Last modified April 2010
*** Supported by DK1TB's SatPC32 program!!***
While not intended to be used with large antenna arrays, it can handle a standard VHF/UHF setup and even a small dish. This is not a hand holding how-to, but rather an attempt to describe how we built an inexpensive effective system suitable for amateur radio satellite chasing.
SAEBRTrack (SST) consists of 1 Parallax Basic STAMP II, a Linear Technology LTC-1298 Serial 12 Bit Analog to Digital converter, and four relays that control +/- drive voltages to a rotor (or pair of rotors)--either the popular (albeit expensive) Yaesu G-5500 or the inexpensive Orbit 360 TV rotors.
The development systems used inexpensive TV rotators. With minor to no modifications, you should be able to drive any az/el system that uses +/- drive voltages and a variable resistor to indicate position.
The SST unit samples the position voltages from the two rotators. These analog voltages are converted into digital numbers by the LTC-1298 A/D converter. The position voltage from the rotators varies between 0 and 5 volts depending on the position of the rotator. This value is converted into a digital number that varies between 0 and 4095. The theoretical resolution of this system is 360/4095 or .087 degrees. This digital number is serially sent from the A/D unit to the Basic STAMP II. The STAMP is is also getting new-position data from the tracking program which it uses to compare the last known position of the rotators to the new-position coming from the track program. The STAMP compares these two voltages in order to decide whether the rotator needs to be run clockwise or counter-clockwise in order to make the rotator position value equal to the satellite's new-position value.
The Orbit 360 rotators (maybe sold as Gemini rotors perhaps?) are DC motors. This means that reversing the power leads reverses the direction of rotation. This makes direction control fairly easy using 2 relays per rotator. One relay serves as a switch to turn +12 volts on or off. The other relay switches -12 volts. The outputs from these 2 relays are tied together then go to one power lead of the rotator. The other rotator lead is connected to ground. Since both relays are never turned on at the same time, there are no problems with the two relay outputs being tied together...in a perfect world. To protect the system in the event of a 'computer crash' on the STAMP, current limiting resistors are placed in both the +12 and -12 volt power lines just in case the unthinkable happens. The STAMP II code is located in the files below. It is neither clever nor elegant and can benefit from knowledgeable programmers. The only part that might need explanation is the 'software brake'. The prototype unit exibited a fair amount of 'antenna glide' at the end of a repositioning cycle. This was due to the mass of the antennas and the lack of a physical brake in the rotators. In order to minimize this coasting effect, at the end of each positioning cycle, there is a 100 ms burst of the opposite voltage to quickly stop the coasting of the antennas. With small arrays this seems to work nicely. With large arrays, this could be hard on the gears in the rotators. You should consider your particular situation then decide whether you want the 'software brake' or you should remove it from the STAMP code.
The theory of operation for the Yaesu G-5500 is even simpler. The tracker is simply grounding the relevant pins on the rear DIN connector of the factory dual controller box. See the relevant schematic for further details.
Lastly, the name SAEBRTrack is a combination of our call signs. Gene Brigman, KC4SA & Mark Hammond, formerly KC4EBR (now N8MH). It's pronounced 'sa' as in say, and 'ebr' as in ber. You know, saber the sword. Just be thankful we didn't call it the Brigman/Hammond Serial Satellite Tracker and Radio Doppler Correction System. We can be reached via email: firstname.lastname@example.org or email@example.com. We will try to answer any questions as time and commitments allow.
The common/popular LCD displays have 3 rather obvious connections---ground, + voltage, and "Serial In." Merely connect the proper pin of the stamp (P12) to the Serial In line of the LCD display. I used a backlit one (with the serial backpack) from Scott Edwards Electronics: http://www.seetron.com/
The STAMP is simply sending out data in a serial manner to the display. Again, it's quite flexible as far as which display you pick and what the actual output looks like. Be creative!
Other folks from around the world have built tracking systems based on our SAEBRTrack design.
Ollie Eisman, AJ10, has built another fine looking SAEBRTrack box! He has some nice information on his page, so be sure to check it out here: http://www.earthrobot.com/amateur_radio_station_aj1o.html
Michael Tondee, W4HIJ, sent me a couple photos of his SAEBRTrack box (outside view, inside view) to control a pair of Orbit360 rotators. What a nice looking SAEBRTrack box!! I especially like the trim around the LCD display.
Bob Barnes, G4BGQ, an avid HRPT weather satellite enthusiast from the UK, sent us a modified version of the BASIC Stamp 2 code that he uses for his tracker. His code also displays the name of the satellite being tracked (in addition to the normal display of the AZIMUTH and ELEVATION values). He is publishing an article about HPRT and his tracking system in the Remote Imaging Group journal.
Our SAEBRTrack project has even inspired an ARRL tracking project by WA8SME that also uses the Parallax BASIC Stamp! You can read about it here: http://www.arrl.org/FandES/tbp/articles/2004/1102/index.html. Apparently there is an article by WA8SME in the September 2005 issue of QST, and FAR Circuits even carries a circuit board for it.
DISCLAIMER OF LIABILITY: With respect to documents available from this server, neither Mark Hammond nor Gene Brigman nor any of their employees, makes any warranty, express or implied, including the warranties of merchantability and fitness for a particular purpose, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.