HAARP TEC Receivers

HAARP TEC Receivers


Total Electron Content (TEC)The ionospheric plasma is made up of positive ions and negative electrons. One of its most important parameters is plasma density, measured in electrons/meter3. Adding up plasma density in small increments along a line gives its "line integral" which is called total electron content (electrons/meter2), or TEC. A plasma reduces the speed of a radio signal traveling through it. The greater its density and/or the lower the radio frequency, the more it is slowed. This signal delay introduces an error in measurements made by the Global Positioning System (GPS) and other satellite-borne navigation systems. To permit correction for the error, GPS satellites transmit on two frequencies. The difference in range measured on the two frequecies by some sophisticated GPS receivers (but not all) provides a measurement of TEC and permits correction. At HAARP, an Ashtech Z-FX Continuously Operating Reference Station (CORS) is used to measure absolute TEC on the raypaths to several GPS satellites.
Because GPS satellites move slowly across the sky, the Z-FX CORS measurements provide essentially snapshots of TEC at a few locations over Alaska. Several "Oscar" Class Transit satellites of the old Navy Navigation Satellite System (now decommissioned) are still in polar orbits. They move rapidly over Alaska, transmitting phase-coherent signals at 150 and 400 MHz, as do several other U.S. and Russian satellites. The Transit-like satellites do not permit direct measurement of absolute TEC, but the phases of their two signals (locked together when they leave the transmitter onboard the satellite) do shift relative to one another as they travel. The differential (or dispersive) phase shift is proportional to TEC, permitting a latitudinal scan of relative TEC. This measurement is relative because the phase shifts by many cycles along each instantaneous path and so is not known absolutely. At HAARP, we convert the measurement to absolute TEC when the track passes near a GPS raypath. So long as a receiver stays locked to the Transit-like signals during a satellite pass the change in dispersive phase provides a scan of TEC across the sky.
In the daytime, the sun's ultraviolet radiation usually produces more plasma (and, therefore, TEC) at middle latitudes than in Alaska. At night, the auroral ionosphere in Alaska often has more plasma than does the mid-latitude ionosphere. The auroral ionosphere often is patchy, made up of "clouds" of ions and electrons separated by less-dense patches. The density, size, and motion of these plasma patches has much to do with the interaction between the sun, the solar wind, and the earth's magnetic field and upper atmosphere. Measuring TEC as Transit-like satellites move in polar orbits (at about a thousand km altitude) is an important way of finding out where most of the ionospheric plasma is and how patchy it is.
The NWRA ITS10 Coherent Radio Receiver Northwest Research Associates (NWRA), under contract to the Air Force Research Laboratory, has developed an instrument (the ITS10) specifically for measuring dispersive phase between the two phase-coherent signals received from a Transit-like satellite. Using a simple dipole antenna (seen over an elevated ground screen in the foreground of the photo to the left), the receiver automatically locks to the UHF (400 MHz) signal when it "hears" such a Transit-like satellite rise above the horizon. It then continuously measures the dispersive phase between that reference signal and the satellite's VHF (150 MHz) signal until the satellite sets, producing a TEC scan across Alaska, usually an approximately latitudinal one, during an interval of minutes.
A larger photo is available
A totally passive system (not emitting radio waves), the ITS10 is located in the HAARP optical shelter, which can be seen in the background of the above photo. Its antenna collects a signal with a power of -115 dBm at UHF and -109 dBm at VHF. (This is about 10-17 times the power in a 100-watt light bulb). Low-noise preamplifiers mounted beneath the antenna deliver those signals, via shielded coaxial cables, to the shelter. There, the ITS10 amplifies them and converts them to 50-KHz baseband signals, which are sampled and processed digitally to record the strength of the received signals and the dispersive phase between them. An on-line desktop computer converts the dispersive phase to TEC, sends instrument status and diagnostic information over the Internet to NWRA in Bellevue, WA, and stores the TEC data on HAARP's World-Wide-Web server here in Gakona.

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Last updated June 24, 1999.