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Spread Spectrum Modulation Techniques

As an ex military satellite communications engineer I certainly remember working with spread spectrum modulation and also frequency hopping technology in the 1980's. Wireless Local Area Networking technology today exploits a technology which was thitherto mostly hidden inside this shadowy domain of military communications and radar. This technology comprises a collection of ideas which are termedĀ Spread Spectrum Techniques (SST). Spread Spectrum techniques have some powerful properties which make them an excellent candidate for networking applications. To better understand why, we will take a closer look at this fascinating area, and its implications for networking.

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The Chirpsounder / Ionosonde

ionosphereAnybody who has ever set up a working international HF link will know it can be a tricky business. You see there's a pesky movable thing called the ionosphere which is pretty fundamental to the whole business.
Communicating with a point halfway round the planet using HF is like trying to play that old 70's children's game called Rebound. Since radio links are usually close to or distinctly line of sight links, communicating with a point on the other side of a sphere would seem like a fairly insurmountable problem. I'd think the first time this problem was solved using the ionosphere it was probably an accident caused by some early radio pioneers receiving signals for their fellow pioneers some way round the planet and beginning to wonder why and how it was happening.

The reason it was and does happen is because of a thin layer of the Earths atmosphere called the ionosphere. The ionosphere is a region of the upper atmosphere, from about 85 km (53 mi) to 600 km (370 mi) altitude, and includes the thermosphere and parts of the mesosphere and exosphere. It is distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth. This is the reason we as Telecommunications Engineers are interested in it.

The ionosphere is a layer of electrons and electrically charged atoms and molecules in the upper Earths atmosphere, ranging from a height of about 50 km (31 mi) to more than 1,000 km (620 mi). It exists because of the Suns ultraviolet radiation which causes these gases to ionise and develop a charge. Because of the boundary between this layer and the relatively uncharged layer below, wave diffraction occurs. This phenomenon takes place at different incidences with different frequencies and, with clever utilisation of this property, the ionosphere can be utilized to "bounce" a transmitted signal down to the ground. Transcontinental HF-connections can rely on up to 5 of these bounces, or hops.

aerials-takeoff-anglesIt is the process of determining the appropriate frequencies and their respective bounce points around the planet that is the focus of this post. The applied physics involved in this refraction are beyond the scope of this post but, in a nutshell, what they do produce is a spread of frequencies which bounce at different incident angles to the boundary layer such that different distant points on the surface of the planet can be reached when the bounced radio wave returns to the ground. This is shown more clearly in the diagram on the left.

Unfortunately it is not quite as straightforward as the diagram above suggests as the strength and location of the ionosphere is always changing as day becomes night and also as cosmic radiation from the Sun changes over time. This presents those wishing to use this phenomenon with the constant problem of determining which frequencies are workable and usable between any two given points on the Earth.

The problem of determining these usable frequencies was the driving force behind the invention of the Chirpsounder (also known as an Ionosonde). The Chirpsounder, or rather a pair of Chirpsounders operate in tandem using a Chirp transmitter in one location and a Chirp receiver in another. The job of the transmitter is to transmit a sweep of radio output from one predetermined frequency to another over a given amount of time. A Chirp receiver situated close to the transmitter would, if synchronised to match the sweep timings, receive all of the sweep from the beginning to the end but the same Chirp receiver placed two thousand miles away over the Earths horizon may not fare so well. This is where the technology really comes into its own.

Screenshot_3When a Tx/Rx pair of Chirpsounders are running a synchronised sweep between two distant locations, the receiver will receive from the transmitter only during those parts of the sweep that are conducive to a working link between the two. This information is gathered by the Chirp receiver and is used to provide the user with a graph showing frequency on the x-axis and receive delay on the y-axis. There will also often be a display of receive signal strength incorporated in the output. A sample Chirpsounder output is shown on the right.

As can be seen there are a number of elements shown on the trace and each of these represents a successful reception of the signal from the transmitter. The more solid the line, the more reliable the link and this information, when used in parallel with the received power information can enable telecommunications professionals to choose the most appropriate frequency. Once the decision had been made the operational transmitters and receiver could be set appropriately and the operational radio channel could begin to pass its traffic using the ionospheric bounce. Quite amazing really.

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