Research: High Frequency Radar and Robot Perception

Alex Foessel

Radar holds the prospect for modeling, safeguarding, and navigation from a space robot with advantages of operating in and through dust, in vacuum and atmosphere. Size and resolution are major drawbacks for this technology. Non-mechanical scanning antennas or antennas with simple designs reduce the
size. I have researched to identify technologies and vendors that develop non-mechanical scanning antennas or very simple mechanical ones.

Software narrowing techniques reduce it further by enhancing wide beam (i.e.. small antenna aperture) sensor data. The adoption of the "evidence grids" technique promises the use of wide beam sensor data to produce very detailed maps, without characteristic problems of radar perception (for example, rejection of side lobes). Another approach is the adoption of Synthetic Aperture Antenna techniques. Addressing these issues is the agreed thesis topic with my advisor.

Comprehensive testing of an existing system has been the major activity of the pre Antarctic weeks. A thorough test to characterize the sensor was recently completed. The testing included range and angular precision and resolution measures, mixed pixel phenomena, minimum detectable gap, beam shape and grazing angles for various soil types. A formal report of this activity will be available shortly.

Specifically for this Antarctic season, I work to produce a test of millimeter wave radar to validate it as an adequate perception modality in all Antarctic conditions. Performance for different grazing angles and different terrain in Antarctica (blue ice, snow, sastruggi) and atmospheric penetration under severe visibility conditions are main topics to explore.

Introduction

Imaging radar research at Carnegie Mellon University explores the use of this sensing modality for the perception of field robots, both in terrestrial and planetary applications. Millimeter wave radar is preferred to sense in environments that are dusty and have changing light conditions.

The sensor size and mechanical complexity challenge the applicability. The size is driven mainly due to the minimum antenna aperture necessary to achieve a narrow beam for high angular resolution. The need to scan a beam to provide the imaging capability requires several moving parts, compromising the reliability and space worthiness of the sensor.

The current research seeks to answer these shortcomings by adopting techniques from other perception fields that have wide beam sensors (synthetic aperture radar, ground penetrating radar, sonar) to achieve comparable resolution maps using smaller antenna apertures. The mechanical complexity is addressed by embracing non-mechanical or one-moving-part scanning antennas.

Scanning Antenna Technologies

The research activities will in the identification of the most promising technologies. The selection criteria include the mechanical simplicity, a potential low production cost, and the actual existence of demonstration hardware. The following list includes technologies that are explored.
  1. Optical beam forming uses a photo conductor where the illumination changes the diffraction properties of the material, thus attenuating incident millimeter wave radiation. Because the material responds rapidly to changes in the incident light, it is possible to form a scanning beam without moving parts. It is possible to construct an antenna capable of 60 degrees of angular scanning in two axes. The beam width at 94 GHz is 1.6 degrees in both planes. The antenna might operate at any frequency in the 2 to 100 GHz range. Size: 6 - 11 inches. Fall back: very low efficiency.
  2. Ferrite control changes the magnetic field along a dipole array antenna, thus changing the phase shift between each consecutive dipole. A simple current control scans of the beam. There are linear one-dimensional scanning antennas available for 77 GHz and planar two dimensional antennas for 38 GHz. The 77 GHz antenna scans a range of 15 degrees, and it is possible to double the figure by double feed. The antenna is non-reciprocal (transmission and reception patterns are not the same), therefore two are needed for radar applications. This requires a more complex control strategy to synchronize the two antennas.
  3. Spinning grating antennas are capable of providing fast linear scan while requiring only continuos speed rotation. They are characterized for very low cost and simple design. Current applications are automotive collision avoidance and  aircraft autonomous landing systems. This technology uses one moving part for one axis scanning.

Software Narrowing Techniques

"Evidence grids" technique uses a learned model of the sensor to weight the evidence of presence or emptiness. This evidence is integrated into a grid if cells, so the certainty of emptiness or occupancy grows with the number of observations. The ongoing development of the radar beam model pursues the application of this technique to radar beams. The technique has the potential to sharpen the images and correct for erroneous side lobe information. The field experiments with the FMCW imaging radar will provide the information to build the sensor model and apply this technique to real scenes. The rejection of side lobe ghost targets is a major expected improvement of the application of this technique.

Synthetic aperture radar techniques use the relative motion of the sensor with respect to the target to synthesize a larger antenna. Field robots are generally mobile and that feature shows promise to increase the accuracy of the terrain maps.

Experimental Activities

The first stage of radar testing finished recently. The selected area has no undesired reflections and enough area to lay targets and vary the geometry of the scene. The completed  phase includes the following tests: An early conclusion of this experimental activity is the need for better range resolution. A market research for state of art technology in short range, high-resolution radar transceivers is ongoing.

Current Sensor Description

The radar imaging research is undergoing an intensive experimental phase. A 77 GHz FMCW imaging radar was made available for this research. The
sensor scans horizontally four vertically stacked beams. Each beam is one degree wide and two degree high. The horizontal scan range is 64 degree and the four beams cover 8 degree in elevation. The angular range is mechanically scanned at 2.4 times per second and the angle increment is one degree. The radar uses FMCW (Frequency Modulated, Constant Wave) range estimation with a maximum range of 64 meter and 0.5 meter range increments.

Plans for Antarctica

In November 1998, a MMW imaging radar will be deployed at Patriot Hills, West Antarctica.  MMW radar is a preferred imaging sensor modality because it provides precise range measurements for the environmental imaging needed to perform autonomous operations in dusty, foggy, falling snow occluded (white out, blizzard) and poorly lit environments.

The backscatter test will measure the energy reflected in a direction opposite to the incident wave. This test will explore the energy returned at different grazing angles and on different surface types (blue ice, flat ice and snow). These results will be compared to the performance on a similar geometry of regular dirt.  This data will help to build a database of performance of MMW imaging radar on different terrain.

The atmospheric penetration test will measure the degradation of performance of the sensor under severe visibility conditions. The same scene will be sensed under clear air conditions and also under heavy flying snow (whiteout, blizzard). Visual conditions will be recorded by photography of the scene. A laser scanner will scan the same image for comparison purposes. A quantitative estimation of performance in severe polar vision conditions will support the application of this sensing modality.

Burying metallic targets and scanning with a frontal beam will test solid snow penetration estimation. The targets will have increasing depths up to a few centimeters. Only shallow penetration is expected.

Short Term Plans during and after Antarctic Deployment

  1. Select the best scanning antenna technology and characterize the beam pattern. Assemble the antenna to an integrated millimeter wave transceiver with high range resolution.
  2. Demonstrate functionality of software beam narrowing techniques. Specifically, apply the "evidence grids" principle to solve the side lobe issue. Establish metrics to relate application requirement with maximum beam width tolerable with the software narrowing techniques.
  3. Determine the applicability of synthetic aperture radar to field mobile robots. Specify requirements of position estimation that would enable the utilization of this technique.
Find out about the millimeter wave radar experiment in the current Antarctic expedition.

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Robotic Search for Antarctic Meteorites 1998
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This document prepared by Michael Wagner.