Pre-Flight SAOCOM-1A SAR Performance Assessment by Outdoor Campaign

Technical Note Pre-Flight SAOCOM-1A SAR Performance Assessment by Outdoor Campaign

Davide Giudici 1, Andrea Monti Guarnieri 2 ID and Juan Pablo Cuesta Gonzalez 3,* 1 ARESYS srl, Via Flumendosa 16, 20132 Milan, Italy; [email protected] 2 Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan, Italy; [email protected] 3 CONAE, Avda. Paseo Colon 751, 1063 Buenos Aires, Argentina * Correspondence: [email protected]; Tel.: +39-02-8724-4809

Academic Editors: Timo Balz, Uwe Soergel, Mattia Crespi, Batuhan Osmanoglu, Daniele Riccio and Prasad S. Thenkabail Received: 23 May 2017; Accepted: 12 July 2017; Published: 14 July 2017

Abstract: In the present paper, we describe the design, execution, and the results of an outdoor experimental campaign involving the Engineering Model of the ﬁrst of the two Argentinean L-band Synthetic Aperture Radars (SARs) of the Satélite Argentino de Observación con Microondas (SAOCOM) mission, SAOCOM-1A. The experiment’s main objectives were to test the end-to-end SAR operation and to assess the instrument amplitude and phase stability as well as the far-ﬁeld antenna pattern, through the illumination of a moving target placed several kilometers away from the SAR. The campaign was carried out in Bariloche, Argentina, during June 2016. The experiment was successful, demonstrating an end-to-end readiness of the SAOCOM-SAR functionality in realistic conditions. The results showed an excellent SAR signal quality in terms of amplitude and phase stability.

Keywords: SAR; pre-ﬂight testing; SAR performance

1. Introduction The forthcoming Argentinean mission Satélite Argentino de Observación con Microondas (SAOCOM) [1] is constituted by two identical Low-Earth-Orbit (LEO) satellites, SAOCOM-1A and -1B, carrying as the main payload an L-band Synthetic Aperture Radar (SAR). The SAOCOM-SAR has an active array antenna with elevation and azimuth steering capability for frequent-revisit, wide-swath, and medium resolution Earth observation. The mission’s target applications [2] require a tight overall amplitude and phase stability performance, which is imposed to a large extent by the instrument critical elements, such as the chirp generator, the distribution network, the Transmit-Receive Modules (TRMs), the clock, and the antenna. The pre-launch assessment and verification of the phase and amplitude performance is paramount in order to limit the calibration and verification efforts to be done in-flight (e.g., during the commissioning phase). Typically, the instrument elements can be tested on a singular basis, and the antenna far-field radiation pattern can be reconstructed from planar-near-field-scanner (PNFS) measurements of the near field radiation patterns of the elements of the antenna by means of an accurate antenna model [3]. In the latter case, extreme care is needed in the measurement setup to avoid biases in the measurements. The far-field pattern is not available on-ground and the antenna model can be verified only in-flight, exploiting e.g., homogeneous targets for the elevation case and recording transponders for the azimuth. A typical example is the acquisition over the Amazonian rainforest. However, the in-flight data intensity profile depends on a list of other factors (e.g., pointing, time-variant instrument gains, processor gains), also to be calibrated or verified, so that it might be difficult to isolate the antenna contribution from the others, as reported for the case of Sentinel-1 in References [4,5]. Furthermore,

Remote Sens. 2017, 9, 729; doi:10.3390/rs9070729 www.mdpi.com/journal/remotesensing Remote Sens. 2017, 9, 729 2 of 12 the in-flight verification allows a limited verification of the elevation antenna pattern, within the angular range used by the imaging swath. One way to check the mid- and long-term stability of the system, say from seconds to minutes, is an outdoor experiment where SAR acquisitions are carried out with the full antenna (or a part of it, e.g., one tile) pointed to targets of opportunity or man-made calibrators in the far-field. In addition, the collected power measures for different antenna pointing can be correlated to the theoretical antenna model to obtain a first far-field verification, complementary to what could be obtained with PNFS measurements.

2. Outdoor Experiment Concept, Design, and Simulations The well-known far field condition on the distance R for an antenna with size L operating at central wavelength λ, is: λ R >> L (1) L In the case of the SAOCOM-1A SAR instrument, we refer to the SAR antenna, a planar array with the total size along azimuth L equal to 9.94 m. The antenna is made by seven tiles with the size c of 1.42 m × 3.48 m (azimuth × elevation), and the center frequency is 1.275 GHz (λ = f is equal to 23.53 cm). The far field distance results are in the order of 40 m. Moreover, the SAR instrument cannot receive echoes returning to the antenna while the chirp transmission is still active. Considering a minimum chirp length of 10 µs, and another 10 µs of guard time between the end of transmission and the start of echo reception, the minimum distance of a “visible” target is 3.3 km. Compared to the SAR acquisition in space, here the antenna is not moving along an orbit, so there is no synthetic aperture. The area illuminated by the antenna is limited by the range resolution ρg and by the real antenna aperture in azimuth λ R. Considering a homogeneous reflective scenario (e.g., bare La 0 soil or short vegetation) in front of the radar, with reflectivity equal to σT, the radar cross-section (RCS) of each resolution cell is computed as: ρgλR 0 σc = σT (2) La Considering the bandwidth range equal to 50 MHz and one tile of the antenna with a length equal to 1.42 m, the resolution cell at the SAOCOM central frequency is as large as 1600 m2. Assuming a reflectivity of −15 dB, the obtained RCS is 17 dBsm. (We indicate with dBsm the ratio of the area with respect to 1 m2, expressed in logarithmic scale). This large backscattered power can be considered either as a signal—if stable—or as a noise—if unstable. Water and leaves are highly unstable; however, their contribution is effectively reduced if averaged for seconds. Slower-moving targets could provide long-term noise contributions that affect the measure. We then decided that the observed scenario might not be stable enough for an accurate measurement, so we inserted a trihedral corner reflector in the scene. Figure1 shows the schematic representation of the overall concept of the SAOCOM outdoor test (ODT): the SAOCOM SAR antenna points towards a known point target (trihedral corner reflector), moving along the line of sight, with a linear motion, thanks to a linear actuator. The acquired data are downloaded and processed to extract signal characteristics and to assess the phase and gain stability of the instrument. Remote Sens. 2017, 9, 729 3 of 12 Remote Sens. 2017, 9, 729 3 of 12

Figure Figure1. SAOCOM 1. SAOCOM outdoor outdoor test test (ODT) (ODT) concept concept overall overall view. view. In the In ﬁgure,the figure, P represents P represents the moving the moving target totarget be acquired to be acquired and and ϑ theϑ the inclination inclination of of the the Radar Radar Line Line of Sight of withSight respect with torespect the local to normal the local to normal the terrain. to the terrain.

The reason for target motion is to effectively limit the size of the corner reflector to reasonable The reason for target motion is to effectively limit the size of the corner reflector to reasonable values. The target, moving with controlled and known motion, can be separated from the background values. andThe other target, moving moving targets with provided controlled that the and motion known is significant motion, with can respect be separa to halfted the from wavelength. the background and other movingThe RCS targets of the trihedral provided corner that reflector the motion with length is significantd can be modeled with respect as: to half the wavelength.

The RCS of the trihedral corner reflector with length4 d can be modeled as: 2 4 d σx = (3) 3π4λ2 d 4 σ 2 = (3) In order to distinguish it from the backgroundx andπ λ to2 have sufficient accuracy in the phase and 3 2 σx amplitude measurement, we ask for a signal to clutter ratio 2 of at least 30 dB. With the computed σc In RCSorder of theto distinguish clutter as above, it from this would the background lead to a target and with to a sizehave greater sufficient than 8 m.accuracy in the phase and Let us then assume that the corner reflector is mounted onσ a2 rail, and moves with time, say of amplitude measurement, we ask for a signal to clutter ratio x of at least 30 dB. With the computed an extent much less than a range resolution cell. The return fromσ 2 successive echoes measured at the target range can be modeled as a mono-dimensional time-variant signal,c given by the superposition of RCS of thethe distributed clutter as target above, (or this clutter), would the signal lead fromto a thetarget trihedral with target, a size and greater the noise thanw, due8 m. to thermal and quantization: Let us then assume that the cornerZ reflector is mounted on a rail, and moves with time, say of an jφ (ξ,τ) jφ (ξ ,τ)+φr(τ) extent much less than a rangey(τ) =resolutionace APS cell.dξ +Thaese returnAPS 0 from +successivew(τ) echoes measured(4) at the ρ target range can be modeled as a mono-dimensionala time-variant signal, given by the superposition of the distributedwhere the first target integral (or is clutter), extended the to the signal unfocused from azimuth the trihedral aperture, target, the resolution and theρa .noise In the w, due to equation above, τ represents the time and ξ the spatial coordinate. The phase noise term φ accounts thermal and quantization: APS for the propagation within the troposphere and it is discussed later on in the text. The two terms ac and as are the returns, respectively, fromφ theξ τ distributed targetφ andξ τ the+φ trihedral.τ The second term is y(τ ) = a e j APS ( , )dξ + a e j APS ( 0 , ) r ( ) + w(τ ) the signal related to the target. Thec phase term φr accountss for the motion of the target and represents (4) ρ an equivalent of the “phase history”a of the target, to be compensated for (“focused”). In the case of linear motion along the line of sight with velocity v, the phase history corresponds ρ where theto a linearfirst integral phase ramp: is extended to the unfocused azimuth aperture, the resolution a . In the 4π 4π φr(τ) = − R(τ) = − (R0 + v · τ) (5) φ equation above, τ represents the time andλ ξ the spatialλ coordinate. The phase noise term APS accounts for the propagation within the troposphere and it is discussed later on in the text. The two terms ac and as are the returns, respectively, from the distributed target and the trihedral. The φ second term is the signal related to the target. The phase term r accounts for the motion of the target and represents an equivalent of the “phase history” of the target, to be compensated for (“focused”). In the case of linear motion along the line of sight with velocity v , the phase history corresponds to a linear phase ramp: 4π 4π φ (τ ) = − R(τ ) = − ()R + v ⋅τ r λ λ 0 (5)

Remote Sens. 2017, 9, 729 4 of 12

And hence the observed data sequence at the target range R0 is a sampled sinusoid with frequency:

2 f = − v (6) λ The term f is the Doppler Frequency created by the target motion. Unlike a spaceborne SAR, in which every target in the scene has its own Doppler, here all the ﬁxed targets appear at Doppler zero except for the one of interest. Assuming a very good knowledge of the phase φr, the “focused” target response is retrieved by compensation of the phase on the signal y(τ), performing what in SAR focusing is called “backprojection”:

1 −jφr yF = ∑ y(τi) · e (7) N i In the following, we study the content of y(τ), making different hypotheses on the disturbing phase noise φAPS and on the clutter statistics. The impact of propagation (the phase noise term φAPS), accounts for the additional delay introduced by the non-free space propagation within the troposphere. If the monochromatic approximation holds (a small bandwidth compared to the carrier frequency), the phase term φAPS is linearly related to the delay, which in turn depends on the integral of the refraction index along the path. 4π 4π Z → φ = d = N(r( l ))dl (8) APS λ ATM λ Lp where N is the space- and time-variant refractivity index of the atmosphere, a function of the space r and the time of acquisition. The statistics of the atmosphere have been widely studied in the past [6]. Limiting the temporal scope to one day (far longer than one SAR acquisition), we can well consider the Kolmogorov turbulence statistical model of the delay, represented by the power law:

∆t α E[d(t + ∆t) − d(t)] = ad (9) τ where τ is a temporal constant depending on the local atmospheric conditions (typical values are in the order of hours). The exponent α is also defined as slope, as the function is a line in a log-log space, assuming values experimentally determined to be close to 2/3. The applicability of the model was verified with experimental ground-based radar measurements in Reference [6]. Numerical simulations were carried out to investigate the impact on the obtained signal-to-clutter ratio due to the atmospheric delay spatial and temporal variation. We now analyze the clutter contribution (the first term in Equation (4)), assuming sufficient stability of the atmosphere in the time interval of one acquisition. This assumption was verified by analyzing the Doppler spectrum of the acquired clutter echoes (see Section4). The result of the focusing operation in (7) is a stochastic process with variance depending on the clutter spatial and temporal covariance matrix. This will depend on the scenario (e.g., rocks or vegetation) and on its stability (e.g., due to wind). To show the principle of the outdoor test, we analytically derive the case of clutter perfectly correlated in time (frozen scene). In this case, the clutter ac is a constant along time, so its contribution to the focused signal yF is given by:

2 σ2 = σ2sinc2 L (10) c,F c λ where L is the total length of the target motion. The relation above shows that the contribution of the clutter to the focused moving target is a cardinal sinc, which depends only on the total motion Remote Sens. 2017, 9, 729 5 of 12

2 2 2  2  σ = σ sinc L c,F c  λ  (10)   where L is the total length of the target motion. The relation above shows that the contribution of the clutter to the focused moving target is a cardinal sinc, which depends only on the total motion extent λ L. With a motion extent multiple of , the clutter power is (ideally) completely cancelled, as the 2 Remote Sens. 2017, 9, 729 5 of 12 total signal-to-noise ratio is only thermal-noise limited. In this ideal case, the velocity of the target is completely irrelevant (provided that the radar Pulse repetition Frequency (PRF) is sufficient to extent L. With a motion extent2 multiple of λ , the clutter power is (ideally) completely cancelled, as the sample the sinusoid at f = v , which, for2 typical PRFs in the order of KHz, is practically always total signal-to-noise ratio is onlyλ thermal-noise limited. In this ideal case, the velocity of the target is true).completely irrelevant (provided that the radar Pulse repetition Frequency (PRF) is sufﬁcient to sample 2 the sinusoid at f = λ v, which, for typical PRFs in the order of KHz, is practically always true). 3. SAOCOM Outdoor Experiment Setup 3. SAOCOM Outdoor Experiment Setup The SAOCOM-1A outdoor test (ODT) took place in the Investigación Aplicada (INVAP) facility close Theto the SAOCOM-1A Bariloche Airport, outdoor Patagonia, test (ODT) Argentin took placea. The in thefacility Investigaci consistedón Aplicadaof a radome (INVAP) hosting facility one tileclose of tothe the SAOCOM Bariloche antenna Airport, and Patagonia, the SAOCOM Argentina. electronics, The facility connected consisted to ofa control a radome and hosting data- processingone tile of room. the SAOCOM antenna and the SAOCOM electronics, connected to a control and data-processingThe moving room. target was hosted in a radome placed at a distance of 3755 m in the plane in front of theThe facility. moving target was hosted in a radome placed at a distance of 3755 m in the plane in front of the facility.Figure 2 shows the setup on Google Earth (a) and an aerial view of the area (b). The picture was takenFigure standing2 shows on a small the setup hill of on approximately Google Earth 50 (a) m and altitude an aerial above view the of large the areaplane. (b). In The the foreground, picture was theretaken is standing the area on hosting a small the hill shelter of approximately with the guard 50 m and altitude the power above generator, the large plane. connected In the to foreground, the small radomethere is thehosting area hostingthe moving the sheltertarget. withThe fixed the guard target and was the placed power on generator, the right, connected approximately to the small20 m awayradome from hosting the radome. the moving Far beyond, target. The the ﬁxed facility target hosting was placedthe SAOCOM on the right, with approximately the large radome 20 m can away be seen.from the radome. Far beyond, the facility hosting the SAOCOM with the large radome can be seen.

(a) (b)

FigureFigure 2. 2. SAOCOM-1ASAOCOM-1A outdoor outdoor test test setup. setup. ( (aa)) View View on on map; map; ( (bb)) aerial aerial photo. photo.

Three trihedral corner reflectors were available during the ODT campaign: Three trihedral corner reflectors were available during the ODT campaign: • 75 cm •• 10075 cm cm (visible in Figure 3b) •• 150100 cm cm (visible in Figure3b) • 150 cm The first two corner reflectors could be mounted on the moving actuator, while the large one was placedThe first to twobe kept corner immobile reflectors at approximat could be mountedely 20 m on from the moving the moving actuator, target while radome. the large one was placedThe to Advanced be kept immobile Remote at Sensing approximately Systems 20 (ARESY m fromS) the corners moving are target trihedral-shaped radome. aluminum reflectorsThe with Advanced modular Remote faces that Sensing were Systems assembled (ARESYS) on-site. corners are trihedral-shaped aluminum reflectorsTable with1 below modular reports faces the that RCS were in assembled dB square on-site. meters (dBsm) of the corner reflectors at the SAOCOMTable 1center below wavelength reports the RCS(0.2353 in dBm). square meters (dBsm) of the corner reflectors at the SAOCOM center wavelength (0.2353 m).

Table 1. Radar Cross Section (RCS) of the corner reﬂectors.

75 cm Corner RCS 100 cm Corner RCS 150 cm Corner RCS 13.79 dBsm 18.79 dBsm 25.83 dBsm Remote Sens. 2017, 9, 729 6 of 12 Remote Sens. 2017, 9, 729 6 of 12

(a) (b)

FigureFigure 3. ((aa)) Radome Radome hosting hosting the the SAOCOM SAOCOM antenna antenna tile; tile; ( (b)) Radome Radome hosting the moving target.

Table 1. Radar Cross Section (RCS) of the corner reflectors. The accurate pointing of the corner reflectors mounted on the moving rail was ensured by the alignment of the75 rail cm itself Corner with RCS the line of100 sight, cm Corn whicher RCS was carried 150 cm out Corner by Comisi RCS ón Nacional de Actividades Espaciales13.79 (CONAE) dBsm with the use 18.79 of a dBsm differential Global Positioning25.83 dBsm System (GPS). The pointing of the large corner reflector was instead performed with the use of a compass and an inclinometerThe accurate (resulting pointing thusof the in corner a coarser reflectors pointing). mounted This method on the wasmoving employed rail was because ensured the by large the alignmentcorner reflector of the was rail thought itself with to have the sufficientline of sigh RCSt, which so as to was be leftcarried movable out by around Comisión the scene, Nacional in order de Actividadesto ease the execution Espaciales of (CONAE) preliminary wi visibilityth the use tests. of a differential Global Positioning System (GPS). The SAOCOMpointing of antenna the large was corner one single reflector tile ofwas the instead total antenna. performed One with tile is the composed use of a ofcompass 20 elements and anin theinclinometer elevation (resulting direction thus and onein a elementcoarser pointing). in the azimuth This method direction, was with employed a total because size of 3.48the large m in cornerelevation reflector and 1.424 was thought m in azimuth. to have Each sufficient TRM RCS was transmittingso as to be left 50 movable W with around an efficiency the scene, of 75%. in order The toexcitation ease the coefficientsexecution of were preliminary set according visibility to a Taylortests. (amplitude only) tapering, in order to shape the side-lobes.The SAOCOM The pointing antenna in was elevation one single and tile azimuth of the total was antenna. possible One thanks tile tois composed the mechanical of 20 elements support inequipment the elevation which direction could be and steered one element with steps in ofthe approximately azimuth direction, 1 degree with and a total the knowledgesize of 3.48 ofm thein elevationpointing was and of 1.424 approximately m in azimuth. 0.1 degree. Each TRM The linkwas budget,transmitting accounting 50 W forwith all an the efficiency gains and of losses 75%. fromThe excitationthe transmitter, coefficients through were the set medium, according and to to a the Taylor receiver, (amplitude and the only) corresponding tapering, in signal-to-noise-ratio order to shape the side-lobes. The pointing in elevation and azimuth was possible thanks to the mechanical support (SNR) computation, are reported in Table2 below. The noise figure N F term is used to define the equipmentequivalent noisewhich temperature could be steered of the with receiver steps and of approximately to then compute 1 degree the thermal and the noise knowledge power with of the pointingclassical equation:was of approximately 0.1 degree. The link budget, accounting for all the gains and losses from the transmitter, through the medium, and to the receiver, and the corresponding signal-to- PNoise,Thermal = KBTeqB = KB NFTaB (11) noise-ratio (SNR) computation, are reported in Table 2 below. The noise figure NF term is used to definewhere theKB isequivalent the Boltzmann noise temperature constant, B isof thethe signalreceiver bandwidth, and to then and computeTa is the the ambient thermal temperaturenoise power with(290 K).the classical equation: The acquisition parameters were set in order= to ease= the execution of the outdoor test by putting: PNoise,Thermal K BTeq B K B N FTa B (11) • a short chirp to allow close range where• the KB maximum is the Boltzmann bandwidth constant, to reduce B is the the signal resolution bandwidth, and Ta is the ambient temperature (290 K).• a reduced sampling window length to avoid unnecessarily large datasets The acquisition parameters were set in order to ease the execution of the outdoor test by putting: The main SAOCOM settings are described in Table3 below. • a short chirp to allow close range • the maximum bandwidth to reduce the resolution • a reduced sampling window length to avoid unnecessarily large datasets

Remote Sens. 2017, 9, 729 7 of 12

Table 2. Link budget computation and Signal to Noise Ratio (SNR) computation (ideal propagation media).

Parameter Value (Linear) Value (Log-Scale) Peak power 750.0 W 28.75 dBW Antenna area transmit (TX) 4.87 m2 6.88 dBsm Instrument and antenna TX losses 1.17 0.70 dB TX path loss (R = 3755 m) 5.64 × 10−9 −82.48 dB RCS corner (1-m corner case) 75.66 m2 18.79 dBsm Receive (RX) path loss 5.64 × 10−9 −82.48 dB Instrument and antenna RX losses 1.17 0.70 dB Received power 6.4 × 10−12 W −111.94 dBW Noise ﬁgure 2.14 3.3 dB Noise power at receiver 4.28 × 10−13 W −123.69 dBW SNR raw 14.96 11.75 dB Number of focused steps 1000 30 dB SNR focused 14,962 41.75 dB

Table 3. SAOCOM 1A acquisition parameters for outdoor test.

Parameter Value Acquisition mode Stripmap/TOPSAR Center frequency 1,274,140,000 Hz Bandwidth 50 MHz Sampling window start time 23 µs Chirp duration 11 µs Sampling window length 20 µs Acquisition duration variable 1 min–10 min Polarization Quad Pol Pulse Repetition Frequency (PRF) 4545 Hz Chirp Down

In addition to the moving target, a sampling equipment was placed to measure the transmitted chirp and to check the PRF.

4. SAOCOM Outdoor Experiment Results The raw data, provided by a dedicated implementation of the CONAE User Segment Service (CUSS), called mini-CUSS, were non-Block-Adaptive-Quantizer (BAQ) compressed and modulated (at 30 MHz) data. The processing software had then to perform the following steps:

• digital down conversion • range compression • azimuth compression

The digital down conversion step performs the demodulation of the signal to baseband. In the employed software, it also allowed the sub-sampling of the input dataset in order to speed up the processing for a fast analysis of the input data. In fact, the PRF of the SAOCOM is quite high compared to the Doppler content of the illuminated scene. The real input samples are also converted into complex samples during this step. The range compression performs the matched ﬁlter either with an ideal chirp (synthetically generated by the routine itself, or with the chirp replica coming from internal calibration. It is noted that the chirp replica is close to the ideal chirp, except for an amplitude tapering in the upper part of the chirp (Figure4a). The effect of the imperfect ﬂatness in frequency is seen in the impulse-response-function (IRF) side-lobes level, which are lower than the ideal level of the perfectly rectangular spectrum of the ideal chirp (Figure4b). The effect on resolution is of about 4% Remote Sens. 2017, 9, 729 8 of 12 Remote Sens. 2017, 9, 729 8 of 12 Remote Sens. 2017, 9, 729 8 of 12 lossresolution (see Table loss 4), (see which Table is4), almost which completely is almost completely recovered recoveredwhen the whenreplica the is replicafocused is with focused the ideal with losschirp.the ideal(see Table chirp. 4), which is almost completely recovered when the replica is focused with the ideal chirp. 0 0 Rep-Rep -20 0 Ideal-Ideal -5 Rep-RepRep-Ideal -4 -2 Ideal-Ideal -5 Rep-Ideal -4-6 -10 -6-8 -10 -10-8 -15

-10-12 IRF range [dB] range IRF -15 -20 -12-14 IRF range [dB] range IRF Power Spectrum [dB scale] -20 -14-16

Power Spectrum [dB scale] -25 Replica V -16-18 Ideal -25 Replica V -18-20 -30 -30 -20 -10 0 10 20Ideal 30 -15 -10 -5 0 5 10 15 Frequency [MHz] Range [m] -20 -30 -30 -20 -10 0 10 20 30 -15 -10 -5 0 5 10 15 Frequency(a) [MHz] (Rangeb) [m] (a) (b) FigureFigure 4. 4. ((aa)) Comparison Comparison between between the the ideal ideal chirp chirp and and the the chirp chirp extracted extracted from from internal internal calibration; calibration; Figure((bb)) Range-compressed Range-compressed 4. (a) Comparison Impulse Impulse between Response Response the ideal Function Function chirp and (IRF) (IRF) the comparison. comparison. chirp extracted from internal calibration; (b) Range-compressed Impulse Response Function (IRF) comparison. TableTable 4. 4.RangeRange IRF IRF analysis analysis results.results. Table 4. Range IRF analysis results. IRF Parameter Replica-Replica Case Ideal-Ideal Case Replica-Ideal Case IRF Parameter Replica-Replica Case Ideal-Ideal Case Replica-Ideal Case IRFResolution Paramete r Replica-Replica 2.77 m Case Ideal-Ideal 2.66 m Case Replica-Ideal 2.68 m Case Peak-to-Side-LobeResolutionResolution ratio 2.77− 2.7716.2 m mdB − 2.66 2.6613.3 m mdB −14.98 2.68 2.68 m dB m Peak-to-Side-LobePeak-to-Side-Lobe ratio ratio −16.2−16.2 dB dB −−13.3 dB −−14.9814.98 dB dB The range power profile of one acquisition is shown in the following figure, for the VV polarization.TheThe range range power power profile profile of ofone one acquisition acquisition is shown is shown in the in thefollowing following figure, figure, for the for VV the polarization.VV polarization.The high backscatter from the small hill and three main peaks can be recognized, corresponding to theTheThe fixed high high corner backscatter backscatter reflector from from (closest), the the small small a smallhill hill and and heap th threeree of maindirt main (mid), peaks peaks canand can bethe be recognized,recognized, moving target corresponding corresponding inside the toradometo the the fixed fixed (at 3755 corner corner m). reflector reflector (closest), (closest), a a small small heap heap of of dirt dirt (mid), (mid), and and the the moving moving target target inside inside the the radomeradomeThe (at (atsignal 3755 3755 tom). m). clutter ratio of the latter is approximately 13 dB. Considering the 100-cm corner RCS TheasThe in signal signal Table to 1, clutterwe can ratioratio estimate ofof thethe a latter latterclutter is is approximatelyRCS appr inoximately the resolution 13 13 dB. dB. Considering cell Considering in the order the the 100-cm of 100-cm 5 dBsm. corner corner This RCS RCSvalueas in as Tableis in 12 Table 1dB, we lower 1, can we estimatethan can estimatethe a assumption clutter a clutter RCS made in RCS the resolutionduringin the resolution the celldesign in thecell phase order in the (clutter of order 5 dBsm. RCS of 5 This ofdBsm. 17 value dBsm) This is valueand12 dB reported is lower 12 dB than in lower Section the assumptionthan 2. theWe assumptioncan made then during refine made thethe during designclutter the phasesignal design (clutter level phase to RCS − 27(clutter of dB. 17 The dBsm) RCS analysis of and 17 reported dBsm) in the andDopplerin Section reported domain2. We in canSection (Figure then 2. refine5b) We of can thethe clutterthenclutter refine signal shows the level aclutter very to −low signal27 dB.impact level The of analysisto the −27 wind, dB. in theThe with Doppler analysis an estimated domain in the Dopplerstable(Figure to5 b)domaintime-variant of the clutter(Figure components shows 5b) of athe very ratio clutter low (also impact shows called ofa theveryDC/AC wind, low ratio) impact with of an 40of estimated dB.the wind, stable with to an time-variant estimated stablecomponents to time-variant ratio (also components called DC/AC ratio ratio) (also ofcalled 40 dB. DC/AC ratio) of 40 dB.

(a) (b) (a) (b) Figure 5. (a) Range-compressed data intensity profile and interpretation; (b) Doppler analysis of the clutter. FigureFigure 5. 5. (a()a Range-compressed) Range-compressed data data intensity intensity profile proﬁle and and interpretation; interpretation; (b) Doppler (b) Doppler analysis analysis of the of clutter.the clutter.

Remote Sens. 2017, 9, 729 9 of 12 Remote Sens. 2017, 9, 729 9 of 12

The effect of the target motionmotion is clearlyclearly seenseen inin thethe range-Dopplerrange-Doppler map,map, showing the Doppler spectrum for each range. The two “targets” are the focused responses of the moving target. The bright stripe at the higher range is the backscatter ofof movingmoving peoplepeople insideinside thethe controlcontrol shelter.shelter. The azimuth impulse impulse response response function function of of the the syst systemem can can be beextracted extracted as a as horizontal a horizontal cut cutof the of therange-Doppler range-Doppler map map and and is shown is shown in inFigure Figure 6.6 The. The importance importance of of the the result result below below is toto havehave thethe possibility to analyze in advance the impulse responseresponse of the Synthetic Aperture Radar, normally obtained only once the instrument is ﬂyingflying andand thethe datadata areare properlyproperly processedprocessed onon ground.ground.

-10

-20

-30

-40

-50

-60

Focused Response [dB] Response Focused -70

-80

-90

-100 -3 -2 -1 0 1 2 3 Doppler Frequency [Hz] (a) (b)

Figure 6. ((aa)) Range-Doppler Range-Doppler map map with the focused moving target clearly visible; (b) Azimuth IRF.

Concentrating now on the moving target range bin, we can assess the end-to-end overall Concentrating now on the moving target range bin, we can assess the end-to-end overall amplitude and phase stability, which will be a combination of the instrument and of the propagation amplitude and phase stability, which will be a combination of the instrument and of the propagation medium stability. medium stability. Figure 7 shows, on the left, the reconstructed phase of the signal corresponding to the moving Figure7 shows, on the left, the reconstructed phase of the signal corresponding to the moving target bin. The expected sawtooth trend can be seen, corresponding to the movement of the corner target bin. The expected sawtooth trend can be seen, corresponding to the movement of the corner reflector back and forth during the acquisition. The image in the center shows the corresponding reﬂector back and forth during the acquisition. The image in the center shows the corresponding time-variant concentration of the signal energy in the azimuth frequency domain, moving from time-variant concentration of the signal energy in the azimuth frequency domain, moving from positive positive to negative frequencies depending on the direction of motion. The location of the peaks in to negative frequencies depending on the direction of motion. The location of the peaks in frequency frequency allows one to estimate with high precision the actual motion velocity and to synthesize an allows one to estimate with high precision the actual motion velocity and to synthesize an ideal linear ideal linear motion and the corresponding phase trend. The rightmost plot shows instead the residual motion and the corresponding phase trend. The rightmost plot shows instead the residual phase after phase after linear motion compensation. linear motion compensation. By collecting the phase of all the peaks from the acquired 10 min of long data, the amplitude and By collecting the phase of all the peaks from the acquired 10 min of long data, the amplitude and phase stability results reported in Table 5 were obtained. phase stability results reported in Table5 were obtained. Table 5. Amplitude and phase stability results over 10 min. Table 5. Amplitude and phase stability results over 10 min. Parameter Value (VV) Value (HH) AmplitudeParameter stability better Value than (VV) 0.1 dB better Value than (HH) 0.1 dB AmplitudePhase stability stability better 3.9 than deg 0.1 dB better 3.4 than deg 0.1 dB AmplitudePhase stability drift <0.1 3.9 dB/min deg <0.1 3.4 dB/min deg AmplitudePhase drift drift 0.19<0.1 deg/min dB/min <0.1 0.35 dB/mindeg/min Phase drift 0.19 deg/min 0.35 deg/min The correctness of the antenna excitation setting and the validation against the theoretical antennaThe pattern correctness calculation of the antenna was carried excitation out by setting repeating and the the validation data acquisition against with the theoretical different antenna patternpointing calculation in elevation was thanks carried to outthe bymechanical repeating steering the data of acquisition the antenna. with different antenna pointing in elevationFigure thanks 8 below to the shows mechanical an example steering of the of obtained the antenna. results, where each dot on the plot represents the averageFigure8 power below at shows the moving an example target of range, the obtained estimated results, on one where data each acquisition, dot on the either plot VV represents or HH. theThe averagedots are powersuperimposed at the moving over the target theoretical range, estimatedantenna pattern on one shape, data acquisition,corresponding either to the VV applied or HH. Thetapering dots areon the superimposed antenna (Taylor over thetapering). theoretical The antennagood agreement pattern shape, of the correspondingmeasures with to the the expected applied taperingpattern can on be the seen antenna up to (Taylor the second tapering). side-lobe. The goodThe agreement agreement is of better the measures for positive with angles the expected than for negative angles. This can be explained considering the known interaction of the antenna with the ground, introducing a ripple on the whole pattern.

Remote Sens. 2017, 9, 729 10 of 12

pattern can be seen up to the second side-lobe. The agreement is better for positive angles than for Remote Sens.negative 2017, 9, angles.729 This can be explained considering the known interaction of the antenna with the 10 of 12 ground, introducing a ripple on the whole pattern.

-10

-20 Moving target phase [rad] phase target Moving -30

-40

-50 0 10 20 30 40 50 60 Time [s] (a)

(b)

-1 Residual phase [rad] phase Residual

-2

-3 0 1 2 3 4 5 6 7 8 Time [s] (c)

Figure 7.Figure Analysis 7. Analysis of the of moving the moving target target signal. ( a()a Phase) Phase versus versus time; (time;b) Doppler (b) Doppler frequency versusfrequency time versus time (colorscale(colorscale in dB,dB, normalized normalized to the maximum);to the maximum); (c) Residual phase(c) Residual after linear phase motion compensation.after linear motion compensation.

Figure 8. Far-field elevation antenna pattern validation against the theoretical model.

Remote Sens. 2017, 9, 729 10 of 12

-10

-20 Moving target phase [rad] phase target Moving -30

-40

-50 0 10 20 30 40 50 60 Time [s] (a)

(b)

-1 Residual phase [rad] phase Residual

-2

-3 0 1 2 3 4 5 6 7 8 Time [s] (c)

Figure 7. Analysis of the moving target signal. (a) Phase versus time; (b) Doppler frequency versus time (colorscale in dB, normalized to the maximum); (c) Residual phase after linear motion Remote Sens. 2017, 9, 729 11 of 12 compensation.

FigureFigure 8.8. Far-ﬁeldFar-field elevationelevation antennaantenna patternpattern validationvalidation againstagainst thethe theoreticaltheoretical model.model.

5. Discussion The execution of the ODT and the processing of the collected data provides an important set of results that give the first significant insights on the overall system performance. First of all, the transmitted signal, after its replica were provided by internal calibration and their similarity to an ideal chirp were checked, showed an almost ideal IRF in range. The compensation of the known phase function, corresponding to the target motion, allowed us to check the phase stability of the system over a long time interval spanning up to 10 min. The analysis over datasets acquired with different azimuth and elevation pointing of the SAOCOM antenna tile allowed the successful validation of the antenna pattern pointing and shape over a large interval including the first and second side-lobes. The main limitations remain the single antenna tile used, thus allowing us to test only the beamforming in the elevation direction, and the target used, which responded only with the co-pol channels. Nevertheless, the collected results allow us to state that the key functionalities of the SAR system are verified, even with the limitations that an on-ground setup unavoidably brings.

6. Conclusions The SAOCOM-1A outdoor test took place in Bariloche, Patagonia, Argentina, during June 2016. The main objective of the outdoor test (ODT) was to provide an end-to-end validation of the SAOCOM-SAR functionality, in a realistic condition, where the SAR pulses are radiated by the antenna, reflected by a target, and then received by the antenna and recorded as SAR data. The test setup was made up of two main elements: the SAOCOM SAR engineering model, including one of the seven antenna tiles, and the moving target. It was proven to work as expected throughout the full duration of the tests. Several datasets were successfully acquired through the setup and processed to L0B data. Overall, the ODT objectives were met and the SAOCOM-1A proved to show an excellent signal quality, both from radiometric and interferometric points of view. The ODT provided the unique occasion to obtain a set of pre-flight far-field measurements. The results can be considered representative for SAOCOM-1A and for SAOCOM-1B as well, so no additional experiments are foreseen. The conclusive and formal verification of the SAOCOM performance is left to the laboratory pre-flight tests and to the commissioning phase.

Acknowledgments: The authors would like to thank CONAE and INVAP for the fruitful cooperation before, during, and after the execution of the outdoor test campaign. Author Contributions: Andrea Monti Guarnieri originally conceived the experiment. Davide Giudici carried out the feasibility study, coordinated the moving target design and development and processed the acquired SAR data. Juan Pablo Cuesta Gonzalez provided a thorough review of the feasibility study and coordinated the actual setup and execution of the experiment. Conﬂicts of Interest: The authors declare no conﬂict of interest. Remote Sens. 2017, 9, 729 12 of 12

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