CoEP Satellite Initiative: Ground Station Ground Station Shreevallabh Gharote Amit Phadke Aniket Sirsath Suraj Chafle College of Engineering Pune Shivajinagar, Pune 411005
2 CoEP Satellite Initiative: Ground Station Preface The primary aim of the Communications Sub-system is to establish reliable and efficient twoway communication between the satellite, main ground-station and the auxiliary groundstations. The Power, Structure and ADCS sub-systems place constraints on the Communications sub-system. This report presents the progress made by the sub-system. Communications Sub-system CoEP Satellite Initiative Main Objectives Establish one Main Ground-Station and at least two Auxiliary Ground-Stations. Design a high bit-rate (1.2kbps) uplink and downlink of data. Implement a low bit-rate (480 bps) Beacon transmission system which will be functional for the entire lifetime. Ground Station A. Location: a. Height of the site b. Interference from surroundings c. Horizon to horizon angle d. Ease of accessibility e. Availability of power, LAN etc On the basis of above factors we selected the roof of Metallurgy department of CoEP as the site for our ground station. A thorough survey using theodolite was undertaken in order to determine interference from surroundings. Data available from a previous BSNL survey was used to determine the RF interference.
3 CoEP Satellite Initiative: Ground Station B. Block Diagram: C. Antenna The choice of the Ground Station antenna has been dictated by two factors: 1. The system must necessarily establish communication. 2. Faraday rotation due to ionosphere. Both factors are taken care of by a Crossed Yagi Uda Antenna. The Crossed Yagi is circularly polarized whereas the monopole on the payload is a linear omni-directional antenna. This implies that the Crossed Yagi will always pick up the signal from the payload although with a 3-4 db loss. The circular polarization also counters the effects of Faraday rotation. The results of simulation of Crossed Yagi Antenna at 435MHz in 4NEC2X are as follows:
4 CoEP Satellite Initiative: Ground Station
5 CoEP Satellite Initiative: Ground Station As can be observed, the antenna impedance is 50.2 + j0.04 ohms and its SWR is 1. The Beam width is about 65 degrees. The Crossed Yagi was constructed and tested. The test results were as follows: SWR at 435MHz = 1.2562 Input impedance = 46.2 ohm Further experimental characterization of the antenna is pending. While we could crudely establish the gain the to be around 9 to 12 db, testing of gain and radiation pattern can only happen in a anechoic chamber which is unavailable to us.
6 CoEP Satellite Initiative: Ground Station D. Antenna Mast Design Rotor Booms of the Crossed Yagi antennas Height of the antenna tower = 2.5 meters (follows that the height of the tower should be greater than lambda divided by 6 to avoid reflection) Size of support on which the two antenna booms have been mounted = 1.5 meters (distance is greater than lambda divided by 2 for the 145 MHz antenna) E. LNA We have selected ZX60-33 from Minicircuits as our LNA. The LNA offers a gain of 20 db with a noise figure of 1 db. To achieve amplification of 60dB we shall cascade 3 LNAs together. Considering a Crossed Yagi antenna, 6 LNAs will be required for each of the Yagi antennas.
7 CoEP Satellite Initiative: Ground Station F. Transceiver The transceiver that we have selected is Kenwood s TS-2000. The transceiver is popular in the HAM community because of its functionality and the various features it offers. We know that this is overkill but it offers the students of our college to become well versed with several concepts of RF communication and observe them in real life. The primary concerns in the choice of transceiver were, 1. Doppler Shift compensation 2. RF attack time 3. Noise Figure 4. Modulation schemes it supports 5. Output power 6. Sensitivity 7. Bandwidth TS-2000 allows for automatic Doppler Shift compensation. It has a RF attack time of 30msecs which suits our system well since it can easily access our beacon which has duration of 1 sec. The noise figure of the transceiver is a bit high at 7.5 but that does not cause major trouble. Its sensitivity is 0.11 micro volts It can emit signals of 50W power and works at 145 and 435MHz bands. G. Antenna Rotor System We have selected the Antenna Rotator System from ARS, Spain as to serve as our tracking system. The ARS system consists of, 1. RCI-SE hardware board which has relays which control the rotor 2. ARSWIN software which controls the hardware. The RCI-SE circuit fulfils the two following objectives: Read the current antenna position by means of the incorporated A/D Converter, and on other hand, Controlling the turn (right or CW and left or CCW) by means of relays. The RCI-SE has the following connectors: J1: The azimuth antenna rotation is controlled by means of this connector. It s is attached to 3 relays at the RCI-SE Board. One of the relays (AUX) is able to control a brake or speed control if it s applied. J2: Similar than J1, it s used for elevation control. J3: Power input. It requires a power supply: 12-14 VDC.
8 CoEP Satellite Initiative: Ground Station J4: Input to the A/D converters for the azimuth & elevation rotators. This input is used to read the antenna position. This point will be connected in parallel with the wires attached to the rotor s potentiometer allowing the antenna position readout. DB-25: Female DB25 connector to be attached with the parallel port at our computer. As can be observed, ARS system offers both Azimuth and Elevation control based on orbit data from a computer. We intend to use ARSWIN and SATPC32 as the software controlling the rotor. H. Rotor The rotor selected for our Ground Station is Yaesu G5500. The Yaesu G-5500 provide 450 azimuth and 180 elevation control of medium- and large-size unidirectional satellite antenna arrays under remote control from the station operating position. The two factory-lubricated rotator units are housed in weatherproof melamine resin coated die-cast aluminium, to provide maintenance-free operation under all climatic conditions. The rotators may be mounted together on a mast, or independently with the azimuth rotator inside a tower and the elevation rotator on the must.
9 CoEP Satellite Initiative: Ground Station Specifications: Voltage requirement: 110-120 or 200-240 VAC Motor voltage: 24 VAC Rotation time (approx., @60Hz): Elevation (180 ): 67 sec. Azimuth (360 ): 58 sec. Maximum continuous operation: 5 minutes Rotation torque: Elevation: 14 kg-m (101 ft-lbs) Azimuth: 6 kg-m (44 ft-lbs) Braking torque: Elevation: 40 kg-m (289 ft-lbs) Azimuth: 40 kg-m (289 ft-lbs) Vertical load: 200 kg (440 lbs) Pointing accuracy: ±4 percent Wind surface area: 1 m2 Control cables: 2 x 6 conductors - #20 AWG or larger Mast diameter: 38-63 mm (1-1/2 to 2-1/2 inches) Boom diameter: 32-43mm (1-1/4 to 1-5/8 inches) Weight: Rotators: 9 kg (20 lbs) Controller: 3 kg (6.6 lbs) The rotor has to bear a weight of around 20 kg in our system. We can observe that this is easily manageable. The rotation torque and braking torque are capable of handling the mentioned load at a height of only 2.5 meters. The motor s control unit allows for external control by means such as Antenna Rotator System.
10 CoEP Satellite Initiative: Ground Station I. TNC The TNC selected for Ground Station is the PacComm Picopacket. This is a standard TNC for Amateur Satellites. It handles AX.25 data packetization and depacketization by itself. The TNC can support baud rates from 300 to 9600 bps. It is connected to the computer via a RS- 232 port. Testing of the TNC is still pending. The above document covers all the major aspects of the Ground Station in detail. The detailed design down to the last connector is complete. Construction and implementation of certain parts of the Ground Station is still pending. However, the design has been completed by conducting a thorough survey of all the available literature and by soliciting help of experts such as Dr. K.P.Ray and Dr. Shevgaonkar apart from CoEPs faculty. We would especially like to thank the IITB Pratham team for their invaluable inputs and guidance. The Communications team is confident that all the experimentation shall be complete in the near future and the design shall be validated by practical results.