Groundstation v.2.0

An initial block diagram is in development and will be posted here before Wednesday. For now, I will outline this version of the ground station, highlighting the difference between this version and those in the 1.x family, as well as what makes this project unique among ground stations. This version of the ground station will use the user's ham radio(s) for communication rather than include a software defined receiver (SDR) as versions 1.0 and 1.1 have. The reason for this decision is that we do not expect to provide the computer for use, as doing so would blow any attempt at a budget. I am using two radios, a Feidaxin FDC-150 VHF transceiver, and a Baofeng UV-5R dual band (vhf/uhf) transceiver. I may go back to the SDR for receive, but with the current setup, I see no reason to do so until we start to mess around in the L-band (microwave).

What this means is that since the user will be providing the computer for prediction as well as the radio transceiver(s), the ground station project itself is physically comprised of an azimuth elevation mount and two X-yagi circularly polarized antennas, one for VHF and one for UHF. The software will be Gpredict and hamlib installed on a Panasonic CF-50 laptop running Linux. Gpredict will find the satellite and pass the azimuth and elevation information to Hamlib, the ham radio interface library. Hamlib will talk to the Az-El mount's microcontroller (an arduino) and handshake as though it were an expensive commercial mount such as a Yaesu. This is a vastly simplified approach on a systemic level than even the original ground station concept.

Using a 3 rpm gear motor to rotate a 24" aluminum beam, azimuth is fairly straightforward. To take the load, the beam will sit on a ball bearing turntable rated for several hundred pounds. While not a very strong solution, the amount of mass at 12" from the shaft is trivial. What is unique is the method used to determine azimuth position. Rather than calibrate the mount's direction to true north and add complexity and drag to the mount by adding some sort of positional transducer, this design uses a digital compass module to determine the magnetic heading of the array. The true azimuth will be determined by a set offset (magvar). Future revisions of this design will contain a GPS module and mathematically extract magvar from the GPS position so that the ground station may be used anywhere in the world without calibration.

Elevation for each antenna is accomplished using a ordinary hobby servo. What isn't ordinary is that the servo is mounted in the aluminum channel using an aluminum mount for the servo as well as a pillow block (bearing) to support the weight and torsional loads of the antenna. In keeping with the no calibration design of the azimuth axis, the elevation axis receives positional feedback from an accelerometer. This device will determine zenith from the vector of the gravitational pull, and determine elevation from there. The accuracy of the selected sensors is higher than the selectivity of the antenna's pattern, so while the sensors may give noisy data, this can be sanity checked and mathematically smoothed to yield a stream of good data in a noisy environment.

Additional programming will be in place to prevent sudden motion of the array. Rather than simply turn the motor on in order to rotate the azimuth axis which could impact the longevity of the transmission, the controller will ramp the speed up according to a programmed rate of acceleration (soft start) and slow it down based upon a mathematical process called PID. PID stands for proportional-integral-derivative, which simply means that the speed at which the motion occurs is proportional to the distance it must travel to hit the set point. As it approaches the set point, the motion slows reducing wear on the components and enhancing accuracy. These systems should make the mount have a long productive life chasing satellites.

While all this fancy programming helps the ground station work well for satellite operation, these capabilities will enhance the project's utility to other types of users such as FPV (first person view) aircraft (drone) pilots, high altitude balloon crews, and emergency communications for first responders, disaster response, and government (military) applications. All of these are possible with minor adjustments to the mechanism. Even full mobile use (mounted to a moving vehicle) is possible, thanks to the use of sensors which constantly report real time azimuth and elevation data.