Projects:

Definition & Development
of Galileo

Signal Structure

Galileo Test Bed GATE

ESA Galileo Test Bed GSTB_V1

GNSS Software
Simulation

C-Nav Study

Discussion 

Increased Transmit Power vs. Phased Array Antenna

The use of phased array user antennas and the construction of high-end C-Band receivers with very low implementation losses may not be necessary if the satellite antenna input power is significantly increased. In the following, both approaches (increase of satellite transmit power and use of phased array antennas) are discussed with respect to their benefits and drawbacks.

Increase of Satellite Transmit Power. Many drawbacks of C-Band navigation could be compensated by increasing the satellite transmit power by 10dB (minimum). By means of this approach, the following enhancements can be achieved:

• Compensation of the increased free space loss

• Increase of (C/N₀)_eff within the tracking loops, thereby reducing the influence of thermal noise, the cycle-slip probability and enhancing the DLL/PLL performance

• Compensation of the increased tropospheric attenuation, thereby increasing availability

• Use of omni directional user antennas, resulting in a relatively simple receiver architecture, moderate power consumption, low manufacturing costs and enhanced mass-market suitability

• Less attention must be paid to the development of high-quality C-Band receivers with low implementation losses, so that simple low-end C-Band receivers using simple 1bit-quantization techniques are feasible

However, increase of the satellite transmit power results in additional problems:

• Increased power consumption (satellite)

• Necessity of additional and/or larger solar panels

• More space required within the satellite-launching rocket

• Increased weight of the satellite-launching rocket

• Increased launch cost

Use of Phased Array Antennas. At first sight, the use of phased array antennas seems to be a suitable approach to limit the required satellite transmit power and to compensate the occurring signal losses at C-Band. The main advantages of this approach are the increased antenna gain compared to an omni directional antenna and the ability to null out multipath and/or interfering/jamming signals by means of beam forming. However, the use of such antennas results in the following drawbacks:

• Phased Array Antennas will presumably be larger, heavier, and more unwieldy and complex than omni directional antennas. Due to their increased size, they will not be suitable for certain applications

• Since a phased array antenna consists of several antenna elements, a corresponding amount of front ends will be necessary (one front-end per antenna element). Additionally, a beam forming and beam steering unit will have to be implemented. In contrast to an omni directional receiver, the phased array approach thus results in a complex receiver architecture, thereby increasing size, weight, power consumption and manufacturing cost.

Conclusion

C-Band navigation offers both benefits and drawbacks. Although it might be feasible to overcome the technical issues, it is uncertain that a (future) C-Band navigation system can compete with current sophisticated L-Band equipment. Furthermore, the L-Band performance will be permanently upgraded in the near future (GPS modernization, Galileo L-Band). Therefore, satisfactory acceptance of a C-Band system by the SatNav community is doubtful. However, a future C-Band signal might be an interesting option in combination with L-Band signals. Moreover, technological progress might balance some of the disadvantages and might allow C-Band navigation within a future generation of Galileo.

Further information (german): 
Forschungsdatenbank der Universität der Bundeswehr: Satellitennavigation im C-Band

Introduction (CNAV start page)
Signal Propagation and Tracking
Impact on Satellite Payload
Impact on Future C-Band Receivers
Conclusions