The main objective of the activity is to review and investigate the most promising RF-Front-End and Antenna technologies relevant to the avionic Satcom terminal in order to assess and evaluate their applicability as well as to highlight their advantages/disadvantages.
In particular the following aspects shall be tackled:
- Use of combined antenna for Iris and other services (currently the Iris baseline is to use an omnidirectional L-Band antenna).
- Number and position of antenna(s) for optimum performance.
- Redundancy aspects, in particular with respect to multiple antennas.
- Since the RF Diplexer is a cost-driver for Satcom avionics, analysis of the feasibility to cost-reduce the diplexer and its subsequent impact on the satellite communication system design.
- Feasibility and use of advanced power amplifiers as a performance improvement enabler.
Because aircraft avionics design is key for future acceptance of any new satcom system by end users, particular care shall be taken to ensure that design features guarantee that they will be as low cost and easy to install in aircraft as possible. To this extent, any technological advances that could allow integration of equipment with other elements of aircraft avionics, or allow co-location of the communication antenna with other existing antenna would be beneficial.
Channel congestion in the current VHF frequency band is foreseen to happen if no migration of the ATM system from voice to data is adopted. A new datalink system will alleviate saturation by additional capacity in congested regions on the one side, and the need for improved operational efficiency in oceanic and remote airspace on the other side.
Satellite communications is considered in this context as a potentially valuable resource and shall be hence regarded at as an integral part of the foreseen global long-term solution.
To this end, a new satcom terminal needs to be defined by taking care in particular that design features guarantee the terminal to be as low cost and easy to install in aircraft as possible.
The study was addressing 2 major key aspects of the satcom terminal: the RF-front-end technologies and the antenna technologies.
An RF-Front-End usually comprises some filtering (usually a diplexer), a high power amplifier (HPA) for the transmit chain and a low noise amplifier (LNA) for the receive chain. The cost of the diplexer is often dictated by the number of resonators in the design.
The cost of the LNA is not a major contributor to the overall cost of the terminal. However there are some benefits in using new technologies like Gallium Nitride (GaN) devices.
The cost of the HPA is much more significant contributor to the overall cost of a satcom terminal. This is not just because of the cost of the devices, but also the efficiency is a major factor as it dictates the size of the heatsink/enclosure, which has a significant cost impact. The cost and ease of installation to multiple aircraft types can be maximised by: (a) reducing the output transmit power, (b) using a device with good efficiency like GaN, (c) choosing a modulation scheme with low peak-to-mean ratio, (d) limiting the design to one transmit carrier as any time, and (e) using a low transmit duty cycle.
The study has shown that Low Gain Antennas can be used to provide a low cost low weight solution that will be acceptable to airframe and airlines. However, limitations with respect to coverage are a consequence with the link being unavailable for up to 2 or 3 minutes during certain aircraft manoeuvres when near the satellite edge of coverage.
Using two LGA antennas can have a number of advantages (and disadvantages).
The study recommends a multiple offset antenna installation resulting in increased coverage at low satellite elevations whilst satisfying availability criteria.
The following tasks have been performed:
- Task 1 - Avionic SATCOM Terminal RF-Front-End Technologies;
- Task 2 - Avionic SATCOM Terminal Antenna Technologies.
For each Task, an assessment of the cost drivers has been also performed.
The two reports have investigated in depth practical RF-Front-End and Antenna designs, and come up with THALES recommended G/T and EIRP values for use in link budget design.
On the receive side, we recommend that the link should be dimensioned for a G/T value of –27.7dB/K. This is a worst-case number based on a satellite elevation of 5o, and an aircraft banking at 25o. It includes sky noise, antenna gain, LNA noise temperature and other losses.
On the transmit side, we recommend that the link should be dimensioned for an EIRP value of 7dBW if a QPSK-like modulation is used or 10dBW if a constant envelope modulation is used. These are worst-case numbers based on an aircraft banking of 25o. It includes a cable loss between HPA and antenna of <0.3dB, an antenna gain of –3dBic and other losses. The HPA output power is chosen such that a small (e.g. around 2MCU) enclosure can be used for the HPA/DLNA, without requiring forced air-cooling. However some aircraft installations may require an integral fan to be used.