This is known generally as out-of- band emissions. Since these undesired signals may be on the desired receive frequencies, they can only be eliminated by filtering the offending transmitter or moving the offending transmitter to another location. Some types of natural phenomena that affect electromagnetic waves propagation. The radio frequency is reflected for long distances through this layer and returns to earth, causing harmful interference between uses in areas separated by great distances.
This phenomenon is much more prevalent at frequencies below 30 or 40 MHZ and is time dependent. The interference usually occurs more at night and in the evenings. Electromagnetic waves are reflected to all ranges at these layers and travel much farther than normal, causing harmful interference to frequencies similar or close in frequency to the long distances, and this phenomenon occurs early in the morning and ends by noon.
General RF Noise Devices other than radio equipment can cause radio frequency energy. Typical of items that can cause interference are arc welders, electric motors, faulty spark plug wires, lightning, and even rusty tower bolts. Regions where RFI removal is successful share two primary characteristics:. Though successful, it is important to point out that most of the RFI mitigation efforts at the L-band have been largely reactive in nature in that the interference environment was not fully understood until the sensor was in orbit.
In the case of the JAXA sensors, for instance, it is fortuitous that both the design of the sensor and the precise nature of the interference environment have combined to allow RFI mitigation to be successful.
Noteworthy cases include some urban areas of East Asia. Although L-band remote sensing systems have not experienced the severe transmit restrictions that have occasionally been imposed on P-band systems, they appear to be coming under increased scrutiny in the spectrum approval process. Although no proposed spaceborne system has so far been denied the ability to operate, the burden of proof imposed on such systems to demonstrate that they will not adversely impact incumbent systems appears to be growing heavier in recent years.
Airborne systems have likewise been impacted. For instance, although UAVSAR has generally not had difficulty operating over the United States, the full Stage 4 certification approval has nevertheless been delayed due to concerns of agencies that operate incumbent systems. Although these measures can be seen as justifiable means to demonstrate that critical incumbent services will not be interfered with, the perception of many in the science community is that the criteria for interference are often ill defined, and, consequently, extreme conservatism is invoked at the expense of science measurements.
Again, it is noted that no instances or reports of L-band science sensors actually having interfered with incumbent services have come to the attention of the committee. The S-band is a good intermediate frequency that has many of the long-wavelength advantages of the L-band good penetration of low vegetation, low temporal decorrelation, low tropospheric propagation losses , as well as some of the advantages of higher frequencies smaller antennas, lower degree of ionospheric effects, ability to measure rain when operated at high power in ground-based applications.
Whereas the. No information regarding RFI experienced at the S-band has come to the attention of the committee. Little information regarding the Chinese SAR observations has been available to the committee. No ongoing difficulties with spectrum access at the S-band have come to the attention of the committee.
The choice of the C-band offers a suitable compromise between the antenna aperture size required to achieve a desired beamwidth which decreases with frequency and the attenuation of the radar signal as it propagates through rain and clouds which increases with frequency. The C-band is also used for Doppler weather radar, which detects the motion of rain droplets in addition to the intensity of precipitation. Examples of satellite SAR systems operating at the C-band include the Canadian Radarsat-1, which was launched in , and Radarsat-2, which was launched in Examples of satellite.
C-band ground-based weather radars operate in the primary radiolocation band of MHz. The biggest concern for C-band science radar interference is the proliferation of wireless access systems, which include radio local access networks RLANs. There has been a subsequent push by the wireless industry, however, for the addition of the MHz band for RLAN use. A major concern is that extension of mobile services to the MHz band would eventually be applied worldwide.
As an interference mitigation approach, it has been proposed that radar and wireless access system WAS technologies will coexist in the same environment through a frequency abandonment protocol implemented in the RLANs. Before using a channel, the RLAN would check for the presence of radar signals. If any signal is detected, the RLANs would vacate the channel for a minute period. In addition, before reusing the channel, the RLANs must continuously monitor.
Furthermore, the expected density of RLANS in a suburban environment will exceed the maximum interference limit by over 24 dB. This presents a significant degradation to C-band SAR performance over major areas of coverage. It has also been well documented that RLANs can severely interfere with the operation of ground-based weather radars.
The study identified a set of problems with the commercial DFS implementations, which were remedied in subsequent versions of the RLANs. Compatibility studies with simulation results have shown that the EESS-Active does not cause harmful interference to the other services in the C-band. Overall, no ongoing difficulties with spectrum access at the C-band have come to the attention of the committee. The X-band is another important band for scientific radar imaging. X-band science sensors share the band with terrestrial radio navigation radars and radiolocation tracking radars operating between MHz.
Although some anecdotal discussion of RFI observed by X-band SAR came to the attention of the committee, this has not been reported to be much of a current problem. At least over North America, most of the current radio navigation and radiolocation radars at the X-band have either unmodulated pulses or frequency modulated pulses with relatively narrow bandwidths of 1 MHz or less.
As previously discussed, mitigation techniques have been demonstrated for this type of interference at the L-band and the C-band. There are no known restrictions on spectral access for science sensors in the X-band. The pulsed radars of the EESS-Active are generally very compatible with the pulsed radars of the radio navigation and radiolocation service radars.
Driven by the desire to achieve higher spatial resolution for science and applications, there is an Agenda Item AI 1. This is proposed to be accomplished by expanding in a contiguous lower band, in a contiguous upper band, or in a combination of lower and upper bands.
Although there has been interest in creating new allocations for EESS-Active in other regions of the spectrum, this proposal at the X-band represents perhaps the most significant near-term effort to expand the EESS-Active allocation rather than simply defend it. The K u -band is an important band for spaceborne altimeters, scatterometers, and precipitation radars, as well as many airborne systems making similar measurements on a more regional basis over the years.
The K u -band is also being used for a relatively new measurement of snow depth and snow water equivalent by airborne systems. There is interest among European and U. No reports of significant RFI at the K u -band have come to the attention of the committee. It is largely unknown if future sensors seeking to make more sensitive measurements could observe RFI in the future, but that is certainly possible.
K u -band radar interference could come from aeronautical radio navigation radars operating in the MHz band and radiolocation radars operating in the MHz band and the MHz band. An NTIA report indicates that emitters at the K u -band show occupancy by high-power, long-range air search radars and terrestrial point-to-point communications.
There is an Agenda Item AI 1. There is also an Agenda Item AI 1. Moreover, there is also an Agenda Item AI 1. These services represent a potential threat to science sensors either as future interference sources or as future spectral co-occupants that could impose transmit restrictions on active science systems. Sanders, B. Ramsey, and V. Although compatibility with existing services must always be established before flying any new sensor, no current significant barriers to operating active sensors at these frequencies have come to the attention of the committee.
The K a -band is employed for altimeters, SARs, and cloud and precipitation radars. Multiple airborne systems exist to make these measurements for regional science or as demonstration platforms for space missions. Because of the relatively compact electronics and antennas, as well as the sensitivity to atmospheric hydrometeors associated with this high RF, the K a -band is expected to be a significant growth area, seeing more and more sensors flown in the near future.
No reports of significant RFI at the K a -band have come to the attention of the committee. Like other frequencies, there are some known, very-high-power ground radars that K a -band sensors must take care to protect themselves against.
Similar to the K a -band, many current sensors either predominantly operate over the ocean or significantly process the data onboard, making a characterization of low to moderate RFI difficult. As with the K u -band, it is currently difficult to assess the prospects of future measurements with respect to RFI.
There are some restrictions on transmitted signals for science sensors at the K a -band. Modern sensors such as KaRIN on SWOT meet this criterion, but not with a large amount of margin, perhaps suggesting that significant operational limitations may exist if the science community wishes to employ higher-power or higher-gain systems in the future.
The W-band is used primarily for cloud profiling radar CPR systems. W-band radar interference could potentially come from radiolocation radars operating in the MHz band and the MHz band. There are restrictions on spectral access for science sensors in the W-band related to possible interference with radio astronomy a passive radio science technique.
Another Radio Regulations footnote 5. These bands have been proposed for possible spaceborne use for more sensitive cloud radars, but no satellite remote sensing EESS-Active system currently operates in this spectral region. These bands, however, are a potential new frontier for science sensors, and there is danger of potentially losing these bands to other services if they are not used.
Interference in the mm-band could potentially come from fixed, intersatellite, and mobile service systems operating in the MHz band. There are restrictions on spectral access for science sensors in the mm-band.
The lower mm-band at The upper mm-band at There is no such restriction in the lower band at Finding 8.
Like passive sensors, active sensors can experience RFI from other radio services. Conversely, and unlike passive systems, active systems also transmit signals and are hence subject to operational restrictions to ensure that they do not interfere with other services.
With demand for and use of the spectrum growing rapidly, both of these spectrum issues are generating concerns for the successful operation of current and planned active science sensors. Conservative interference standards in some bands can make science operations difficult. Restrictions imposed in the lower-frequency UHF and L-bands are increasing with time. The only documented instance to come to the attention of the committee of an active science sensor actually interfering with the operations of another service was the CloudSat radar, which can interfere with the radio astronomy service another science service.
Within the heavily used and well-studied L-band allocation of MHz, the amount of RFI observed worldwide has steadily increased over time. The more that sources, or aggregates of sources, resemble broadband white noise, the more difficult the interference is to mitigate with known techniques.
Consequently, active remote sensing is able to more effectively share with some services than others. The proposed MHz. The broadband, noiselike nature of RLAN emitters is difficult or impossible to mitigate. Consequently, RFI measures are often only fully implemented reactively when the true nature of the interference environment is encountered after a new sensor is deployed.
This is due to both the incompleteness of information regarding current emitters worldwide and the evolving nature of the RFI environment over time. This, coupled with Finding 8. Such instruments are described as being rich in tone color. And even the best choirs will earn their money when two singers sing two notes i.
Music is a mixture of sound waves that typically have whole number ratios between the frequencies associated with their notes. In fact, the major distinction between music and noise is that noise consists of a mixture of frequencies whose mathematical relationship to one another is not readily discernible.
On the other hand, music consists of a mixture of frequencies that have a clear mathematical relationship between them.
While it may be true that "one person's music is another person's noise" e. To demonstrate this nature of music, let's consider one of the simplest mixtures of two different sound waves - two sound waves with a frequency ratio. This combination of waves is known as an octave. A simple sinusoidal plot of the wave pattern for two such waves is shown below. Note that the red wave has two times the frequency of the blue wave. Also observe that the interference of these two waves produces a resultant in green that has a periodic and repeating pattern.
One might say that two sound waves that have a clear whole number ratio between their frequencies interfere to produce a wave with a regular and repeating pattern. The result is music. Another simple example of two sound waves with a clear mathematical relationship between frequencies is shown below. Note that the red wave has three-halves the frequency of the blue wave. In the music world, such waves are said to be a fifth apart and represent a popular musical interval. Observe once more that the interference of these two waves produces a resultant in green that has a periodic and repeating pattern.
It should be said again: two sound waves that have a clear whole number ratio between their frequencies interfere to produce a wave with a regular and repeating pattern; the result is music. Finally, the diagram below illustrates the wave pattern produced by two dissonant or displeasing sounds.
The diagram shows two waves interfering, but this time there is no simple mathematical relationship between their frequencies in computer terms, one has a wavelength of 37 and the other has a wavelength 20 pixels. Observe look carefully that the pattern of the resultant is neither periodic nor repeating at least not in the short sample of time that is shown.
The message is clear: if two sound waves that have no simple mathematical relationship between their frequencies interfere to produce a wave, the result will be an irregular and non-repeating pattern.
This tends to be displeasing to the ear. A final application of physics to the world of music pertains to the topic of beats. Beats are the periodic and repeating fluctuations heard in the intensity of a sound when two sound waves of very similar frequencies interfere with one another. The diagram below illustrates the wave interference pattern resulting from two waves drawn in red and blue with very similar frequencies. A beat pattern is characterized by a wave whose amplitude is changing at a regular rate.
Observe that the beat pattern drawn in green repeatedly oscillates from zero amplitude to a large amplitude, back to zero amplitude throughout the pattern. Points of constructive interference C. When constructive interference occurs between two crests or two troughs, a loud sound is heard.
This corresponds to a peak on the beat pattern drawn in green. When destructive interference between a crest and a trough occurs, no sound is heard; this corresponds to a point of no displacement on the beat pattern. Since there is a clear relationship between the amplitude and the loudness, this beat pattern would be consistent with a wave that varies in volume at a regular rate.
The beat frequency refers to the rate at which the volume is heard to be oscillating from high to low volume. For example, if two complete cycles of high and low volumes are heard every second, the beat frequency is 2 Hz. The beat frequency is always equal to the difference in frequency of the two notes that interfere to produce the beats.
So if two sound waves with frequencies of Hz and Hz are played simultaneously, a beat frequency of 2 Hz will be detected. A common physics demonstration involves producing beats using two tuning forks with very similar frequencies.
If a tine on one of two identical tuning forks is wrapped with a rubber band, then that tuning forks frequency will be lowered. If both tuning forks are vibrated together, then they produce sounds with slightly different frequencies. These sounds will interfere to produce detectable beats. The human ear is capable of detecting beats with frequencies of 7 Hz and below. A piano tuner frequently utilizes the phenomenon of beats to tune a piano string.
She will pluck the string and tap a tuning fork at the same time. If the two sound sources - the piano string and the tuning fork - produce detectable beats then their frequencies are not identical.
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