath the nose radome are two separate TFR antennae, each providing individual information to the dual-channel TFR system. In case of a failure in that system, the F-111 has a back-up radar altimeter system, also connected to the automatic pilot. Then, if

3024-625: The sinusoidal carrier is x c ( t ) = A c cos ⁡ ( 2 π f c t ) {\displaystyle x_{c}(t)=A_{c}\cos(2\pi f_{c}t)\,} , where f c is the carrier's base frequency, and A c is the carrier's amplitude, the modulator combines the carrier with the baseband data signal to get the transmitted signal: where f Δ = K f A m {\displaystyle f_{\Delta }=K_{f}A_{m}} , K f {\displaystyle K_{f}} being

3108-407: The speed of light . Radar altimeters required a simple system for measuring the time-of-flight that could be displayed using conventional instruments, as opposed to a cathode ray tube normally used on early radar systems. To do this, the transmitter sends a frequency modulated signal that changes in frequency over time, ramping up and down between two frequency limits, F min and F max over

3192-407: The 1910s, Bell Telephone was struggling with the reflection of signals caused by changes in impedance in telephone lines, typically where equipment connected to the wires. This was especially significant at repeater stations, where poorly matched impedances would reflect large amounts of the signal and made long-distance telephony difficult. Engineers noticed that the reflections appeared to have

3276-419: The F-111 ever dips below the preset minimum altitude (for example, 15 meters) for any reason, its automatic pilot is commanded to put the F-111 into a 2G fly-up (a steep nose-up climb ) to avoid crashing into terrain or water. Even in combat, the hazard of a collision is far greater than the danger of being detected by an enemy. Similar systems are used by F/A-18 Super Hornet aircraft operated by Australia and

3360-723: The German invasion approached the labs in Paris. The labs were deliberately destroyed to prevent the research from falling into German hands. The German teams found the antennas in the rubble and demanded an explanation. The IT&T director of research deflected suspicion by showing them the altimeter unit on the cover of a magazine and admonishing them for not being up-to-date on the latest navigation techniques. Radar altimeters are frequently used by commercial aircraft for approach and landing, especially in low-visibility conditions (see instrument flight rules ) and automatic landings , allowing

3444-472: The Heaviside layer and back again, the skywave. Accurately measuring the distance travelled by the skywave, proving it was actually in the sky, was necessary for the demonstration. This was the purpose of the changing frequency. Since the ground signal travelled a shorter distance, it was more recent and thus closer to the frequency being sent at that instant. The skywave, having to travel a longer distance,

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3528-755: The SNR is much improved over AM. The improvement depends on modulation level and deviation. For typical voice communications channels, improvements are typically 5–15 dB. FM broadcasting using wider deviation can achieve even greater improvements. Additional techniques, such as pre-emphasis of higher audio frequencies with corresponding de-emphasis in the receiver, are generally used to improve overall SNR in FM circuits. Since FM signals have constant amplitude, FM receivers normally have limiters that remove AM noise, further improving SNR. FM signals can be generated using either direct or indirect frequency modulation: Many FM detector circuits exist. A common method for recovering

3612-555: The UK and Arthur Kennelly in the USA independently postulated the existence of an ionized layer in the upper atmosphere that was bouncing the signal back to the ground so it could be received. This became known as the Heaviside layer . While an attractive idea, direct evidence was lacking. In 1924, Edward Appleton and Miles Barnett were able to demonstrate the existence of such a layer in

3696-620: The United States. The International Telecommunication Union (ITU) defines radio altimeters as “radionavigation equipment, on board an aircraft or spacecraft, used to determine the height of the aircraft or the spacecraft above the Earth's surface or another surface" in article 1.108 of the ITU Radio Regulations (RR). Radionavigation equipment shall be classified by the radiocommunication service in which it operates permanently or temporarily. The use of radio altimeter equipment

3780-404: The aircraft, only that below it; such functionality requires either knowledge of position and the terrain at that position or a forward looking terrain radar. Radar altimeter antennas have a fairly large main lobe of about 80° so that at bank angles up to about 40°, the radar detects the range from the aircraft to the ground (specifically to the nearest large reflecting object). This is because range

3864-457: The amplitude A m {\displaystyle A_{m}\,} of the modulating sinusoid is represented in the peak deviation f Δ = K f A m {\displaystyle f_{\Delta }=K_{f}A_{m}} (see frequency deviation ). The harmonic distribution of a sine wave carrier modulated by such a sinusoidal signal can be represented with Bessel functions ; this provides

3948-664: The autopilot to know when to begin the flare maneuver . Radar altimeters give data to the autothrottle which is a part of the Flight Computer . Radar altimeters generally only give readings up to 2,500 feet (760 m) above ground level (AGL). Frequently, the weather radar can be directed downwards to give a reading from a longer range, up to 60,000 feet (18,000 m) AGL. As of 2012 , all airliners are equipped with at least two and possibly more radar altimeters, as they are essential to autoland capabilities. (As of 2012 , determining height through other methods such as GPS

4032-402: The basis for a mathematical understanding of frequency modulation in the frequency domain. As in other modulation systems, the modulation index indicates by how much the modulated variable varies around its unmodulated level. It relates to variations in the carrier frequency : where f m {\displaystyle f_{m}\,} is the highest frequency component present in

4116-405: The binary state 0 or 1 of the modulation signal. If h ≫ 1 {\displaystyle h\gg 1} , the modulation is called wideband FM and its bandwidth is approximately 2 f Δ {\displaystyle 2f_{\Delta }\,} . While wideband FM uses more bandwidth, it can improve the signal-to-noise ratio significantly; for example, doubling

4200-575: The carrier frequency which would result in a modulated signal that has spurious local minima and maxima that do not correspond to those of the carrier. While most of the energy of the signal is contained within f c ± f Δ , it can be shown by Fourier analysis that a wider range of frequencies is required to precisely represent an FM signal. The frequency spectrum of an actual FM signal has components extending infinitely, although their amplitude decreases and higher-order components are often neglected in practical design problems. Mathematically,

4284-482: The change in the carrier frequency is about the same as the signal frequency, or as wideband if the change in the carrier frequency is much higher (modulation index > 1) than the signal frequency. For example, narrowband FM (NFM) is used for two-way radio systems such as Family Radio Service , in which the carrier is allowed to deviate only 2.5 kHz above and below the center frequency with speech signals of no more than 3.5 kHz bandwidth. Wideband FM

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4368-470: The demodulation may be carried out by using the Hilbert transform (implemented as a filter) to recover the instantaneous phase, and thereafter differentiating this phase (using another filter) to recover the instantaneous frequency. Alternatively, a complex mixer followed by a bandpass filter may be used to translate the signal to baseband, and then proceeding as before. When an echolocating bat approaches

4452-418: The development of commercial radio systems. Espenschied's patent was not granted until 1936, and its publication generated intense interest. Around the same time, Bell Labs had been working on new tube designs that were capable of delivering between 5 and 10 Watts at up to 500 MHz, perfect for the role. This led Sandretto to contact Bell about the idea, and in 1937 a partnership between Bell Labs and UAL

4536-670: The distance to the end of wires. The two used it as the basis for a joint senior thesis in 1929. Everitt disclosed the concept to the US Patent Office , but did not file a patent at that time. He then approached the Daniel Guggenheim Fund for the Promotion of Aeronautics for development funding. Jimmy Doolittle , secretary of the Foundation, approached Vannevar Bush of Bell Labs to pass judgment. Bush

4620-411: The distance to the reflectors in the wire. In 1929, William Littell Everitt , a professor at Ohio State University , began considering the use of Appleton's basic technique as the basis for an altimeter system. He assigned the work to two seniors, Russell Conwell Newhouse and M. W. Havel. Their experimental system was more in common with the earlier work at Bell, using changes in frequency to measure

4704-447: The height " Above Ground Level " (AGL). As of 2010 , all commercial radar altimeters use linear frequency-modulated continuous-wave (LFMCW or FMCW) and about 25,000 aircraft in the US have at least one radio altimeter. The underlying concept of the radar altimeter was developed independent of the wider radar field, and originates in a study of long-distance telephony at Bell Labs . During

4788-579: The information signal is through a Foster–Seeley discriminator or ratio detector . A phase-locked loop can be used as an FM demodulator. Slope detection demodulates an FM signal by using a tuned circuit which has its resonant frequency slightly offset from the carrier. As the frequency rises and falls the tuned circuit provides a changing amplitude of response, converting FM to AM. AM receivers may detect some FM transmissions by this means, although it does not provide an efficient means of detection for FM broadcasts. In Software-Defined Radio implementations

4872-464: The instantaneous frequency deviation , i.e. the difference between the frequency of the carrier and its center frequency, has a functional relation to the modulating signal amplitude. Digital data can be encoded and transmitted with a type of frequency modulation known as frequency-shift keying (FSK), in which the instantaneous frequency of the carrier is shifted among a set of frequencies. The frequencies may represent digits, such as '0' and '1'. FSK

4956-444: The luminance ("black-and-white") component of video to (and retrieving video from) magnetic tape without distortion; video signals have a large range of frequency components – from a few hertz to several megahertz , too wide for equalizers to work with due to electronic noise below −60  dB . FM also keeps the tape at saturation level, acting as a form of noise reduction ; a limiter can mask variations in playback output, and

5040-711: The main objective of the Sentinel-6 mission is to measure sea surface topography with high accuracy and reliability to support ocean forecasting systems, environmental monitoring and climate monitoring. The mission definition is driven by the need for continuity in provision of TOPEX/Poseidon mission and Jason satellite series ( Jason-1 , OSTM/Jason-2 , and Jason-3 ) with improvements in instrument performance and coverage. ESA, NASA, and EUMETSAT will provide mission management and system engineering support. EUMETSAT and NASA will be responsible for long-term archives of altimetry data products. All partners will be involved with

5124-402: The modulating signal x m ( t ), and Δ f {\displaystyle \Delta {}f\,} is the peak frequency-deviation – i.e. the maximum deviation of the instantaneous frequency from the carrier frequency. For a sine wave modulation, the modulation index is seen to be the ratio of the peak frequency deviation of the carrier wave to the frequency of

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5208-536: The modulating sine wave. If h ≪ 1 {\displaystyle h\ll 1} , the modulation is called narrowband FM (NFM), and its bandwidth is approximately 2 f m {\displaystyle 2f_{m}\,} . Sometimes modulation index h < 0.3 {\displaystyle h<0.3}  is considered NFM and other modulation indices are considered wideband FM (WFM or FM). For digital modulation systems, for example, binary frequency shift keying (BFSK), where

5292-448: The modulation index. The carrier and sideband amplitudes are illustrated for different modulation indices of FM signals. For particular values of the modulation index, the carrier amplitude becomes zero and all the signal power is in the sidebands. Since the sidebands are on both sides of the carrier, their count is doubled, and then multiplied by the modulating frequency to find the bandwidth. For example, 3 kHz deviation modulated by

5376-511: The ocean stores and redistributes heat, water, and carbon in the climate system. The two satellites, Sentinel-6 Michael Freilich and Sentinel-6B, will extend this legacy through to at least 2030, which will provide a nearly forty-year record of sea level rise as well as changes in ocean currents. The Sentinel-6 was developed by European Space Agency (ESA) in the context of the European Copernicus Programme led by

5460-516: The only ones working on the concept. Seeing that the idea was already not a secret, the Mission introduced the NRC to its production quality designs. The Bell-based design was abandoned in favour of building the fully developed British ASV Mark II design, which operated at much higher power levels. In France, researchers at IT&T 's French division were carrying out similar experiments on radar when

5544-505: The power of a frequency-modulated signal lies within a bandwidth B T {\displaystyle B_{T}\,} of: where Δ f {\displaystyle \Delta f\,} , as defined above, is the peak deviation of the instantaneous frequency f ( t ) {\displaystyle f(t)\,} from the center carrier frequency f c {\displaystyle f_{c}} , β {\displaystyle \beta }

5628-411: The ratio is around 10,000. Consider, for example, a 6-MHz carrier modulated at a 3.5-MHz rate; by Bessel analysis, the first sidebands are on 9.5 and 2.5 MHz and the second sidebands are on 13 MHz and −1 MHz. The result is a reversed-phase sideband on +1 MHz; on demodulation, this results in unwanted output at 6 – 1 = 5 MHz. The system must be designed so that this unwanted output

5712-619: The regenerative circuit in 1914, the superheterodyne receiver in 1918 and the super-regenerative circuit in 1922. Armstrong presented his paper, "A Method of Reducing Disturbances in Radio Signaling by a System of Frequency Modulation", (which first described FM radio) before the New York section of the Institute of Radio Engineers on November 6, 1935. The paper was published in 1936. As the name implies, wideband FM (WFM) requires

5796-510: The same line. During this same period there was a great debate in physics over the nature of radio propagation. Guglielmo Marconi 's successful trans-Atlantic transmissions appeared to be impossible. Studies of radio signals demonstrated they travelled in straight lines, at least over long distances, so the broadcast from Cornwall should have disappeared into space instead of being received in Newfoundland . In 1902, Oliver Heaviside in

5880-428: The selection of science investigators. ESA EUMETSAT NASA NOAA CNES Radar altimeter As the name implies, radar ( ra dio d etection a nd r anging) is the underpinning principle of the system. The system transmits radio waves down to the ground and measures the time it takes them to be reflected back up to the aircraft. The altitude above the ground is calculated from the radio waves' travel time and

5964-404: The sensitivity of the frequency modulator and A m {\displaystyle A_{m}} being the amplitude of the modulating signal or baseband signal. In this equation, f ( τ ) {\displaystyle f(\tau )\,} is the instantaneous frequency of the oscillator and f Δ {\displaystyle f_{\Delta }\,}

6048-575: The signal-to-noise ratio in the user's ear. They are also called auditory trainers , a term which refers to any sound amplification system not classified as a hearing aid . They intensify signal levels from the source by 15 to 20 decibels. FM systems are used by hearing-impaired people as well as children whose listening is affected by disorders such as auditory processing disorder or ADHD . For people with sensorineural hearing loss , FM systems result in better speech perception than hearing aids. They can be coupled with behind-the-ear hearing aids to allow

6132-427: The system worked as advertised, but stated that it would have to work at higher frequencies to be practical. Espenschied had also been considering the use of Appleton's idea for altitude measurement. In 1926 he suggested the idea both as a way to measure altitude as well as a forward-looking system for terrain avoidance and collision detection. However, at that time the frequency of available radio systems even in what

6216-408: The time. It had been known since the late 1800s that metal and water made excellent reflectors of radio signals, and there had been many attempts to build ship, train and iceberg detectors over the years since that time. Most of these had significant practical limitations due to the use of low-frequency signals that demanded large antennas to provide reasonable performance. The Bell unit, operating at

6300-459: The two complementary principal methods of angle modulation ; phase modulation is often used as an intermediate step to achieve frequency modulation. These methods contrast with amplitude modulation , in which the amplitude of the carrier wave varies, while the frequency and phase remain constant. If the information to be transmitted (i.e., the baseband signal ) is x m ( t ) {\displaystyle x_{m}(t)} and

6384-434: The value of Δ f {\displaystyle \Delta {}f\,} , while keeping f m {\displaystyle f_{m}} constant, results in an eight-fold improvement in the signal-to-noise ratio. (Compare this with chirp spread spectrum , which uses extremely wide frequency deviations to achieve processing gains comparable to traditional, better-known spread-spectrum modes). With

6468-445: Was delayed, and was thus the frequency as it was some time ago. By mixing the two in a frequency mixer, a third signal is produced that has its own unique frequency that encodes the difference in the two inputs. Since in this case the difference is due to the longer path, the resulting frequency directly reveals the path length. Although technically more challenging, this was ultimately the same basic technique being used by Bell to measure

6552-399: Was formed to build a practical version. Led by Newhouse, a team had a working model in testing in early 1938, and Western Electric (Bell's manufacturing division) was already gearing up for a production model. Newhouse also filed several patents on improvements in technique based on this work. The system was publicly announced on 8 and 9 October 1938. During World War II , mass production

6636-519: Was known as shortwave was calculated to be fifty times lower than what would be needed for a practical system. Espenschied eventually filed a patent on the idea in 1930. By this time, Newhouse had left Ohio State and taken a position at Bell Labs. Here he met Peter Sandretto , who was also interested in radio navigation topics. Sandretto left Bell in 1932 to become the Superintendent of Communications at United Air Lines (UAL), where he led

6720-488: Was on the order of 9%, but this might be as high as 10% when flying over rough terrain like the built-up areas of cities. During early flights of the system, it was noticed that the pattern of the returns as seen on an oscilloscope was distinct for different types of terrain below the aircraft. This opened the possibility of all sorts of other uses for the same technology, including ground-scanning and navigation. However, these concepts were not able to be explored by Bell at

6804-506: Was renamed in honor of the former director of NASA Earth Science Division , Michael Freilich , who was instrumental in advancing space-based ocean measurements. It follows the most recent U.S.-European sea level observation satellite, Jason-3 , which launched in 2016, and is currently providing high-precision and timely observations of the topography of the global ocean. Since the launch of TOPEX/Poseidon on 10 August 1992, high-precision satellite altimeters have been essential to monitor how

6888-440: Was skeptical that the system could be developed at that time, but nevertheless suggested the Foundation fund development of a working model. This allowed Newhouse to build an experimental machine which formed the basis of his 1930 Master's thesis, in partnership with J. D. Corley. The device was taken to Wright Field where it was tested by Albert Francis Hegenberger , a noted expert in aircraft navigation. Hegenberger found that

6972-406: Was taken up by RCA , who produced them under the names ABY-1 and RC-24. In the post-war era, many companies took up production and it became a standard instrument on many aircraft as blind landing became commonplace. A paper describing the system was published jointly by Espenschied and Newhouse the next year. The paper explores sources of error and concludes that the worst-case built-in scenario

7056-415: Was working at Bell Labs when he conceived using this same phenomenon to measure distances in a wire. One of his first developments in this field was a 1919 patent (granted 1924) on the idea of sending a signal into railway tracks and measuring the distance to discontinuities. These could be used to detect broken tracks, or if the distance was changing more rapidly than the speed of the train, other trains on

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