Transmitting Live Aircraft Security Data by 3G Steve Lane, Commercial Director at electronic design consultancy Triteq, talks about how commercial 3G mobile phone technology has been adapted to monitor security through live data links onboard planes. Unlocking the Potential of 3G Air traffic across Europe is at an all time high and consequently, the aviation industry is running short of radio spectrum for communications. Additionally, onboard security has presented a new challenge to airlines. To help find a solution to this, the European Commission and Eurocontrol, the organisation in charge of air navigation in Europe, has explored the potential of 3G wideband technology as an alternative medium for communications intended for security. Having had successful results with previous trials of 3G communications in various aeronautical spectrum bands, Eurocontrol has recently been looking at 3G as a potential solution for Air Traffic Management (ATM) security. Developing an Air Traffic Management (ATM) Security System One option in developing an ATM security system was to provide a high capacity air-ground downlink that would support the transmission of encrypted voice, flight data and onboard video. This could be transmitted from the cockpit of an aircraft during a security alert. Such information would provide ground based decision makers with a clearer picture of the situation onboard the plane. The EU TEN-T (Trans European Network-Transport) funded project, known as European Aviation Security based on 3G technology (EAS-3G), is based on a C-band air-ground link operating at a frequency of around 5GHz. The International Telecommunications Union has allocated aeronautical spectrum for this purpose. The project aims to adapt existing 3G technology to avoid the high costs of developing an entirely new system. EAS-3G System Overview The concept of the project s system is that traditional node B and UE elements of the UMTS TDD system are replaced by ground stations (reconfigured node Bs) and air stations (reconfigured 3G PCMCIA modem). A data link is established and maintained between an air station and a ground station, and the system is capable of performing handovers across the cell boundaries between the ground stations. In effect, the 3G UMTS TDD technology provides an IP bit-pipe between the ground segment and the air station.
Figure 1 shows the basic EAS-3G system concept with Ground Stations (GS), Air Stations (AS) and Integrated Network Controllers (INCs). Figure 2 shows a top-level functional breakdown of the system into the ground station, air station and ground network sub-systems. The air interface operates in the 5090 5150 MHz frequency band, thus requiring frequency converters at both ends; the Ground Frequency Converter (GFC) and the Air Frequency Converter (AFC). The TDD equipment (indicated in blue) consists of node Bs, Integrated Network Controllers (INCs) and PCMCIA Modems, plus elements of the core network. Figure 2 System Components and Technical Challenges Triteq became involved in the project in 2006, after initial equipment concept trials had been carried out by Eurocontrol. There was a requirement to develop a working test system based on a commercially available 3G modem, enclose this in an avionics box and conduct flight trials. Triteq provided electronics design support to the project, which meant overcoming several technical challenges in selecting the right components. A key aspect of the project was to adapt a commercially available modem and ensure the system would operate at aircraft speeds.
An industry standard ARINC 4MCU enclosure, as illustrated above, was used. The complete design had to be certified for air-worthiness to allow flight trials to be conducted, so this was an important consideration in the selection of both the enclosure and other system components. Ground Station The ground station was based on industry standard UMTS-TDD equipment working at 1.9 GHz with a subsequent frequency conversion to the 5GHz aeronautical band. The converter was a separate development carried out by Triteq in parallel with the avionics circuit board manufacture. However, one of the main technical challenges in the project was implementing the avionics equipment particularly because of size, power and weight constraints. This part of the system is further described below. Air Side Configuration The basic configuration of the avionics system was conceived as a PCMIA 3G modem with the receive and transmission signals being converted between 2GHz and 5GHz for the air interface link. A PC104 module was used to provide control functions including the compensation for Doppler shift and the correct timing advance for the RACH (Random Access Channel) transmissions. Modifications to Avionics Modem A commercially available 3G modem was selected to minimise development costs for the project. This modem had to be modified to gain access to specific signals and to split the transmit and receive signals from the modem. In addition, the extended timing advance mechanism within the modem had to be controlled. The range limitation (due to the RACH configuration) was solved by sacrificing a timeslot and by modifying the RACH burst type and mapping it to a normal burst. These changes to the modem, and to Node-B in the ground station were implemented with collaboration from the UMTS equipment supplier. The 1.9 GHz transmit signal from the modem is up-converted to 5 GHz before transmission from the antenna and the 5 GHz signal from the antenna is down converted to 1.9 GHz to the modem. Overcoming Doppler Shift UMTS is not designed to cater for aircraft velocity; 250 km/hr is about the maximum velocity possible, but planes can cruise at ground speeds in excess of 1000 km/hr. As a result, there is a Doppler shift which is well outside of the normal operating tolerance of the 3G system. Triteq had to enable the modem to compensate for both Doppler shift and reference frequency tolerance using an Automatic Frequency Control (AFC) methodology. AFC was determined from the decoded received signal and used to control the reference crystal oscillator, which in turn was used to provide the clock for digital processing and to phase lock the local oscillator for both the received and transmitted signals. It corrected the frequency error of the reference oscillator. The 3G Modem originally used an analogue AFC applied to a 15.36 MHz voltage controlled crystal oscillator (VCXO). To obtain maximum performance it was necessary for the mobile modem to appear near stationary with respect to the fixed ground station. Hence the Doppler shift compensation applied to the transmitted signal had to be equal and opposite to that of the received signal. So the transmit (Ftx) and receive (Frx) frequencies for a nominal channel frequency (Fchannel) were corrected by the Doppler frequency (Fdoppler). Frx = Fchannel + Fdoppler Ftx =
The Doppler frequency was estimated based on the position and velocity of the aircraft relative to the base station the information being provided from the Aircraft Navigation Systems through an ARINC interface to the module. Doppler Correction The frequency error introduced by Doppler shift depends on the speed of travel and the frequency of operation. For a worst-case scenario the highest operating frequency of 5150 MHz and a speed of 1225 km/h (Mach 1 at sea level) was assumed. This produced a Doppler shift of +/- 5.8 KHz. The frequency synthesiser therefore had to allow for this range of Doppler correction. Frequency Drift The initial frequency error between the modem and the base station needed to be corrected. The AFC mechanism in the modem was designed to correct for the frequency error introduced by a crystal oscillator in the modem. A higher specification part had to be substituted in place of the modem oscillator. This improvement had to be sufficient to allow for the error introduced by the frequency translation. Phase Noise and Spurs The system does not have to fully meet the 3GPP standards because it is not connected directly to a commercial network. It is also subject to different types of interference, both cochannel and adjacent channel, so it is not possible to fully specify the phase noise and spur requirements. Good RF filtering was provided so out of band spurs did not need to be suppressed. An initial target of non harmonic spurs of <-60 dbc was achieved. Phase noise performance similar to a typical 3G system was also specified:- <-100 dbc at 100 khz <-130 dbc/hz at 10 MHz Switching Time and Frequency Control A special allocation of time slots was required in the avionics TDD system to allow for the large potential range between the aircraft and each base station. One time slot was allocated to switch between transmit and receive channels. This gave a target of 600 us maximum to carry out the switching on the airside. For the initial prototype system a lowest risk option was followed. A separate TX and RX VCTCXO were used to drive a separate synthesiser for TX and RX. This allowed very rapid switching between transmit and receive without the need to vary the VCXO frequency or allow the PLL to settle. A high stability fixed frequency oscillator was used as a reference to measure VCXO frequency. The VCXO for the modem was independent of the rest of the frequency control system. The TX and RX VCXO were both nominal 10 MHz frequency. At turn on a calibration of the tuning slope of each VCTCXO was performed. This was used to provide a course guide to the control voltage required to achieve a particular output frequency. During normal operation the output frequency of the VCTCXO was measured by the control system and the control voltage adjusted to achieve the correct output frequency. As part of the system development a PC104 control was implemented. This platform was used to develop software for modem, system and Doppler control. It also allowed interface to other wireless systems including 802.11a/g which can be installed onboard the aircraft for communication when the aircraft is on the ground and for in-flight application to enabled devices.
Test and Future The prototype system has been successfully tested by Eurocontrol, both on the ground and in the air with ground to air communication achieving high-speed live data and video transmission. Using Triteq s technical expertise, the project demonstrated that working with commercially available products can significantly reduce system development costs. Since the completion of the prototype design Triteq has been working further with Eurocontrol on the evolution of system design, including improved spectrum utilisation and frequency synthesis/control. This next stage supports Eurocontrol s move towards a working Air Traffic Management security system. Steve Lane is Commercial Director at Triteq (email: steve.lane@triteq.com, Tel: +44 (0)1488 684554, www.triteq.com).