DELEGATES from the railway industry, global navigation satellite system (GNSS) technology partners, and public mobile telecommunication companies gathered in the picturesque Transylvanian ski resort of Poiana in Brasov, Romania, on April 24 and 25. The meeting was not for a ski event, but instead an opportunity to share the culmination of 28 months of work in a pioneering railway research project called Satloc. Unlike many previous sponsored research projects, a working system has been created which proves the concept. Delegates were therefore able to ride on one of the trains involved in the project and witness the traffic control centre (TCC) in operation.

Satloc is intended to assess the development of innovative GNSS-based safety systems and demonstrate their application to rail transport. Satloc introduces GPS positioning technology and its enhancement by the European Geostationary Navigation Overlay Service (Egnos), the first pan-European satellite navigation system and a precursor to Europe's Galileo GNSS, to train control, train position monitoring, speed supervision, traffic control and traffic management on low-traffic lines. Another important objective was to reduce the total cost of deploying and maintaining a Satloc railway line.

A final requisite from the main sponsors of Satloc was to ensure compatibility with current railway signalling and standards, especially ETCS Level 2 and precursors to Level 3.

The main target for Satloc is the easily-overlooked market of low-traffic lines (UIC Line Category E), which account for around 40% of the European rail network, while the global percentage is estimated to be even greater. The reduction in costs afforded by Satloc is intended to attract investment to these lines, thereby facilitating their survival and revival.

To this end, Satloc uses widely-used and commercially-available technology including public mobile systems for train communications, RFID tags instead of balise groups or an absolute position reference (APR), a device similar to a balise which is used on metros, and off-the-shelf GPS receivers for position monitoring.

Satloc's core subsystems are the onboard unit (OBU), the mobile telecommunications system, the TCC, and the trackside infrastructure.

The OBU was built in Austria by Siemens Rail Automation in collaboration with F H Wels University. The OBU is a step forward from a previous project deployed by the OBU team for the narrow-gauge Pinzgau line in Salzburg province, which was revived through a public-private partnership scheme after being threatened with closure, and which resulted in the development of Siemens' Trainguard STC. Many of the principles of Satloc were already deployed in Pinzgau, and Satloc was a natural progression for Trainguard STC, especially with the introduction of the ETCS Level 2 and Level 3 standards.satloc

The OBU computes a train's position by using both Egnos/GPS position sensors and Deuta high-accuracy odometer sensors and transposes them over an onboard map of the railway. The OBU can also access the train's emergency brakes and uses the standard ETCS braking curve algorithms in its calculations. This function is carried out on a small-footprint industrial-grade PC running the VX Works operating system.

The OBU uses ERTMS language to communicate with the trackside system, specified by Unisig. The core of this specification is the positional references based on offsets and directions from balise groups, which in Satloc are virtual. The virtual balise groups are transposed on the railway map at strategic points specified by absolute latitude and longitude coordinates. The OBU is able to reference its position with respect to these virtual balise groups.

For further reinforcement, RFID tags are deployed on the trackside at strategic points such as diversions and shunt area limits. The tags are also transposed on the railway map as balise groups, with their absolute latitude and longitude positional references.

Each position report from the OBU is accompanied by a packet 44 containing the GPS fix of the position in the corresponding Unisig position report packet 0.

The ETCS Level 2 and Level 3 core principle of having an RBC in control of the railway and issuing movement authorities is deployed by Satloc within its TCC.

The remaining task for the OBU is to communicate with the TCC to enable the train to move under its supervision. The TCC and OBU each require a UMTS or similar modem to establish communication.

There was a dilemma for the Satloc team: the need to ensure compatibility with ETCS was at odds with the requirement to use publicly-available technologies. In order to embrace the continuing progress in public mobile telecommunications systems, Satloc decided to trade off this requirement. Indeed, Unisig has ETCS working groups developing packet-switched technologies to supersede or complement the current circuit-switched GSM-R technology. As European Union sponsorship is common to Satloc and ETCS, this provides a platform for sharing knowledge between projects.

Satloc deviates from the ETCS specification in that it can use any available mobile communications media. In the initial trial sites in Romania, 3G UMTS provided by Vodafone Romania is in use. To produce a communications subsystem with high availability, Vodafone Romania enhanced its offer to Satloc by providing local roaming with all public networks in Romania. For security reasons, Vodafone Romania also provided a high-end VPN for exclusive Satloc use.

A further deviation is that Satloc used TCP/IP packet-switched communications transport. This differs from GSM-R and its circuit-switched technology currently used by ETCS.

One of the challenges of introducing packet-switched data was that packets share the same media channel, and therefore need to be tagged with source and destination information. In contrast, circuit-switched data has dedicated channels for each source and destination pair obviating the need for identity data.

Unisig Euroradio safety and application layers are deployed in Satloc according to the relevant Unisig specifications.


The TCC was built by Siemens Rail Automation UK. Given the remit to use ETCS technology, Siemens decided to reuse its established technology for providing ETCS movement authorities. These include host-based versions of its Trackguard Futur ETCS radio block centre (RBC) and Trackguard Westrace interlocking. These host-based versions contain the blueprint from their parent high-integrity systems, but without the accompanying high-availability and high-integrity target hardware and infrastructure. Despite the remit for a low-cost solution, safety is always paramount, and the use of these otherwise SIL4 systems enables the project to focus on developing and integrating the new and challenging technologies, without compromising safety.

The TCC subsystem also used Siemens Trackguard Westcad, a widely used and highly-configurable control system interface.

While existing technology is used for the RBC, interlocking, and control system interface, new developments for the TCC are:

• to provide a diverse check of the position reports between the GPS packet 44 and associated packet 0

• to provide an interface to the dispatcher by transposing the position reports on the map shared with the OBU

• to implement the TCP/IP communications technology, and

• to provide a juridical recording database of every event passing to and from each subsystem.

These functions are integrated in a single off-the-shelf industrial PC running Microsoft Windows.

Tasks are carried out by a new component, the diverse monitor and communicator (DMC). This transposes the train's position onto a map drawn and managed using Trackguard Westcad, which provides the interface to the dispatcher who monitors the train's position and controls routes by interfacing with the interlocking. The train's position is transposed onto the Trackguard Westcad display by the DMC, and the interlocking interfaces with the RBC.

The DMC also checks the train's actual speed against the speed profile and initiates an emergency stop message to the train if it violates the speed profile. Similarly the DMC will also send an emergency stop message to the train if the diverse check between the packet 44 and packet 0 displays incoherent movement beyond a configurable threshold.

Satloc aims to minimise trackside infrastructure as far as possible. To achieve this, trailable points are used, reinforced by the use of RFID tags at diversions, to compensate for the margin of error and limited accuracy of the GPS Egnos system at these prime locations. A train that it is not anticipated to traverse an RFID tag will be tripped immediately. In addition the TCC sends an emergency stop message to the train after detecting such an incorrect movement.

To enhance safety further, and in line with Romanian railway rules, these sections are protected by an Indusi magnet system.

As established interlocking components are used in the TCC, Satloc should be able to provide a means of controlling trackside infrastructure such as point machines when a railway is unable to operate without them. To facilitate this, a means of communication between the TCC and the point controllers will need to be established. Mobile communication, which is proven in the track-to-train link, is a very good candidate for solving this problem.

The TCC can connect to level crossings either to provide updates to the dispatcher's display, or to control the level crossings based on the train position information, or indeed both.

Although the first implementations of Satloc use UMTS and 3G technology, the amount of data being transmitted to and from the train and trackside systems is relatively small, so it would be possible to use GPRS over 2G systems such as GSM-R, or even bespoke radio systems such as those deployed by ham radio enthusiasts.

At the other extreme, using 4G, which is able to transfer data from end to end at high speed, could facilitate the use of an enhanced version of Satloc on higher speed lines, and possibly on lines with a higher traffic density.

The successful showcasing of Satloc running on the Transylvanian plains in April is only the beginning. This break from tradition is set to continue with several very real and tangible ideas to enhance or customise Satloc to various conditions.