SOLAR FLARE AND RADIO BLACKOUT

Earth-orbiting satellites have just detected an X1.1-class solar flare from sunspot AR3217 (Feb. 11th @ 1548 UTC). Extreme UV radiation ionized the top of Earth's atmosphere, causing a strong shortwave radio blackout over South America:

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Ham radio operators, aviators, and mariners may have noticed unusual propagation effects at frequencies below 30 MHz for as much as an hour after the flare.

This was an impulsive flare, intense and fast. It may not have lasted long enough to lift a CME out ofthe sun's atmosphere. Confirmation awaits fresh data from SOHO coronagraphs. Stay tuned. Solar flare alerts: SMS Text.

See: https://spaceweather.com

Radio propagation due to solar activity and is real. During the mid-1960's to the very early 1970's, radio propagation was especially noticeable in the low 30 MHz spectrum. At that time the Los Angeles Fire Department's primary frequency for West Los Angeles and the San Fernando Valley was 33.900 MHz. At the same time, the primary dispatch frequency of the Broome County, NY, Emergency Services and EMS was also 33.900 MHz. The radio skip for various periods in that time period was strong enough to skip the LAFD's Motorola 100 tones, for fire station alerts, and the same tones from Broome County to each other as if the two departments were adjacent to each other in the same location. Broome County's tones alerted LAFD units and vice versa. Dispatch calls and unit responses were equally loud during brushfire seasons for both departments. Over the intervening years both departments have moved to high frequency trunked radio systems*, with he LAFD moving to 800 MHz and Broome County to 400 MHz. High frequencies limit this atmospheric skip to negligible levels.
Hartmann352

* Trunked Radio Systems - centrally manage a pool of radio channels in the same frequency band, and intelligently switch, by a centralised computer which controls the frequency use and the pre-programmed radios, users to whatever channel is open at a given time. This is often a difficult concept for scanner enthusiasts to understand, as trunking represents a paradigm shift in design of the system. Trunked radio systems are one of the most complex types of radio systems in use today. Trunked radios use several channels or frequencies, and allows those channels to be shared by a large number of users, in multiple talkgroups, without their conversations interfering with each other. The more frequencies in use enables fewer busy signals and reduces user interference.

Talkgroups are defined groups of users. For example, all police officers and dispatchers would belong to a police talkgroup; all fire fighters and fire employees would belong to a fire talkgroup, and all municipal waste collectors would be in a waste management talkgroup.

Trunked systems use a control channel, called the trunk, which transmits data packets which allow a talkgroup to carry on a conversation by telling radios of a talk group which frequency to communicate on when they key up. This allows for a large number of users to communicate using only a small number of frequencies, and more efficient use of those frequencies.

Trunked radio systems operate on the assumption that not all talkgroups will be in use at once. This allows for the channels to be shared by multiple talkgroups, achieving a more efficient use of the band. Rather than requiring all radios to be set to same frequency, thus monopolizing that frequency whether or not it’s actively in use, trunked systems allow for many users to share multiple frequencies. This means that far fewer frequencies are required to support the radio traffic, and therefore greater efficiency of spectrum allocation.

Talkgroups also allow for more granular assignment of user groups. This keeps conversations relevant to the group that needs to hear them. Consider a simple example of municipal refuse collection. In a traditional non-trunked system, the refuse collection department would be assigned a frequency, and all users would hear all transmissions. With talkgroups, several distinct groups can be created. There could be one talkgroup for all trash collectors, and another talkgroup for an overlapping subset of trash collectors such as those who only pick up recycling. When a communication is only relevant for the recycling group, the recycling talkgroup can be used, sparing the wider refuse collection group from having to hear the communication.

Trunked systems generally utilize a bank of channels for talking, and a control channel of some type. Depending on the type of system, the control channel may be either fixed (dedicated), or a random channel which changes (dynamic). Regardless of the type of trunking system and the control channel type, all trunked systems need a way to let users know on which frequency their talkgroup is on. The control computer sends a signal to all the radios which are part of the talkgroup, which instructs the radios assigned to that talkgroup to tune to a specific frequency.

In many, if not most, trunked systems, the transmission and reception will remain active on the same frequency until the communication sequence is complete. With these types of systems, even without a trunk-capable radio at your disposal, you can at least follow the conversation if you should come across one. Keep in mind, however, that without trunking support, it won’t be possible to know which talkgroup you have found, and you are in essence in the dark.

Other systems will change the frequency used for communication each time a user keys up. In these types of systems, it is generally very difficult, if not impossible, to follow a conversation unless you have a trunk-capable radio.

An additional advantage of trunking systems is that, should the allotted frequency become saturated, the central controller can prioritize traffic by updating the computer. This is especially useful in municipal systems where there is limited spectrum available for multiple services. For example, in a given city, the Fire Department may have Priority over the Police Department, who may have priority over Animal Control, who may have priority over the Service Department, and so on.

Many trunked systems also have a feature where the radios (mobile, handhelds, and central dispatch) have an emergency button. When the emergency button is pressed, the system will drop lower priority users and traffic from the system to allow the emergency traffic to immediately get through to the dispatchers showing defined information in inluding the radio's number.

See: https://www.sciencedirect.com/topics/computer-science/trunked-radio-system
 

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