Wolfgang Keydel DLR Oberpfaffenhofen Microwaves and Radar Institute
D-82230 Wessling GERMANY
e-mail: wolfgang.keydel@dlr.de Correspondence Address
Mittelfeld 4 D-82229 Hechendorf
GERMANY
Tel.:+49-8152-980 523, Fax:+49-8152-980 525 e-mail: wolfgang@keydel.com
ABSTRACT
Based on both the states of the art and the expected developments of antennas, RF micro electronics, and SAR techniques and technologies an outlook in the future of SAR antenna development shows that the SAR Antenna will mutate to an Antenna SAR.
1. Introduction
The antenna is the most essential SAR system component, it is the greatest weight driver for space borne SAR, and there is no relevant SAR design equation which does not content antenna parameters.
Goal of the paper is to present the expected development of future antennas for both airborne and space borne SAR based on both the state of the art confirmed by experimental results and on a roadmap for the expected development of microelectronics.
2. The Antenna as System Determining Element for SAR
A Synthetic Aperture Radar, principally, produces a long linear array antenna by means of computer technique moving a small antenna along a straight line and collecting and storing all signals with respect to amplitude, phase, frequency, polarisation, and running time for gaining special desired information with special processing algorithms. Therefore, a SAR is an antenna, Fig.1 shows the principal SAR scheme.
The most important equations which combine different radar-, processing-, and antenna parameters for the basic strip map SAR technique as well as for both along track and cross track interferometry are given in Table 1 [2,3].
Some specialities of SAR with respect to the antenna dimensions become evident especially in rows 1 to 6. These self explaining equations establish principal technological limits for the development and applications of airborne and space borne SAR and point out the importance of the antenna dimensions clearly.
Paper presented at the joint RTO IST & SET Symposium on “Smart Antennas”,
From Radar Antenna to Antenna Radar Perspectives for Future
Antenna Development for Airborne and Spaceborne SAR
1 Point Target Signal/Noise
δ π λ
σ
az p 3 o
2 ave 2
kT F)2V R ( ) (4
A
= P N
S
2 Best Azimuth Resolution δaz = 3 Synthetic Aperture Length L = λ R/
4 Swath Width S = c p
5 Minimal Antenna Area 4vpλRtanθ)/c
6 PRF 2vp ≤ (PRF) ≤ c/2Rmax
7 Slant Range Resolution δrg = c/2 τp/2 8 Azimuth Pixel Number. Naz = Rλ/2 9 Range Pixel Number Nrg = δrg
10 Data Rate DR = Nrg
11 Pixel Rate Q = nv δrg
12 Cross Track Interferometry
Altitude Accuracy dz≈(λRsinθδϕ)/2π
13
Along Track Interferometry
Target Azimuth Velocity vta=-[vpl(ϕ+n2π)]/ 4π 14 Target Range Velocity,
Target Displacement
Method
Vtr=1,8Fgr[2vt∆x+n λ]/R
A :Antenna Area, B:
Bandwidth,
Bl = Basis Length, c = Light Velocity , D = Antenna Length, Fgr= Slant/Ground Range Factor kToF = Noise Characteristic, L=Synthetic Aperture Length n=Nr. of Looks, Pave=Mean Power,
PRF=Pulse Repetition Frequency,
Q = Pixel Rate
R=Distance SAR/Pixel, Rmax=Maximum Distance
SAR/last Pixel, S = Swath width
vp = Platform Velocity, vt = Target Velocity,λ = Wave- length, σ = Radar Cross Section, τp = Pulse
Length,ϕ=measured Phase,
∆x = Target Displacement θ = incidence angle δaz =Azimuth Resolution δrg = Range Resolution δz = Alitude Accuracy δϕ= Phase Accuracy Factor 1,8 = adaptation m/s to km/hr
Table 1: Important Relations for Strip Map SAR, SAR Interferometry, Moving Target Indication;
parameter containing antenna specifications bold printed, arrows indicate respective row number [2,3]
D/2 D D/4v Amin>(
/D B = c
δaz = L/δazÎ 2,3 S/ Î 4
PRF Î 6 S/δaz Î 2, 4,7
Bl
Bl
(PRF)R
3. Main Requirements
Future SAR systems have to be cost effective, small, lightweight, easy to handle, and must bring economically usable products. These economic user requirements for SAR entail requirements for SAR antennas with respect to size, weight, structure, etc. The operational user requirements like resolution, swath width, calibration, determine RF requirements with respect to bandwidth, diagram shape, half power beam width, side lobe level, diagram switch capability, temperature stability etc..
Furthermore, special SAR modes and systems entail special requirements for the antenna also; examples are SAR Interferometry, Bistatic and Multistatic techniques, SAR Cluster consisting of many SAR satellites in defined formation flight etc..
For space borne SAR , mainly, the application requirements together with equation 5 in Tab.1 determine the antenna area as well as other antenna specifications. Examples: SAR for topography, hazard, and biomass observation requires L-Band , full polarization and a 3m x 10m dish; subsurface probing requires 50 MHz – 200 MHz, dual polarization, high power, and a dish diameter of at least 25m; space borne SAR Interferometry from Geo stationary Orbit (GEO) requires L-Band, high power, and extreme pointing accuracy.
Military requirements, for example, claim for superb information i.e. extremely high resolution, wide swath, permanent coverage, real time information etc.. Mostly favorable, presently, is X-Band.
In airborne systems the antenna dimensions possible are limited due to the size and shape of the platform.
This claims for conformal arrays and smart skin antennas.
4. State of the art 4.1 Antenna technology
States of the art for operational space borne SAR are Phased Arrays with T/R-Modules equipped with semiconductors in P-, L-, and C-Band. Due to higher losses X-Band SAR systems are equipped with TWT’s. The SRTM antenna, 10m x 3m, mechanically deployed, flown in 2000, is an example. The antenna dimensions for space borne SAR are at present of about 10m x 1m to 15 m x 1 m for micro strip array antennas (ENVISAT [4], Radarsat [5]); for Radarsat 2 an array antenna with about 800 kg mass and a stowed volume approaching 7 cubic metres is planned [6].
Reflector antennas, presently, are not out of consideration [7].Their main advantage is the possibility to realize full polarization and high bandwidth with low technological risk at low costs. The main drawbacks are the limited instantaneous swath width and the lack of an electronically beam steering capability.
Inflatable deployable dish antennas are demonstrated principally, however, the surface smoothness obtained is still inadequate and the rigidization attainment of design shape in space has not yet been demonstrated. For L-Band a mesh antenna of 12.5 m in diameter is space qualified, which is extendable in size and frequency.[8].
Inflatable deployable phased arrays are in a laboratory demonstration stage. A 3,3m x 1m inflatable array with 0.5mm to 1mm flatness has been built successfully as well as an engineering model of a 3m x 5m single wing inflatable array. Major challenging technology elements are in the active control of structure, surface profile, and roughness. Presently, a dual polarized SAR antenna using a three layer membrane with a ground-plane of 12m x 3m is under consideration. It can be stored within a volume of less than 1 m3 and will have a total mass of about 175 kg :Membrane material and spacers 45 kg; sunshield and insulation 27 kg, deployment mechanism 100 kg,; stowage container 3 kg. These figures point out the importance of the deployment mechanism [6].
Presently, the weights for all planned space borne SAR antennas are in the order of several 100 kg up to 1000 kg.
For airborne SAR the antenna dimensions, principally, are limited due to the size and shape of the platform.
4.2 Innovative techniques
For SAR phased arrays digital beam forming (DBF), presently, is on the brink of operational introduction.
[9,10]. Each phased array element will consist of a radiator followed by filter, low noise amplifier, mixer, digitizer, and, in the next future, an own computer, i.e. a conventional T/R-Module with an additional MPU. Therefore, each array module is forming a Synthetic Aperture by its own as shown in Fig.1.
However, an additional central computer unit summarizing and combining all the signals will form the antenna beam of the whole real array digitally Fig. 2. This method has been verified experimentally by replacing the MPU’s in Fig.2 by very fast switches [9].
An exceptional meaning has DBF for Multistatic SAR where one central illuminator, feed by a TWT for instance, is radiating the transmitter power over the whole area to be observed by regularly ore statistically distributed receivers. The method of digital beam forming in the receiver allows focusing the whole illuminated area simultaneously. In subsequent computer processing, multiple beams and strategically placed nulls can be created. Low noise figures can be obtained and due to the relative high efficiency of the microwave vacuum sources the system efficiency can be improved drastically by using one central illuminator only. The small sub array elements can be formed to a conformal array easily.
4.3
Technology drivers are both RF performance and “mechanical” performance:
RF performance: RF bandwidth, losses, cross-polarization isolation, beam efficiency, side lobes high power (500 watt), T/R Modules, solid state and chip technology, etc..
Mechanical performance: Deployment, rigidization, gas release, structural accuracy and stability, vibration and thermal shock resistance, sub reflector and focal plane compensation, membrane shape, smoothness and surface roughness, material characterization survivability, packaging and deployment control, flexible structure dynamics, pointing accuracy and control, effects of outgasing and thermal loads etc.
4.4 Miscellaneous
Space borne as well as airborne systems house beside a SAR a wide variety of other electronic systems for communication and navigation additionally each equipped with an antenna.
This, however, holds especially for military systems. which, principally, have electronic systems for air to ground as well as air to air reconnaissance, communication, navigation, fire control, self protection, ECM, ECCM, data links etc.. Fig 3 shows an schematic example for the variety of microwave links on a fighter aircraft. The extremely wide bandwidth necessary to cover all microwave links in airborne weapon systems for example reaches from 500 MHz up to 18 GHz. Presently, each single system has its own, separate antenna. The large number of antennas on an aircraft as well as on a satellite is a large driver for overall weight, volume, and cost and has to be reduced drastically. Integrated antenna systems with multifunction shared apertures are required. The most promising candidates are conformal patch arrays consisting of spiral ore cone patches etc. which can cover a wide bandwidth. State of the art is to cover the whole RF spectrum from 500 MHz up to 18 GHz by three multifunction apertures: 500 MHz to 2 GHz, 2 GHz to 6 GHz, and 6 GHz to 18 GHz [11].
5. Future Technique and technology development 5.1 Future Electronic Development
The so called “Moore’s Law” gives a macro trend of the progress to be expected for the next decades meaning that market demand and semiconductor industry response for the functionality per chip (bits, transistors, MMIC’s, MPU’s etc.) doubles every 1.5 to 2 years. A “corollary to Moore’s Law” suggests that the “cost-per-function” (micro cents per bit or per transistor) decrease by about –29% per year.
Table 2 shows exemplary a representative roadmap for the next 15 years [12].
Year Item 2001 2004 2007 2010 2013 2016
Improvement Factor
Chip Frequency/GHz 1,7 4,0 6,7 11,5 19,5 29 17
DRAM:
Functions/Chip/Gb
2.2 4.5 17 34.4 69 136 62 DRAM Gbits/cm2 0,6 1,5 3,0 6.1 18,5 37 67 ASIC: Transistors /cm2 89 178 357 714 1,4 106 2,9 106 32 ASIC: Transistors/chip 714 1020 2041 4,1 109 8,2 109 16 109 23 Logic: Transistors/cm2 40 106 77 106 0,2 109 0,3 109 0,6 109 1,2 109 30 SRAM:MTransistors/cm2 185 106 0,4 109 0,8 109 1,7 109 3,5 109 7,2 109 40 Transistor Cost /10-9cents 0.2 0.1 0.05 0.02 0.01 0.005? >50
Table 2: Exemplary Roadmap Data for Special Semiconductors Furthermore the following technical and technological developments are expected:
Within the time frame of about two decades the power efficiency of solid-state devices will exceed 60 %.
This, in connection with higher miniaturization, will drastically reduce the mass and volume of RF-
systems including the antenna down to 10 % and less. A high degree of automation of the radar and operation functions will reduce the effort for post launch mission operation.
High temperature super conductors will be applicable for extremely small filtering as well as for transmit/receive modules with high signal to noise ratio, progress in time synchronization to nsec- accuracies is expected, the efficiency of solid-state power devices will exceed 60 %.
Future SAR , mainly, will consist of the antenna with a small number of more ore less peripheral elements only (solar cells, GPS, power supply etc.). The present SAR Antenna mutates to a complete Antenna SAR. The expected progress in miniaturization (Table 3) is essential. The proposed Terra SAR configuration is a first step in that direction, Fig. 4 [13], Tab.3 and 4 give rough, respective roadmaps for both Satellite SAR Antennas and expected figures of merit for front end devices.
Satellite ERS-1 RadarSAT ENVISAT TERRASAR AntennaSAR
Launch 1991 1995 2002 2004 2015 ?
Nr. of Sensors 6 SAR only 10 SAR only SAR only
Mass 2500 kg 2750 kg 2145 kg 1000kg ? 200kg ?
Antenna Dimension 1m x 12m 1,4mx15m 1m x 12m 0.7m x 4,8m Tbd.
Tab.3 Roadmap of space borne SAR wit respect to mass and antenna dimension
2001 2004 2007 2010 2013 2019
Low Noise Amplifiers (LNA) 1 1,5 2,5 3…4 4…5 5…7 Voltage Controlled Oscillators
(VCO)
1 1,2 1,4 1,6..1,8 2,0..2,2 2,4..2,8
Power Amplifier (PA) 1 2 4 5…6 13…15 17..21
Analog Digital Converter
(ADC) 1 2 2,5..3,3 4…6 6…10 10..25
Tab.4 Expected Figure of Merit development for Front End Devices with respect to 2001
6. Perspectives and visions for future systems as basis for future antennas
Future SAR systems will be software based multi static systems characterized by multi-polarization and multi-frequency capability, and multiple operation modes as well. They will have one or more central illuminators together with a synchronized fleet of airborne, space borne, or ground based receivers which enable continuous availability with a nearly global coverage [14]. Further characteristics will be:
extremely high resolution down to cm, dedicated information transfer to specific users in real time based on onboard data processing, extremely accurate time synchronisation, and very effective data and communication links.
The operational requirements for wide coverage, real time information, and high resolution are in conflict with requirements for smaller and cheaper systems. The present communication and navigation systems have small and cheap user units (to a wide extent standardized) and more or less centralized transmitters.
Space division multiple access (SDMA) is a keyword on the receiver (user) side. Hence, it is necessary to learn from these already existing systems and to take over respective technologies, techniques and even components. This predestinates in some cases the frequency range (L-Band for example by using GPS).
This leads to SAR systems with wide angle beam illumination realized by highly efficient reflector antennas fed from high power microwave vacuum sources with high efficiency on the transmitter side and a fleet of receiver satellites which will be organized as an intercommunicative web which is a macro instrument concept that allows for coordinated efforts between multiple numbers and types of sensing platforms, including both orbital and terrestrial both fixed and mobile. Information gathered by one sensor is shared and used by other sensors in the web [15]. Each sensor communicates with its local neighbours and thus distributes information to the instrument as a whole. Principal system components are: geo stationary , low orbit based ore ground based illuminators on towers respectively, receivers in satellites,
aircrafts, and on the ground (cars with distance measurements for collision avoiding as example) Fig.5.
This will end up with an extremely large Phased Array Antenna SAR with DBF where each satellite is an array element. The software will play the most important part. The intelligence of such radar systems is put to the board computers totally, which enables also parallel data processing as well as real time classification on board considering the present rapid development of computer technology. A standardization of both frequencies and respective components will reduce the cost. A dual use of respective frequency bands should allow applying the same modules for reconnaissance as well as for navigation and communication purposes. However, an exact synchronization as well as an interconnection between all airborne, space borne, and ground based system components is conditio sine qua non. This claims for multifunction integrated antenna apertures.
Precision formation flying technology will allow deployment of a large number of low cost, miniaturised spacecraft and introduction of new members to the formation over time. The formation flying system must act collaboratively as a single collective unit i.e. an extremely large antenna Fig.6. The guidance and control system for formation flying must be on board with autonomous formation position determination capability, autonomous navigation, formation estimation, and path planning functions capability.
Extreme pointing and attitude stability is required. Especially the control of spacecrafts in low orbits poses significant challenges due to several non-uniform perturbations that can potentially destabilize the
formation geometry and decrease the measurement accuracy of the system.
The Cartwheel concept [16], a parasitic SAR system, with 3 small satellites (300 kg or less) rotating on the same orbit around each other with permanent interferometry capability is an example for present possibilities and considerstions Fig. 7 [14].
7. Conclusions
Future space borne SAR payloads will consist mainly of the antenna with a small number of peripheral elements only, like solar cells, GPS, power supply, downlink equipment etc.. The present space borne SAR Antenna will mutate to a complete Antenna SAR that means a SAR which is primary an antenna where all radar components inclusive the AD-Converter and the image processing computer are integrated. A substantial step to an Antenna-SAR is the expected progress in miniaturization shown in a respective roadmap. The miniaturization, will drastically reduce the mass and volume of the antenna including the RF system down to 10 % of the today’s value. Ultra-light-weight antennas with large structural components, such as deployable ore inflatable booms, and membranes with very low power consumption will be available during the next decade.
For a senor fleet in space consisting of many satellites in a well known and controlled formation flight each satellite receiver (and transmitter respectively) may be considered as a single element of a very large DBF-Array.
For airborne SAR the antenna dimensions are limited due to the size and shape of the platform. Here digital beam forming allows to form small sub arrays to a conformal array, a so called smart skin. For the next two decades broadband arrays are expected which are able to share between SAR, other radars, forward looking radar for example, electronic support as well as electronic countermeasures, and communication purposes.
This will increase the effectiveness and the applicability of future SAR systems by reducing the overall mass, volume, and cost which are indispensable requirements for the future.
8. References
[1] Moreira, A,: SAR principle, RTO - Seminar on Advances in Radar Techniques, 6-8 June 2001, Gdansk, Poland
[2] Morchin, W., "Radar Engineers Source Book", Artech House, Boston, London, 1993.
[3] Keydel, W., "Basic Principles of SAR", "SAR Peculiarities, Ambiguities and Constraints", in
"Fundamentals & Special Problems of Synthetic Aperture Radar(SAR)",AGARD LS 182, 1992.
[4] Zink M., Torres R., Buck C. H, .Rosich B., Closa J. (2002): The Advanced SAR System on ENVISAT: Mission Status, Proceedings 4th European Conference on Synthetic Aperture Radar, EUSAR 2002, 4 – 6 June 2002, Cologne, Germany, pp.72
[5] Evans N., Lee P., The RADARSAT-2&3 Topographic Mission, Proceedings 4th European Conference on Synthetic Aperture Radar, EUSAR 2002, 4 – 6 June 2002, Cologne, Germany, pp.37ff
[6] Wood P., Seguin G., Canadian Space Membrane Antenna for a SAR Application. Proceedings
4th European Conference on Synthetic Aperture Radar, EUSAR 2002, 4 – 6 June 2002, Cologne, Germany, pp.307.
[7] Schröder R., K.-H. Zeller, T. Neff, H. Süß:Potentials of Reflector Antenna Technology for a fully Polarimetric L-band Space borne SAR Instrument Targeting Land Applications, Proceedings 4th European Conference on Synthetic Aperture Radar, EUSAR 2002, 4 – 6 June 2002, Cologne, Germany, Nr.5 pp.41- 44
[8] Shaubert D., Kakar R., Lou M.:Large, Lightweight. Deployable Antennas, Earth Science Enterprise Technology Planning Wokshop, JPL, 23 –24 Jan. 2001available at http://nmp.jpl.nasa.gov/workshopeo4/proceedings/ESE_Wkshp_antennas.pdf
[9] Buckreuß S., Krieger G., Mittermayer J. ,Moreira A., Sutor T., Wendler M., Witte F., (2000): Final Report: SIREV-Development of a Functional Model, DLR-Forschungsbericht 2000-44, ISSN 1434- 8454
[10]Younis M., Fischer C., Wiesbeck W. (2002): An Evaluation of Performance Parameters of Software Defined Radar Sensors, Proceedings 4th European Conference on Synthetic Aperture Radar, EUSAR 2002, 4 – 6 June 2002, Cologne, Germany, pp. 91
[11] Fuchs U., Bleichrodt H.: Integrated Anna systems “IAS”, Proceedings German Radar Symposium, 33. –5. Sept. 2002, Bonn, Germany, Nr.151,
[12] N.N. ”International Technical Roadmap for Semiconductors”, Overall Roadmap http://public.itrs.net/Files/2001ITRS/ExecSum.pdf
[13] Suess M, Riegger S., Pitz W. , Werninghaus R.: TERRASAR-X – Design and Performance Proc. of the 4th EUSAR Conference, 4-6 June 2002, Cologne, Germany.nr.7, pp.49-52
[14] G. Krieger, G., M. Wendler, H. Fiedler and A. Moreira: "Interferometric Performance Analysis for Several Spaceborne Bistatic SAR Configurations.
[15] Delin, K.A., Jackson, S.P (2001).: The Sensor Web: A new Instrument Concept. SPIE Symposium on Integrated Optics, San Jose, CA, January 2001.Also: http://sensorwebs.jpl.nasa.gov/
[16] Massonnet, D. 1999: Capabilities And Limitations Of The Interferometric Cartwheel, Proceedings of the CEOS Workshop, October 1999. Manuscript available at
http://www.estec.esa.nl/confannoun/99b02/index.html
9. Images
Fig. 1 Scheme of SAR principle, courtesy Alberto Moreira [1]
Fig 2 Scheme of future Digital Beam Forming (DBF) SAR. Each single array module has its own Microwave Processing Unit ( MPU) with storage capability and may produce a zoomed SAR image following the procedure in Fig 1 (upper right corner) whilst the whole array may act as a conventional SAR with a switch able beam (blue).
T/R ADC MPU
T/R ADC MPU
Arra y Antenna
T/R ADC MPU
T/R ADC MPU
Central Proces U sor Image nit
Processor
Whole Array Strip Map
Whole Arrray
T/R ADC MPU
T/R ADC MPU
Arra y Antenna
T/R ADC MPU
T/R ADC MPU
Central Proces U sor
nit
T/R ADC MPU
T/R ADC MPU
Arra y Antenna
T/R ADC MPU
T/R ADC MPU
T/R T/R ADC ADC MPU MPU
T/R T/R ADC ADC MPU MPU
Arra y Antenna
T/R T/R ADC ADC MPU MPU
T/R T/R ADC ADC MPU MPU
Central Proces U sor Image nit
Processor Image Processor
Whole Array Strip Map
Whole Arrray
Single Array M odule
Zoom DBF
Single Array M odule
Zoom DBF
I nstitut für Hochfrequenztechnik und Radarsysteme
Air to Air Radar
Missile Aproach Warning Communication Navigation
(GPS)
WeatherR adar
Self protect/
Escort Jamming
Air to Ground Radar
Weapon Data
Side Looking Radar (SAR)
Communication
I nstitut für Hochfrequenztechnik und Radarsysteme
Air to Air Radar
Missile Aproach Warning Communication Navigation
(GPS)
WeatherR adar
Self protect/
Escort Jamming
Air to Ground Radar
Weapon Data
Side Looking Radar (SAR)
Communication
Fig. 3.Example for the variety of RF data links for a fighter showing the benefits of an integrated multifunction antenna system
ER [13]
Fig 4. TerraSAR-X satellite configuration without thermal hardware seen from nadir direction for the nominal looking configuration. Courtesy EADS DORNI
Fig 5. Schematic example for a sensor web. Communication-, TV-, special Remote Sensing-, and GPS satellites as well as earthbound transmitter are acting as different central illuminators, receive only satellites on a Low Earth Orbit (LEO) form a large array wit DBF.
Intercommunication between all web elements and a careful control of position, attitude, time, etc. for and between all web elements is indispensable
Fig. 6 Schematic examples of Multistatic arrays where the single receiver array modules are complete satellites (left configuration). The master satellite acts as a transmitter
Fig 7 Cartwheel scheme a an example for a three boom space borne receive antenna for SAR interferometry applications [14].
