Antennas
5.13 ANTENNAS MOUNTED ON AIRCRAFT
Some of the first radar antennas mounted on aircraft were part of the airborne radars in operational use in 1940 for the detection of shipping (air to surface vessel, ASV), and other aircraft (aircraft interception, AI). The radars were manufactured to detect single targets for the aircraft to attack and their successors can be found in the radars installed in current fighter and maritime attack aircraft.
For airspace surveillance, ground surveillance radars can only see above the earth’s surface so that elevated siting gives a better view out to further in range but also increases the range where ground clutter is present. Greater surveillance volumes extending to low altitudes can only be achieved by mounting the radar under a balloon or on an aircraft. There are two major types: those that are made to search the airspace above the ground that have to suppress the large amount of ground clutter and radars that map objects on the ground.
There are a number of successful radar systems that are hung under balloons or rotate within the gas envelope.
Balloons are difficult to move quickly and radars mounted on airframes are more flexible. Small radars have radomes that are carried underneath aircraft and helicopters. For longer ranges, larger antennas are necessary that are mounted
Flight path
Distant point
P
Ground
Swath width
R
A B
C
The general term is synthetic aperture. If the aperture can be considered as being made up of an array of elements,
1
then the term synthetic array is also used.
Figure 5.88 An aircraft flying by a distant point P.
inside “flying saucer” radomes to reduce aerodynamic drag and to have the same drag at all antenna positions when the antenna is rotated. Ground clutter from the vertical sidelobes is a problem and has to be reduced to an acceptable level using an airborne moving target indicator processor (Chapter 11) or a high pulse repetition pulse frequency Doppler processor and antennas with low vertical and horizontal sidelobes. Another form of antenna is to have a fixed linear array along the length of the fuselage or on top of it as a dorsal fin. Phased array scanning gives a coverage in azimuth of ± 60 degrees on each side of the aircraft with limited coverage towards the nose and tail of the aircraft using the end-fire antenna gain. Again good ground clutter suppression, using an airborne moving target indicator or pulse Doppler processing is required.
Using a fixed antenna looking sideways and allowing the movement of the aircraft to achieve the scanning was used initially for military reconnaissance to look over a front line at objects on the ground, in contrast to the signal processing in Chapter 11 devoted to suppressing ground echoes. For this use the time frame between observation and use is quite different and measured in hours, not seconds as required by air traffic control. The radar signals were recorded on film during flight, after the flight the aircraft landed, and the film was developed and evaluated. Though range resolution can be improved by using shorter pulses, the azimuth resolution depends on the beamwidth of the antenna. The length available for narrow enough beamwidths was too short so that synthetic aperture radar was developed.
5.13.1 Synthetic apertures1
In Figure 5.88 an aircraft is flying in a straight line and illuminating the point P with its sideways-looking radar. The pattern of the antenna mounted on the aircraft that illuminates the ground is defined in range by the vertical beamwidth.
The range extent depends also on the height of the aircraft and is called the swath, a term used in mowing grass or cereals with a scythe or mower. The point P is illuminated by the radar in the aircraft within the horizontal beamwidth when the aircraft flies from A to C.
Discrimination in range is achieved by using small pulse widths, if necessary using pulse compression. The width illuminated is proportional to the range from the radar as the radar flies by. The coordinates of the “picture” are expressed as Cartesian coordinates projected on the ground plane in range and cross-range (alternatively across the line of flight and along the line of flight, respectively) in contrast to the polar angle coordinates and range in conventional surveillance radars.
For a long time the offset bipolar coherent video from a ring demodulator (Section 9.2.1) was recorded on film with the range direction across the film. Optical lens systems were used to process the film image after the flight that allowed the vector summing of the many echoes from each object that the radar had seen, as shown diagrammatically in Figure 5.89.
Range = 0
Maximum range Range bins
Aircraft positions
Window for analysis Film sprocket holes
Area on the film covered by echo signals from objects at different ranges.
Synthetic array Physical antenna
Store
Combining network in signal processor d
Store
Store Store
Signals out
Store Store
-N/2
0
+ N/2 Synthetic aperture
Store
d ' v
prf m
Figure 5.89 A film record of video for later analysis.
Figure 5.90 The physical antenna in N+1 positions to give a synthetic array in a synthetic aperture.
(5.112) As semiconductor and magnetic memory became faster and larger, it became possible to store the radar echo vector data using them. Smaller, faster computers allow evaluation during flight and the results can be passed to the ground using radio links. Figure 5.90 shows the physical antenna and a device for storing the signal vectors as solid lines.
The elements shown dotted are the other positions of the antenna and storage components as the aircraft flies by the broadside position of point P.
The aircraft flies with a velocity, v m/s, and has a constant pulse repetition frequency, prf, thus the distance between the synthetic elements, d, is
The velocity of the aircraft and the pulse repetition frequency must be chosen so that the positions of the synthetic elements are less than w/2 (w is the width of the antenna on the aircraft), much less gives oversampling.
The feed system of a physical array adds the echo signals together vectorially and if the record of the echo signals at the same range within the physical beamwidth of the antenna on the aircraft are added vectorially, the same process occurs. The left-hand part of Figure 5.91 shows an aircraft radar illuminating the point P and on the right 31 idealized echo signals at the same zero phase angle. If these signals are added together vectorially, as they would be in a feed system, the synthetic azimuth beamwidth is much narrower than the physical beamwidth.
-10 10 20 30
Distant point
P
Flight path
Antenna patterns for a primitive antenna and the synthetic pattern from the
sum of 31 pulses Signals from 31 echoes
0.2 0.4 0.6 0.8 1
-15 -10
-5 5
10 15
Physical array Synthetic array
R
x d
Rt
Rr
Rt = Rr
P P
R
d
x
O O
L1 ' R2 % x2 % R2 % (x & nd)2 Pythagoras
L1 ' R (1 % x2 R2 & nd
R2 % n2d2 2R2 % ...)
Figure 5.91 Signals taken along a line and the synthetic antenna pattern.
Figure 5.92 Comparison of the path lengths with physical and synthetic apertures.
(5.113)
(5.114) The phase shift from synthetic element to element is double the phase shift between elements in a physical array.
With a physical array the objects of interest (P in Figure 5.92) are assumed to be illuminated by a wavefront coming nominally from the phase center of the transmitting antenna (O in Figure 5.92).
The echo signals are scattered back from the point P and enter the array elements with a phase angle proportional to the two-way path length to an element, L , given by (Figure 5.92)1
where R is the distance from the center to the plane containing the point P;
x is the distance from the center-line in the same units;
d is the distance between two elements or aircraft positions;
n is the number of the element counting from the center.
The phase difference there and back is L 2B/8 radians. Expanding L using the first terms of the binomial series1 1
L2 ' 2 R2 % (x&nd)2
L2 ' R (2 & x2 R2 & 2nd
R2 % n2d2 R2 % ...)
Standard beamwidth, 1 ' 8
2W radians
Resolution ' R1 ' 8R
2W ' 8R
2 w
8R ' w 2
N ' R 2B
8 (sec2&1)
' R 2B
8 22 2! % 524
4! % ...
(5.115)
(5.116)
(5.117)
(5.118)
(5.119) With the synthetic array there is physically only one element so that the transmitting phase center moves with each pulse along the synthetic array so that the lengths out and back are the same, namely,
Expanding as the first two terms of a binomial series
Notice the linear term for the total path length in L (2 nd/R² ) is twice that of L giving twice the phase change across2 1 the aperture compared to a physical array to give a beamwidth half of that of the physical aperture, width W.
More simply, the echo signals reaching a physical antenna have originally come from its phase center. With the synthetic array the phase angles of the echo signals have an additional phase change caused by the fact that the antenna used for transmission has moved in contrast with the phase center of the physical antenna.
The standard beamwidth of the physical antenna of width, w, is 8/w and it covers a length on the ground of R8/w.
The resolution of the synthetic aperture is 1R
The resolution is limited by the amount of signals that are able to be collected and processed coherently, the tapering of these signals to reduce sidelobes, and irregularities of the motion of the aircraft. Part of the problem is caused by geometry in that the echoes to be integrated do not lie on a circle but on a straight line, this is called unfocused processing.
Most physical antennas are “focused at infinity” and assume that all objects appear in the far field or Fraunhofer zone. Equation (5.8) gives the range necessary to assume that far field (Fraunhofer zone) conditions exists, namely 8/16 or 8/32 distance error at the edge of the aperture, B/8 or B/16 radians phase error. Different authors give maximum errors for synthetic apertures before phase correction becomes necessary, for example, [34, p. 922, Fig. 12.1] 8/8 one way, two-way phase shift = 90°.
The length of synthetic apertures and the ranges used place the objects of interest in the near field so that the range and phase angle to the object changes during the fly past, as shown in Figure 5.93.
The distance of P from the flight path varies and is R sec2 so that there is a phase change along the flight path, N, of
where R is the range, 8 is the wavelength measured in the same units, and sec2 has been expanded as a series.
Line of flight
R 2
P
A B C
R sec 2 R sec 2
Flight path Distant
point P
Antenna patterns for a primitive antenna and the sum of 31 pulses
Phase angles of the 31 pulses
0.2 0.4 0.6 0.8 1
0.2 -15
-10 -5
5 10
15
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
-15 -10
-5 5
10 15 B/2 = 1.571
Signals from 31 pulses
-5 5 10 15 20 25
-0.6 -0.4 -0.2
0.2 0.4 0.6
-20 -10
10 20
(R % 8
8)2 ' W2 4 % R2
W ' R8
Figure 5.93 Simple geometry for a synthetic aperture.
Figure 5.94 The signals and synthetic antenna pattern from a train of signals with phase shifts due to range difference.
(5.120)
(5.121) Figure 5.94 shows the echoes returned to the physical antenna flown with a parabolic phase shift of the signals at a maximum of B/2 or 90 degrees at the ends. The same processing has been carried out as in Figure 5.91 and the phase errors in the echoes cause the complex beam synthetic pattern to warp and wilt.
The modulus of the beam pattern is in the common form in Figure 5.95. With B radians or 180 degrees phase shift at the ends, the synthetic pattern starts to break up. When the beam is focussed, the phase along the record of echo signals is corrected to remove the phase errors.
The resolution without phase correction and having a phase error of B/2 or 90 degrees at the start and end of the recording corresponding to a one-way distance of 8/8, is then
giving the maximum width of the synthetic aperture of
0 5 10 15 20 25 30
-0.3 -0.2 -0.1 0.1 0.2 0.3
u
Synthetic beam patterns with no phase shift
B/2 radians (90 degrees) phase shift
B radians (180 degrees) phase shift
8
2W ' 1
2 8 R
1R ' 1 2 8R
8/R /2 8R /2
Figure 5.95 Synthetic patterns with parabolic phase shift.
(5.122)
(5.123) with a standard beamwidth of
and resolution
The unfocused resolution is independent of the size of the physical antenna and greater resolution is only obtainable by shortening the wavelength.
The three types of antenna using a physical broadside antenna 2 m wide at X-band are compared in Table 5.6.
Table 5.6
Comparison of standard beamwidths and resolutions for an antenna 2 m wide with 3 cm wavelength
Standard beamwidth Resolution at 20 km
Physical antenna 8/w 15.0 mrad R 8/w 300.0 m
Unfocused antenna 0.61 mrad 12.25 m
Focused antenna 8/2W W/2 1.0 m
5.13.1.1 Focusing
Table 5.7 shows the improvement in a typical case that can be obtained by correcting the phases of the echoes before summing. The correction is different and must be calculated for each range.
Reference [35] gives the approximate characteristics of the AN/APQ-102A radar and the characteristics are quoted as an example.
Table 5.7
Approximate characteristics of the AN/APQ-102A radar [35]
Characteristic Value
Wavelength 3 cm
Transmitter
Pulsewidth 1 µs
Bandwidth 15 MHz
Average power 90 W
Antenna length 1.2 m
Standard beamwidth 1.43 degrees
The number of cross-range samples collected [35] are shown in Table 5.8.
Table 5.8
Amount of data sampled by the AN/APQ-102A radar [35]
Characteristic Values
Range
18.5 km 55 km
Processed synthetic aperture 30.8 m 92.3 m
p.r.f 1 800
Synthetic aperture time 0.11 s 0.33 s
Number of samples collected 3 042 2 281
The performance after signal processing [35] is shown in Table 5.9.
Table 5.9
Resolution performance of the AN/APQ-102A radar [35]
Characteristic Value
Range
18.5 km 55 km
Range resolution 10 m 10 m
Cross-range resolution 9 m 9 m
Number of samples processed 202 606
5.13.1.2 Spotlight synthetic array radar
Where flying by an area of interest does not give enough resolution, it is possible to have a clearer picture by flying an arc around the area of interest, but with greater processing effort per unit area [35, 36].
5.13.2 Mapping
Three-dimensional ground mapping may be carried out when a second receiving antenna is mounted above or below the original antenna as an interferometer. The phase differences between echoes received give an elevation angle (sometimes ambiguous) that allows contour maps to be produced for areas of the world that are normally inaccessible or have so much fog that optical measurements are not possible.
5.13.3 Radars on satellites
Synthetic aperture radars are used on satellites for, among other uses, maritime surveillance. The oceans are large and the objects of interests, ships, do not move quickly.
Satellite in orbit
Swath
Figure 5.96 A radar in a satellite illuminating the Earth’s surface.
Figure 5.96 shows a radar in a satellite illuminating the ground obliquely in order to have a range dimension. The coverage depends on the orbit and the ability to direct the antenna in angle on the satellite. A number of satellites are in orbit with heights of around 250 km or 900 km. The angle of the antenna is set to give swath widths of the order of 50 to 100 km and the resolutions have been of the order of 15 m to 100 m. The pulse repetition frequencies are chosen to cover the swath width so that there will be a number of pulses on their way to the earth and back at the same time. The radio frequencies from L- to X-band have been used.
5.13.4 Other considerations
It has been assumed that the cross-range samples (range sweeps) are collected equally spaced in time along a straight line or arc of a circle. Aircraft in flight react to buffeting in flight inversely proportional to their mass. With wavelengths of 3 cm, random movements can be an appreciable proportion of a wavelength. Signals recorded from the aircraft’s navigation system may be used to be able to correct for these phase shifts. As aircraft are flexible, an inertial measurement unit (IMU) may be mounted behind the antenna to signal any short term positional changes.
In [36, p. 448] a number of methods are described for correcting a run of data after it had been collected by being able to estimate the quadratic phase errors and applying corrections.
The discussion above has been limited to sideways looking antennas. Synthetic aperture techniques have also been applied to antennas squinted off broadside.