DRS05 in Odometry and Speed & Distance Unit

Interview with Dr. rer. nat. Rainald Koch, Diplom Phys.
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The precise positioning of rail vehicles is the necessary prerequisite for a higher utilisation of the route and plays a central role in ETCS train control systems. The speed and distance must be determined at SIL4 level. This can only be achieved with diverse sensors and a high level of expertise – the field of speciality is called odometry. With 3 billion hours, the DRS05 Doppler radar sensors are the world’s most tried and tested radar sensors and an integral part of odometry.
Interview with the DEUTA radar experts Dipl.-Phys. Wulf Alexander Kolbe and Dr. Rainald Koch After focusing on the calibration of DRS05 radar sensors in our first interview with Wulf Kolbe, our radar experts answer questions about the Doppler radar sensors’ operating hours and conditions of use as part of the odometry in the second part of the interview. They explain the task of a speed & distance unit and clarify the advantages of this library for customers.

Let us start our interview with an incredible record: The DRS05 radar sensors in use have reached 3 billion operating hours over the past ten years. Congratulations on this world record. How do you achieve this record high?

Dr. Rainald Koch:

The DRS05 radar sensors are based on a proven technology and meet the highest quality standards. The housing shape and the housing thickness withstand stone impact even at maximum speed. They are equipped with two microwave modules whose signals are combined by a patented algorithm. This improves the accuracy on changing track ground while the comparison of the signals evaluated in parallel contributes to the safety. If one of the modules fails or if the antenna receives a weaker reflected signal, the firmware changes to a special “single mode algorithm” to increase the availability of the speed signal.

How does a Doppler radar sensor work?

Dr. Rainald Koch:

An antenna module emits a fixed-frequency continuous wave (24 GHz, 5 mW) in a slanted beam towards the track ground, which ideally scatters the wave diffusely. The back-scattered wave whose frequency is shifted by the movement is multiplied by the transmitted signal to yield the Doppler signal of difference frequencies. Its spectrum contains a line whose position is proportional to the speed and whose width reflects the directional characteristic of the antenna – ideally. In fact, the track ground also influences the angle dependence of the backscattering; but only minimally with the DRS05 as said before. By the way, each antenna module provides a signal pair like a two-channel incremental pulse generator for determining the driving direction (DOT).

Is the installation position of the DRS05 under the car body also decisive for the longevity?

Dr. Rainald Koch:

Typically, we mount two DRS05 units under the car body close to the bogie – but not on the bogie because the vibrations would shorten the service life of the sensor.

The DRS05 is an integral part of the ETCS odometry. What is the function of the odometer in the ETCS system?

Dr. Rainald Koch:

Typically, the odometer is an algorithm executed on the EVC (EVC = European Vital Computer, core of the ETCS on-board equipment). In a safe and highly available manner, it is to provide the speed and the distance travelled from multiple sensor signals which individually are insufficient. And because the signals of similar sensors are often disturbed simultaneously, safety can only be ensured using diverse sensors.

What is the advantage of the diverse measuring principle using a DRS05 Doppler radar sensor?

Dr. Rainald Koch:

Typical diverse systems count on axle- mounted sensors in combination with our Doppler radar sensor DRS05 – a simultaneously occurring error of radar sensor and axle-mounted sensors is deemed unlikely. The DRS05’s precision being stable for decades allows to calibrate wheel diameters dynamically. It allows to increase the balise distances on high-speed routes and decrease train headway in metros. Although there are some ETCS projects at the moment where attempts are made to establish diversity by using axle-mounted sensors and an alternative sensor technology. The results are not at all satisfactory: Compared to optical sensors, radar in the K-band is insensitive to soiling; compared to satellite navigation, a radar sensor is highly available, even in street canyons and tunnels; and the signals of acceleration sensors cannot be integrated over a sufficiently long period of time due to the uncertainty of the inclination of the track (clue ’cross-sensitivity to acceleration of gravity’). Therefore, the high market penetration of the Doppler radar DRS05 continues to grow.

Why has DEUTA developed the speed & distance unit?

Dr. Rainald Koch:

The motivation for developing the SDU library to be integrated in the customer software was to help our customers to maximise the benefits of our sensor products. Odometry is not an easy craft. The measurement uncertainty cannot be described adequately by white noise if the target is SIL4. There often is a narrow Gaussian part of the distribution which greatly underestimates the actual frequency in case of major deviations. In case of two grossly different measured values, a Gaussian fault model would provide a mean result which would be clearly different from the two measured values, although it is more likely that only one of the two measured values is grossly incorrect – or both in the same direction; e.g., in case of slip or slide. In addition, there are correlations in time, namely error states. Finally, diagnostic information is to be taken into consideration which is already provided to some extent by the sensor in case of DRS05. In general, however, diagnostic information shall be a by-product of the odometer to take advantage of the other sensors' signals. Some proposals of customers for extended functionalities of the DRS05 turned out to be a workaround for special situations which the odometry failed at.

Gaussian pitfall Assuming Gaussian error models (as do unscented Kalman filters) means taking the mean even in case of conflicting information.

How does such an odometer work?

Dr. Rainald Koch: 

In the railway sector, the state-of-theart technology used were Kalman filters; i.e., Bayes estimators for the train’s state of movement with Gaussian fault models for the sensors, in combination with various heuristics for the selection of the sensor signals. The state machine is complex to analyse, and with each addition to the fault model of a sensor, the entire algorithm needs to be reconsidered. Furthermore the width of the confidence interval to be issued is defined a priori depending on the remaining sensor configuration resulting from the application of the heuristics (keyword: degraded mode). Afterwards, when analysing the records, it is often found that the confidence interval was sometimes too small; i.e., uncertain, sometimes too large; i.e., bad for the performance.

How does the DEUTA SDU work and what are its concrete advantages?

Dr. Rainald Koch:

I have replaced the heuristics by a Bayesian treatment of the outliers as well as the error states. By declaring the fault models of the sensors – one or two DRS05s and two at least two-channel axle-mounted sensors – to be an input of the algorithm, its correctness could be mathematically proven which greatly simplified the safety case. With a Bayes filter that can also represent multimodal probability distributions for the speed and acceleration of the train, we managed to calculate the confidence interval adapted to the situation which, as a result, usually is narrower than with a conventional odometry but safer.

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Odometrie and Speed Distance Unit