The 50th anniversary of man’s landing on the moon was celebrated roughly a year ago. But space adventure is not just about putting a man in space. The conquest of space has prompted human beings to devise missions that go beyond the confines of our own solar system. In order to be successful, all of them have combined the talent and work of engineers from different branches of engineering.
Communicating with Earth, regardless the distance, is an essential part of all these missions. The communication system that makes this possible is known as Deep Space Network (DSN).
What is DSN concept?
The DSN supports spacecraft, NASA and other space agency missions. The DSN is a basic element of the missions, and is based on a reliable, robust telecommunications system that is capable of communicating with Earth through long-distance space probes. It promises to be the most sensitive radio communications system in the world, exceptionally versatile and able to communicate, for example, with the small Curiosity robot (more than 58 million kilometers away), or with Voyager 1, already outside our solar system.
The DSN relies on three observation points at three different locations around the globe: California (USA), Canberra (Australia) and Madrid (Spain). This provides automatic coverage for all space mission telecommunications. Each observation point is about one third the length of the Earth’s surface from the other two. This is to ensure that any spacecraft/probes/devices are always visible to at least one of the three centers, whose antennas are about 120° from each other.
All the antennas used in the DSN are quasi-parabolic reflectors. This achieves high directivity and gain with electromagnetic waves in the microwave range, with frequencies greater than 1 GHz.
The parabolic antennas are able to concentrate all the incident rays travelling parallel to each other in a single point, called a focus. The shape of the antenna dish also has the advantage of forcing the entire incident wave front that bounces on the different points to travel the same distance until reaching this focus, so the “phase” between them is maintained. In addition, some of the dishes at the three centers are Cassegrain. By having two reflectors (a primary and secondary one), the Cassegrain configuration avoids having to place the transmitter/receiver at the focal point. It also facilitates access to the electronic devices in the secondary reflector for easy maintenance. Another advantage compared to single-reflector parabolas is the greatly reduced size of the main disk, which means less volume, weight and effort when it is moved.
The emission and reception signal is similar to a common microwave radio link. The microwave signals are transmitted through space, by an unguided means. The vacuum waves propagate in a straight line at the speed of light (i.e., at about 300.000 km/s, although this speed isn’t constant).
Signal transmission antenna.(1)
Although you might think that electromagnetic waves travelling through space assume a constant propagation speed, this isn’t really the case. The speed will vary when the waves pass through more dense media, such as planetary atmospheres, conglomerates of gas or cosmic dust. This means that, given the long distances between the emitters and receivers and the various media crossing the paths of the electromagnetic waves, distant probes will be more affected.
In order to correctly interpret the weak electromagnetic waves coming from the space probes, the ingenuity of the aforementioned parabolic antennas is combined with a microwave amplification system based on a Microwave Amplification by Stimulated Emission of Radiation (maser). This amplifier focuses on microwaves and its operation is based on the stimulated emission phenomenon described by Albert Einstein .
The Deep Space Network handles the following types of data:
• Mission tracking: Communication sessions start with an exchange of carrier waves between the antenna situated on Earth and the one on the spacecraft. Pure tones are used to track the position, a key factor for the engineers in charge of navigation. Ranging data helps determine (within a 1 meter error margin) the distance to spacecraft. Doppler shift, measured on the downlink carrier wave, determines speed (within factions of a millimeter per second).
• Telemetry: Telemetry is the digital engineering data gathered. It includes temperatures of key spacecraft parts, scientific data, and imposing images of places like Saturn and Titan, captured by the Cassini space mission, or of the surface of Mars, sent by the Curiosity rover.
• Command: Orders sent to spacecraft. Instructions for new experiments or course corrections for spacecraft on route.
• Radio reception: Signals change slightly as they pass through the atmosphere of a planet, moons or the sun, or through planetary rings or gravitational fields. Radio receptors register these changes for study.
For over half a century, the Deep Space Network has allowed thousands of scientists to develop their space research, which undoubtedly demonstrates the quality and reliability of the network. However, continuing social change is forcing scientific communities to develop new communication systems that can transmit data at much higher and faster rates. Likewise, at Teldat group, we are constantly evolving to adapt new technologies to the world of communications.
(1) Source: https://www.nasa.gov/directorates/heo/scan/communications/outreach/