A Ka Band Imaging Radar: DRIVE on board ... - Semantic Scholar
A Ka Band Imaging Radar: DRIVE on board ONERA Motorglider Nouvel J.F., Jeuland H., Bonin G., Roques S., Du Plessis O., Peyret J. Electromagnetism and Radar Department Office National d’Etudes et de Recherches Aérospatiales Salon de Provence, F-13661 [email protected] Abstract— Following previous studies, a concept of low-cost imaging Ka-Band radar is presented in this paper. This radar is integrated into under-wings pods that are fixed on a STEMME S10VT motorglider. This radar concept combines real aperture in the cross-track direction, by the antennas geometrical aperture, and synthetic aperture in the along-track direction, realized with the aircraft motion. Radar front-end uses FMCW (for Frequency Modulated Continuous Wave) technique which allows to reduce the power emission to a few Watts. In addition, the use of the millimeter band induces antennas size reduction, and makes possible the radar integration into pods. Thus, radar particularities are a lowsize, a low-weight and a low-cost basis, making this radar suitable for future integration on board small vehicles, such as UAV (Unmanned Aerial Vehicle). The radar definition and specifications will be exposed, together with the first results obtained on April 2006. Two ways of operation will be exposed: An application as vertical sounder, using cylindrical horn antennas, and an application as SAR radar, using rectangular antennas. The two cases will be illustrated by samples of results. I. INTRODUCTION UAVs from different countries and with different payloads have proven their capabilities within military applications. Future UAV employment in the civilian areas of surveillance (pollution, natural risks prevention, fire prevention), monitoring (traffic control, environmental or urban monitoring, earth observation) and communication relays becomes unavoidable. Many payload configurations may be used in UAV operations, but compared to systems working in other spectral regions (such as optical or infrared sensors), radar has the main advantage to be able to operate in all-weather condition. ONERA, and more precisely the Electromagnetism and Radar Department, has developed a great experience in radar applications for years. ONERA major projects are for instance NOSTRADAMUS, a transhorizon radar, GRAVES, to detect and monitor low altitude satellites, MERIC, for in-flight measurement of aircraft's electromagnetic signature, RAMSES, a SAR station which operates 8 frequency bands and lastly DRIVE, an under-wing pod embedded radar in Ka band. The DRIVE radar has been developed and built to fly on board an UAV vehicle. Indeed, this radar is designed and will be used by ONERA as a UAV radar test bench.
DRIVE Project started at ONERA on February 2005, and first flights are planned for April 2006. This radar will be operated in two main modes: Vertical sounder and SAR imagery radar. II. DEMR AXIS OF INTEREST EM sensors represent a key element of UAV payloads as they can perform missions in all-weather and day/night condition. For the last few years, the Electromagnetism and Radar Department of ONERA (DEMR) has been leading a special R&D and experiment program for UAV payloads with a focus on payloads that are closely integrated on the UAV structure. The aim of this program is to conduct research studies on new concepts of EM payloads and to validate their concept and feasibility by demonstrator developments and flight tests onboard a dedicated test bench. This carrier offers a large wingspan, characteristic of particular interest for the payload integration. Indeed, this can lead to high classes of performance, for example, in term of direction of arrival estimation performed by a localization system (interferometer array along the wing) or in terms of directivity and covered area for an antenna array (array elements along the wing). On this way, we focus presently on two projects which take advantage of the wings area : The first, named "large vibrating antenna" studies the impact of wings’ deformations and vibrations on the antenna’s performance due to their integration in the wings. For this purpose, the project focuses on the analysis and the compensation of these non-desirable effects by means of mechanical sensors and auto-calibration technique. The second, named “DRIVE”, is presented in this paper. It is a MM-band nadir-looking imaging radar dedicated to the imagery of non-smooth areas (urban, forested or mountainous) where conventional SAR generally suffers from shadowing effect and geometric distortions. The radar will be composed of a linear array of nadir-looking receivers spanning the wings of the UAV and it will use both beam forming and SAR technique with a frequency-modulated waveform for the 3D-image synthesis of the area under the flight path. III. 3D RADAR IMAGERY CONCEPT Objective of DRIVE Project is to study the 3D imagery concept with vertical geometry. In mountainous, forested and mainly in urban areas, conventional SAR technique suffers
from shadowing effects which cause information gaps in the resulting image. Moreover, geometric distortions due to layover and foreshortening effects may appear in the image. The use of a nadir-looking imaging radar is a way to complete the SAR information. Indeed, such a radar system seems to be very powerful and promising because, in addition to its allweather and day/night capabilities, this radar system is adapted to all types of terrain (urban, plain, hills, mountains) as it looks downward the flight-path (limited shadowing, layover and foreshortening effects). It also leads to advantageous power budgets because ground strongly reflects the energy in the incoming direction at normal incidence. A very good target-toclutter discrimination can also be obtained as most targets may be seen as plane reflective surfaces for angles of sight equal or close to the specular angle. 3D imagery will be performed using distance compression on vertical axis and SAR technique on motion axis [1]. To access the transversal resolution, we will use at least a FFC based technique, with conformable antennas fixed below the airplane’s wings (Fig. 1). The use of the conformable antennas array technique will allow to assess the question of wings bending and vibrations, and to develop techniques in order to compensate these unfavorable motions. Indeed, the project main objective is to operate a large antennas array fixed under-wings, together with the SAR imagery techniques and a vertical line of sight. Thanks to the combination of these techniques, we will make up innovating 3D radar images, rich in terms of information. It will be the first time this combination is performed. DRIVE radar (Fig. 2) uses FMCW techniques and operates in Ka Frequency Band. The central frequency is 35GHz and the bandwidth is about 800MHz width.
Plan
VV Along-track processing
Plans
SAR
z
x0 = 0
Figure 2. Ka radar integrated into an under-wing pod.
IV. PLATEFORM PERFORMANCES Current airplane used for testing applications is a STEMME S10-VT motorglider (Fig. 3) as the geometry of this airplane may be representative of a UAV’s one: wingspan is 23m large and weigh is about 900 kilograms. The SV10-VT motorglider is a low-cost, very flexible platform. Wings have been reinforcing and four under-wing hard attach points have been adding per wing. Cable channels have been provided into the wing, from pods to fuselage, in order to connect the payload to on-board power supply and control-command system. Two composite under-wings pods have been developed, each pod can accommodate up to 60kg/80 liters payload. The table below summarizes the principal performances of the platform : TABLE I. BUSARD PERFORMANCES Wing span
23 m
Length overall
8.42 m
Wings area
19 m2
Max speed
270 Km/h
Manoeuvring speed
180 Km/h
Service ceiling
9145 m
Weight empty
660 Kg
Max landing weight
980 Kg
Number of pod
2
Pod volume
80 l
Pod diameter
35 cm
Pod payload max weigth
60 kg
xn =v.n Cross-track processing Digital Beam Forming x
∆x ∆y
xN =v.N y
Figure 1. UAV 3-D Downward-loohing imaging Radar principle
B. The development of a complete 3D radar The second phase integrates the development and the integration of a conformal linear array. The objective is to give the radar its full 3D capacity. Preliminary studies are done to design a first antenna array, about 1 meter large, which will be integrate under the wings. With such a demonstrator, the objective is to assess the feasibility of a complete 3D SAR radar and to test the 3D SAR image formation using a vertical geometry. Figure 3. ONERA BUSARD motorglider.
The development of such a demonstrator is currently under definition and first flights are scheduled during the year 2007.
V. SCHEDULE The DRIVE radar development is divided in the following two phases. A. The development of a first radar sensor in Ka band The objective of this first phase is to develop and operate a radar sensor embedded into under-wing pod and that can be operated following two modes: • An application as vertical sounder, using circular horn antennas, • An application as SAR radar, using rectangular horn antennas (Fig. 4). This first phase is now fully completed (Fig. 2) and first flights are planned for April 2006. Currently, this radar has been tested from end to end at laboratory level in the two modes: Vertical sounder and SAR imagery radar. A first measurement campaign is planned for the end of 2006.
Figure 4. Circular horn antennas (left) and rectangular horn antennas (right).
REFERENCE [1]
R. Giret, H. Jeuland and P. Enert : « A Study of & 3D-SAR Concept for a Millimeter-Wave Imaging Radar onboard an UAV », European Radar Conference 2004, Amsterdam, Netherland, pp. 201-204.
DRIVE Project started at ONERA on February 2005, and first flights are planned for April 2006. This radar will be operated in two main modes: Vertical sounder and SAR imagery radar. II. DEMR AXIS OF INTEREST EM sensors represent a key element of UAV payloads as they can perform missions in all-weather and day/night condition. For the last few years, the Electromagnetism and Radar Department of ONERA (DEMR) has been leading a special R&D and experiment program for UAV payloads with a focus on payloads that are closely integrated on the UAV structure. The aim of this program is to conduct research studies on new concepts of EM payloads and to validate their concept and feasibility by demonstrator developments and flight tests onboard a dedicated test bench. This carrier offers a large wingspan, characteristic of particular interest for the payload integration. Indeed, this can lead to high classes of performance, for example, in term of direction of arrival estimation performed by a localization system (interferometer array along the wing) or in terms of directivity and covered area for an antenna array (array elements along the wing). On this way, we focus presently on two projects which take advantage of the wings area : The first, named "large vibrating antenna" studies the impact of wings’ deformations and vibrations on the antenna’s performance due to their integration in the wings. For this purpose, the project focuses on the analysis and the compensation of these non-desirable effects by means of mechanical sensors and auto-calibration technique. The second, named “DRIVE”, is presented in this paper. It is a MM-band nadir-looking imaging radar dedicated to the imagery of non-smooth areas (urban, forested or mountainous) where conventional SAR generally suffers from shadowing effect and geometric distortions. The radar will be composed of a linear array of nadir-looking receivers spanning the wings of the UAV and it will use both beam forming and SAR technique with a frequency-modulated waveform for the 3D-image synthesis of the area under the flight path. III. 3D RADAR IMAGERY CONCEPT Objective of DRIVE Project is to study the 3D imagery concept with vertical geometry. In mountainous, forested and mainly in urban areas, conventional SAR technique suffers
from shadowing effects which cause information gaps in the resulting image. Moreover, geometric distortions due to layover and foreshortening effects may appear in the image. The use of a nadir-looking imaging radar is a way to complete the SAR information. Indeed, such a radar system seems to be very powerful and promising because, in addition to its allweather and day/night capabilities, this radar system is adapted to all types of terrain (urban, plain, hills, mountains) as it looks downward the flight-path (limited shadowing, layover and foreshortening effects). It also leads to advantageous power budgets because ground strongly reflects the energy in the incoming direction at normal incidence. A very good target-toclutter discrimination can also be obtained as most targets may be seen as plane reflective surfaces for angles of sight equal or close to the specular angle. 3D imagery will be performed using distance compression on vertical axis and SAR technique on motion axis [1]. To access the transversal resolution, we will use at least a FFC based technique, with conformable antennas fixed below the airplane’s wings (Fig. 1). The use of the conformable antennas array technique will allow to assess the question of wings bending and vibrations, and to develop techniques in order to compensate these unfavorable motions. Indeed, the project main objective is to operate a large antennas array fixed under-wings, together with the SAR imagery techniques and a vertical line of sight. Thanks to the combination of these techniques, we will make up innovating 3D radar images, rich in terms of information. It will be the first time this combination is performed. DRIVE radar (Fig. 2) uses FMCW techniques and operates in Ka Frequency Band. The central frequency is 35GHz and the bandwidth is about 800MHz width.
Plan
VV Along-track processing
Plans
SAR
z
x0 = 0
Figure 2. Ka radar integrated into an under-wing pod.
IV. PLATEFORM PERFORMANCES Current airplane used for testing applications is a STEMME S10-VT motorglider (Fig. 3) as the geometry of this airplane may be representative of a UAV’s one: wingspan is 23m large and weigh is about 900 kilograms. The SV10-VT motorglider is a low-cost, very flexible platform. Wings have been reinforcing and four under-wing hard attach points have been adding per wing. Cable channels have been provided into the wing, from pods to fuselage, in order to connect the payload to on-board power supply and control-command system. Two composite under-wings pods have been developed, each pod can accommodate up to 60kg/80 liters payload. The table below summarizes the principal performances of the platform : TABLE I. BUSARD PERFORMANCES Wing span
23 m
Length overall
8.42 m
Wings area
19 m2
Max speed
270 Km/h
Manoeuvring speed
180 Km/h
Service ceiling
9145 m
Weight empty
660 Kg
Max landing weight
980 Kg
Number of pod
2
Pod volume
80 l
Pod diameter
35 cm
Pod payload max weigth
60 kg
xn =v.n Cross-track processing Digital Beam Forming x
∆x ∆y
xN =v.N y
Figure 1. UAV 3-D Downward-loohing imaging Radar principle
B. The development of a complete 3D radar The second phase integrates the development and the integration of a conformal linear array. The objective is to give the radar its full 3D capacity. Preliminary studies are done to design a first antenna array, about 1 meter large, which will be integrate under the wings. With such a demonstrator, the objective is to assess the feasibility of a complete 3D SAR radar and to test the 3D SAR image formation using a vertical geometry. Figure 3. ONERA BUSARD motorglider.
The development of such a demonstrator is currently under definition and first flights are scheduled during the year 2007.
V. SCHEDULE The DRIVE radar development is divided in the following two phases. A. The development of a first radar sensor in Ka band The objective of this first phase is to develop and operate a radar sensor embedded into under-wing pod and that can be operated following two modes: • An application as vertical sounder, using circular horn antennas, • An application as SAR radar, using rectangular horn antennas (Fig. 4). This first phase is now fully completed (Fig. 2) and first flights are planned for April 2006. Currently, this radar has been tested from end to end at laboratory level in the two modes: Vertical sounder and SAR imagery radar. A first measurement campaign is planned for the end of 2006.
Figure 4. Circular horn antennas (left) and rectangular horn antennas (right).
REFERENCE [1]
R. Giret, H. Jeuland and P. Enert : « A Study of & 3D-SAR Concept for a Millimeter-Wave Imaging Radar onboard an UAV », European Radar Conference 2004, Amsterdam, Netherland, pp. 201-204.
