1. Seeing Beyond: Over The Horizon Radar Systems and HF Propagation Nathaniel Frissell Lyndell Hockersmith 2 December 2008 ECE-5635: Radar System Design

2. Outline Why Over the Horizon (OTH) Radar? Brief History HF Propagation
Technical Considerations
Modern Implementations
Summary

3. We face challenges for reasons…
Why OTH Radar?
We face challenges for reasons…

4. Why OTH Radar?
Most radar systems have limited range due to the horizon.
Military and law enforcement wants early warnings.
Scientists want to take measurements over a large area.
Airborne EWR provides a good alternative to OTHR. [Wikipedia]
OTH radar is less accurate but also less expensive than Airborne EWR. Now that the cold war is over, missile-tracking accuracy is less important.
OTHR systems have strong applications in maritime reconnaissance and drug enforcement. [Wikipedia]
Source: ARRL (

5. Basic Principles Most radars operate at VHF or Higher (> 30 MHz)
Waves primarily propagate in a Line-of-Sight manner.
HF Radio Waves (3 – 30 MHz) can be reflected by both the Ionosphere and the Earth’s Surface.
Source: Lusis, 1983.

6. Uses of HF Shortwave Broadcasting Military Communications
Amateur Radio Communications
Radar Applications

7. Nothing happens in a vacuum…
Brief History
Nothing happens in a vacuum…

8. Early History HF was used in very early radar designs.
United States and United Kingdom developed systems independently of each other.
Development began prior to World War II.
Clark 1997 notes that Henrich Hertz was the first to notice that surrounding objects interfere with radio waves. In 1900, Tesla was the first to suggest that reflected radio waves be used for object location and ranging. He compared the idea to the well known phenomenon of a sound echo.
Radar could not be developed initially because the technology to produce powerful signals and detect week ones did not exist. The 1904 invention of the electron tube addressed this issue.
In 1904 the British government issued a patent to German inventor Christian Hülsmeyer for the telemobiloscope, described as a “hertzian-wave projecting and receiving apparatus…to give warning of the presence of a metallic body such as a ship or a train.” He successfully demonstrated this on May 10, 1904 at Rhine Bridge in Cologne. In time, he even equipped a ship with a system that could detect other ships up to 5 km away. In spite of this success, companies were not interested due agreements with the Marconi company and the idea that this simply duplicated results of radio communications equipment. In addition, the German Navy did not see a use for this system. Hülsmeyer dropped radar development and pursued other work.
Radio communications was embraced by naval military organizations immediately after its development. It’s value was easy to see.
After Hülsmeyer’s work, the next major wave in radar development came in the period between WW1 and WW2. While the US and UK were major players in this development, Germany also had its own independent radar development programs. However, the Germans discounted the use of HF for radar very early on, and therefore do not contribute to the design of over-the-horizon radar.

9. US HF Radar Development
Discovered in 1922 at the Naval Reseach Laboratory
Primary Engineers / Scientists:
Albert Taylor
Robert Page
Leo Young
In 1935, Young and Page demonstrated a pulse radar operating at 25 MHz.

10. UK HF Radar Development
Originally headed by Robert Watson-Watt.
Experienced in radio communications.
UK Air Ministry charged him with developing a death ray.
Collaborated with Arnold Wilkins
Demonstrated Radar in 1935 with the Daventry Experiment
Received a 6 MHz BBC Broadcast signal reflected off of an airplane. [Clark, 1997]

11. Radar and Communications
By the 1920s, radio communications was fairly well developed.
Radar technology grew out of 1920s radio communications technology.
HF equipment was widespread at this time.
Note: Use of HF does not necessarily imply OTH Radar.
Even though HF does not imply OTH, early designs could provide inspiration for later OTH systems.

12. British Chain Home System
British WWII early warning radar.
First effective HF Radar System.
Could detect planes 120 miles away.
Did not make use of OTH, but used HF because that is what was available.
[Clark, 1997]
Source: J. M. Briscoe / GNU Free Documentation License (

13. HF Radio Wave Propagation
The Physics of Radio Wave Travel

14. The Key
The key to understanding and designing OTH radar systems is understanding the underlying propagation processes.

15. The Basics: A Closer Look
HF is great because signals can propagate long distances, even over the horizon (1000 – 3000 km).
Names for this type of propagation:
Ionospheric Skip
Sky-Wave
Multi-Hop
[Lusis, 1983]
Source: Lusis, 1983.

16. The Ionosphere
The Ionosphere is a region of the upper atmosphere which contains free charged particles (ions and electrons).
[Lusis, 1983]
Source: ARRL (

17. Radio Waves and the Ionosphere
When radio waves encounter the ionosphere, three things can happen. The radio wave can:
Be absorbed.
Pass through the ionosphere.
Be refracted and reflected back to Earth
These phenomena are a function of three basic things:
Ionization levels.
Wavelength.
Angle of Incidence (measured from normal to the ionospheric plane).
Click to show 2nd major bullet.
[Lusis, 1983]

18. Absorption Absorption: Increases with ionization.
Increases with wavelength.
Increases with angle of incidence.
[Lusis, 1983]

19. Transmission
The probability that a radio wave passes through the ionosphere:
Decreases with ionization.
Decreases with wavelength.
Decreases with angle of incidence.
[Lusis, 1983]

20. Refraction Refraction (bending of waves): Increases with ionization.
Decreases with wavelength.
Decreases with angle of incidence.
[Lusis, 1983]
Source: Lusis, 1983.

21. Controlling Our Variables
As engineers, we have reasonable control over 2 of our 3 variables:
Wavelength
Angle of Incidence
We have no control over the last variable:
The Ionosphere
Time to think like a scientist!
Click to bring up 2nd major bullet.
“Time to think like a scientist!” appears after delay.

22. What drives ionization?
Source: UV Solar Radiation
Sink: Collisions with other particles
Sun appears automatically after delay.
Click to bring up words.
[Lusis, 1983]

23. Ionospheric Structure and Variations
Due to the source and sink mechanisms, the ionosphere experiences:
Diurnal (day/night) variations
Stratification
Source: ARRL (
[Lusis, 1983]

24. Ionospheric Structure and Variations
Layer
Height (mi) D
30 – 60 E
60 – 70 F1
140 F2
200
[Lusis, 1983]
Source: ARRL (

25. Ionospheric Layer Characterizations
Characteristics D
Exists during daylight hours only.
Frequent collisions of ions and neutral particles.
Has very poor refraction/reflection properties.
Absorbs most frequencies below approximately 10 MHz.
Remember: “Darn D” Region E
Some ionization due to X-rays and meteors.
Supports sporadic long distance VHF propagation.
Absorbs some energy of lower frequencies. F1
Merges with F2 layer at night.
Similar daytime characteristics as E layer. F2
Merges with F1 layer and descends in height at night.
Gas is highly rarefied; collisions are infrequent.
Best support for long-range communications and radar work.
[Lusis, 1983]

26. Additional Variations
Sunspots
Are correlated with ionization levels.
Occur on an 11 year cycle.
Source: NASA SWPC (

27. Other Disturbances Coronal Mass Ejections (CME’s) Solar Flares
Coronal Holes
Geomagnetic Storms
Geomagnetic Substorms
These phenomena are non-cyclical; however, their effects may repeat due to a 27 day solar rotation.

28. Propagation Summary HF Propagation is a function of:
Wavelength and Frequency
Angle of Incidence on the Ionosphere
Ionization of Ionosphere
Diurnal Variations
Sunspot Variations
Geomagnetic Disturbances

29. Rules of Thumb
If you don’t know anything about current space weather conditions, try this for long-distance communication, detection, and ranging:
Frequencies in the middle of the HF band (~14 MHz) are a great place to start.
During the day, try frequencies between 14 and 30 MHz.
During the night, try frequencies between 3 and 14 MHz.

30. Technical Considerations
What is a radar engineer to do?

31. Stop and Think (Like an Engineer)
What are some challenges that HF propagation will cause when designing and operating HF radars?
How would you address these issues?
Reminder: HF Propagation is a function of
Wavelength and Frequency
Angle of Incidence on the Ionosphere
Ionization of Ionosphere
Diurnal Variations
Sunspot Variations
Geomagnetic Disturbances
Click to bring up questions.
Click to bring up reminder box.

32. Problem 1: Unreliable Range
The largest problem with HF OTH radar systems is the fact that is has unreliable and unpredictable performance characteristics.
Primarily due to ionospheric variability and unpredictability.
Possible solutions:
Frequency Agility
Environmental Awareness

33. Frequency Agility
It is almost always possible to find some frequency that will support sky-wave propagation.
A system that has the ability to quickly change frequencies anywhere within the HF band (3 – 30 MHz) is desired.
Frequency agility has the additional potential benefit of providing the ability to defeat jammers.

34. Frequency Agility Challenges
HF wavelengths range from 10 to 100 m.
It is difficult to create an effective antenna system to handle this range of wavelengths.
The HF band is only 27 MHz wide.
This bandwidth is shared with the whole world.
Military
Shortwave Broadcasters
Ships and Airplanes
Ham Radio Operators
Frequency agility is only useful if you know what frequency to change to.

35. Environmental Awareness
Operators need to choose correct operating frequencies. This is done by:
Maintaining awareness of cyclical diurnal and solar variations.
Maintaining awareness of current geomagnetic conditions.
Direct measurement (active and passive).
Use of models and prediction software.

36. Direct Measurement
Real-time, direct measurement is the best way to know what current ionospheric conditions are.
This can be:
Designed into the radar system.
Obtained from the government, a company, or scientific body (e.g. NASA, SWPC, NOA, and NIST)
Active systems send out signals to measure ionospheric properties.
Passive systems can listen for beacons at known locations and frequencies.

37. Modeling and Prediction
By using both current and past data, indices and modeling software can predict how HF radio waves will propagate.
Statistical Packages [Tanyer and Erol, 1998]:
IONCAP (Ionospheric Communication Analysis and Prediction)
VOACAP (Voice of America Communication Analysis and Prediction)
ICEPAC (Ionospheric Communication Enhanced Profile Analysis and Circuit Prediction Program)
Physics Based Packages:
Jones and Stephenson Ray-Tracing Package [Jones and Stephenson, 1975]
CAPS (Communication Alert and Prediction Software) [Phillips, 2008]

38. CAPS and Google Earth - MUF

39. Problem 2: Unwanted Backscatter
Backscatter from the ground and ionospheric irregularities appear as targets to the radar.
Often are unwanted targets.
Use of Moving Target Indicator Doppler and knowledge of the desired target can filter out unwanted targets.

40. Unwanted Backscatter Example
These plots are a sample of SuperDARN data, which looks to measure ionospheric irregularities.
Unwanted ground scatter has a low Doppler velocity and narrow spectral width.
Therefore, velocity measurements with these characteristics have been grayed out.
Source: SuperDARN Data Archive (

41. Modern Implementations
Theory into practice…

42. Two Current OTH Systems
Jindalee
Aboriginal for “Bare Bones”
Australian OTH System for National Defense
SuperDARN
Super Dual Auroral Radar Network
International Radar Network for Space Science Research

43. Jindalee Needed to protect a 7.7 million square mile country.
Development began in 1974.
Typical detection capability of up to 3000 km.
Bi-static system
Due to large antennas and use of FMCW instead of pulse techniques, there is a high probability of interference between TX and RX systems.
Therefore, a bi-static system is used.
Often called OTH-B.
Source: Australia Defence Materiel Organisation ( gif)

44. Jindalee (Continued)
Utilizes a network of radars all tied to a central processing and command facility.
Frequency Agile System.
Incorporates a Frequency Management System (FMS)
Actively takes ionospheric measurements.
Also listens to a beacon network.
[Colegrove, 2000; Wise, 2004]
Wise 2004 states ionosonde receivers function between 5 and 30 MHz. Exact frequency ranges for actual radar transmitters were not given.
Source: Colegrove, 2000.

45. SuperDARN North Pole Radars
(Source:
OTH Radar System for measuring ionospheric convection.
Network of 21 radars funded, operated, and maintained by independent research institutions.
Radars look toward the polar regions.

46. SuperDARN (Continued)
Coherent Radar System
Frequency agile between 8 and 20 MHz.
Frequency choices determined by PI of radar or by committee.
Construction of VT Blackstone Radar
(Source:

47. SuperDARN (Continued)
OTH System needed because:
Required large coverage area.
Ionospheric F Layer velocity measurements required in polar regions.
F region measurements are required because they directly correlate to magnetic field line velocities due to infrequent collisions of particles.
To make a measurement, target irregularities my be perpendicular to the radar wave propagation vector.
Magnetic field lines are vertical in these regions.
Ionospheric refraction is the only way to satisfy the orthogonality condition.
[Chisham, et al., 2007]

48. Summary

49. Summary
OTH Radar is a long range radar system that typically uses HF radio waves.
HF propagation is affected by a number of variables. One variable, ionospheric conditions, is entirely outside of human control.
A thorough understanding of HF propagation is necessary to design, implement, and operate a HF OTH radar system.
Effective OTH systems operate today.

50. References
G. Chisham, M. Lester, S. E. Milan, M. P. Freeman, W. A. Bristow, A. Grocott, K. A. McWilliams, J. M. Ruohoniemi, T. K. Yeoman, P. L. Dyson, R. A. Greenwald, T. Kikuchi, M. Pinnock, J. P. S. Rash, N. Sato, G. J. Sofko, J.-P. Villain, and A. D. M. Walker. A decade of the Super Dual Auroral Radar Network (SuperDARN): scientific achievements, new techniques, and future directions. Surveys in Geophysics, pages 28:33–109, G. C. Clark. Deflating British radar myths of World War II. Master’s thesis, Air Command and Staff College, Maxwell, AL, March S. B. Colegrove. Project Jindalee: From bare bones to operational OTHR. In IEEE International Radar Conference, pages 825–830. IEEE, IEEE, R. M. Jones and J. J. Stephenson. A versatile three dimensional ray tracing computer program for radio waves in the ionosphere. OT Report 75-76, US Office of Telecommunications, D. J. Lusis. HF propagation: The basics. QST, pages 11–15, December T. Phillips. 4D ionosphere. URL: Accessed on: 1DEC APR2008. S. G. Tanyer and C. B. Erol. Broadcast analysis and prediction in the HF band. IEEE Trans. on Broadcasting, 44(2):226–232, June J. C. Wise. Summary of recent Australian radar developments. IEEE A&E Systems Magazine, pages 8–10, December 2004.

51. Additional Credits
Background Photo: Woodin. Marconi Tower at Sunset. Accessed from: _Tower_at_Sunset.jpg. Public Domain. Accessed 29NOV2008.