Radar interferometry can be used to obtain sub-kilometre resolution over a swath at the expense of additional transmit power and a sufficiently long baseline to accommodate at least two antennas. This paper reports an innovative concept called AltiCube+, a low-cost long fixed-baseline interferometric radar altimeter based on CubeSats on-orbit assembly. The AltiCube+ concept consists of multiple 16U CubeSats. After an early operation and commissioning phase, these CubeSats will perform autonomous rendezvous and docking with each other via deployable booms to establish a long fixed-baseline, and then deploy antennas for an interferometric altimeter configuration. The uniqueness of AltiCube+ is on the potential scientific opportunities brought by two left and right looking interferometric altimeters with around 6 m baseline (total system length is more than 8 m) and the sustainability due to its significantly low cost and short development lifecycle. If budget allows, multiple AltiCube+ systems with same or different altimetry capabilities can form a constellation to dramatically reduce the revisit time and, therefore, provide much better spatiotemporal coverage.

Radar interferoinetry can be used to obtain sub-kilometer resolution over a swath at the expense of additional transmit power and a sufficiently long baseline to accommodate at least two antennas. This paper reports an innovative concept called AltiCube+, a low-cost long fixed-baseline interferometric radar altimeter based on CubeSats on-orbit assembly. The AltiCube+ concept consists of multiple 16U CubeSats, After an early operation and commissioning phase, these CubeSats will perform autonomous rendezvous and docking with each other via deployable booms to establish a long fixed-baseline, and then deploy antennas for an interferometric altimeter configuration. The uniqueness of AltiCube+ is on the potential scientific opportunities brought by two left and right looking interferometric altimeters with around 6 meter baseline (total system length is more than 8 m) and the sustainability due to its significantly low cost and short development lifecycle. If budget allows, multiple AltiCube+ systems with same or different altimetry capabilities can form a constellation to dramatically reduce the revisit time and, therefore, provide much better spatiotemporal coverage.

Multiple CubeSat altimeters can work independently or corporately to form altimeter constellations. Different configurations of the constellations can acquire distinguished advantages: improved spatial/temporal sampling and high cross-track resolution, which will be helpful for observations of oceanic small-scale structures and weather forecasting. Compared to single conventional altimeters, CubeSat altimeter constellations may achieve better performances with lower costs. To fully understand these systems, this article focuses on the performance analysis and methodology for CubeSat altimeter constellations. Besides the typical analyses of the resolution, revisit, and absolute sea surface height (SSH) accuracy, the performance analysis was conducted by considering the characteristics of multiple measurements provided by CubeSat altimeter constellations. Local and global spatial sampling performances are investigated for various constellations and compared by sampling density and swath size. Moreover, relative SSH accuracy is introduced and evaluated based on the spatial structure functions of errors to effectively evaluate the measurement performance. Related system requirements on power, delta-v, etc., to achieve the performance are also discussed, which ensures that the analysis fits the boundary conditions of implementation. Finally, different concepts of the CubeSat altimeter constellations are compared, where their limitations and possible solutions are also discussed.

Distributed SAR systems provide imaging capabilities that cannot be achieved by traditional monolithic satellites, thanks to the multiple angles and times of observation. In this paper the opportunities offered by a swarm of small satellite nodes operating in S-Band are discussed. The nodes fly in a close formation and operate in MIMO mode. All the $N$ satellites transmit and receive the $N$ pulses at the same time, following a Frequency Division Multiplexing (FDM) scheme. The work focuses in particular on the impact of the cross-track baselines on the resolution and on the quality of the signal reconstructed from the $N^{2}$ channels. A first iteration of a distributed target processor based on adaptive frequency and channel treatment is hence proposed with the aim of effectively accounting for the slope-induced spectral shifts.

Radio frequency interference (RFI) has become a growing concern for weather radar, distorting radar variable estimation. By simultaneously or alternately transmitting the horizontal and vertical polarized waves, polarimetric weather radar can be referred to as SHV radar or AHV radar. The SHV radar can mimic the AHV radar by discarding either H- or V-channel measurements, which leads to an alternating scheme. In this research, the real RFI measurements from an operational C-band SHV radar are used to characterize the RFI temporal, spectral, and polarimetric features. Then, the RFI is simulated to quantify the performance of the object-orientated spectral polarimetric (OBSPol) filter in RFI mitigation. The OBSPol filter has been previously proposed by the authors to mitigate the narrowband clutter (both stationary and moving) and noise. This work extends the application of the filter to remove the RFI for SHV radar. Specifically, by taking advantage of the low copolar correlation of the RFI signal measured in AHV radar, the RFI mitigation method is designed, and its effectiveness is proven by qualitative and quantitative analyses. In particular, in the case of RFI overlapped to weather echoes in the time domain, the RFI can be mitigated, also when the duty cycle of the RFI is high. However, this work does not provide a full evaluation of the RFI mitigation performance on all radar data outputs but a proof of concept to show the effectiveness of the proposed filter for RFI mitigation.

This paper discusses the design details of a high resolution, low "Size, Weight, Power and Cost" (SWaP-C) radar altimeter (RA) system. Operating frequency of the radar is chosen within the Ka-band to achieve the desired size and weight requirements, that are highly demanded for the small satellite missions in a cost-efficient way. We propose a system design such that, an intended radar altimeter can be built by using the Commercial off the Shelf (COTS) components. The simulation results show that the proposed RA has high potentiality for realization.

The paper introduces the principles and the technical elements supporting the so-called SwarmSAR concept, consisting in a close formation of simple nodes cooperating in a MIMO-like frame to boost their imaging flexibility and performance. The philosophy of the swarm consists in employing extremely basic but self-sufficient nodes, each one guaranteeing sufficient image quality even when used individually. The costs are hence diverted from the node to the formation launching and maintenance aspects. We promote in this paper the use of S-Band as a convenient frequency both for the single node and for the formation requirements and resourceful for applications. An outline of the envisioned cooperative illumination modes, including high resolution imaging and the interferometric modes, and a preliminary discussion on their expected performance and challenges is provided.

We investigated the sensitivity of fully focused SAR (FF-SAR) processing of Cryosat-2 altimeter data to Earth rotation. Earth’s rotation causes scatterers at varying cross-track locations to have a different relative velocity with respect to the satellite. This second-order effect of Earth rotation on the phase is currently not corrected for in FF-SAR processing of altimetry data. The difference is largest near the poles, where the satellite flies parallel to the equator. Not correcting for the second-order effect yields a parabolic shape in the counter-rotated phase, which increases with the cross-track distance. Its effect is, however, limited by the time-in-view of the scatterer, which is shorter at the edge of the altimeter footprint, and therefore destructive interference will not occur when using Cryosat-2 data. For Cryosat-2, the only expected effect is a reduction in power and along-track resolution in the waveform tail and in the grating lobes. If the FF-SAR processor focuses on one point, and there is a bright scatterer at another, then there is a residual parabolic phase, whose sign and shape depend on the cross-track distance and whether the signal is left or right of the chosen focal point. In theory, if the viewed scene only has few bright coherent scatterers, then it might be possible to determine the cross-track position of each. In practice, however, natural targets are rarely coherent over the integration time.

The tremendous progress of small satellite technologies in recent years brings unique opportunities for satellite radar altimetry. Flying radar altimeters onboard a group of small satellites could provide better swath observation, an equivalent or even improved performance and coverage at a much lower price, while complementing large satellite missions. To this end, a feasibility study of a Ka-band altimeter CubeSat constellation for ocean monitoring, called AltiCube, is being carried out by the Delft University of Technology, under the support of the European Space Agency. This paper provides an overview of the AltiCube mission study. First, the potential observation products and performance of five constellation/formation concepts are analyzed. Then the requirements on the platform are discussed based on these concepts. The main focus is on the payload performance and the needed delta-V for the constellation/formation maintenance. Based on the assessment with respect to performance, platform, technological readiness level, cost and other factors, the along-track comb formation (consisting of five identical CubeSats) is selected. Further details of its system design is given in the paper. The selected CubeSat altimeter along-track comb formation shows the capability to map the ocean sub-mesoscale structures, which is extremely hard to monitor using traditional large spaceborne radar altimeters.

We investigated the sensitivity of Fully-Focussed SAR (FF-SAR) processing of CryoSat-2 altimeter data to Earth rotation and exploit it to decontaminate waveforms in coastal zones. Earth's rotation causes scatterers at varying cross-track locations to have a different relative velocity with respect to the satellite. This second-order effect of Earth rotation on the phase is currently not corrected for in FF-SAR processing of altimetry data. The difference is largest near the poles, where the satellite flies parallel to the equator. We show when not correcting for this second-order effect it yields a parabolic shape in the counter rotated phase, which increases with the cross-track distance. If the FF-SAR processor focusses on one point, and there is a bright scatterer at another, then there is a residual parabolic phase, whose sign and shape depend on the cross-track distance and on whether the signal is left or right of the chosen focal point. In theory, if the viewed scene only has few bright coherent scatterers, then it might be possible to determine the cross-track position of each. In practice though, natural targets are rarely coherent over the integration time. The incoherent property of natural targets can, however, also be exploited to decontaminate waveforms in coastal tracks from land signals, by differencing two radargrams computed with two different focal points. If the cross-track distance between the focal points is set correctly, the speckle signal from the ocean becomes quasi-independent, while the land signal is similar in both radargrams and can subsequently be removed.

The work investigates staggered and random PRF (Pulse Repetition Frequency) strategies for a close formation of small Synthetic Aperture Radar (SAR) satellites operating in a multistatic configuration. The satellites are positioned within a fraction of the along-track critical baseline, hence allowing for the application of Displaced Phase Center image formation approaches. The performance of regular and random pulse sampling schemes is in particular assessed for a single-input multiple-output (SIMO) S-Band constellation, whose feasibility is further analyzed in relation to the number of satellites and their antenna size.

Weather radar is well recognized as an effective sensor for obtaining the microphysical and dynamical properties of precipitation at high spatial and temporal resolution. Radar calibration is one of the most important prerequisites for achieving accurate observations. In this article, a portable, cost-effective and repeatable radar calibration technique, namely, unmanned aerial vehicle (UAV)-aided radar calibration, is proposed. A UAV serves as the stable aerial platform carrying a metal sphere, flying over the radar illumination areas to complete the calibration process. The flying routine of the UAV can be pre-programmed, and thus, the antenna pattern regarding different elevation and azimuth angles can be retrieved. To obtain the position of the sphere, the real-time single-frequency precise point positioning-type global navigation satellite system solution is developed. In addition, the radar constant is calculated in the range-Doppler domain, and only the data where the metal sphere separates from clutter and other objects are selected. The S-band polarimetric Doppler transportable atmospheric radar (TARA) is used in the calibration campaign. The experiments demonstrate the following results: 1) antenna pointing calibration can be completed and 2) antenna pattern can be retrieved and weather radar constant can be accurately calculated.

Satellite wind scatterometers like the MetOp SG (Second Generation) SCA of Eumetsat are not designed to measure ocean currents, yet if they could, it would improve the wind vector product and provide important additional information to the oceanographic community for various applications. Previous publications showed this possibility, but only with modifications to the system. In this paper the possibilities of measuring the ocean current vector simultaneously with the wind vector without modifications to the SCA system, are investigated through simulation studies. The measurement principle relies on phase change in a pulse pair measurement. The results indicate that it should be possible with SCA to measure the ocean current vector simultaneously with the wind with an accuracy of better than 1 m/s on a 50 x 50 km grid.

A new technique to enhance the indication of moving targets is proposed, which makes use of randomized Stepped Frequency Continuous Wave (SFCW) modulations, where the frequency order is changed from pulse to pulse. This kind of signals gives the possibility to discriminate target responses whose Doppler spectrum is folded back into the clutter region. The discrimination is performed directly during the range compression, so avoiding spectrum aliasing in the Doppler domain. For the range compression, the deramping technique, commonly used with linear SFCW, is extended to the case of non linear stepped modulations. In this way, the sampling constraints can be relaxed also when using randomized SFCW signals, allowing the same sampling frequency as with deramped linear SFCW, and reducing therefore the system complexity. Compared to linear modulations, randomized SFCW signals give also the advantage to suppress range ambiguities and therefore the capability to look at further range.

Remote visibility (Vis) estimation by radar is of interest to aviation, road traffic, and other fields. Millimeter-wave radars are suitable candidates because of such advantages as high spatial resolution and sensitivity to small droplets in reflection and attenuation. To investigate remote Vis estimation and to develop physics-based models and algorithms, a 35 GHz cloud radar at the Cabauw Experimental Site for Atmospheric Research (CESAR) in the western part of the Netherlands has acquired data during fog periods in 'fog mode.' The advantage in using millimeter-waves for remote sensing of fog is that they interact strong enough with fog for sensing it, but not too strong so that they can penetrate the fog allowing to sense fog top. Simultaneously, fog drop size distribution (DSD) and Vis are continuously automatically measured by the in situ optical sensors at CESAR. Radar reflectivity (Z) and Vis can be linked theoretically since they are related to the sixth and the second moments of an assumed Gamma-shaped DSD. However, in reality the fog DSD is not always Gamma-shaped, leading to errors in the Vis estimation. A further development of the Vis-Z model includes the attenuation factor (La), which is proportional to the liquid water content at a given radar wavelength. This improves the estimated accuracy of Vis, in the theoretical moments-based model. Finally, we were able to arrive at a higher accuracy by introducing an empirical exponential model, estimating Vis from Z and La. A test based on DSD data sets for various fog types in the literature showed robust performance of the Vis-Z-La model for large variations in DSD. The Vis-Z-La model is also validated with the actual fog data sets that were collected by the in situ and remote sensing instruments synergy at CESAR.

In recent years, synthetic aperture radar interferometry has become a recognized geodetic tool for observing ground motion. For monitoring areas with low density of coherent targets, artificial corner reflectors (CRs) are usually introduced. The required size of a reflector depends on radar wavelength and resolution and on the required deformation accuracy. CRs have been traditionally used to provide a high signal-to-clutter ratio (SCR). However, large dimensions can make the reflector bulky, difficult to install and maintain. Furthermore, if a large number of reflectors are needed for long infrastructure, such as vegetation-covered dikes, the total price of the reflectors can become unaffordable. On the other hand, small reflectors have the advantage of easy installation and low cost. In this paper, we design and study the use of small reflectors with low SCR for ground motion monitoring. In addition, we propose a new closed-form expression to estimate the interferometric phase precision of resolution cells containing a (strong or weak) point target and a clutter. Through experiments, we demonstrate that the small reflectors can also deliver displacement estimates with an accuracy of a few millimeters. To achieve this, we apply a filtering method for reducing clutter noise.

A radar scatterometer operates by transmitting a pulse of microwave energy toward the ocean’s surface and measuring the normalized (per-unit-surface) radar backscatter coefficient (r8). The primary application of scatterometry is the measurement of near-surface ocean winds. By combining r 8 measurements from different azimuth angles, the 10 m vector wind can be determined through a Geophys- ical Model Function (GMF), which relates wind and backscatter. This paper proposes a mission concept for the measurement of both oceanic winds and surface currents, which makes full use of earlier C-band radar remote sensing experience. For the determination of ocean currents, in particular, the novel idea of using two chirps of opposite slope is introduced. The fundamental processing steps required to retrieve surface currents are given together with their associated accuracies. A detailed description of the mission proposal and comparisons between real and retrieved surface currents are presented. The proposed ocean Doppler scatterometer can be used to generate global surface ocean current maps with accuracies better than 0.2 m/s at a spatial resolution better than 25 km (i.e., 12.5 km spatial sampling) on a daily basis. These maps will allow gaining some insights on the upper ocean mesoscale dynamics. The work lies at a frontier, given that the present inability to measure ocean currents from space in a consistent and synoptic manner repre- sents one of the greatest weaknesses in ocean remote sensing.

The microphysical processes in fog are examined based on an analysis of four fog events captured by the in-situ and remote sensing synergy at the Cabauw Experimental Site for Atmospheric Research (CESAR) in the western part of the Netherlands. A 35 GHz cloud radar at CESAR has been used in "fog mode" for the first time in the campaign. In this paper, the microphysical parameterization of fog is first introduced as the basis for analyzing the microphysical processes in the lifecycle of fog. The general microphysical characteristics of the four fog events are studied and key microphysical parameters (droplet number concentration, liquid water content, mean radius, and spectral standard deviation) related to fog are found lower than those in other sites due to the low aerosol concentration at Cabauw. The dominant processes in fog are investigated from the relationships among the key microphysical parameters. The positive correlations of each two parameters in lifecycle stages of a stratus-fog case suggest the dominant scheme in fog is droplet activation with subsequent hygroscopic growth and/or droplet evaporation, which is also supported by the combined observations of visibility and radar reflectivity. The shape of fog drop size distribution regularly broadens and then narrows in the whole lifecycle. However, other mechanisms could exist, although not dominating. Collision-coalescence is a significant factor for the continued growth of big fog droplets when they have reached certain sizes in the mature stage. In the datasets, the collision-coalescence process could be distinguished from the unusual negative correlations among the key microphysical parameters in the lifecycle of another stratus-fog case, and the temporal evolutions of droplet number concentration, mean radius, spectra width, visibility and radar reflectivity show the evidence of it.

In The Netherlands a demonstration small SAR instrument mission is prepared under the ESA Prodex program. Its launch is expected in 2017. The overall objective of this project is to demonstrate an alternative means to systematically assess the structural health of large volumes of infrastructure (built environment), in order to avoid hazardous situations. The assessment is based on an X-band interferometric Synthetic Aperture Radar with high resolution in the order of 2 meter. The radar system will be using the FMCW principle and will be realized within stringent Size, Weight and Power (SWAP) requirements in order to allow future missions of this type to be flown on small satellites. These future missions will possibly exploit the full FMCW capability in a formation flying sensor suite. The current demonstrator will consist of one platform with the instrument operating in Interrupted FMCW mode. This back-up mode is foreseen also for the future missions, in order to keep imaging capability in case one of the instruments in the formation has a serious malfunction. The paper describes the mission goal, the radar instrument and its performance.