Pointing an EME Antenna
K0YW and K5GW
Click on callsign for e-mail

The Tracking Problem

1.  Target size and speed:

  •  Moon is approx ˝ Degree Wide.
  •  It Moves at an approx. rate of 14.4 degrees/hour.
  •  Its Declination can range plus and minus 25 deg above and below the Celestial Equator during the Lunar Month.
2.  Required accuracy of Tracker:
  •  Depends on antenna pattern width.
  •  Should be able to hold center of pattern on the target center.
3.  Mount Requirements:
  •  Must physically support the antenna.
  •  Must maintain target boresight in all expected environmental conditions.
4.  Polar vs AZ/EL tracking:
  •  AZ/EL has primary advantage of wider tracking range.
  •  Most Polar Mounts are restricted in Hour Angle and Declination range.
Tracking System Options

1.  Manual vs Auto-track:

  • OK for low gain systems where frequent updating is not required.
  • Good accuracy of readouts is still necessary.
  • Set and Forget Auto tracking is still the only effective way to go for large antennas and narrow beam width microwave EME systems.
2.  Position Sensing Systems:
  • Analog systems, based on potentiometers and op-amp, wheatstone bridge, or analog/digital voltmeter readouts work well in low to medium precision systems.
    • Repeatability is an issue as well as thermal drift.
    • Frequent calibration is necessary for real precision.

  • Selsyn based systems also work, but are typically limited to accuracies of > 1 degree.
  • Digital systems employing encoders are now the method of choice for high precision systems with auto track capability with high precision and repeatability.

3.  Digital Encoders, Incremental vs Absolute:

  • Incremental Encoders are non-contacting rotary position sensors which report a change in shaft angle to external decoding electronics.
  • The A2 family of Absolute Digital Encoders use a non-contacting optical rotary position sensor which reports the shaft angle over a 360 degree range. but possess on board µP and counter electronics that establish the absolute position of the device.
    • Since the position data output is dependent upon the position of an optically scanned disc, the position data is not lost under power off conditions.
    • An internal EEPROM keeps track of the encoder's programmable parameters, such as: Resolution, origin, zero degree origin, rotation direction, data rate and scaling.

4.  Computer vs µP Control:

  • Most auto tracking systems still require a computer to house the software to read the antenna position as supplied by the decoders and compare it to the target position established by the programs target position algorithm.
  • The computer then determines the differences in AZ and El Position and commands the control portion of the system to operate the AZ and EL tracking drive systems to bring those position differences to zero.
  • Most µP based systems do not have enough available memory to house the target position algorithms. They are still employed to provide accurate position readout info to a manual tracking system.
  • The GW Moon Track System Utilizes the US Digital SEI BUS (serial encoder interface bus) to directly read out the encoder position information within the computer and compare the resulting position data to the target position info being generated by the internal position algorithms.
  • The result is a plug together system that works extremely well with little external electronics.
System Implementation

1.  Absolute Digital Encoders -Antenna Position Sensing

2.  GW-Moontrack software - Antenna Position Decoding, Target Location, Target Tracking, Antenna Control Housekeeping.

3.  W5LBT µP card - Gates position update data to Positioner Motor Control card.

4.  Computer Screen - Target Position Readout, Antenna Position Readout, Control Menu and Target Selection Menu Options.

5.  Control Box - µP and Motor Control Cards, Card Power Supply, Manual Control Switches.


          1.  AZ Encoder: US Digital, A2-S-S

          2.  EL Encoder: US Digital, A2I or A2T ("T" version
                is more a recent, improved and lower cost design)

          3.  Serial Encoder Interface (SEI) Bus

          4.  AD2B interface adapter (9 pin- RS-232)

          5.  AD2A interface adapter (25 pin- RS-232)

          6.  PS-9V SEI BUS 9VDC Wall wart Supply

          7.  RJ-12 Modular shielded data bus cable

          8.  RJ-12 Modular “Y” Jack (CON-MD6-2J)

          9.  GW-Moontrack adapter cable.

          10.   W5LBT 9600 Baud US Digtal Control Card
                     click to e-mail W5LBT to order this card

All component Part Numbers are available from US Digital Inc, Vancouver, WA
http://www.usdigital.com/ or tel: (800)736-0194


GW Moon Track RS-232 Interface Cable

Motor Control Driver & Relay Circuit

The circuit to the left is available in kit form from Down East Microwave (DEM) in a times 5 configuration. Order their P/N: DEM PTT-X5 KIT. For control of dish azimuth and elevation, use 4 of the circuits (#1 for CW-AZ, #2 for CCW-AZ, #3 for UP-EL, #4 for DOWN-EL). The remaining circuit can be used with your transceiver to key an external amplifier (or whatever)! Don’t forget that the relays are small DPDT units that are only rated to switch 0.6 amps per section. This is plenty to key the 12 or 24VDC coil voltage of your real motor control relays up on the antenna, next to your AZ and EL drive motors. Make sure these relay contacts are properly rated for the motors used. I use DPDT relays with contacts rated at 10A with both sections wired in parallel to increase my capacity to 20 Amps, just for insurance. The circuit to the right is a typical relay connection diagram for the DEM relay control board. Four are used, as indicated (CW, CCW, UP, DWN).

This is a typical relay connection diagram for the DEM relay control board; Four such are used, as indicated (CW, CCW, UP, DWN).

Moontrack Software & System Set-up

Software by K5GW
Click on callsign for e-mail


The Initial Screen is what you see when you first start the program. It may be called the Program Start screen. It is pretty intuitive. Its purpose is to let you run the program or go to the portion of the program that lets you set the time and date for your location. The system reference exits the program.


The SET-UP data screen is complicated looking, but is actually pretty straight forward. Your call and the LAT-LON data is easy; +/- refers to W/E and N/S, with no sign entered for "plus". In fact, ALL numbers have an implied plus sign unless they are negative. For example, 29 deg NORTH lat is "29" while 29 deg SOUTH lat is "-29". Note that lat-lon of your position is entered in DECIMAL DEGREES.

You can set the AZ and EL resolution at 3600 (0.1 degree) or go higher to 0.01 degree 36000 to 1. All that really does is give you a blinking digit, as most drive systems are not tight enough to do much better than 0.1 deg.

The AZ and EL rev is used in cases in which the encoder mounting causes the rotation direction to be backwards from the directioin of the drive. For example, if a gear is used to drive the encoder, or if the encoder is belt-driven and mounted and mounted upside down for better rain protection, this "reverse" will restore proper readout direction. This parameter can also be stored in the encoder setup EEPROM.

The AZ cal and EL cal should be initially set at Zero. These numbers can be changed during calibration to make the readout rate agree with antenna position data. The <Z>ero command provides a way to calculate the Cal numbers and enter them in the setup automatically.

The AZ spd and EL spd are numbers used to determine the time required to run the AZ and EL motors while in the autotrack mode. The numbers are approximately equal to the drive speed in degrees per second; they will be fine-tuned in operation to alloow proper position changes without overshoot or undershoot. Numbers too small will make the motor run longer than necessary, resulting in overshoot; numbers too large will cause the motors to cut off early resulting in undershoot

UTC is negative for E lon and positive for W lon. All numbers in hours ie: 5 for CDST. The rest is simple setup; remember to select the right comm port!

The cntl A and B rev swaps the cw/ccw action of the AZ-EL drive motors. It could, for example, be used if the drive wiring was done backwards; this setup item allows the user to run the system properly. Most users would likely prefer to correct the wiring error rather than leave it as-is, so this feature may be deleted from future versions.

The AZ and EL scale allows use of encoder drive arrangements that are not exactly 1 to 1. An example would be use of a belt drive between the AZ mast and a pully attached to the AZ encoder. The numbers used here can be determined by carefully measuring the diameter of the AZ shaft and the pully, or by observing the aiming error in a westerly direction after calibrating at an easterly direction. Sun noise measurements are invaluable in checking the results of these tests and adjustments.

The bottom line is up to you; your GMT correction for your computer clock. If GMT time is used for everything, set it to zero.

The next 4 entries are electronic limit positions. The autotrack will disengage when the antenna position reaches any of these limit numbers. The motors will still run, but only under manual control.

The Hysterisis windows allow you to set the width of the update window in each drive axis in both the Manual and Auto track modes. That is when the target position and antenna position difference exceeds the programmed amount, the dish will update. This number depends on your dish size. A 10-12 foor dish can use a number on the order of 0.9 degrees. My 30 ft dish needs 0.3 degrees. Too narrow a window will cause the dish to update, overshoot too far and try to recover by going backwards til it stops. If it coasts back past the lower window edge, it will drive forward again, and so on, ‘til you break something.


The business end is the Run Screen via an initial "averaging on/off" screen. It displays the actual tracking data, your antenna position, the doppler shift for the band you have selected. The three test lines at the lower left really only are pop ups at the screen initialization to let you know that your encoders are working OK along with a good comm port.

The line across the bottom of the screen is the tool bar or command line. It gives you the options that you might want while you are tracking.

<esc> lets you stop drive rotation during the manual aiming process (see <M>anual below) or in case there is a runaway situation. Theoretically the latter should never happen, but better safe than sorry!!

<E>xit will close the program when you want to leave and shut it down.

<B>and selects the band that you are operational on. It is also displayed on the doppler line as shown (1296, 144, 432 etc).

<A>uto toggles the autotrack on and off. The antenna position must be within 10 degrees of the target for the autotrack to engage. An error message will appear if this key is pressed and the aiming error is greater than 10 degrees.

<T>arget toggles the target selection screen on and off. This command allows selection of the Moon, Sun, noisy areas of Cassiopia (Cas), Cygnus (Cyg) and Saggitarius (Sag), Leo for cold sky, Aquarius (Aqu) for cool sky, and the park position.

<M>anual allows the user to run the drive motors to a desired AZ/EL position in preparation for engaging the autotrack function, parking the antenna, or making noise and calibration tests. When first selected, the AZ and EL numbers associated with the selected target are displayed; these can be selected as "manually targeted" AZ-EL by pressing the <enter> key once for AZ and once for EL. If different AZ-EL numbers are desired, they can be typed into the data fields over the displayed values. The motors will automatically start and run until the antenna is within a degree of the chosen AZ-EL. Pressing the <esc> key will abort the action and return AZ-EL control to the user.

<U>pdate forces the autotrack to run the drive motors immediately without waiting for the normal twice-per-minute run cycle. this command is useful while testing and adjusting the AZ and EL speeds during setup. It can also be helpful when the autotrack is first engaged and the system is adjusting drive positions for minimum tracking error.

<Z>ero is used to automatically compute the AZ-EL cal numbers in the setup. A typical use would be pointing the antenna to peak sun noise while observing the sun position AZ/EL data. If the AZ-EL numbers are not identical, this command will alter setup data to force equality. The user is given the option of storing the correction for subsequent system use or not. This command will not compensate for problems in drive ratios or out-of-plumb issues issues or other mechanical fualts with the drives or dish support structure.

<O>stn stands for "Other" station; it allow the user to look at another station's window. Pressing <O> opens a smaller box that lets you enter the other station's data. The first choice is a callsign. If the callsign is in the uhfmic.dta file it will extract his grid locator and then reverse calulate the lat/lon for use in the program. If the callsign is not in the data base then you are prompted to enter a grid locator. If you don't know the grid locator, press <enter> and it will prompt you for a lat/lon entry. You can also press <enter> to skip the callsign entry. The <O> functions toggles on and off like the <T> function. When it is on you can <E>xit the box and use the <P>lan function to check for common moon windows.

<P>lan allows one to look at where the selected target is for various dates, showing target rise and set times for each date selected with the up/down arrow keys.

System Performance

1)  Tracking Accuracy:  Plus/Minus 0.1 degree
2)  Readout Resolution:  Programmable from 0.01 to 1.0 Deg.
3)  Repeatability:  Absolute encoder position accurately reads to limit of the programmed resolution and limit of the mechanical stability of the drive/mountings.
4)  Hysteresis:  The amount of “Motion Window” allowed before the system will update the antenna position. This parameter is programmable in the System Setup. Typically, it is set to 0.3 deg for antenna beamwidths from 1.0 to 1.8 degrees.
5)  Calibration:  Az-EL calibration is done initially by positioning the antenna to a known position and setting the encoder positions to agree via use of the USD SEI programming software. The AZ-EL calibration can be fine tunes by peaking the antenna on sun noise and using the <Z>ero command. This procedure will compensate for small errors introduces by drive geometry and the feed position.

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