GLENAIR
SATCOM The RF communications subsystem is an essential part of every spacecraft. It is required to transmit important health and telemetry data down to Earth, as well as receive commands from ground operators. As with all spacecraft subsystems, there are power and mass constraints placed on the comm system, and engineers must make numerous compromises in performance and efficiency.
beyond the highest RF frequency bands, it manifests as light—from infrared radiation (IR), to visible, ultraviolet, X-rays, and gamma rays. The physical design of RF circuits is generally determined by the wavelengths present within the radiation band of interest. The two basic design strategies are either 1) to keep all the devices and features so small (say smaller than
When designing a RF comm system, the first trades performed are for data rate, power consumption, and total mass. For example, a mission with high data rate needs would select a high frequency such as X-band for downlink and a directional high-gain antenna. Based on the ground station locations available, engineers would perform link budget analyses to determine the minimum power needed for a specific ground station antenna. This analysis would factor in rain and atmospheric attenuation, as well as modulation and coding. A few different link budget trades will be run, varying antenna size, RF output power and data rate. Each link will return a different margin of decibels, representing the reliability of the system. The engineers will proceed to calculate the final mass and power for each configuration. The mission designer will have a limit on mass and power constraints for the communications subsystem. Each configuration traded will compare data rate, power, and mass. A high data rate downlink may cost a high amount of mass for the antenna and power for the amplifier and radio. Conversely, a low-power, low- mass system may have a lower data rate. Another factor that is considered in the design phase is pointing. Depending on the orbit of the satellite and whether the link is UL/DL or XL, the system may have a specific pointing requirement. Large satellites frequently use gimbals—platforms that can pivot to point their antennas. The addition of a gimbal will increase the overall mass and power draws of the system. CubeSats frequently trade high- gain antennas for low-gain, omni-directional ones to maintain the link regardless of directionality. CubeSats may also change their attitude to point a body-mounted antenna, rather than use a gimbal.
1/10th of a wavelength) to ensure there is minimal opportunity for the radiation to escape or couple in unwanted ways or 2) to work on a larger scale and guide or manage the radiation fields in an environment optimized for their propagation. Examples of the former approach would be a transistor on a computer chip circuit, where the dimensions are so small compared to the natural wavelength propagation that size can be neglected to first order. A waveguide or coax cable on the other hand has dimensions optimized to “guide” the radiation modes between metallic surfaces. The performance of such devices is highly sensitive to variance in physical size. Designing a system capable of operating over a broad range of frequencies and scale can, in this regard, be a significant challenge. The division of the electromagnetic spectrum into bands is helpful, as one can typically design a device optimized for the largest possible area within each band, and then work to minimize the damage in the remaining frequencies. A classic example of this challenge is the design of an antenna, a device used to convert a current/ voltage on a conductor into an electromagnetic radiation. In its simplest form, an antenna consists of a symmetric disposition of two conductors, both fed from their center. This is called a dipole or Hertz antenna. Dipole antennas create a relatively homogeneous radiation strength-pattern in the plane perpendicular to the arms and will work over a reasonably broad spectrum centered around the half wavelength of the system (the length of the two arms equals half a wavelength). But for a fixed point-to-point communication system, it is not optimal because radiation is not concentrated along the direction of the link. This illustration points to two key figures of merit for an antenna: directivity
QwikConnect • January 2023
3
Powered by FlippingBook