Mitigating the Effect of Direct Signal Interference in Passive Bistatic Radar Christopher Coleman School of Electrical and Electronic Engineering The University of Adelaide Adelaide, Australia
[email protected] Abstract— Passive bistatic radar uses target illuminations by transmitters of opportunity (often commercial broadcasts). Unfortunately, the strength of the direct signal interference (DSI) can often be so strong as to mask target returns. Furthermore, when illumination is provided by a DAB(Digital Audio Broadcasting) network, the problem can be exacerbated by several transmitters operating on the same frequency. The present paper discusses strategies for reducing DSI and, in particular, the use of array nulls, cross polarization and shielding by topography. The effectiveness of these strategies is investigated through modelling. Keywords- bistatic radar; passive radar; DAB; propagation; adaptive arrays; cross polarisation
I.
INTRODUCTION
Figure 1. Topographical map showing radar (red X), illuminators (blue +) and airport (red O) positions.
With the advent of digital broadcasting, passive radar has become an effective and economical option for applications such as aircraft traffic control [1]. In particular, DAB (Digital Audio Broadcasting) provides a signal of opportunity with excellent properties [2]. The transmitters have high power (typically 5kW), the signal has an almost thumbtack ambiguity function (its modulation makes it noise like) and its wide bandwidth (1.536MHz) ensures a range resolution of better than 200m. As with all passive bistatic radar, however, there is the potential for the direct signal interference (DSI) to swamp the return from the target. In the case of DAB, this can be exacerbated when there is a single frequency network of transmitters (transmitters broadcasting virtually identical signals on the same frequency). Multiple illuminators open up the possibility of better target location, but they can also exacerbate the DSI problem. The University of Adelaide have built a system [3-6] to test the potential of DAB for radar and have located it at the university of Bath in the UK (pending the establishment of a DAB system in Australia). Fig.1 shows the region around Bath with the locations of the radar (red X), the DAB illuminators (blue +) and Bristol airport (red O). Bristol airport is a major source, and destination, of air traffic with most flights being holiday charter and using aircraft of the Boeing 737 variety.
Figure 2. Propagation loss from Bath (at 0km) to Wenvoe (at 64km).
Figure 3. Propagation loss from Bath (at 0km) to Pur Down (at 20km).
Figure 5. Bistatic X-section for a small passenger jet, illumination to side.
From fig.1, it will be noted that potential sources of DSI are located all around the radar with one source (Wenvoe) in the direction of the Bristol airport (the intended source of targets). A traditional means of mitigating DSI has been the blocking effect of buildings and/or terrain [7]. Figures 2 and 3 show propagation losses towards the DSI sources of Wenvoe and Pur Down. Pur Down exhibits strong blocking by terrain, but this effect is negligible for Wenvoe. Another approach to DSI reduction is to arrange target antenna nulls in the direction of DSI sources and this has been used in the Adelaide sytem. In addition, Saini et al [8] have suggested the reception of target returns in cross polarisation from that of the DSI sources and this also been used to great effect in the Adelaide radar [6].
When using our radar for observing targets at Bristol airport from Bath, it will be noted that the airport is bracketed by the DSI sources of Wenvoe (51º27’33’’N,3º16’54’’W) and Mendip (51º14’12’’N,2º37’31’’). Consequently, we need to arrange the first antenna nulls towards these DAB sources. Fig. 4 shows the gain pattern for a 3 element array of 4 element Yagi-Uda antennas with the length of the array adjusted to place the first nulls at an angle of 45degrees (the angle between the these DSI sources when viewed from the radar). Such an arrangement considerably reduces the DSI in the target antenna whilst retaining a large volume of air traffic around Bristol. All other sources of DSI are contained within the back lobes and are therefore also considerably reduced.
The Adelaide system has been described elsewhere [6] and so this report concentrates on an investigation of potential DSI mitigation stratagies and their effectivenes. We do this through detailed numerical modelling of antennas, target cross sections and propagation. II.
CROSS POLARISATION AND ANTENNA NULLS
Figure 4. Gain pattern of Yagi-Uda array towards target.
A useful strategy for avoiding DSI is to observe the target in horizontal polarization, cross to that of the illuminator [6,8]. The DSI comes in at almost grazing and so even the sides of the array will show little response to DSI. The array, however, will respond to the considerable cross polar component of the radar returns. Figures 5 and 6 show cross polar radar cross sections for a small passenger jet (19m long and 15m wingspan) and indicate a typical level of around 10dB.
Figure 6. Bistatic X-section for a small passenger jet, illumination to fore.
III.
Figure 7. Cross polar performance of the target array.
If cross polarization be part of the DSI reduction, target arrays of the above type will best remove cross polarisation in the plane of the antenna. DSI, however, arrives very close to this plane (even after ground reflections) and an angle of 2º is typical. Figure 7 shows the cross polar response of the array at this angle and it can be seen that the response is very small. Whilst azimuthal null formation in the direction of illuminators is an effective DSI mitigation strategy, it can also null out desired target returns. At this point, it is worth noting the effect of ground upon signals that enter the receive antenna. For returns close to grazing incidence, the direct and reflected signal from a source will tend to cancel, due to the imperfect nature of the ground, whilst at higher angles of incidence the cancellation will be very much less. The effect can be seen in fig.7 where the ground reflected signal has been subsumed into the gain pattern of the target directed array. (Note that the array is one wavelength above the ground and the DAB illuminator operates at around 222MHz). Since signals from the DSI sources will come in at close to grazing incidence, they will therefore be severely attenuated. Targets on the other hand will, in general, be at high altitudes and will therefore come in at elevations where the antenna gain is high. For vertically polarized antennas the effect is very much stronger, but such antennas would not have the advantage of cross polarization.
Figure 8. The effect of imperfect ground upon the target array pattern.
HIDING FROM THE DSI
Our radar is at present sited at the campus of the University of Bath on the roof of the electrical engineering building. This site is in direct view of the DAB transmitters of Wenvoe, Bath and Mendip and so suffers from considerable DSI. Up to now, this location has been dictated by the need for extensive support services for the radar system. We have, however, developed a portable version of the radar and so this introduces the possibility of relocating at a position which is hidden from the DSI sources. To investigate this, the same algorithms used to simulate the propagation of figures 2 and 3 have been developed into an algorithm that allows the calculation of the total power (from all illuminators) at any position. This allows the construction of a map of DSI that can be used to calculate the optimum position for the radar. Figure 9 shows a map of total DSI power for a radar with its antenna at 10m above the ground. It will be noted that there are considerable areas where the level of the DSI is sufficiently low (a level of -80dBW can be accommodated by the hardware) for there to be no need for analogue DSI cancellation. Furthermore, several of these areas are located close to the university (51º22’44’’N,2º19’41’’W).
Figure 9. Example of a figure caption. (figure caption)
It is important to note that the the power in figure 9 is made up of several illuminations, but that there is a common volume in which all DSI is relatively low. Figure 10 shows the separate illuminations from Mendip and Naish Hill. These figures shows the expected characteristics of radiation (from Mendip to the south west and Naish Hill to the east) after it has been interupted by the hills that surround Bath. If we locate a radar receiver at one of the positions with low DSI, the important question is as to whether the radar will now be able to see any targets. Figure 11 shows the propagation losses towards Bristol airport when the radar receiver is located at the low DSI spot of (51.35ºN,2.31ºW). It will be noted that there is visibility for targets at elevations above 1000m for ranges out to 20km (120dB loss is low enough for passenger jet observations) and further at higher altitudes..
For observations of aircraft at low altitudes, the use of a low DSI site is clearly problematic. For a typical site, some directions will be more of a problem than others. Figure 12 shows the one way propagation loss in all directions around the low DSI site of figure 11 and for a height of 900m. This figure shows that to the north and south there are large swathes where the signal loss is virtually that of a free path. Noting, from figure 9, that there is considerable choice in low DSI sites, a suitable strategy might be to place receivers at a variety of these sites to gain a more comprehensive air picture (and further information for target positioning). Figure 13 shows the losses at a height of 900m for a nearby low DSI site (51.385ºN,2.315ºW). It will be noted that, whilst there is significant common visibility between the sites, some additional coverage is provided by the new site. If a network of passive radars is contemplated, the use of low DSI could be an effective option.
Figure 10. DSI from Naish Hill and Mendip.
Figure 13. One way loss at 900m around alternative low DSI site.
REFERENCES [1] Figure 11. One way loss from low DSI site to airport.
[2] [3]
[4]
[5]
[6]
[7]
[8] Figure 12. One way loss at 900m around low DSI site.
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