Antenna design for Space Applications M. Sabbadini European Space Agency, Noordwijk, The Netherlands

Transcription

1 Antenna design for Space Applications M. Sabbadini European Space Agency, Noordwijk, The Netherlands

2 Day 3 overview Array antennas in space Antennas for Earth observation Antennas for Science and Exploration 2

3 Array antennas in space

4 Array basics Small aperture θ d sin(θ) d 0 N-1 x x E ( θ, ϕ) = I( γ J( x, y) ) E θ, ϕ = E θ, ϕ + E θ, ϕ e E jk0d sinθ ( 2)( ) ( ) ( ) θ, ϕ = E θ, ϕ ( )( ) ( ) N n= N 1 0 e jk 0 nd sinθ If k 0 d > 2π, i.e. d > λ, there is at least one θ 0 such that k 0 d sinθ=2π and the summation is equal to N. Therefore there will be more than one peak in the array radiation pattern E (N) (θ,φ). 4

5 Array pattern Pattern of a linear array with 11 elements of size d and spaced by d Visible range (-90 θ 90) - Function of k 0 and d d <λ d >λ E θ, ϕ = E θ, ϕ ( )( ) ( ) N n= Array pattern Element pattern Element pattern Function of k 0 d Envelope for the array pattern N 1 0 e jk nd sinθ 0 Array factor Grating lobe Alias of the main lobe under the sampling of the radiating aperture with period k 0 d -1 0 sinθ sinθ k0 d sinθ 5

6 Array design issues High number of degrees of freedom (layout, elements, excitations) High aperture efficiency but High losses in BFN Low power-efficiency (if active) Thermal problems (passive & active) High mass Reliability issues (complexity) Limits in element spacing to avoid grating lobes Limits in feasible aperture size L-band array for mobile applications (courtesy of RUAG Aerospace Sweden) Arrays are used only when unavoidable 6

7 Partial cures for array limitations The drawback of arrays in space applications is complexity. There are various way to tame complexity: Simplify the structure to match the number of degrees of freedom required by the design Use of sub-arrays Sparse arrays Magnified arrays (or imaging reflector antennas) Exploit manufacturing technologies to easily produce a large number of similar elements Printed arrays Modular structures 7

8 Sub-arrays Groups of elements with fixed beam forming network Limitations Under-sampling of aperture (d>λ) Scan loss Grating lobes Complexity still high Partial cure Randomization Orientation Phase Pattern φ φ subarray 8

9 Sparse arrays Regular array A A x Equivalent aperture Instead of sampling the aperture illumination, fix the element excitation and space the elements to cover areas corresponding to the same power flux. A 2 dx x x 9

10 Pattern of a sparse array normalised gain (db) k 0 dsinθ 10

11 Magnified arrays Use two reflectors to generate an enlarged image of the (feed) array Normally two parabolic reflectors, but could be two elliptical ones Limitations Scan loss and grating lobes Need of oversize the reflectors to accommodate most scanned beams Alternative: a single reflector fed by an array out of the focal plane (typically closer to the reflector) 11

12 Beam forming networks Parallel feeding Wide band Difficult to match Corporate feeding Good compromise Limit cases Series feeding Narrow band Easy to match Rotman lens Directional coupler Slotted waveguide 12

13 BFN technology Waveguide Low loss High power Stable vs T Bulky Dispersive 3D paths Easy to assemble Square coax Bar-line Higher loss Lower power Quite Stable vs T Compact but heavy Non-dispersive 2.5D paths Complex to assemble Microstrip Stripline High loss Low power Not stable vs T Light Non-dispersive 2D paths only Easy to assemble X-band and above UHF to C-band UHF to L/S band* *) also C and X band for sub arrays 13

14 BFN complexity BFN layout for a tile of a Synthetic Aperture radar 14

15 Multilayer arrays ~ ~ Simple patch Electromagnetically coupled patch Courtesy of Thales Alenia Space ~ ~ Slot-fed patch Stacked patch Courtesy of RUAG Aerospace Sweden 15

16 Giove-A L-band antenna Courtesy of Thales Alenia Space 16

17 Earth Observation 17

18 Satellite orbits Polar* orbit Geostationary orbit * Sun-syncronous Km typical 18

19 Instrument types Passive instruments Radiometers: to measure the thermal emission (soil, clouds, etc.) Sounders: to measure differential attenuation of signals produced by external sources Synthetic Aperture Radiometers: same as radiometers Active instruments Altimeters: to measure distance to ground Synthetic Aperture Radar: to image the Earth Scatterometers: to measure surface roughness Ground Penetrating Radars: to see below the surface Rain and Cloud Radars: to monitor rain and clouds 19

20 Typical beam geometry orbit satellite velocity vector nadir vector antenna pointing vector swath antenna beam footprint ground track 20

21 Fundamental parameters Resolution: minimum angular distance between two element of the scene that are seen as distinct by the instrument. Sensitivity: minimum level of the signal coming from the scene that is distinguishable from the noise generated by the instrument and by the environment. 21

22 The Radiometric equation Radiometers measure the power emitted by objects in their field of view. T τ ( f,0) secθ ( f, θ ) = T e T (, θ ) b b0 + u f zenith θ surface emission + reflection of downward atmosphere radiation f = frequency τ = atmosphere opacity upward atmosphere radiation Key factors Thermal emission of objects Resonant absorption frequencies of atmospheric elements Emittance of Earth surface Reflectivity of Earth surface 22

23 Atmosphere absorption spectrum Water column content sounders oxygen Temperature profiles Water vapour water surface atmosphere Ice clouds imagers 23

24 Operating frequencies Frequency (GHz) Application Surface Radiometry Ocean Salinity Soil Moisture Surface Roughness Polarimetry Biomass Estimation Surface Radiometry Snow Water Equivalent Surface Roughness Polarimetry Atmosphere Radiometry Water column content Water clouds Atmosphere Radiometry Atmosphere Chemistry Temperature profiling Water Vapour content Ice Clouds Rainfalls (rate) 24

25 25 Calibration Compare output of receiver for different known conditions (hot & cold) Remove systematic error in references from other measurements Cold sky Hot load scene ( ) ( ) ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' c h h c c h c h c h a a c c h c h h a c c h c h h a T T T T T T V V T T T T V V V V T T T T V V V V T T T T = = + = + = + = β α β α Measurement Correct result Correction Use of true values of T a, e.g. from target on ground of radiosonde data to derive correction terms.

26 Antenna architectures Scanning Conical scan Wisk broom Push broom 26

27 T a Antenna parameters ( f ) = G( θ, ϕ ) T ( f, θ, ϕ ) = 4π main lobe + b side lobes sinθ dθ dϕ = Beam efficiency Integral of sidelobe level + polarisation to be considered for polarimetric radiometers The physical temperature of the antenna and receiver have to be minimised to increase sensitivity cryogenic receivers 27

28 Antenna design Dynamic Balance Mass Main Reflector CFRP Struts Cold Calibration Dish Top Panel Hot Calibration Target Front Panel Side Panel HDRM 28

29 ENVISAT radiometer 29

30 Limb sounding A signal transmitted by an external source (e.g. a companion satellite or a GPS one) is received by the sounder atmosphere Earth Also the inner part of the atmosphere may be explored in this way, down to a certain altitude. Applications: investigations on atmosphere chemistry 30

31 Metop GRAS Atmospheric limb sounder Courtesy of RUAG Aerospace Sweden 31

32 Synthetic aperture radiometer S V T T i ij b jφi ( ω ) ( ω) = Ai ( ω) e = S ( ω) S ( ω) = A ( ω) A ( ω) i A 2 F V ( ξ ) V ( u) j T e b i j2πuξ Uses interferometry to increase resolution j e jδφ ( ω ) d u =, ξ = sinθ λ ij Real array Δd 3Δd 2Δd S(ω) S(ω)e jδφ(ω) S(ω)e j4δφ(ω) S(ω)e j6δφ(ω) S(ω)e j8δφ(ω) S (ω) 2 e j3δφ(ω) S (ω) 2 S (ω) 2 e -j3δφ(ω) S (ω) 2 e -j2δφ(ω) Synthesised array signals (equivalent element positions) 32

33 SMOS Soil Moisture and Ocean Salinity mission 33

34 SMOS instrument Noise source N H I A/D V Switch Q A/D MUX Cold load Switching commands Common LO Optical fiber link Correlator Data X-band to Ground station 34

35 Radar basics τ R + ΔR R Received energy Received signal SNR = Echo duration = τ PG G t t 2 rλ σ time 3 4 ( 4π ) R ktbfl Two targets are just resolved if the echoes are just non-overlapping, i.e. if they are separated by about one pulse-width τ Hence 2ΔR/c = τ or ΔR = cτ/2 Antenna dependent factors 35

36 Altimeter Accurate measurement are possible only if satellite position is known with sufficiently accuracy ERS altimeter Orbit Orbit determination Altimeter measurement Surface Reference ellipsoid Geoid Accuracy achieved using laser ranging, tracking stations, GPS 36

37 Synthetic Aperture Radars The principle of synthetic aperture processing is to achieve high resolution in one plane by emulating a large phased array synthesizing its wide aperture from a small one moved along it and using the spectral properties of the radar signals. In the orthogonal plane there is still need of a rather large antenna to limit the beam to allow the resolution in time of the echoes arriving from the scene. satellite r 0 r x Point target 37

38 ERS SAR 38

39 ERS SAR Antenna Courtesy of RUAG Aerospace Sweden 39

40 ENVISAT Active SAR antenna Courtesy of EADS-Astrium 40

41 Envisat - active SAR antenna panel Courtesy of EADS CASA Espacio 41

42 Wind Scatterometer A wind scatterometer is a specific type of radar designed to measure the backscatter coefficient (σ 0 ) Over the ocean surface, the backscatter coefficient at oblique incidence is related to the wind speed and direction This is due to Bragg scattering which only occurs when the radar wave front is parallel with the wave crests Hence the sensitivity to wind direction as well as wind speed but with 180º ambiguity λ s sinθ θ λ s 42

43 METOP scatterometer The METOP Scatterometer Geometry The METOP Satellite (EUMETSAT) 43

44 Ice penetrating radars Along -track P-Band Radar Signal Surface 500m depth Bedrock Emitted pulse Received pulse Time Final objective Depth Echogram Bedrock Along-track 44

45 Antennas for space science and exploration

46 Antenna issues Science and exploration missions have very diverse needs from the antenna point of view. Typical examples are: Space radio-telescopes: with large reflector antennas operating over very wide frequency bands and at very high frequencies (up to the THz region). They are similar to radiometers, but there is a great variety among them. Planetary probes: carrying instruments similar to those used for Earth Observation, e.g. altimeters, surface penetrating radars, synthetic aperture radars. 46

47 Planck A mission to map the anisotropy of the cosmic microwave background with a sensitivity of 10-6 K in the frequency range 30GHz to 1THz and with an angular resolution better than 0.5 to 0.1 across the range. Artist view of the Planck satellite Artist view of the focal plane array 47

48 Stray-light To map the anisotropy of the cosmic background the telescope needs to detect minimal variations in the signal. To obtain the desired sensitivity it is necessary to control and quantify the unwanted scattering coming from the whole spacecraft. Direct feed spillover Main lobe Feed spillover reflected on shield 48

49 Modelling issues Many different contributions needs to taken into account in computing the full-sphere radiation pattern of the antenna mounted in its thermal shroud. Double diffraction main reflector+shield Main reflector spillover Sub reflector spillover Approximate calculations can be made with available tools. Accurate modelling would take years. Internal reflections Double diffraction subreflector+shield Future missions will need even higher resolution and accuracy. 49

50 High quality receptors Corrugated horns are the closest possible match to a purely polarised source generating an axially symmetric beam with low side lobes and a wide operating band. The corrugations realise a uniform boundary condition for the electric field all around the horn, emulating a magnetic conductor. Courtesy of EADS Astrium 50

51 75GHz horn 51

52 2.5THz corrugated horns Manufacturing mandrel (top left), horn aperture (top right), radiation pattern (bottom left). Input waveguide: μm Corrugations: ~26 μm 52

53 GHz array 53

54 Optical beam forming Quasi-optical beam multiplexer for radiometry applications. Courtesy of EADS Astrium Horn Solid mirror Selective mirror: frequency or polarisation 54

55 Terahertz radiometer for Exomars 55

56 LISA Pathfinder A precursor mission to verify the feasibility of the LISA gravitational wave detection experiment. The LISA experimental concept A model of the LISA Pathfinder spacecraft 56

57 The LISA PF telemetry and tele-command antennas will have to provide for the RF link with ground until the satellite reaches the final orbit and have sufficient gain for use in emergency situation on-station. The LISA PF spacecraft Science Module The target orbit is around the Earth-Sun L1 Lagrange point Critical phase Propulsion Module Telemetry and tele-command antennas operating simultaneously in opposite circular polarisations The LISA PF trajectory from launch to the final orbit around L1 57