Welcome to the third week of the new Miles Team Weekly Topic round table! This is the last reminder that, going forward, we here at Team Miles would like to bring a weekly, in-depth look at a rotating variety of topics that are essential to our Mission and the NASA Cube Quest in general. These topics focus on the very foundation of our decision-making and variations in any of these areas have the potential to drastically alter the team, the craft design, and the mission.
This week we will be looking at the topic of communication, in particular, communication windows. Last week, the round table focused on cold gas propulsion, the next week will focus on our fellow competitors/ fellow secondary payloads on EM-1, and then we will be coming full circle and starting fresh to look at the Cube Quest competition as a whole.
So, how do satellites communicate? For our readers that need a refresher, most satellites communicate by using radio waves to send signals with a transponder to a receiver either on the earth or on another satellite or an object on the surface of an entirely other planet. Another method of how satellites communicate that is growing in prominence is by transmitting data through a beam of light. For both of these methods, one of the challenges of satellite communications is the dilution of signal or beam of light that intensifies as the distance between communication nodes increases.
For satellites that communicate with radio waves this is called the free-space path loss (FSPL). FSPL is the loss in signal strength of an electromagnetic wave that would result from a line-of-sight path through free space (usually air), with no obstacles nearby to cause reflection or diffraction. FSPL is generally used as part of the Friis transmission equation, which includes antenna gain. Antenna gain increases a satellites ability to transmit and receive over long distances. However, there are limits to the communications systems available.
Current satellite communications systems have requirements in order to be able to have successfully transmissions with their intended sender/recipient. The requirements for successful satellite communication require that the power and antennae gain must allow for coherent communication with an acceptable level of signal loss over the intended distance. When these requirements are all met, they present what is called a communication window.
Laser communications can carry a lot more information do to much higher bandwidth. The draw back of laser communications is that there needs to be a direct line of sight between the two communications network nodes, which is doable in outer-space but requires very agile, rapid, and precise pointing of the laser. Electromagnetic wave bounce and bend around objects and thus does not require line of sight or precise pointing but it is a lot slower, and carries less information.
To maximize the communication windows for satellites that do not stay pointed at one ground position on Earth (in geosynchronous orbit), there are extensive networks of interconnected ground stations (giant radio antennas or optical sensors arrays), placed optimally around the Earth so as it rotates, the intended satellites may be able to communicate with at least one of the ground stations. AMSAT is one such organization with an extensive network of electromagnetic wave sensing and transmitting ground stations spread around the world for this purpose.
The Ragnarok Industries Cube Quest team has partnered with AMSAT and it is providing them with 2,621 communications windows per Earth rotation for their satellite, which gives them a significant advantage for competing for the Cube Quest data communication prizes.
While our current mission plan does not go after any of the data prizes, it is important to always be in contact with Earth. We are also staying flexible and cognizant in this area while planning and improving our mission plan so that we may pursue a communications prize should the ‘window’ present itself. Ha!
To maximize our communication windows, we have solved for a polar lunar orbit, which allows us to always stay in sight of Earth. NASA’s orbit solving tool, GMAT, could not find this orbit alone. Our team leader coded additional solver functionality to work with GMAT to solve for this orbit. To any interested competitor teams, industry vendors, or outside-Cube Quest space missions; our polar orbit is for sale! We very much look forward to implementing this advantage for our mission.