2.2.1.2 : Timing Resolution
Relative time-delay depends on direct (transmitter to target) and indirect (target to receiver) propagation path length differences. The extent to which (relative) time-delays can be resolved by a radar system is commensurate with transmission inverse bandwidth,
\/B, and determines resolvability of target locations. Doppler frequency shift of received echoes depends on the sum of target Doppler motion resolved individually by transmitter and receiver. Doppler resolution in turn depends on integration length of received echoes. Such are important to radar performance, and so are given much attention in Chapter 3.
2.2.1.3 : Echo Detection
Echo signal strength depends on the product of the inverse square of target range from receiver and transmitter. The mean power of echo signals versus the mean power of noise and clutter signals determines how reliably echo signals may be detected. This determines radar detection performance and is related to a maximum detection range.
Threshold detection is a simple method of deciding target presence. The threshold is set at a voltage level, not a SNR level. This precludes GEAR, which adapts the threshold according to clutter near the target. Reliable detection means high detection probability when a target is present (noise does not suppress the echo signal) and low false alarm rate when no target is present (noise does not breach the threshold in the absence of a signal).
There are usually variations in echo signal mean power due to target fluctuation, e.g.,
relative motion of many scatterers or bulk motion across the antenna beam. Since we are characterising a receiver and not targets, we pursue Swerling case zero, i.e., constant RCS. Fluctuations are therefore entirely due to instantaneous voltage fluctuations of thermal noise. Properties of thermal noise are expanded upon later in this Chapter and in Chapter 3. Mean thermal noise power statistically describes noise fluctuation fully, and allows direct comparison of mean powers of echo and noise signals. Hence we shall remain with mean SNR for the rest of this Chapter.
What optimum visibility factor must mean SNR exceed to apply reliable threshold detection of targets? Blake [18] has calculated detection probability and false-alarm rate for a given echo mean SNR in the case of Swerling zero targets. Table 2.1 tabulates visibility factors of linear rectified voltages (in dB) above mean noise power. This shows that a suitably reliable detection performance of P^=90% and Pfa=10'^ in the presence of noise requires the threshold of detection to be set at 13dB (bold) above mean noise power.
CftapterZ TrincipCes oftfie ‘Bistatic i^dar V (d B ) Pd = 0.001 Pd = 0.01 Pd = 0.1 Pd = 0.5 Pd = 0.9 Pd = 0.99 Pd = 0.999 P f a = 1 0 ' < -15dB < -15dB < -15dB +2.5dB +7.2dB +9.6dB + ll.ld B Pfa = 10" < -15dB < -15dB +0.5dB +6.1dB +9.4dB +11.3dB +12.5dB P f a = 1 0 ' < -15dB +2.0dB +4.1dB +8.0dB +10.7dB +12.4dB +13.5dB P f a = 1 0 ‘‘ -3.2dB +2.0dB +6.1dB +9.4dB +11.8dB +13.3dB +14.2dB Pfa = 10" +0.7dB +4.3dB +7.6dB +10.4dB +12.5dB +13.9dB +14.8dB P f a = 1 0 ' +3.1dB +5.9dB +8.7dB +11.2dB +13.2dB +14.5dB +15.3dB Pf^= 10* +6.2dB +8.2dB +10.4dB +12.5dB +14.2dB +15.4dB +16.2dB P fa = 1 0 * “ +8.1dB +9.7dB +11.7dB +13.5dB +15.0dB +16.1dB +16.9dB P fa = 1 0 '" +9.6dB +11.0dB +12.6dB +14.3dB +15.7dB +16.7dB +17.4dB Pfa = 10*" +10.8dB +12.0dB +13.5dB +15.0dB +16.3dB +17.2dB +17.9dB Pfa = 10*" +11.7dB +12.8dB +14.2dB +15.8dB +16.8dB +17.7dB +18.3dB
Table 2.1. Required visibility factor (SNR) versus detection probability, P^, and false alarm rate, Pfa, for Swerling case 0 targets.
2.2.2 : Parasitic Bistatic Radar
One possibility of bistatic radar is to design and construct a receiver that uses an already existing transmitter as an opportunity. This has two side-line advantages; a) a saving in the cost of building a transmitter and b) the passive nature of the receiver makes it immune to radio-location. However, it does not guarantee in-built synchronisation and appropriately modulated transmissions, and the receiver may not be able to achieve the necessary processing as a result.
2.2.2.1 : Satellite-borne Illuminators
Another source of transmitters of opportunity that have come into existence in the past decade are space-based transmitters. One group are the Low Earth Orbit radar satellites. A single satellite may have various instruments on board such as wind scatterometers, radar altimeters, rain radars and cloud radars. These transmit powerful signals with modulations designed for radar purposes. However, a fast fly-by rate (typically 7kms means the transmitter is available only at occasional instances of a few seconds. Yet another group are Direct Broadcast by Satellite Television (DBS TV) satellites. These are space-based, but relatively stationary transmitters with continuous, wideband transmissions covering large areas, and we now review these.
CfmpterZ
2.2.2.2 : O pportunities with A vailable DBS TV Satellites.
Trincip(es of the ‘Bistatic H(adar
Figures 2.1a-d illustrate m ean flux densities across Europe o f satellites providing services over London, UK, derived from published EIRP m aps using Eq. A 1.2. M arco Polo 1 provided a service dedicated to the British Isles, but has been decom m issioned as a DBS TV satellite since 1993. How ever, its flux density m ap provides the specification for a British Isles service, should it be resum ed. The w eakest satellite, A STRA , was the first to be launched, but the subsequent trend to m ore powerful satellites has plateaued, hence
_2
flux densities higher than -95dB W m ” probably will not com e about in the future.
60
(a) ASTRA (19.2'E )
55 -110 50 -110 -110 45 40 -10 65 (b) Marco Polo I (31 °W) -107 60 -104 55 -110 50 -110 -107 45 -15 -10 10 15 Longitude/" (0) TDF1/2 (18.8°W) ■> -104 0 5 10 Longitude/" Longitude/" 60 TV-Sat2 (19.2°W ) -104 55 -101 S -107 50 -101 -104 45 -104 -107 40 -10 Longitude/"
Figure 2.1. Four DBS TV satellite flux density distribution specifications (dBWm' ).
G eom etry concerning DBS TV satellites is presented in A ppendix 1. These appear tow ards the south at elevations typically 20° to 30° over London. TV channels are transm itted on FM carriers covering upper X-band, (10.7G H z to 12.5GHz). M axim um allocated channel bandw idth is 27M Hz, though lOMHz is typical, and allow s us to anticipate a typical tim ing resolution of 50ns, leading to a bistatic range resolution o f the order o f 15m. Perm itted satellite motion is restricted to a 50km box, giving a mean diurnal radial motion of 0.6ms" ^ and a m ean carrier D oppler shift of 24Hz. Diurnal variation in transponder frequency typically approaches several kH z [19], and since this
exceeds greatly the D oppler shift we may regard satellite m otion as negligible, i.e.,
CfmpterZ (Principles oftfie (Bistatic 9(adar
2 . 2 . 2 3 : O utline of the Radar System Concept
Figure 2.2 illustrates the proposed radar system. The DBS TV satellite provides
floodbeam coverage o f its allotted service area, within which receiver and target are located. Tw o antennas separately receive direct reference and indirect echo signals. Target location is related to path propagation relative tim e-delay, Ar=(/?Rx+/?Tx-^)/c, and echo direction.
Reference antenna
Coherent downconverter DBS TV Satellite
Direct reference
signal (~37,000km) High gain, low noise amplifier JD_ Delay, t Illuminating signal (-37,000km) Tx Receiver Output X-band LG High gain, low noise Indirect echo signal \ amplifier
(10km-100km) \ hC
Multiplier Integrator
Target
Coherent downconverter Echo antenna
Figure 2.2. Illustration of the radar receiver system concept.
Selection o f a DBS TV satellite as transm itter of opportunity perm its the use o f a dom estic satellite receiver system to receive a direct reference signal with a typical m ean SN R o f 15dB. A second dom estic receiver system may be devoted to echo signals, and output signals o f both sets may be processed to detect targets by cross-correlating and integrating the outputs to achieve sufficiently reliable target detection.
DBS TV satellites provide transm ission coverage constant to w ithin 3dB over geographic scales o f the order o f 1,000km. Targets and the receiver lie within the sam e footprint, so
signals will be via the same antenna beam and therefore coherent. This is not
considering incoherence introduced by target D oppler m otion, which is a m atter dealt with later. D irect transm issions provide continuous reference signals, thus synchronisation and pulse-chasing are not issues for this concept.