Ground Station part 1
DIY Ground Station, Part 1
By Aaron Harper
Communication is a fundamental part of intelligence; it is one of the things that makes us human. It should come as no surprise that a foundational technology to mankind’s reach into space is his ability to communicate. To communicate with a spacecraft, a specialized set of equipment is required. It requires a computer, radio, antenna, and operator. While this sounds fairly straightforward, space throws us a few curves.
The first issue is literally a curve… the curvature of the earth and to a lesser degree the local terrain. This is an issue because a spacecraft is only visible to any given spot on earth for a small part of it’s orbit. It would really help to know in advance where the spacecraft will be at any given time in order to prepare for the communication. As you would expect, this is possible with the application of mathematics
The second issue is that to remain in orbit, the spacecraft is moving at a fairly high velocity, and thus the time it is visible (called a window) can be quite short if it is in low earth orbit (LEO). At a higher orbit, the craft remains visible for longer as it’s apparent motion is slower until you get to the geostationary altitude of 22,236 miles, when the apparent motion matches the earth’s rotation, making it stationary relative to a fixed point on the earth.
A third issue is the orientation of the spacecraft. While it is generally safe to assume the business end of the antenna will be pointed at the surface of the earth, but what is up, down, left, and right makes a difference in standard antennas. The craft will cross over different parts of the ground at different angles (skew), so a standard vertical or horizontally polarized antenna will require constant fiddling like the rabbit ears on an old TV.
The fourth issue relates to the apparent (relative) velocity of the spacecraft. Like anything else in motion producing a waveform, the doppler shift applies. As a train approaches the sound of the horn is higher than when it departs because the sound waves are compressed by the motion of the train relative to the listener (you). The satellite, which is moving at a good clip relative to the ground station shifts the radio frequency as well, making tuning rather challenging.
The final issue is that radio signals become weaker as the distance increases (inverse-square law). A very bright flashlight will be barely visible, if at all, on a distant mountain. This is because as the light travels outward, less and less photons reach our eyes until it is below our ability to perceive it. Spacecraft are a fairly long way away when in orbit, not to mention when they are visiting distant worlds, so receiving their signals becomes quite challenging.
Without solving these issues, stable radio communication with space assets is impossible. Fortunately, these problems have already been solved for us, and it is these solutions working in concert that become a 21st century ground station. Today a ground station designed to receive voice and data traffic from spacecraft such as ISS may be constructed using common components for under $200.00, not the millions it cost NASA.
A computer running software to predict a satellite pass is the first component of a ground station. This will easily predict satellite passes, giving us the craft’s precise location in the sky at any given time, though it generally will not take terrain into account. GPredict is a free, open source program that has an intuitive interface, displaying the data on a table or the view on a map or polar graph. With some plug-ins, it also solves a few of the other issues as well.
The skew issue is solved by using circular polarization which only cares if the signal is sent with a right hand or left hand polarization (imagine a spiral from the spacecraft to the ground station), not which way the transmit and receive antennas are oriented. This is a function of antenna design, and a bit of a “black art” compared to the rest of the solutions. This brings us to a decision… to point or not to point.
There are plenty of omnidirectional circular polarized antenna designs, but they have a weakness. An antenna which points in all directions at once can only increase the signal (gain) by a factor of 8 as a theoretical maximum (+9dB), while antennas which focus on one direction (directional antennas) can go much higher, bringing in the weak signals. The disadvantage is that the higher the antenna gain, the more directional the pattern, and the more precisely the antenna must be aimed. This increases complexity, mass, and expense. Always a tradeoff.
The ability to point a directional array, while technically optional for LEO spacecraft, is mandatory for anything in geostationary orbit or beyond. The mechanism used to point the antenna or array of antennas are largely up to the imagination of the engineer, but they must be made to point accurately enough so that the spacecraft stays within the peak gain area (lobe) of the antenna and it is able to do so in high wind without damage. Keep in mind that flat panel antennas as well as dishes make excellent sails on blustery days.
Now, wouldn’t it be nice if the prediction software such as GPredict were able to sent the direction of the spacecraft to the pointing assembly (Az-El mount)? Most can! In GPredict, a module called hamlib may be added which facilitates the communications between the computer running GPredict and equipment including Az-El mounts. That said, for the sub-$200.00 ground station, an omnidirectional antenna will be used.
Since the position and velocity of the craft are known, the prediction software may be used to calculate the anticipated doppler shift during the satellite pass. Using this information in GPredict, some radios may be tuned directly using the hamlib plugin. This makes running a modern, well integrated ground station a relatively simple process. As a spacecraft comes into view, simply select it on the software and the hamlib plugin will point the antenna and keep the radio in tune. This solves all but the last issue in setting up a ground station, that of signal strength.
Major factors which contribute to the ability of a signal to reach from the transmitter to the receiver are the output power of the transmitter, the gain of the transmitter antenna, the distance (inverse square of the distance, as mentioned before), the gain of the receive antenna, and the sensitivity of the receiver. Unless we designed it, we don’t have much control over the transmitter output power, antenna gain, or the distance (orbital altitude) of the spacecraft. This leaves the receive antenna gain and receiver sensitivity as areas the builder of a ground station can optimize things.
Fortunately for us, modern radio receivers have really improved. Back in the day, we were lucky to get a sensitivity figure of -84dB, but today a $20.00 USB dongle is capable of -114dB. To put this into perspective, every 3dB difference essentially doubles the measurement in this logarithmic scale. This means that the 30dB difference represents a real improvement of 2 to the 10th power, or 1024. In English, a modern USB dongle receiver available on Ebay or Amazon is over 1000 times more sensitive than those used in the 60’s that communicated with our astronauts on the moon!
Sensitivity and low cost isn’t the only thing these receivers have going for them. those same receivers which had the 84dB sensitivity were capable of tuning only within a fairly narrow band (406 – 549 Mhz). The dongle (a Realtek RTL2832u TV receiver) is capable of tuning 24MHZ to roughly 1850MHz by way of comparison. Simply put, this dongle makes the bridge between a modern computer and an antenna, turning it into the ground station Apollo era engineers could only dream of. The only wildcard is the antenna.
While there are many antenna designs, to keep the ground station simple and below $200.00, we must select the best omnidirectional solution instead of building (and paying for) an Az-El mount. A little research has shown a simple design with excellent gain characteristics that can be built by a hobbyist; the “eggbeater” antenna. As it’s name suggests, this antenna’s design looks like an eggbeater with two wire loops at 90 degrees to one another. This antenna is circularly polarized, and has a gain of around 8dB. Construction details are available at here.
This leaves one final component. The operator is a person with the responsibility and/or interest to operate the ground station. They have the knowledge of how the systems work, and get usable audio and/or data from the system. While a license (FCC amateur radio, ham license) is not required for reception in the United States, local, homeowner association, and national regulations vary. Check if in doubt. That said, a ham license will be required for the next step: transmitting.
Transmitting voice and data is required for most use of space based assets and real communication. This will be the subject of the next $200.00 project write up, and as said before, the use will require an FCC license. A technician class ham radio license is quite easy to get, with no requirement to learn morse code. The concepts you will learn in getting one will serve you well as an operator of a full fledged ground station. Transmitting capability is an upgrade to the ground station that will take your equipment to the next level and will let you use space for your communication needs. Stay tuned!