Design of a millimetre Synthetic Aperture Radar (SAR) onboard UAV’s

Raquel Ruiz Saldaña Department of Signals, Systems and Radiocommunications

Polytechnic University of Madrid. Madrid - SPAIN

raquel_ruiz_saldana@yahoo.es

Félix Pérez Martínez Department of Signals, Systems and Radiocommunications

Polytechnic University of Madrid. Madrid – SPAIN

felix@gmr.ssr.upm.es

Abstract— This paper describes the design for an experimental short-range (2 Km) high-resolution (30x30 cm2) Synthetic Aperture Radar (SAR) sensor in the millimetre wave band to take it onboard Unmanned Aerial Vehicles (UAV). The novelty lies on the establishment of new and very demanding requirements induced by the need of being onboard low cost aerial platforms and so, susceptible of been employed in numerous military and civil applications, providing the basic advantage of these sensors against electro-optics: the possibility of using them at any time (night and day) and practically under any meteorological condition.

I. INTRODUCTION This paper presents some results of a study developed

for analyze the viability of a SAR sensor (Synthetic Aperture Radar) with a weight, volume and consumption that permit take it onboard unmanned aerial vehicles (UAVs) with very low payloads. Therefore, very demanding requirements are established accordingly both the need of being onboard aerial platforms with the exposed limitations, and the high required provisions by terms of resolution and quality at images.

To be precise, the designed prototype will work at millimetre band (at 35 GHz). This band presents the following advantages for realising High Resolution Radar (HRR) Systems opposite lower frequencies or optics: - Possibility of night and day use, with fog and adverse

meteorological conditions (short range with rain). This aspect offers the possibility of observation through obstacles like clouds, fog, etc.

- Reduction of antennas and equipment sizes, enabling being onboard smaller dimension platforms.

- High resolution and precision, because great bandwidth signals can be used.

- Mature and affordable technology. All these characteristics convert this technology in the

perfect candidate to the proposed application [1]. A resolution capability of 1 meter range and azimuth resolution involves the use of great bandwidth signals and

synthetic aperture antennas. The principal advantage achieved with high resolution image radars is the increase of the available information at the system to detect, localize and identify the presence of all types of objectives, because the multiple reflectors which forms the real targets can be divided at the reception process [2].

II. DESIGN TOOL To calculate parameters of SAR system wave forms it

has been developed a design tool that follows the process line shown at Fig 1.

Fig. 1. Calculus Algorithm

1-4244-1378-8/07/$25.00 ©2007 IEEE. 1

So, once chosen the system work frequency, the first step is determinate the PRF (pulse repetition frequency). At the beginning, it’s obtained from an approximation where the transmitted pulse length (Tx) is not consider (not defined yet at this moment), due to suppose it like a little contribution in relation with other factors of the PRF expression. Next, a first value of Tx is estimated from the PRF obtained. Later, the pulse repetition frequency is recalculated and, subsequently, the transmitted pulse length. This iterative calculus converges rapidly in one or two iterations.

Similar feedback cycles are used in the determination of other factors, like sampling frequency or intermediate frequency bandwidth, due to the dependency that some of them present between themselves.

III. PROPAGATION MODEL When modelling radiation phenomenon in a system of

this characteristics, it has to take account of the influence of work frequency variation in system parameters. This supposes a direct modification of rest of parameters which take part in radar equation (1) but has indirect influence too through propagation loss factor as Fig. 2 shows.

2 2

4max 3

min 0(4 )tx

n tot

P GRSNR k T B F L

λ σπ

⋅ ⋅ ⋅=⋅ ⋅ ⋅ ⋅ ⋅ ⋅

(1)

Fig. 2. Specific Attenuation due to rain

In order to elaborate a tool to design radar parameters valid at hole frequency range, it has been implemented a propagation model that allows obtaining propagation losses depending on frequency following the UIT-R P.530-9 and UIT-R P.838 recommendations. Next, it is shown step by step the development: Step 1: Obtain loss factor from rain intensity R (mm/h) by exponential law:

R k Rαγ = ⋅ (2)

Considering lineal or circular polarization, and any geometry of the way, k and α values are obtained from (3) and (4) expressions, at which αH, kH, αV and kV are result of a spline interpolation of values from table 1 for each work frequency.

2[ ( ) cos ( ) cos(2 )] / 2H V H Vk k k k k rθ= + + − ⋅ ⋅ (3)

2[ ( ) cos ( )

cos(2 )] / 2H H V V H H V Vk k k kr k

α α α α α θ= ⋅ + ⋅ + ⋅ − ⋅ ⋅⋅

(4)

Step 2: Calculate effective length of the way by multiplying the real distance, d, by a factor r given by

0

11

rdd

=+

(5)

Where,

0.0150 35 Rd e−= ⋅ (6)

Step 3: Calculate propagation losses with the expression:

prop eff R RL d dγ γ= ⋅ = ⋅ (7)

This model has been used to calculate system losses, which value is analyzed at next point [3].

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Table 1: Interpolation constants

IV. SIMULATIONS AND RESULTS It is expected to design a SAR system in millimetre

band at 34 GHz frequency that fulfils certain requirements imposed by the platform at which SAR will take it onboard. Its mode of operation is analyzed under different hypothesis of rain conditions to guarantee its correct operation at any condition.

Initial characteristics imposed to sensor are collected in the following table:

Azimuth beam width θAz-3dB ≤ 6º Elevation beam width φEl-3dB ≤ 8,7º Gain at Tx (with losses) GTX0 ≥ 28,74 dB Gain at Rx (with losses) GRX0 ≥ 28,74 dB Maximum transmitted power Ppeak ≤ 6 W Transmitted pulse length Tx < 1000 µs Instantaneous bandwidth Bi < 1000 MHz Maximum antenna noise figure Fant < 6 dB Antenna TOI TOIant > -39 dB TOI CE+RFC TOI CE+RFC > -23 dB

Table 2: Emission parameters of millimetre radar

System geometry is defined by following parameters:

Nominal flight height 100 m Nominal flight velocity 90 Km/h Range Resolution 0,3 m Azimuth Resolution 0,3 m Range over ground 2000 m Maximum squint angle 0º

Table 3: System Geometry

Given these premises, at following lines it’s shown the parameters of sensor designs obtained without considering rain effect (0 mm/h):

SYSTEM PARÁMETERS

Parameters Units Value Fc GHz 34 BRF MHz 675,843 PRF Hz 1300 Tx Μs 630 PCR 426024 Γ MHz/µs 1,0722 TS Μs 651,34 BFI MHz 22,5 FS MHz 27 Ppeak Kw 0,002 Paverage W 1,64 DC % 81,94 GTx dB 28,74 GRx dB 28,74 NRG 70344 Ti Μs 592.34

Table 4: System parameters

BEHAVIOUR AGAINST NOISE Squint angle (º) (dB)

0 20 45 CNR (ymax, s0 = -25 dBm2/m2 ) 6,84 6,57 5,33 SNR (ymax, s = 10 m2 ) 54,74 54,47 53,23 SCNR (ymax, s0 = -25 dBm2/m2, s0 = 10 m2)

47,08 47,04 46,78

NESZ (ymax) -31,84 -31,57 -30,33

Table 5: Behaviour against noise

SYSTEM RESOLUTION Range resolution δr = 0,3 m Azimuth resolution δa = 0,05 m

Table 6: System Resolution

Fig. 3. SNR

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Fig. 4. Signal to Clutter+Noise Ratio

Fig. 5. Clutter to Noise Ratio

Fig. 6. NESZ

It can be observed that with absence of rain (0 mm/h), the needed transmission average power to achieve a 2 km range is around 1.64 W. Lets see what happens if we consider different propagation effects depending on rain rate.

Given a rain intensity of 15 mm/h, to maintain established range it has to triple initial value of transmission power, adjustment that is no feasible with these systems which have limited capacities because of volume and weight platform at which will be integrated, that is, we can not increase transmission power all we want because it has a superior limit imposed by antenna sizes. So the affected parameter is system range that, specifically, reduces its value of about 200 m. This way, for the

established rain conditions, with a power of 1.64 watts, it would reach a distance of about 1800 m.

Fig. 7. Rain effect at transmission power for 1 Km range

Fig. 8. Rain effect at transmission power for 2 Km range

In case of 25 mm/h intensities, this effect is aggravated by a considerable way being necessary much higher powers to cover initial range. By a similar way, if transmission power is maintained invariable it would obtain a range up to 1100 m.

We can conclude that system losses, due to rain attenuation, can make worse range in a very damaging way with normal conditions at systems of these characteristics.

V. EXPERIMENTAL PROTOTYPE Actually, a high resolution radar sensor prototype of

these characteristics is being developed at Microwave and Radar Group of Polytechnic University of Madrid. The novelty of this project lies on demanding requirements associated with the necessity of being onboard aerial low cost platforms and, so, susceptible of being used at numerous civil and military applications.

Figure 9 presents the appearance of such prototype. It can be appreciated that given the size and dimensions of the prototype, its embarkation on UAVs can become feasible.

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Fig. 9. SAR Prototype developed at Microwave and Radar

Group (GMR)

VI. CONCLUSIONS During present paper, it has been described the design

for an experimental short range, high resolution Synthetic Aperture Radar (SAR) sensor in the millimetre wave band to take it onboard UAV such that can be onboard at small dimension platforms.

As well, it has being developed a design tool that evidences the rain importance over range at these bands.

ACKNOWLEDGMENTS

This work has been subsidized by I+D national plan projects TIC02-04569-C02-01 and TIC02-02657-C02-01.

We acknowledge to Isabel González Hervás her participation with the tool used in this work.

REFERENCES [1] A.W. Doerry, D.F. Dubbert, M.E. Thompson, V.D. Gutierrez “A

portfolio of fine resolution Ka-band SAR images”.. SPIE Defense and Security Symposium. March 28- April 1, 2005

[2] A. Blanco, A. Asensio, P Dorta, J. Gismero, D. Ramirez. "Millimeter Radar Demostrator for High Resolution Imaging". European Radar Conference (EuRAD2004). Amsterdam, The Netherlands. pp 65-68

[3] W.G.Carrara,R. S. Goodman. “Spotlight Synthetic Aperture Radar”. 1995

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