Vantage Point International Inc.
Motion sensing synthetic aperture radar uses two or more physical antenna phase centers aligned with the platform flight vector in order to provide a means of detecting and/or measuring the radial component of velocity within the observed scene. This scheme permits time and azimuth position to be partly decoupled, which allows radial velocity of a scatterer to be distinguished from an offset in azimuth position of that scatterer.
The technique has been demonstrated for airborne SAR systems, but should also be adaptable to spaceborne SAR. At orbital altitudes and practical antenna beamwidths the synthetic beam is valuable to resolve small moving scatterers from other stationary or moving scatterers within the mainbeam. Moderate-cost spaceborne motion sensing radar systems will likely have to be synthetic aperture systems.
II. Status of the Instrument Technology
A number of systems have been described in the literature, notably at recent IGARSS meetings and within the open research literature oriented towards military radar. The US, Germany, Canada and the UK are known to have developed experimental airborne motion sensing SAR systems.
No consensus on the terminology used to describe this class of technique has been reached. Some names for specific approaches which have appeared in the literature include
- Displaced Phase Center Antenna (DPCA) SAR
- Along-Track Interferometry (ATI)
- Space-Time Adaptive Processing (STAP)
- Moving Target Indication (MTI) SAR
- Delta Time Interferometry
The author proposes the general term motion sensing SAR because of its simplicity and independence from any of the several competing analysis approaches.
The major incremental radar instrumentation requirement for motion sensing SAR is multiple antenna phase centers arranged along the flight vector with separate, closely matched receiver channels. Recent efforts in support of SAR polarimetry and across-track interferometric systems (e.g. TOPSAR) are directly applicable in that these all aim at providing multiple (usually two) closely matched channels. Multiple closely matched coherent channels are also used for digital beamforming systems. Although instrumentation cost and reliability improvements are badly needed, these improvements are already being driven by many technology trends.
Research opportunities specific to motion sensing SAR are therefore primarily in the derivation of a good theoretical framework and investigation of suitable digital signal processing algorithms. At present the techniques are either very simple (direct phase comparison of two independent SAR images) or so general that they are difficult to implement (matrix inversion formulations to suppress signals from stationary scatterers).
III. Application Areas
Potential applications of motion sensing SAR include the following:
2. Maritime traffic monitoring
3. Monitoring of vehicles on land
4. Air traffic monitoring
Many of these are of interest for dual-use (civilian and military) missions, which are in turn of interest given the need to pool and conserve resources in scientific programs.
Ancillary benefits which might be enabled by the technology are not strictly applications, but advantages which add further interest to this technology:
- Low Pulse Repetition Frequency (PRF) / Wide Swath operation
- Motion Compensation
The low PRF benefit is essentially an alternate (non motion sensing) mode which would use the multiple apertures to allow wider swaths at a given azimuth resolution. The idea is to collect N-tuples of aperture samples at one time on the N along-track channels, thus achieving the same synthetic aperture with 1/N as many pulses. This translates into a potential swath width increase by a factor of N, since the lower PRF allows a larger unambiguous range.
Motion compensation might be assisted by the availability of radial motion information through analysis of the multi-channel data.
IV. User Requirements
Oceanographers working with SAR data must infer hydrodynamic parameters indirectly through the complex influence of those parameters on the SAR image formation process. Availability of radial motion information through motion sensing SAR has been shown to have promise for estimation of ocean currents.
Applications which require detection of moving objects are complicated by the azimuth displacement effect of SAR. Motion sensing SAR can be used to compensate this effect, and can also serve as a means of suppressing stationary “clutter” relative to the small scatterer of interest, thereby improving detection in applications where moving scatterers are of interest.
The formation of the synthetic aperture can be disrupted when the relative motion of a scatterer is unknown. Motion sensing SAR could be used to estimate target motion and coherently focus the energy from a moving scatterer. This kind of problem must be solved for observation of aircraft using SAR, or for ship imaging where the aperture time is long.
V. Recommendations for Further Activity
Polarimetric SAR systems and along-track interferometric SAR systems (used for single-pass extraction of topographical information) should be examined to evaluate their adaptability as experimental data sources for motion sensing SAR work.
Motion sensing capability should be considered for new generation systems which will often contain multiple coherent channels. The considerable benefits of motion sensing could be made available at relatively small incremental cost if provided for in the initial design work.
Analysis techniques and digital signal processing algorithms should be developed to help define and quantify the benefits of multiple along-track phase centers, and to realize motion sensing capability in balance with other system characteristics such as speckle suppression, resolution, swath width, and cost.
While motion sensing SAR has been studied and demonstrated for airborne platforms, system level work is needed to study the feasibility of motion sensing SAR for spaceborne platforms with realistic system parameters.