Navigation Mode

At its' highest resolution, the RAS family of Radars produce a significant quantity of data. Navigation Mode is a simple way of distilling the whole of the radar’s field of view to a much smaller dataset, containing the range and bearing of the key objects within the view. The processing to distil the radar data down to these key points can be carried out inside the radar, which reduces the network bandwidth requirements. Alternatively, the source code for this functionality is available, which allows customers to implement this processing within their own specific environment.

The information below describes how the “Navigation Mode” output from a radar is derived from the radar’s view of the environment surrounding it.

This mode is a very simple approach to extracting features from the scene that the radar presents; it is ideally suited to extract features linked to solid, well-reflecting objects within the view of radar that do not have the COSEC² fill-in feature to the radar output beam.

The key parameters to configure the operation of this feature extraction process can be configured in a radar by Colossus message type 205 and the navigation mode output from the radar can be requested by Colossus message type 120. When this feature extraction method is appropriate, the network bandwidth utilisation for the radar’s connection is greatly reduced.

Alternatively, the source code for navigation mode feature extraction function is provided for both C++ and Python environments. This approach can be used to process the raw radar data within a customer’s complete control, and dynamic adjustment/adaptation of the key parameters is possible.

A much more robust, but computationally more intensive feature extraction method is CFAR - a “common form of adaptive algorithm used in radar systems to detect target returns against a background of noise, clutter and interference” (ISBN 0-201-19038-9).

There are various implementations of CFAR, and this approach to feature extraction is much more capable in high clutter environments, or radar scenes where a fixed thresholding approach across all ranges and azimuths is inappropriate.

The Navtech RAS family of radars support a CFAR “point cloud” output over an RDP connection - more details are available here. This function is available in radar firmware 3.1.0.325 and later

This chart presents the data representing one single azimuth of a radar’s field of view. This data reports the reflected radar energies from objects within one beamwidth of the radar from a 625µs slice in time. The make-up of one single azimuth of radar data is described here.

The first step in processing one azimuth of data to yield the Navigation Mode output is to
discard data that is below a defined power threshold as shown in the hatched grey area above.

Subsequently, any “close-in” radar returns in range-bins less than a defined range are discarded,
as shown by the additional grey area at the left hand side of the plot, above.

A zoomed-in view of the data set presented in the earlier plots: considering the remaining data using a windowed function, the positions
(bin numbers) associated with the local maxima are identified - as shown by the points marked in orange, above.

By implementing a rough curve approximation using the power returned from the bins either side of each local maximum, an improved range estimate can be generated - with a resolution of less than one range-bin of the original radar data

The range estimate according to the shape of the fitted curve around a peak in return is reported, along with the measured local maximum power return and the associated azimuth angle at which the data was acquired. For each azimuth, the Navigation Mode function can report
a configurable maximum number of identified peaks per azimuth, and peak positions are reported from the shortest range first.

The reported range bin value is converted to range in metres,
to cm-level accuracy, using calibration coefficients that the radar reports.

Examples

The following images demonstrate how reducing the value of “Threshold” for a given field of view from a RAS6 radar affects the distribution of data within the Navigation Mode output.

In the scenes below, the source raw radar data is shown on the left hand side and the resultant Navigation Mode output is shown on teh right hand side.

The radar is mounted around 1.5m above ground level, and has a view of a grassed area, a roadway and some roadside hedges.

Navigation Mode output - threshold value 40

Navigation Mode output - threshold value 30

Video showing Navigation Mode output from a RAS6-054-H radar,
merged with a measure of radar tilt-angle