Germany: At the DLR Microwaves and Radar Institute, a special traffic processor for the TerraSAR-X/TanDEM-X satellite constellation has been developed which does not require a road database or any other prior knowledge. Off-road vehicles and ships at sea can be detected clearly and with high accuracy.
The traffic processor combines the data recorded by TerraSAR-X and TanDEM-X at different times. To achieve the best performance, the distance between the two satellites is required to be 20 kilometres.
The commissioning phase of TanDEM-X was ideal for demonstrating the performance capabilities of the traffic processor. During this phase, the two satellites were positioned precisely 20 kilometres apart, before eventually moving into close formation.
Differentiation between stationary and moving object
Radar differentiates between stationary and moving objects by analysing the change in frequency of the signal caused by the distance between the radar and the change in position of the observed object, which is proportional to its speed. This difference in Doppler frequency means that moving objects shift from their actual position when depicted in a radar image.
Determination of vehicle positions and speeds
If only one radar satellite is available, the traffic processor has to perform a ‘trick’. It needs to know exactly where the roads are in the radar image. It can then determine the offset between the true and apparent positions of each vehicle and calculate the speed, which is proportional to the offset.
In the worst-case scenario, if TerraSAR-X evaluates just the interferometric phase between its two antenna halves, the results can be several hundred metres out. The main reasons for this are phase noise and interference from stationary objects.
Principle for TerraSAR-X / TanDEM-X traffic processor
If the same area is imaged by TerraSAR-X and then by TanDEM-X shortly afterwards (or vice versa), the moving objects appear shifted from their true positions in both radar images. The difference between the images can be measured with high precision and this means that the vehicle or ship’s actual position, its speed, direction and even its acceleration can be calculated accurately.
Preliminary results from the commissioning phase show that the speed estimation error was less than one kilometre per hour and the average position estimation error was about 20 metres.
Output from new traffic processor
The traffic processor operates automatically and provides as its output what is known as a Keyhole Markup Language (KML) file. This file can be viewed in Google Earth.