Over-the-horizon radar

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United States Navy over-the-horizon radar system

The over- horizon radar (also called OTH for O ver T he H orizon ) is a way of receiving radar echoes far beyond the curvature of the earth without quasi-optical visual contact . The frequencies used are mostly in the shortwave range and thus far below the usual radar frequencies ( microwave range ), which reduces the resolution and the positioning accuracy. However, ground waves or reflection phenomena in the ionosphere can be exploited in this way, which only enables localization beyond the curvature of the earth. Over-the-horizon radar technology is used by several countries. A well-known system is the Australian Jindalee Operational Radar Network (JORN); the NATO operates a corresponding plant in Cyprus (PLUTO system of the Royal Air Force). Over-the-horizon radar transmission stations are in Semipalatinsk (Kazakhstan), Alaska (USA) and Mordovia (Russia).

Physical basics

An above-horizon radar is generally an impulse radar . All other radar concepts are not suitable for achieving ranges of several 1000 km. There are three technical requirements for over-horizon radar in order to achieve these desired ranges:

  1. The transmission pulse must meet the energetic conditions for overcoming this distance. It must contain more energy than the sum of all possible attenuations on the way there and back could weaken the echo signal so that it can no longer be registered in the receiver.
  2. The reception time must be long enough to cover the necessary transit times for the echo signal. The echo signal must be able to be clearly assigned to the transmitting pulse causing it in order to obtain a clear measurement result.
  3. The propagation of the electromagnetic waves of this transmission pulse must be able to overcome the curvature of the earth either by diffraction or reflection .

In the past, the only way to meet these energetic requirements was to use an extremely high pulse power for the transmission pulse. In order to achieve an adequate distance resolution, this transmission pulse had to be very short (in the range of a few microseconds ) and in this short time had an extremely high pulse power of up to 10 megawatts (average power: 600 kW). However, more modern device concepts can use intrapulse modulation so that the transmission energy can be distributed over a longer pulse duration. In this case, the transmission pulse requires a lower pulse power with the same energy content. With the pulse compression method , a similarly good distance resolution can be achieved as with a very short transmission pulse.

In order to enable an unambiguous assignment to the respective transmission pulse and thus avoid ambiguities in the transit time and distance measurement , only a very low pulse repetition frequency could be used. The pulse repetition frequency of older radar devices is around 10  Hz , which corresponds to a signal propagation time of around 100  ms , a distance of 30,000 km and consequently a theoretical location range of a maximum of 15,000 km. Intrapulse modulation also offers advantages here, since each individual transmission pulse can receive a different modulation pattern. This pattern enables the echo signal to be assigned to a transmission pulse independently of time and effectively prevents ambiguities in the transit time measurement. The pulse repetition frequency can thus be increased to values ​​around 1 kHz. The possible pulse integration can significantly improve the signal-to-noise ratio in the radar receiver and thus improve the energetic conditions for the above-horizon radar. As a result, the pulse power can be reduced again for the same range.

Phased array antenna of the Soviet over-horizon radar " Woodpecker "

The greatest challenge is overcoming the curvature of the earth. The propagation of the electromagnetic waves must either adapt to the earth's surface by diffraction, or be specifically reflected on the Appleton layer of the ionosphere. This severely limits the usable frequency range. For diffraction, the radar must work in the lower shortwave range; for reflection, the radar must work in the medium to upper shortwave range.

These frequencies, which are very low for radar devices, mean that the antennas have to be extremely large. However, the effectiveness of these huge antennas is quite low due to the relation to the wavelength , so that they can reach geometrical dimensions of several kilometers in order to achieve the desired diagram width ( angular resolution ). Mechanical rotation of these huge antennas is impossible. Electronic swiveling methods with phased array antennas are often used. Due to the high pulse power and the necessary protection of the sensitive receiver inputs, the bistatic radar method is often used, that is, the transmitting antenna and receiving antenna are located far apart from each other.

technology

Two different device concepts are pursued, which are reflected in the antenna construction:

  • OTH-B (Over-The-Horizon - Backscatter) and
  • OTH-SW (Over-The-Horizon - Surface Wave).

Over-The-Horizon (Backscatter)

Principle of the propagation of electromagnetic waves at OTH-B

OTH-B uses the reflective properties of the ionosphere to achieve ranges of more than 2000 km with the help of the sky wave . The transmission frequency for this method is between 15 MHz and 55 MHz. Radar signal processing is very difficult, however, because the reflective layers are not at a constant height, but vary greatly with the time of day. However, this concept is imprecise due to the difference in transit time in the case of multiple reflections. So it is not just a reflection, but a very complicated process of propagation of the electromagnetic waves. The echo signals are also extremely weak after multiple reflections, so that sensitive receivers must be used on the receiving side. The calculation of the distance is therefore difficult and contains inaccuracies. The distance calculation from the transit time measurement must be constantly adapted to these conditions. In practice, mostly multiple reflections between individual layers of the ionosphere and the earth's surface are used, which leads to additional ambiguities in the measured distance and consequently influences the false alarm rate. These inaccuracies are accepted, however, since these radar devices are only used for military purposes and only have the function of early warning. In contrast to the simple reflection, the single-hop , these multiple reflections are named after the number of reflections at the ionosphere, for example two-hop for a double reflection . The radar signal processing then has to adapt the threshold values ​​for signal detection to the reflections on the earth's surface ( clutter ), which are also registered, according to the occurrence of these disturbances by means of a dynamic, time-dependent gain control .

Special ion probes (so-called chirpsounders ) are used to determine the exact height of the Appleton layer of the ionosphere . These are special radar devices that are displaced approximately at a distance from the first reflection and in the frequency range up to 30 MHz can display the height of the reflecting layer as a function of the transmission frequency in a so-called ionogram. In addition to the amplitude and the transit time (corresponds to the apparent reflection height), a possible Doppler shift, polarization changes and the most favorable angle of incidence can also be derived from the echo signals. With the help of this data, the exact propagation path can be determined in a computer simulation and, on this basis, the measured transit time can be converted into a distance above the ground.

Over-The-Horizon (Surface Wave)

OTH-SW radar devices use a carrier frequency of 2… 3 MHz up to a maximum of 20 MHz. Frequencies above could also be used if necessary, but do not form such strong bumps and therefore have losses in the maximum range. The ground wave adapts to the curvature of the earth's surface and enables more precise range measurements than OTH-B due to the known propagation path. With very long wavelengths, the electromagnetic waves are coupled to the well-conducting surface of the sea and thus overcome the curvature of the earth. With these radar devices, the accuracy of the location cannot be better than the wavelength used.

Comparison of the detection areas of an air traffic control radar (red) with an OTH-SW radar. (Units of measure are nautical miles in distance and feet in height)

application

Possible applications of OTH-SW radar devices are:

  • extensive reconnaissance of ship movements for coastal protection;
  • Vessel traffic monitoring for maritime safety;
  • Recovery and rescue in the event of ship accidents;
  • Disaster warning of tropical cyclones and tsunamis;
  • Observing the drift of icebergs;
  • Supervision of construction activities off the coastal area;
  • Monitoring of environmental protection in the coastal region.

This procedure is often used in coastal defense to detect smugglers . It can be tuned so sensitively that individual swimming people can be safely located. However, it does not achieve the ranges of OTH-B.

This method is also used to measure oceanographic measurements (wave heights, speed, distances and directions of flow) with the help of the Bragg equation and for the detection of environmental pollution and possibly its cause. In this case, the FMCW radar method is used and ranges from 50 km to 250 km are achieved in the frequency range from 5 MHz to 55 MHz. This method is limited also for a tsunami warning suitable

example

The company Raytheon developed such a radar on behalf of the Canadian military, which became known under the name HF surface-wave radar SWR-503 . This is a maritime surveillance radar to combat illegal activities such as drug trafficking, smuggling, piracy, fish robbery and illegal immigration. The drift of icebergs in the region can also be observed. It consists of a 660 meter long antenna field made up of quarter-wave dipoles that are about 50 meters apart. This corresponds to half the wavelength in the used radar band of around 3 MHz. The antenna field can observe targets within the 200 mile zone in a sector of 120 °. The accuracy of the position determination is about 100 meters (corresponds to the wavelength used). The manufacturer declares that this system can also detect low-flying cruise missiles if it operates in a frequency range from 15 MHz to 20 MHz.

Electromagnetic compatibility

The powerful pulse-shaped frequency bandwidth signals of the over-the-horizon radar disrupt shortwave broadcasting in the frequency range from 10 MHz to 20 MHz over a large area and sometimes considerably. In the radio-technical slang such interferers are called "woodpecker ((because of the striking transmission pattern) woodpecker hereinafter)".

Today better technologies are being used for over-the-horizon radar. Mutual interference is reduced by using intra-pulse modulation and noise modulation . However, weaker frequency bandwidth interference still occurs.

For example, the over-the-horizon radar Pluto in the British Akrotiri in Cyprus is observed by radio amateurs on the frequencies 10.13 MHz and 18.13 MHz with the respective frequency bandwidth of 40 kHz (corresponding to four shortwave radio channels each). A Chinese over-the-horizon radar can also be heard in Germany on the frequencies from 6.93 MHz to 7.10 MHz.

The OTH-SW-Radar Wera, on the other hand, tries to find a less-used frequency range by providing information (“ listen before talk ”) before transmitting. Because of this, and because of the unusually low transmission power of 30 watts, mutual interference is low at Wera.

Known stations

Footprint of the ROTHR-TX, -VA and -PR operated by the USA

literature

  • JM Headrick, HF Over The Horizon Radar Chapter 24 in MI Skolnik, Radar Handbook ( PDF , 1.8 MiByte)

Web links

Commons : Over-the-horizon radar  - collection of images, videos and audio files
Wiktionary: Above horizon radar  - explanations of meanings, word origins, synonyms, translations

Chernobyl-2 (2014) photo report (German)

  • Photos of the TULELAKE AFS AN / FPS-118 OTH-B RADAR FACILITY in English

Individual evidence

  1. ↑ The AN / FPS-95 over-the-horizon back scatter radar (Radar system description) , Part 1 of 4 Parts, in FOIA documents on the AN / FPS-95 Cobra, ( online )
  2. ^ Daniel Möller, Over-the-horizon radars on shortwave at www.fading.de
  3. ↑ Kenneth Davies: Ionospheric Radio . Peter Peregrinus Ltd (on behalf of the Institution of Electrical Engineers), London 1990, ISBN 0-86341-186-X .
  4. ↑ at 4 MHz, the maximum range is specified with 250km (Source: Radar Tutorial ) at 30 MHz only 45 km (source: WERA characteristics of the University of Hamburg)
  5. ↑ A. Dzvonkovskaya, D. Figueroa, K.-W. Gurgel, H. Rohling, T. Schlick, HF Radar WERA Observation of a Tsunami near Chile after the Recent Great Earthquake in Japan , International Radar Symposium IRS 2011, Leipzig, Germany, 2011 Proceedings International Radar Symposium 2011, ISBN 978-1-4577 -0138-2 , pp. 125–130, September 2011 ( abstract )
  6. ↑ Press release from HELZEL Messtechnik GmbH ( online ; PDF; 1.8 MB)
  7. ^ Norman Friedman: The Naval Institute Guide to World Naval Weapon Systems . Naval Institute Press, Annapolis, MD. 2006, ISBN 1-55750-262-5 , pp. 18 (English, limited preview in Google Book Search).
  8. ↑ Technical data of the Wera system ( http://www.radartutorial.eu/19.kartei/karte712.de.html )
  9. ↑ Chinese Over-the-Horizon Backscatter Radars (OTH-B) on http://www.globalsecurity.org/ (in English)
  10. ↑ Radar Systems on Shortwave (PDF; 10.0 MB)
  11. ↑ Philippe Dorey et al. a., Nostradamus: The radar that wanted to be a seismometer in GEOPHYSICAL RESEARCH LETTERS, VOL. 37, (online PDF, 0.5 MB)
  12. ↑ Russia's new radar to monitor all Europe including Britain, article in Pravda of November 28, 2011 online
  13. ↑ Russia puts radar system into operation in Kaliningrad, article in Vorarlberg Online from November 29, 2011 online