How to Align your Polar TVRO Dish


    Due to the many requests on the internet for guidance with respect to how to properly align a polar TVRO dish mount, I have put together the following recommendations. There are other techniques which will yield acceptable results, but if followed properly, this technique will provide accuracy far better than is required to track the Clarke belt, and generally better than most consumer systems are capable of resolving.

    First a few comments and definitions. The geostationary TVRO sats are located about 22236 miles above the surface of the earth
(ie about 26200 mile radius) in the plane of the equator. Some of  the TVRO sats are geosynchronous, but not geostationary, ie their
longitude is nearly constant, but their orbit is inclined so that they drift north and south of the equator once per day. Since a
properly aligned TVRO dish only looks at the sats in the equatorial  plane, this means that without some form of elevation control, that
these will only be visible twice a day for about an hour each, as they cross the equator. Most of the sats can be tracked without elevation
control.

    If the sats were infinitely far away, a TVRO mount would be aligned like an astronomical telescope, ie the rotation axis of the mount
would be aligned parallel to the earth’s axis (ie pointing toward the north star), and the dish would be aiming perpendicular to this axis,
ie parallel to the equatorial plane. Since the TVRO sats are not infinitely far away, you cannot sight your dish parallel to the
equatorial plane, but you must tilt the dish down from this plane slightly, by an angle referred to as the DECLINATION ANGLE. {Sorry for amateur graphics}



 For a sat due south of you, this declination angle can be estimated by the arc tangent of the height of your position on earth above the equatorial
plane "h", divided by the distance parallel to the equatorial plane to the sat , which is 22236 + (3963 - "a") where a is the distance from
your position on earth to the polar axis of the earth. This method (ie aligning your mount rotation axis parallel to the earth's axis, and dropping the aim angle
by this declination angle below the equatorial plane) is adequate for C-band with a 10' dish, and is OK for Ku with a 3' dish, but unfortunately is NOT satisfactory for tracking the Clarke belt for Ku with a 10' or larger dish. The reason for this is that for sats to the east or west, the distance to the sats is greater (ie 22236 + 3963), and thus the declination angle will be less. 
    Typically, for temperate latitudes, the declination of a sat to the south might be around 6.4 degrees, and the declination of a sat to the
east or west might be around 5.7 degrees (assuming a latitude of 41 degrees).  So you see, that if you set the inclination of the motor axis to 90-your latitude, which is the most common recommendation, then, you will be off the arc by some 0.7 degrees when aimed at a sat near your eastern or western horizon. This amount isn't too significant for C-band with a big dish, or Ku with a small dish, but is VERY significant for Ku with a big dish.
     Sats in between are of course intermediate. One popular technique is to use a declination in between these two extremes, ie
about 6 degrees in the above example. This will provide accuracy to about 0.3 degrees or better in most cases, which is good enough to get
excellent results with C-band satellites, and good to fair results on most KU sats, and is in fact one of the most popular installation
techniques.
     HOWEVER, if you want to track the belt more accurately, it is necessary to compensate for the changing declination angle. The easiest
way to do this, is to set your declination angle to that of a westerly sat (ie 5.7 degrees in the above example). If you do this, you will be
in alignment for westerly sats, but pointed too high for southerly sats by about 0.7 degrees.  To compensate for this, all you need to do is to tilt the rotation axis of your mount down slightly towards the southern horizon, ie in the above example, you would tilt the axis by about 0.7 degrees. See the figure for
clarification, in which the downward tilt gives you the "Adjusted mount axis". This adjustment will bring the southerly sats into focus, and since you are in effect rotating your mount axis around an axis pointed east/west, it will NOT affect the alignment previously obtained for westerly or easterly sats, and intermediate sats will be affected by amounts between zero and the full 0.7 degree amount. It has been demonstrated that using this technique properly should allow you to have alignment errors of less than a few hundredths of a degree, ie much more precision than you need to get excellent results on both C and KU bands.

    The first step to aligning your dish is to determine the angles for your latitude, ie the declination angles and the amount of tilt required to
align your mounts rotation axis with the earths polar axis, (which I call the "latitude" angle, and the adjusted latitude angle). It is also
important to very accurately determine "TRUE SOUTH" at your site. True south is NOT the same as magnetic south that is observed on a compass,
but rather that value plus or minus the magnetic deviation. You can determine your magnetic deviation using pcgeomag3.exe, a dos program
available from the previous page in this site, or at http://www.ngdc.noaa.gov/seg/geomag/data/geomag/geomag_v53.zip, or by looking at most
USGS maps of your area. However the best way to find true south is to find out the time of true NOON at your location, and look at the direction the
shadows from your pole are aiming. At true noon, the shadow should be aiming north, so if you extend a line through this shadow through the pole
towards the sun, it will be aiming south. This is the direction that the MOUNT should be aligned with (ie find a flat surface along the side of the
mounts rotation axis, and make sure that it aims exactly south. This is a VERY critical step, but it will be tweaked later if you are off a bit. To find the time of
true noon, you can either (a) bisect the time between local sunrise and sunset for your location (b) use the server at http://aa.usno.navy.mil/data/docs/AltAz.html to find the time when the sun is at an azimuth of 180 degrees, or (c) use the program TVCALC,
available at http://megalink.net/~`wejones/tvcalc43.zip , which will calculate the azimuth/elevation of the sun as well as sunrise/sunset and
true noon for your latitude and longitude. The program TVCALC will also determine the proper declination angles as well as the latitude angle and
adjusted latitude angle. . If you cannot run TVCALC, the following table can be used to estimate your angles:

Latitude		27.5	30	32.5	35	37.5	40	42.5	45	47.5	50
90-latitude		62.5	60	57.5	55	52.5	50	47.5	45	42.5	40
Decl	South	4.6	5	5.3	5.65	6	6.3	6.6	6.8	7.1	7.3
Decl	west	4	4.36	4.7	5	5.3	5.6	5.9	6.15	6.4	6.64
Delta	Decl	0.6	0.64	0.6	0.65	0.7	0.7	0.7	0.65	0.7	0.66
psuedo	lat	28.1	30.6	33.1	35.7	38.2	40.7	43.2	45.7	48.2	50.7
psuedo	90-lat	61.9	59.4	56.9	54.3	51.8	49.3	46.8	44.3	41.8	39.3

    After your mount is aligned to the south, to get your mounts rotation axis parallel to the earth’s axis, you use the latitude adjustment on the mount
to tilt the rotation axis back from the vertical by an angle of 90-latitude (or actually eventually 90- the adjusted latitude). If this is done
carefully (often before the mount is even on the pole), it may never have to be touched again. If done with the mount on the pole, the best way is
to use an inclinometer mounted on some flat surface of the mount that is parallel to the rotation axis. Some inclinometers measure angles vs
the vertical, some vs the horizontal, and some both, so make sure whether you are measuring the 90-lat angle or the latitude angle.  Note, it is usually quite easy to fine a surface parallel to the mount rotation axis with a big TVRO dish, however with the new smaller (3' and less) offset dishes, this is not easy.  For these, please refer to http://www.megalink.net/~wejones/Small_offset_issues.html after reading this page.

    At this point, you should set the declination angle on the mount to the "Decl west" value. If the dish is on the mount and pole, you can
often find a flat surface either parallel or perpendicular to the surface of the dish, and use an inclinometer as above. What you are
looking for is an angle for the surface of the dish that is less than the 90-lat value (that you just set for the rotation axis ) by
a value equal to the declination west value. (Again, this will not apply to small offset dishes, as above)  On some dish mounts, the declination angle is set via an adjustable nut/bolt. On other dish mounts, the declination angle is set via multiple locations of the bolts that connect the dish to the mount. On the Orbitron "spinclination" type mounts, the declination angle is adjusted by loosening the bolts at the dish hub assembly, and rotating the dish around pointing axis.
This is possible because the pipe coming out of the dish hub is not perpendicular to the dish surface, but rather tilted by about 8 degrees,
allowing the dish to aim anywhere from 8 degrees above the equatorial plane to 8 degrees below the equatorial plane. Ie a 90 degree rotation
of the dish will provide 8 degrees of declination and 45 degrees rotation gives 4 degrees, etc. Spinclination allows a VERY accurate adjustment of declination, but is annoying because it also affects polarity.

    Once the declination is set, don’t change it again. If it is pretty close, it will be OK, especially on the spinclination type mounts. At this point, you should aim your dish south via the IRD motor controller, and scan around that area with the motor, until you find the sat which is closest to being due south of yout location (ie the longitude of the sat is close to your longitude, or the azimuth angle of the sat is 180 deg). When you find such a sat, peak it as best you can with the IRD, THEN perform final tweaking of the latitude adjustment on the mount to get the best signal. This in effect will perform the operation
of tilting the rotation axis by an angle of about 0.6 degrees toward the south horizon, to give the proper 90-adjusted latitude angle. Doing this adjustment while looking at the TVRO signal is the most accurate way to adjust this angle.

    At this point, if you had accurately aligned your mount toward the south in the very first step, you would be done, and would be tracking
all the sats in the arc, however, as mentioned above, the south pointing alignment is VERY critical, and hard to get accurate using noon shadows
or a compass, so when you try to scan east or west via the IRD motor control, you will probably find that the further east or west you scan,
the worse your reception will be. What you want to do, is scan to the sat which is the most distant from your southerly sat, but which you can
still see with the current adjustment, ie it will be coming in with a very poor signal. Loosen the bolts holding the mount to the pole, and
VERY carefully make VERY SMALL adjustments to the south alignment of your mount (ie rotate the mount on the pole). This is a VERY fine
adjustment, usually about the thickness of a pencil line drawn on the pole for alignment (this is recommended to help get back to your old
adjustment if necessary). After each small adjustment, re-tweak the position along the arc with the IRD motor control. This is VERY important, because often if you make these adjustments, the signal doesn't always get better, even though you are adjusting in the right direction.



 Ie, as you see above, if you make this e/w adjustment, it can sometimes actually take you away from the sat you are trying to peak, even though you are getting closer to the arc, which is really the purpose. So don't expect improvement as you rotate on the pole, but look for improvement AFTER you run the motor back and forth through the signal.   It would be best not to save the sat positions yet. If they have already been saved, you can usually "re-sync" the arc with most IRD’s, which changes the positions of all your sats by the same amount. This re-sync is necessary because adjusting the south alignment of the mount will change the apparent position of the sats in the arc. Once you have repeated these small south alignment adjustments followed by arc re-syncing a couple times, you will probably have adjusted the sat you are on so as to give very good reception. At this point, if you are not already at the sat closest to your horizon, continue on to the most easterly or westerly sat, and repeat this process of peaking by rotation on the pole, however this often will not be necessary.
   At this point, you should be done.  In theory, at this point, if you have followed the instructions properly, you should be able to go back to the southerly sat, and it will still be aligned properly. If it is not, it is usually correctable by re-saving its position in the arc at the IRD. If you cannot get
acceptable reception on the southerly sat, it either means that you  weren’t following the instructions properly, or started too far off
in your initial estimates of the various angles, OR, it could be that your pole wasn't plumb. Check the various angles on the mount with the inclinometer to make sure they are correct to the best you can determine with the inclinometer. If they are significantly different, make the appropriate adjustment (ie to the
latitude adjustment or declination angle), and start over from scratch. Often, however, you can just make a minor adjustment to the latitude
adjustment, which will bring in the sourherly sats, then proceed directly to the most east or west sat, and re-adjust the south mount alignment
(re-syncing along the arc each time) to bring in the east or west sats, but if your pole is significantly out of plumb, it could take a few back and forth repititions between your south sat and your horizon sat to get aligned. At this point, you should be done. Tighten up all the bolts on the mount, and it is often advisable to drill a hole through the mount into the pole, to permanently fix the south alignment of the mount on the pole, but either leave some room for future adjustment, or be sure you have it right before making this permanent.
    Note, the above assumes that the orientation of the feedhorn has been set properly so that the polarity of the sat signals is nearly correct.
If you have an Orbitron spinclination type mount, you should remember that if you should ever re-adjust your declination angle (this really
shouldn’t be necessary), that this adjustment will change all polarity angles you may have set. Some people trying to make fine adjustment to
the declination angles end up making things worse because often they are changing the polarity in addition to the declination. Also the
spinclination declination adjustment changes the apparent position of the sats in the arc, so if you have saved sat positions before
performing the adjustments, you will have to re-sync the arc after any change to the declination angle.
    The above recommendations do not include any discussion of positioning the feedhorn relative to focal length, etc. The program
TVCALC mentioned above does the simple calculations to determine what the focal length should be, from measurements of the dish width and
depth, but it is best to follow the recommendations of the dish manufacturer in this respect, as sometimes the dish shape may make
accurate determination of the parameters difficult. If the dish manufacturers recommendations are not available, use the calculations
in TVCALC as a starting point, and make adjustments to the focal length to get the best signal possible.

Good Luck