1 INTRODUCTION
1.1 Radar
The word radar is an acronym for RAdio Detection And Ranging, in other words a system that uses radio signals to detect and locate objects. Initially these objects were moving targets such as aircraft and ships, but nowadays may also include fixed objects such as terrain and even crops in applications collectively referred to as remote sensing. Typically radar has employed frequencies above 1000 MHz which are referred to as microwave signals. These signals are typically line of sight, although some refraction (bending) of radar signals can occur. On a planetary surface this limits the radar horizon to a little more than the optical horizon. Objects past, or over this horizon cannot in general by detected or located.
The general principle of all radars is to transmit a radio signal and then to receive a small fraction of this signal as it reflected back from the various objects in the path of the radar beam. The time taken to travel the distance from the transmitter back to the receiver allows calculation of the range of the objects. Directional information is given by transmitting a directional radio signal which is successively scanned through the space in which detection is required.
1.2 Over the Horizon Radar
Over the Horizon Radar, as it name implies, is a radar system that is able to detect and locate objects that are beyond the horizon of normal (microwave) radar systems. It does by making use of the ionosphere, a global layer of plasma in the upper atmosphere (at altitudes of 100 to 500 km) that reflects radio signals that lie in the HF (High Frequency) spectrum (typically 3 to 30 MHz).
The principle might be regarded as one of using a mirror held high in the sky which enables one to see beyond the normal horizon. In the OTHR case the mirror not only reflects back signals from the desired target, but also acts as a reflector to illuminate the target (from the transmitter). Because the ground (particularly sea water) also acts as a mirror to radio waves, and because the ionosphere exists all over the world, it is sometimes even possibly to use in essence what are multiple mirrors to look out to even greater distances. Thus, whereas the range of microwave radars is typically measured in hundreds of kilometres (at least for ground based radars), an OTHR may have a range of thousands or even tens of thousands of kilometres.
1.3 Space Weather
The ionosphere is created by the interaction between solar electromagnetic emissions and the molecules of the Earth's upper atmosphere. Below an altitude of 100km the atmosphere is relatively homogeneous. That is, it consists of approximately 20% oxygen and 80% nitrogen. Above this level, the various constituents tend to separate and the atmosphere become heterogeneous with different molecules and atoms existing at different levels. High energy ultraviolet rays (known as extreme UV or EUV) and low energy X-rays from the sun have enough energy per photon to ionise some of the molecules and atoms that exist in the upper atmosphere. This interaction forms the (ionised) plasma that we refer to as the ionosphere. There are different layers in the ionosphere that are named by letter. The layer around 100 km only substantially exists during daylight hours, and is termed the E layer. Above this, at around 150 to 200 km is the F1 layer, which again only exists during daylight. Above this is the F2 layer, which exists all the time, although it shows substantial variability in density of ionisation.
Because the ionospheric layers are formed by radiations from the sun and because these radiations show substantial variations with time, we would expect that the ionosphere shows a similar variability, and indeed it does. Variations in the ionospheric plasma occur with time of day, with season and with year. They also occur with geographic location and of course with altitude. All this leads as to the observation that while the ionosphere allows the propagation of radio signals over large distances, it is not by any means a perfect mirror, and if it is to be used to maximum advantage by an Over The Horizon Radar system, it is essential to have an intimate knowledge of its variations in both time and space.
2 BACKGROUND
It is thought that Marconi and some of the early radio experimenters in the first decade of the 20th Century may have consciously observed signals reflected off ships and other objects. However, no practical development of radar occurred at this time. The first radar was in fact an ionospheric radar, constructed by the physicists Breit and Tuve to determine the existence and measure the properties of the ionosphere in 1926. This radar was of course operated at HF frequencies.
The first widely deployed radars were used by the military and the use of radar greatly increased during the second World War. Initial radars were in the high HF and then VHF range, but with the development of the magnetron, which could be used to produce high power pulses in the microwave, most radar applications moved into the UHF and EHF bands.
It was not until later, probably the early 1960's that interest in going back to lower frequencies (HF), and using the ionosphere to reflect the outgoing and returning radar beams was seriously developed. The US Navy (USN) started experiments in the USA. The Russians deployed and operated very high power OTHRs from the 1970's onward. The USAF built and initially operated an OTHR at Bangalore in Maine, and considered expanding to a three site network across the country. However, the proximity of Maine site to the auroral zone produced so much distortion (due to ionospheric irregularities), that this, together with an increasing interest in higher frequency space based radar, led to the mothballing of the USAF OTHR program. The USN still operates relocatable OTHR systems.
Australia, in the late 1970's decided to develop its own OTHR. This was done by staff at the Defence Science and Technology Organisation (DSTO) just north of Adelaide. Currently this has expanded to a three station network (JORN), with sites near Alice Spring (Northern Territory), Laverton (Western Australia) and Longreach (Queensland). The Australian continent has the benefit of lying beneath a relatively quiet part of the ionosphere, and OTHR provides an economical way to monitor a continent with a low population density and very long coastline.
3 THE RADAR EQUATION
3.1 The Equation
To examine the factors that effect a radar and particularly an OTH radar we can examine what is known as "The Radar Equation". This equation gives us an expression for the maximum detectable range of a radar given specified conditions:
4 / Pav GTx GRx λ2 σ Fp tc Rmax = / ------------------------ \/ (4π)3 No (S/N) Ls
where Rmax - maximum detectable range
3.2 Target Cross Section
The radar cross section of a target relates to the amount of energy that the target reflects back to the radar. It is expressed as a geometrical area in square metres. A target that has a radar cross section of one square metre, means that it reflects back the same amount of energy as does a perfect reflector with a physical cross-sectional area of one square metre in the direction of the radar.
Radar cross section depends on the physical size of the target, the electromagnetic properties of the target and the frequency of the radar signal. The variation in radar cross-section of a metal sphere of radius a is shown as a function of radar signal wavelength in the graph below:
For very high frequencies (wavelength a lot shorter than the size of the sphere) the radar cross section is equal to the physical cross section of the target. For very low frequencies (wavelength considerably longer than the size of the sphere), the radar cross section becomes diminishingly small. It is in the Rayleigh scattering region that most OTH radars operate. A typical OTHR frequency of 10 MHZ has a wavelength of 30 metres. A small unmanned aerial vehicle is thus going to prove difficult for an OTHR to intercept. On the other hand, the normal stealth techniques used to "hide" aircraft such as the USAF F-117 Fighter and B-2 Bomber from microwave radar are ineffective at HF frequencies, and these aircraft can be expected to be readily detectable by an HF OTHR.
3.3 Coherent Processing Time
To obtain a better signal to noise ratio, a radar might in some way combine and process several target returns. The coherent processing time is equal to the number of target returns divided by the repetition frequency of the radar's modulation. It is a value that might typically range from 0.1 to 10 seconds.
It is included in the OTHR radar equation because a Doppler radar requires a dwell time of T seconds if a frequency resolution of 1/T Hz is to be achieved.
A higher processing time means better frequency resolution which in turn means better target discrimination, and better discrimination against clutter. However, if the processing time is too high, the radar may lose agile targets. A high-g maneuver pulled by an aircraft can sometimes allow the pilot to evade radar "lock".
3.4 Propagation Factor
This factor is particularly important for ionospheric radars where there causes of signal reduction other than the normal inverse square law propagation. This factor encompasses:
In the daytime most of the ionospheric energy loss occurs in the D region. This region is maintained by solar extreme ultraviolet and X-radiation. The loss varies inversely as the square of the frequency. Lower frequency signals are thus subject to significantly higher losses than are higher frequencies. At times of large solar X-ray flares the D-region loss increases greatly, sometimes even to the point to render an OTHR unable to acquire any signals.
3.5 Noise Power Density
This noise power per unit bandwidth reflects unwanted radio frequency energy generated by a variety of sources:
In the HF spectrum (3 to 30 MHz), unlike at microwaves, man-made HF signals will always be in excess of receiver noise. This noise is propagated from other transmitters located all over the world via the ionosphere. Thus changing characteristics of the ionosphere elsewhere in the world will affect signal detectability at the OTHR.
4 OTHR MODULATION
An unmodulated radar signal can provide no information about target range (although it can be used to deduce target velocity), and so some form of modulation of the radar transmitter is required. A number of different forms of modulation might used. The two main ones (from which other variants may be derived) are pulse modulation (used by a large number of radars) and frequency modulated continuous wave FMCW.
Pulse modulation has been used in a Soviet OTHR, and the amplitude variation of the transmitted signal with time is shown below. This includes typical parameters such as pulse width (PW) and Pulse Repetition Frequency (PRF) or Period (PRP).
In FMCW, the amplitude or power of the transmitted signal is not varied, but the frequency is swept over a certain range ( <100kHz) in typically a sawtooth function. The diagram below shows diagrammatically what the amplitude and frequency of the waveform look like as a function of time. This type of modulation is used in the Australian OTHR system.
5 OTHR RANGE
The typical range coverage for an OTHR will vary from under 1000 km to almost 4000 km for single hop propagation. Larger distances may be covered using multiple hops but with degraded performance and accuracy, due to increased absorption and ionospheric imperfections.
There is a minimum range to OTHR because at the lowest useable frequency (limited either by D-region absorption of E-layer obscuration) there is a takeoff (elevation) angle above which the transmitted beam will not be returned back to the ground, but which will travel out into space.
Compare the above OTHR ranges with a maximum range of about 400km for a long range conventional microwave radar.
6 RANGE AMBIGUITY
Range ambiguity is a phenomenon that can arise in any radar when the radar repetition frequency fRR is too high. If this is the case then returns from distant targets will not have arrived back at the radar receiver before the next (second) pulse is transmitted. This late return then appears on the radar screen at an anomalously low range. This is what we mean by range ambiguity.
The maximum detectable UNAMBIGUOUS range Rmax in kilometres is then given by the formula:
This formula gives the following ranges:
Repetition Frequency (Hz) Maximum Unambiguous Range (km)
100 1,500
50 3,000
20 7,500 OTHR
10 15,000 Working
5 30,000 Range
The problem of range ambiguity is more acute in an OTHR because of multihop ionospheric propagation possibilities. The multihop returns can often be distinguished because of increased Doppler spread, and other ionospherically caused degradations.
7 RANGE RESOLUTION
The range resolution of an OTH radar is limited, as with any other radar, by the frequency bandwidth of the radar. At HF, the limit on the maximum bandwidth is set by the ionosphere and this seldom exceeds 100 kHz. This limit is because the velocity of propagation of the radar wave within the ionosphere changes with frequency.
The range resolution may be computed from the approximate but simple formula:
The table below gives typical radar range resolution figures:
Bandwidth(kHz) Range Resolution (km)
10 30
20 15
30 10
40 7.5
50 6
60 5
70 4.3
80 3.8
90 3.3
100 3.0
From this we can see that the best range resolution achievable by an OTHR is no better than about 2 km.
Ionospheric irregularities will frequently degrade this figure. Worst case conditions may reduce the resolution to 50 km.
8 ANGULAR RESOLUTION
The beamwidth of the radar antenna is given by the Rayleigh criterion:
Note that λ = c / f
where c = 3 x 108 metres/sec is the speed of light in vacuo
The above equations can be used to produce a table showing some typical antenna beamwidths:
FREQ (MHz) Antenna Beamwidth (Degrees)
D=500m D=1000m D=2000m
5 6.8 3.4 1.7
10 3.4 1.7 0.9
20 1.7 0.9 0.4
30 1.2 0.6 0.3
This resolution is an azimuthal angular resolution. The linear resolution depends both upon his angular resolution and the distance of the target from radar. The formula relating the two is:
The table below shows some typical values for azimuthal linear resolution. Note that this basically represents the minimum distance at which the radar can separate two targets at the specified range. If two targets have a separation which is less than this distance they will appear as one echo (at least in resolution space). If they however, are travelling at different velocities, Doppler information may be able to separate them. Of course, if their velocities are vastly different they will not remain within the resolution separation for very long.
Beamwidth Range (km)
(deg) 500 1000 1500 2000 2500 3000
0.2 1.7 3.5 5 7 9 11
0.4 3.5 7 11 14 18 21
0.6 5 11 16 21 26 31
0.8 7 14 21 28 35 42
1.0 9 18 26 35 44 52
2.0 18 35 52 70 87 105
3.0 26 52 79 105 131 157
4.0 35 70 105 140 175 209
5.0 44 87 131 175 218 262
9 DOPPLER SHIFT
If the radar target is stationary with respect to the radar, then the frequency of the reflected signal will be the same as the frequency of the transmitted signal. However, any relative movement between the two will lead to a change in the received frequency. This change is termed the Doppler effect, and may be calculated via the formula:
where
Note that the radial velocity is the line of sight component of the target velocity. Assuming a stationary radar this is the speed at which the target is approaching or receding from the radar. An approaching target will result in a frequency increase whereas a receding target will produce a frequency decrease.
The table below shows typical Doppler frequency shifts for an OTHR in comparison to microwave radars.
HF Frequency Doppler Shift (Hz)
(MHz) Speed=10km/hr Speed=100km/hr Speed=1000km/hr
5 0.1 0.9 9.3
10 0.2 1.9 18.5
20 0.4 3.7 38.0
30 0.6 5.6 55.6
3000 (C-band) 55.6 556 5556
10000 (X-band) 185 1852 18519
10 DOPPLER SPREAD
A single frequency transmission that exists for an infinite period of time has an infinitesimally small bandwidth.
However, in a pulse radar and even in an FMCW radar, there is a limited "dwell" time of the radar on any single target. This produces a frequency spread in the returned signal proportional to the dwell time.
Thus the ground return, or a return from a target moving at a constant velocity will always have a definite frequency bandwidth.
Further increase in echo bandwidth on the ground return will be due to internal motions within the ionosphere for an OTH radar. Examination of the ground return can thus be used as an internal radar diagnostic of the state of the ionosphere.
In the case of an airborne moving target, a high-G maneuver will cause a substantial change in the Doppler shift of the returned echo during the dwell time. If this change is great enough, the signal will be so spread in frequency that the signal to noise ratio may drop below the point of detectability, and the target will be lost - at least until the maneuver is completed.
Note that increasing the radar dwell time increases the radar resolution in the frequency domain, or what is usually termed Doppler space. This is equivalent to increasing the ability to resolve different targets due to differences in their velocity (even targets that are within the same range and azimuth resolution cell of the radar. The drawback of increasing the dwell time is that the radar becomes less sensitive to targets with changing speeds. As with most systems, a compromise must be accepted and radar parameters optimised for the expected target behaviour.
Even in the absence of man-made targets an OTHR with Doppler resolution has a Doppler clutter spectrum due to ground return, sea return (wave motion spreads out this echo on each sides of a fixed land return), and echoes from natural phenomena such as meteors and aurorae. A typical clutter spectrum in Doppler-Range space is shown below.
11 OTHR REQUIREMENTS
For optimum performance an OTH radar requires:
Unfortunately, not all of these conditions are met anywhere like all of the time. It is thus necessary to understand variations from these ideal conditions, how to determine when they occur and how they affect OTH radar performance.
12 OTHR ENVIRONMENTAL PROBLEMS
12.1 Ionospheric Multipathing
There is often more than a single path through the ionosphere that a transmitted signal may take to reach a target. Several paths are illustrated in the diagram below.
While not all of these paths may occur at a single frequency, it is certainly uncommon to have several paths at a single frequency. This will produce multiple echoes from a single target, echo having a different range dependent upon which path it has taken. The problem of multipath echoes is very much related to the associated practical OTHR problem of coordinate registration. This either requires a very extensive knowledge of exact ionospheric conditions (something that is rarely possible), or else it requires some known targets (often transponders) to allow real-time calibration of the target positions. One way of minimising multipathing is to operate near the maximum useable frequency for the path in question. This usually ensures that only one path (which uses the highest ionospheric return possible) is present. There may however, be reasons why this cannot always be done. Disturbed conditions, during ionospheric storms, may also produce multiple paths, even at the higher useable (and usually depressed) frequencies.
12.2 Frequency Dispersion of the Ionosphere
The ionosphere is a dispersive medium which means that the velocity of propagation of a radio signal through the ionosphere is a function of the frequency of that signal. This limits the frequency bandwidth of the radar and thus the range resolution.
This is discussed in more detail in section 14.3.
12.3 Ionospheric Refraction
We often talk of the ionosphere as a reflector that bounces radio signals back to the ground. However, this apparent reflection is in reality due to refraction (bending) of the propagating signal as it moves through the ionosphere. This may often be regarded as analogous to a point reflection (theorem of Breit and Tuve). Thinking of the ionosphere solely as a reflector can lead to overlooking certain fundamental limitations of ionospheric refraction, even with an ideal ionosphere, let alone one that is disturbed in some way. The approximation of flat horizontal ionospheric layers also breaks down for large distances.
The nature of ionospheric refraction and of the curved Earth (and ionosphere) allows only a specific area to be illuminated by only a limited band of frequencies. Curvature may also result in focusing effects. Continuous refraction also gives rise to multiple ways a signal may reach its target (sec 12.1).
12.4 Changing Electron Distribution within the Ionosphere
The nature of an ionospheric propagation path is subject to continuous change. Solar effects together with changes in the geomagnetic field produce variations in ionisation with time scales from minutes to decades. The most prominent variation is the daily or diurnal variation (caused by the Earth's rotation with respect to the Sun), but much shorter variations (causing signal flutter/fades) is due to movement of discrete patches of ionisation and is caused by upper atmospheric winds.
Longer term variations have periods of about 27 days (due to rotation of a sun that has an uneven longitudinal distribution of extreme-ultraviolet (EUV) radiation producing centres), and decadal variation (10-11 years) due to solar cycle variation in EUV emission. It is this EUV and solar X-ray emission that creates the Earth's ionosphere.
12.5 Clutter
The propagation space of an OTHR is studded with unwanted echoes. These echoes are referred to as clutter. They are produced by reflections from the ground, the sea, auroral ionisation, meteor ionisation and ionospheric irregularities. (A radar operator may typically advise that the radar "is being buzzed" when unexpected clutter appears on the display.) The strength of clutter echoes may be 40 to 80 decibels above the required target echoes. Although discrimination may be achieved through Doppler shift processing, the receivers require a very wide dynamic range to handle this amplitude variation without overload and consequent generation of distortion products. The diagram below shows a typical clutter spectrum for a Doppler HF radar.
12.6 Noise
The performance of a microwave radar receiver is generally limited by the thermal noise generated in the receiver itself. This is not the case in an HF OTHR, where noise sources external to the receiver vastly outweigh any internal receiver noise component. External noise sources can be classified as follows: Natural sources Cosmic (mainly galactic radiation) This comes mainly from the galactic centre. It is only important at the higher frequencies where it may penetrate the ionosphere. Solar radio noise bursts. These are very transitory events. Again only important at the higher frequencies. Terrestrial electrical storms (lightning discharges) These can be very severe in the tropics and in summer. Man-made sources Internal combustion engines. Industrial equipment (eg welders). Anything that generates an electrical arc or discharge. Usually a site will be located to minimise the above types of noise. HF transmitters - there are no frequency allocations exclusively designed for OTHR, and the HF spectrum is so crammed with HF transmissions that there is literally no frequency that does not have some man-made transmission from some part of the world. Deliberate jamming transmissions. In peace-time these can usually be tracked down and decommissioned. In time of war, they may be destroyed if superior force is available. An experienced jammer will jam at a level so that it may not be immediately apparent deliberate jamming is taking place. The predominant noise source to an OTHR is the worldwide cacophony of the ubiquitous man-made HF transmissions, and secondarily (at certain times), noise from global electrical(lightning) storms.
12.7 Ionospheric Irregularities Noise
The ionosphere does not always exist as nice horizontally stratified uniform layers (as often shown in text books). Because the sun's ionising radiation is responsible for creating ionospheric ionisation, there will be irregularities around sunrise and sunset times as the sun's radiation either abruptly appears (sunrise) or disappears (sunset).
Particularly prominent naturally induced irregularities occur in the polar and equatorial zones. These occur because the Earth's magnetic field controls the motion of charged particles in the upper ionospheric layers, and of particles (initially from the sun) which stream down (precipitate) from the magnetosphere. These irregularities effect OTHR by producing clutter, multipathing, Doppler spread and distortion, and range and bearing errors.
Man-made irregularities can be produced by nuclear explosions in the upper atmosphere, spacecraft travelling through the ionosphere. A space based nuclear detonation can create ionospheric absorption over several thousand kilometres which may last for several hours.
IONOSPHERIC FACTORS AFFECTING ALL HF SYSTEMS
13.1 Shortwave Fadeouts
Shortwave fadeouts (SWF's) occur when a flare on the Sun produces an increased output of solar X-radiation. This causes increased ionisation in the D-region of the ionosphere, which results in absorption of HF signals propagating through this region on their way to and from the higher (E and F) "reflective" layers.
Only signals propagating in or through the sunlit sector of the Earth will be affected. Those paths most affected are those whose paths lie closest to the sub-solar point (that point on the Earth's surface where the Sun is directly overhead). Paths that lie fully in darkness will not suffer any effects from increased solar X-radiation (ie they will not experience a fadeout).
Shortwave fadeouts are of different severity and duration according to the magnitude and duration of the solar flare. Typical durations are from minutes to hours. A solar X-ray flare is measured in terms of the peak X-ray output (as measured by a satellite in geosynchronous orbit around the Earth). The flare is classified using a logarithmic scale that has a letter followed by a number. Small flares are referred to as C class, with a sub-division from 1 to 9. Thus a C8 flare would be twice the X-ray intensity as a C4 flare. Medium intensity X-ray flares are classified as M class, where M1 would have the same intensity as a C10 flare. Large X-ray flares as listed as X class. The X class is open ended and starts at X1 (equivalent to an M10 flare) and goes to X10, X11 and beyond.
The current satellite solar X-ray detectors saturate at about X13. The largest flare ever recorded was an X28 (inferred by extrapolation). Most HF systems start to suffer fadeout problems with flares of class M1 and larger. However, sensitive OTHR systems which have a much wider dynamic range than other systems will be affected by C class flares. It is well to remember that the daytime D region is maintained by Lyman alpha radiation from the Sun even in the absence of enhanced X-radiation. The Ly-A emission maintains the sunlit D region at an ionisation level about equivalent to an X-radiation level between C1 and C2. During the nighttime, the D region vanishes as the free electrons which are maintained by both this Ly-A and X radiation recombine within seconds of the radiation source disappearing. The increase in D region ionisation will thus follow the solar X-ray output during a flare very closely.
The prediction of solar flares, and thus the prediction of SWF's is covered in section 15, but it will be noted here that the only forecasting currently possible is on a statistical (probability) basis (ie a 50% chance of an M-class flare in the next 24 hours).
Given a particular X-ray flare class, we know that absorption of HF signals as they pass through the D region is inversely proportional to the square of the signal frequency. Thus higher frequencies are absorbed substantially less than are lower frequencies. For C and M class flares it may thus be possible to use higher frequencies for continued operation during the flare. However, X class flares produce so much absorption that often the frequency required to reduce the D region absorption to usable values is above the frequency that the F layer will reflect. In this case, we have a total fadeout across the entire usable HF spectrum.
13.2 Ionospheric Storms
Ionospheric storms are due to a fluctuating electron density in the F-layer. At the beginning of a storm, the ionisation may actually increase (leading to enhanced critical/reflecting frequencies), but the predominant effect following this initial phase is for a depletion of electron density (ionisation) in the higher (F) layers of the ionosphere. Storms also produce ionospheric irregularities and non-uniformities. The level of depletion depends on the severity of the storm. A mild storm may only result in a 20% depression of critical frequencies, whereas a severe storm may cause criticals to be depressed by 80%. Ionospheric storms affect HF propagation in both the daylight and night-time hours.
Ionospheric storms are usually associated with geomagnetic storms, which are due to an injection of particles into the Earth's magnetosphere from the Sun. Geomagnetic storms are due to a massive ejection of material from the Sun in what is termed a Coronal Mass Ejection (CME), or due to the interception by the Earth of a coronal stream. The later phenomenon is more common during conditions of solar minimum, whereas CME's tend to occur more frequently during solar maximum.
Ionospheric storms also occasionally occur when there is no geomagnetic storm in progress, and the exact nature of ionospheric-magnetospheric coupling is currently a very active research topic. Whatever their origin, ionospheric storms typically last from hours to days. We also know that statistically, ionospheric storms tend to be more frequent and more intense around the equinoxes (March/April and September/October time periods). This is because disturbances in the solar wind (which are caused by variation in the solar particle output) couple more strongly into the Earth's magnetosphere (due to geometrical factors) at these times.
Ionospheric storm forecasting is discussed in section 15, but it will be noted here that warnings for such storms may be possible for several days prior to the event.
Because ionospheric storms affect the higher frequencies, it may be possible to move to lower frequencies during the storm, as long D region absorption is not too strong. Even at lower frequencies, a very severe storm may produce sufficient ionospheric irregularities (according to path geography) that conditions are too poor for most practical purposes.
13.3 Frequencies Affected
In summary: A SWF will affect lower frequencies considerably more so than higher ones. ACTION - move to higher frequencies if available. IONOSPHERIC STORMS usually cause depletion of the F-layer and thus higher frequencies will not propagate. ACTION - use lower frequencies if possible.
14 IONOSPHERIC FACTORS PARTICULARLY IMPORTANT TO OTHR
14.1 Ionospheric Tilts
Tilts occur in the regular ionospheric layers around sunrise and sunset. This is because the solar EUV, which maintains these layers is incident to the ionosphere at around grazing angles, and for a short period of time, may even be coming from "below" the ionosphere. In general, the upper layers of the ionosphere will see the sun first, and thus be ionised first. This thus tends to tilt the ionospheric layer upwards toward the night sector.
Sporadic-E layers may also show tilts from the horizontal (again due to the formation process), and rapidly changing tilts also occur with the passage of travelling ionospheric disturbances (TIDs).
Tilts of any nature will cause both range errors and azimuthal errors. If the ionosphere is titled down away from the radar, then the radar beam which previously illuminated a given spot of the Earth's surface, will illuminate a spot closer to the radar. If the tilt is about an axis that is line of sight from the radar, then a beam from the radar at a given azimuth will in fact illuminate a target to the left or right of this azimuth.
14.2 Ionospheric Irregularities
Ionospheric irregularities are present in two major geographical regions of the world - the polar/auroral regions and the geomagnetic equatorial regions. These irregularities not only cause returns from valid targets to appear at numerous false ranges and azimuths, but they can also generate false targets. Moving irregularities also spread target returns in Doppler space.
Irregularities occur within 20 degrees north and south of the geomagnetic equator (which in certain parts of the world can be displaced considerably from the geographic equator. Their intensity increases with increasing geomagnetic and solar activity. These irregularities are generally much more extensive than polar irregularities although they are generally mostly a problem during night hours. There appear to be certain longitudes at which these irregularities become more troublesome, the Arabian Gulf being one such area.
Polar irregularities are associated with auroral particle precipitation and can be present at all times of day, although more troublesome at night.
Obviously, the location of an OTHR will determine its susceptibility or otherwise to one or the other of the ionospheric irregularity regions. A radar operating mainly in mid-latitude geomagnetic regions will not be as badly affected as a radar operating near the auroral regions. In fact, it has been suggested that at least one of the reasons that the USAF did not continue with its OTHR program was because the initial site at Bangor, Maine suffered from extensive auroral ionospheric irregularities.
In summary, ionospheric irregularities cause multiple echoes from single targets (due to the presence of multiple paths of different lengths), target fading, anomalous clutter, and Doppler spreading.
14.3 Ionospheric Dispersion
As mentioned in section 12.2, the ionosphere is a dispersive medium, which means that radio signals of different frequencies travel at different velocities through the ionospheric layers. This not only limits the radar bandwidth and thus its range resolution , but near the ionospheric critical frequencies (where the velocity of propagation is substantially reduced, apparent target ranges will increase).
A simple way to view this problem it is to note that if different frequencies travel at different speeds, the different frequencies will take different times to travel from the radar transmitter to the same target and back to the radar receiver. The radar display will then show an extended object for the target rather than a more point like feature.
Ionospheric dispersion becomes more of a problem when the radar frequency is near the critical frequency of one or more ionospheric layers. As the critical frequency is approached the speed of the signal in the ionosphere slows down considerably - in fact it theoretically becomes zero right at the critical frequency. From the point of good range resolution and target range accuracy, it is thus desirable not to operate too close to an ionospheric critical frequency. Note that the ionospheric critical frequency will vary as a function of azimuth of the radar beam, so even if target registration through the use of beacons and transponders is undertaken, range accuracy can still be questionable in directions and at ranges sufficiently far away from the calibration points.
14.4 Travelling Ionospheric Disturbances
Travelling ionospheric disturbances (TIDs) originate near the auroral zones during late evening hours, especially if the geomagnetic field is disturbed. They then propagate toward the equator for up to several thousand kilometres at speeds of around 1000 km/hour. They have a width of typically several hundred kilometres.
TIDs are a thus a wave like disturbance of the ionosphere, and at any one fixed location they appear as a sudden enhancement of free electrons followed by a rapid depletion. This causes a wave-like change in the critical frequency values (eg foF2) when plotted as a function of time (this is also how TIDs may be identified from ionograms -as long as the ionosonde soundings are taken frequently enough -at least every 5 minutes).
Whilst TIDs originating in the auroral zones and propagating through high and mid-latitudes create the larger disturbances of their kind, smaller scale TIDs possibly created by thunderstorms and jetstream disturbances are found in the lower latitude regions. These are most often found during daylight hours. TiDs are believed to be related to internal atmospheric gravity waves.
The effect of a TID on an OTHR would be multiple echoes from single targets, Doppler spreading, fading, and range and angular errors (due to the ionospheric tilts that occur).
14.5 Disturbances in a Nuclear Conflict
Nuclear devices exploded high in the upper atmosphere can substantially alter the nature of the ionosphere and affect all HF systems including OTHR.
The essential effect is produced by extra ionisation through the large population of beta and gamma ray emission that occurs following device detonation. This ionisation occurs in the neighbourhood of the explosion and can be particularly intense in the D-region. This can markedly increase the absorption of any HF signals that the HF spectrum becomes totally unusual. At the same time increased ionisation at higher altitudes in the F-layers will raise the maximum frequencies at which the ionosphere can support reflection. Because D-region absorption decreases as the square of the operating frequency, an OTHR that is able to operate at low VHF frequencies (30 to 100 MHz) may be able to negate the effects of this increased D-region absorption.
Beta particles are high energy electrons, which being charged, will spiral around the Earth's magnetic field lines and travel through space to the geomagnetic conjugate point to the explosion. At this location they are again absorbed in the D-region causing anomalous HF radio signal absorption. However, there is no concomitant increase in ionospheric critical frequencies at this location. Because of this effect, an OTHR may be affected even though it lies in the opposite hemisphere from the nuclear explosion.
The area affected by a space based nuclear explosion will depend on the altitude of detonation, but may extend over several thousand kilometres and the effects may last for several hours.
15 SPACE WEATHER FORECASTING FOR OTHR
15.1 The Sun - Primary Source of Space Weather
The definition of space weather that has been adopted by the US National Space Weather Program is:
Space weather refers to conditions on the Sun and in the solar wind, magnetosphere, ionosphere, and thermosphere that can influence the performance and reliability of space-borne and ground-based technological systems and can endanger human life or health.
OTHR, which relies on the ionosphere for its very operation (the OTH part in particular), is thus very much involved with space weather. In turn, it is the Sun that is the primary cause of space weather (just as it is the primary source of terrestrial weather). Electromagnetic radiation from the Sun produces the ionosphere in the first place and is responsible for many of its variations, some beneficial to OTHR and some deleterious. Particle emission from the Sun also perturbs the ionosphere and thus must be considered by any OTHR system. The following sections will explore the various ways in which the Sun acts as the principal driver for space weather, and how knowledge of and prediction of this behaviour may be helpful to the OTHR operator.
15.2 Electromagnetic Emissions
It was discovered very early in the history of long distance radio communication that ionospheric behaviour was closely correlated with solar activity. In particular, a high mathematical correlation can be shown to exist between smoothed sunspot number and mean foF2, the ionospheric critical frequency below which a vertically incident radio signal will be "reflected" back to the ground, rather than pass out into space. This, and other ionospheric critical frequencies are the most useful ionospheric parameters in determining what frequencies may be used to communicate over a given path, and in the case of OTHR, what frequencies may be used to image specified locations. It is the extreme ultraviolet emissions and the soft X-ray emissions radiated by the Sun that interact with the Earth's upper atmosphere and produce the ionosphere. Different frequencies of UV and X-rays interact with different molecules in the upper atmosphere, ionising them by removing their outer electrons, and forming ionospheric layers at different altitudes. It is these free electrons that interact with radio signals causing them to refract and produce apparent signal reflection.
15.3 Particle emissions
The outer atmosphere of the Sun, termed the corona, is very hot (around one million degrees Kelvin (or Celcius)) and this causes violent agitation of the atoms that comprise it. Like a boiling kettle, some of these very energetic electrons have enough energy to escape the Sun's large gravitational pull and stream out into space. This continuous outflux of solar material is called the solar wind. The average speed of the solar wind at the Earth's orbit is about 400 km/sec.
A constant solar wind will not affect normal HF propagation. Unfortunately, the solar wind is not always constant. Large variations in speed and density are noted from time. Coronal holes on the Sun are the source of high speed streams of particles that can cause geomagnetic storms. Large ejections of material from the solar corona (termed Coronal Mass Ejections or CMEs) send massive clouds of plasma into interplanetary space. These clouds can have diameters on the order of millions of kilometers. If they travel toward the Earth they can inject large numbers of particles into the Earth's magnetosphere, where they spiral around the magnetic field lines, causing geomagnetic storms. In general, the geomagnetic storms that result from the impact of a CME are more intense than those that result from high speed coronal streams.
The effect of the ionosphere of geomagnetic storms is to change the density of the ionosphere, and thus produce changes in the critical frequencies. At the start of a geomagnetic storm, the ionisation may show an initial increase, leading to higher frequencies than normal supporting HF signals. However, there are likely to be rapid changes, and significant fading during this time. Following any initial increase, the geomagnetic storm will generally cause a reduction in the free electrons in the ionosphere, and critical frequencies will decrease, sometimes plummeting to only 20% of what they should be in the absence of the storm.
15.4 The Solar Cause of Ionospheric Indices
As we have seen above, it is both the electromagnetic (EUV and X-ray) emission, and the particles ejected by the Sun that are ultimately responsible for major disturbances in ionospheric properties, and thus in the usefulness of the ionosphere in HF propagation and OTHR operations.
Thus continual monitoring of the Sun is essential to be able to identify behaviour that will affect the ionosphere. The problem is that although know what solar activity is likely to be "geoeffective", we cannot always predict it.
Both flares and CMEs are of interest to us. The best we can currently do is to indicate a probability of flare occurrence during a 24 hour period. Sometimes, plage brightness and fluctuations may indicate the greater chance of a flare within a 3 hour time period. However, it is impossible to give an exact time for flare occurrence and magnitude. And once a flare is observed from an Earth based observatory, the X-rays it produces are already acting on the ionosphere (producing radiowave absorption - ie a "shortwave fadeout"). This is because the light from the Sun and the X-rays, both take around 8 minutes to reach the Earth.
On the other hand, when a CME occurs, we have from around 12 to 72 hours before it will hit the Earth. The new STEREO spacecraft, due to be launched in late 2006, should allow us to track the trajectory and velocity of CMEs much more accurately than we have be able to do so far, and thus provide, on average, at least a day's warning of potential HF problems.
15.5 What is Useful?
OTHR operators have access to an ionospheric "Frequency Management System (FMS)" which shows them the state of the ionosphere at the current time. To be of extra value to them, they need advice that will: a) Predict ionospheric disturbances (of the type that will significantly affect OTHR operations) before they happen. However, we should note that even when this cannot be done, it is frequently useful to know the source of a disturbance (ie is this natural or is it due to enemy action?) b) Once a disturbance has occurred, we need to be able to estimate how long the disturbance will last. And to reiterate, information on the source of the disturbance (or the "why this is happening") is also often appreciated by the radar operators. Predictions will invariably be based upon solar, interplanetary and magnetospheric inputs. We try to do this at present, and hopefully the continuing development if various models will assist in this regard. Tilts to the ionosphere can sometimes be predicted from climatology and from a large ionospheric network. For instance, we know that tilts occur around the solar terminator passage (sunrise and sunset), so we have a well known semi-diurnal effect. Also at certain times of years, travelling ionospheric disturbances propagate from certain regions of the Earth to others. Climatology can only give us a probability of these events, but if they occur outside the OTHR region, we may be able to monitor them, and predict when they will move into the OTHR region of interest. This generally requires a geographically spread network of ionosondes (or other ionospheric monitors) that are sampling the ionosphere at least every 5 minutes.
15.6 SWFs
Ionospheric absorption due to solar X-ray flares cannot be predicted on anything other than a probabilistic basis. However, once they are occurring, an indication of frequencies affected and duration can be given from a simple model, which is summarised by the graph below:
It is interesting to note that most HF systems are unlikely to be affected by solar X-ray flares below M1 class (ie peak X-ray output in the 0.1 to 0.8 nm band < 10-5 Wm-2). However, experience has shown that the JORN system can detect and is thus affected by flares as low as C2, which has an effect on the ionospheric D-layer comparable to the normal Lyman-alpha daily flux.
15.7 Ionospheric Storms
We normally base our ionospheric disturbance forecast on magnetospheric effects, assuming that a large geomagnetic storm will produce a large ionospheric storm. There is not always a high correlation between the two, but for the moment, it is probably the best we can do.
Estimates of storm duration could be based upon climatology. For instance, we know that a given solar plasma input around the equinoxes is likely to produce a much greater magnetospheric / ionospheric effect than the same input at the terrestrial solstices.
It is probably worthwhile for OTHR to consider multiple "durations" when considering ionospheric storms. The typical duration might be defined when the critical frequencies deviate by more than a fixed percentage (eg 20%) from median values. Another duration might consider the time span over which significant time spreading (eg F-layer range spread) is present.
Remember that it is the irregularities (inhomogeneities, gradients) that really mess up OTHR, not necessarily just changing critical frequencies (although there is usually coupling between the two).
15.8 Sporadic E
There is one type of sporadic ionospheric phenomena that can be very useful to OTHR. Widespread sporadic E can provide an almost mirror like ionospheric "surface" for the OTHR radar, allowing unprecedented clarity and accuracy in the radar display. Prediction of this type of event , even if only from a climatological viewpoint, could thus be most useful.
15.9 Meteor Showers
Although the Sun is the primary source of space weather, it is not the only one. Macroscopic particles travelling through interplanetary space (meteoroids) may collide with the Earth, ablating through the atmospheric entry process, producing a meteor. Most of the energy of this process produces a plasma (ionisation), which supports the propagation of HF and VHF radio signals. This can be a source of noise to the OTHR operator. It can produce unwanted echoes, and echoes with significant Doppler spread, or it might act (in the case of a meteor shower of very small particles) to raise the overall noise floor of the OTHR receiver, thus reducing its sensitivity.
Meteor showers occur at specific times of the year, although they can also have a much longer period superimposed upon the annual variation. New meteor showers may occur at any time. The mass exponent of the shower is an indication of how the number of meteors varies with the amount of ionisation produced per meteor. Very old showers tend to have a much large number of very small meteors (and thus produce an overall noise increase), than do new showers, which produce a relatively greater number of larger meteors (which causes random artifacts - unreal echoes or "angels"- on the radar display). The average meteor shower shows approximately inverse square behaviour with meteor "brightness". In other words, we find a lot more smaller than larger meteors.
The OTHR operator should be aware of all significant meteor showers that might affect the radar. The table below lists the significant meteor showers during the year.
| Quadrantids Lyrids Eta Aquarids Delta Aquarids Perseids Orionids Taurids Leonids Geminids Ursids | Jan 3-4 Apr 21-22 May 4-5 Jul 28-29 Aug 12-13 Oct 21-22 Nov 3-13 Nov 16-17 Dec 13-14 Dec 21-22 |
Australian Space Academy