In the near future, satellites will be able to acquire almost limitless amounts of data.
However, the amount of data made available to the end user will always be determined by a number of factors including how quickly that data can be downlinked from the satellite to Earth.
A typical pass of a low Earth orbit satellite over a ground station is just 10-11 minutes and all data required must be downlinked in this time. Small satellites are able to image in higher and higher resolution; in the last 10 years SSTL small satellites have gone from a default resolution of 32m (with DMC satellites like NigeriaSat-1) to capable of sub 1m resolution imaging (with the DMC3 constellation). This, along with other platform improvements, has resulted in a huge increase in the image throughput capability and amount of imagery generated. Subsequently, the speed of data transfer between the satellite and ground station must increase.
The gain (amplification) of a satellite’s antenna has a significant effect on the quality and strength of the signals that can be transmitted, and therefore the bandwidth and data rate. Lengthening the antenna increases it’s gain and increasing the horn size on the antenna concentrates the signal, both allowing a higher bandwidth link to the ground station for faster data transfer.
But this required increase in size must be balanced with the need to keep mass down and keep small satellites, small. Faced with this challenge, SSTL’s composites facility developed a new high gain antenna. The use of the advanced lightweight composite material, carbon fibre, allowed a significant jump in performance without changing the mass. The redesigned carbon fibre antenna provides an increased gain of 18dBi (from 15dBi) and a narrower beam-width (3dB) of 18° (from 26°)which effectively doubles the data rate capability compared to SSTL’s existing antenna system.
Using composite materials
meant that SSTL could create a higher capability subsystem that is still compatible with existing satellite platforms and systems. The detailed horn antenna is typically used onboard an Antenna Pointing Mechanism, which is used to maintain the line of sight with the ground station during a pass even allowing for a satellite that changes its orientation during the pass. Through restricting the mass of the horn antenna assembly via carbon fibre materials, in turn the core backbone components of the Antenna Pointing Mechanism did not need to be changed to incorporate the larger sized antenna, such as the motor, transmission and bearings. This approach keeps costs down and allows SSTL to incrementally improve the capability of its satellites, instead of having to redesign entire platforms every time.
In 2000 SSTL satellites downlinked at 38.4Kbps - roughly the same as an old dial-up modem. Dial-up is a thing of the past now. NigeriaSat-2, which was SSTL’s highest performance Earth observation satellite when launched in 2011, had two antennas, each offering 105Mbps data rate. The new carbon fibre antennas can support data rates up to 500Mbps and represent a significant step change in downlink speed since 2011.
Used in conjunction with other newly developed subsystems, like the High Data Rate X-Band Transmitter, High Speed Data Recorder and Flash Mass Memory Unit, the carbon fibre antenna will help increase the capacity of SSTL’s small satellites to acquire, store and downlink high resolution images. These subsystems will all be trialled in orbit on the UK technology demonstration mission, TechDemoSat-1, later this year.