Forum: Virtual Classroom - Tracker Calibration Procedure?

Log In via ThumbSat
via Facebook
Not Registered? Register
Log In
5 + 3 =

Tracker Calibration Procedure?

El-az tracker needs direction calibration. I mean, your software should know north direction in "local stepper coordinates" at least. It's also possible that vertical axe is not really vertical.

Do you have the procedure for calibration or any ideas how to do it? I heard that sun noise tracking technique may give you accuracy of several degrees (1 to 5 degree), but I have never tried this myself. And I don't know, anyway, if it's enough accuracy or not.Yuriy 26th Feb 2016
Hi Yuriy,

Another great question! There are a lot of ways we can calibrate the azimuth antenna for pointing north, but sun noise tracking isn't one I had considered. One of the easiest ways to point to geographic north is to use a magnetic compass and the current magnetic declination (you should find this at the time of setting up your station since it changes with time!). For horizontal alignment the plan is to use a spirit level. But like I said, that's just one way of setting up your antenna alignment, it would be cool to see how accurate you can get using the sun noise tracking technique!


Fraser Fraser James Robinson 26th Feb 2016
Let's me reformulate.

There are two main approaches that may be used for calibration. Let's call them "physical calibration" and "digital calibration".

"Physical calibration" means that you set your actuators to predefined position - to "home" position, most naturally. And then you physically move the whole device to point to north/be vertical.

"Digital calibration" means that you set up your system...somehow, not bothering about accuracy. And then you measuring the offsets and tell them to your software to compensate.

"Physical calibration" normally needs stuff like adjusting screws or something. I do not see any such pieces in your stl's. But your explanation about compass and spirit level looks like you prefer this way.

"Digital calibration" needs special software. And it needs some way to measure the offsets.

So, which approach looks more promising for you?

And, again - do you have any ideas about accuracy needed? Is it something like 5 deg or looks closer to 0.5 deg ? Yuriy 27th Feb 2016
Hi Yuriy

To answer your last question first... remember that we would like ThumbNet to be useful for other things besides ThumbSat, and the required pointing accuracy of the antenna depends on both the type and performance of the antenna, and the overall link budget for the particular use, so there is no one correct answer.

But let's be self-centred and consider that we are talking about a typical ThumbSat mission, and a typical Yagi antenna tuned at the nominal ThumbSat frequency. In which case, our link budget says that a pointing accuracy of +/- 2.5 degrees will be good enough.

Regarding calibration, this to a certain extent will be dependent on the capabilities and skills of each participating station, and will be a combination of physical and software calibration. The physical components that exist in the ThumbPointer components to help with this are the position sensors. So you position the antenna using the position sensors, and this gives the reference. Beyond that, we rely on the mechanical tolerances in the ThumbPointer.

In the long run, we will incorporate more fancy techniques. We will be able to control ThumbPointers remotely (so the station operator doesn't need to do that) and use known passes of satellites to very precisely calibrate the antenna positions.

Having said this, of course we are open to ideas! Thanks for your thoughts.

Shaun Admin 27th Feb 2016
Good enough accuracy of +/- 2.5 degrees is calculated/estimated with assumption of using of LNA or without LNA?

If you already have tested the tracker devices, can you tell me please, how accurate this mechanics are? If I put it to 124 steps from home position, then move to and fro and point to 124 steps again - what will be difference of angles? Is it well below 0.1 degree? Yuriy 28th Feb 2016
Hi Yuriy

The link budget does not assume that an external LNA is used, though for the long run we are working on customised SDR radios that have much higher performance than the current ThumbNet dongles.

The manufacturing error will vary slightly from build to build, but it can be calibrated out (e.g. by counting the steps between home position and 90 degrees from home position). The repeatability of pointing will not be well below 0.1 degrees, realistically it should be more like 0.25 to 0.5 degrees, but it will not be accumulative, i.e. the error should be no greater than 0.5 degrees, no matter how many times or how many steps the pointer is moved - within its normal range.

Yuriy, as I understand it, we may be sending a complete ThumbPointer kit to your group, in which case you will be most welcome to experiment and provide us with feedback from your own tests :-)

Shaun Admin 28th Feb 2016
Most small amateur radio satellites (such as cube-sats) transmit a little more power than the thumbsats, so pointing accuracy is not as important. For instance, with a five element yagi antenna, with point errors of 5 degrees is tolerated with little or no loss of signal. The higher the gain of the antenna, the narrower the beam-width. So, the gain of the antenna will determine the pointing accuracy.

ie: a low gain yagi with a preamp mounted at the antenna will have a wide beam-width compared to a high-gain yagi antenna. The actual cost of either antenna system is approximately the same, with the low gain yagi having an advantage over a high gain yagi of a couple of degrees or more tolerance in pointing accuracy.

bobbie Bobbie 1st Mar 2016
How much increase in cost if the tracking station used gps receiver to determine location and pointing accuracy??? Much like newer telescopes, which calibrate themselves to extreme accuracy.

bobbie Bobbie 1st Mar 2016
Bobbie - regarding the antenna, beam-width, pointing accuracy etc, that's exactly how we see it. However, before we add in an LNA, we're looking at ways to maximise signal-to-noise ratio, so that we're not just amplifying noise. The ThumbNet dongle will be in the ThumbPointer housing, very close to the antenna, but also close to the rest of the electronics, so that needs to be carefully managed.

When designing ThumbNet ground stations, we have worked very hard to drive costs down while maximising performance. This is essential, because we are giving away 250 stations plus spare parts for free to anyone who can demonstrate that they will be useful to ThumbNet! It's also important to keep costs down because we want everyone to have a chance to build their own ThumbNet station, whether or not they participate in ThumbNet.

So... right now, adding a GPS would add about 10% to the cost of a ThumbNet station. I'm not sure we need an integrated GPS for antenna pointing, because position and orientation (to the accuracy that we need) can be determined as well by other means, but if someone asks an interesting question like that, we will always consider it - thanks!

Shaun Admin 1st Mar 2016
Several amateur radio ops are mounting the RTL at the antenna and running 50 feet of USB cable to the computer. Longer distances can be used by adding an USB extender... two types are available.. a 'relay' chip in the USB line, and USB/CAT5 converter chips at the dongle and at the computer. I have used the USB/CAT5 converter up to 150 feet with good results... if needed, I can post a link to amazon or ebay...

This eliminates a lot of RF noise.

If you have time, and patience, a gps pointing system add-on for a fee would be nice...

Bobbie 1st Mar 2016
Hi Bobbie

It took me a while to work out how to answer this because you seem to be talking about what we are planning anyway. By having the ThumbNet dongle (or the planned dongle replacement) within the ThumbPointer housing, we will significantly reduce the sort of noise associated with houses, laptops etc. because the radio is away from those things. We're not concerned about the distance between ThumbPointer and the house/router, because the links (USB, ethernet, wifi etc) are fairly robust, especially if we use the sorts of techniques that you suggest.

However, we *do* want to be careful about the electronics in the ThumbPointer, because the single board computer and stepper motors are close to the antenna, coax and ThumbNet dongle. It's a potentially noisy environment, and it's far better for us to spend time looking at how we reduce that noise rather than just plonking an LNA in and hoping that it will be useful. The brilliant Anthony Stirk explained this far better than I can, here . We have very short distances involved compared to Anthony's example, so it's not instantly clear whether there is much advantage in having an LNA close to the ThumbNet dongle or close to the antenna. We're looking into it.

There's an interesting example of the benefits of an LNA here and a good discussion here .

Shaun 18th Mar 2016
Thanks Shaun

Very well stated..

A couple of points of interest:

You are correct in that the dongle should be as far away from home electronics as possible. Which in most cases is at the antenna feed point. But mechanical problems need to be considered. Also, the dongle is not shielded and will act as an antenna in itself. So, to lower the noise floor and increase the IP3, the antenna driven element should be at ground potential and the dongle shielded with a Faraday shield which is grounded via the same ground as the antenna. This requires a different antenna than the one you are suggesting. A DC grounded antenna with the antenna boom made of conductive metal and the elements electrically/mechanically connected to the boom, which requires a delta feed point of some type... some are very simple. This antenna system will lower the noise floor as most static noise is sent to earth ground at the antenna and not transmitted to the dongle and computer. It is mostly immune to static crashes.

However, the open air yagi that you recommend can be used if a unun is positioned at the feedpoint of the antenna and the winding feeding the dongle has one end grounded. This will effectively drain most static charges to ground and not to the dongle.

The dongle can be placed inside the ThumbPointer if it is properly shielded. A Faraday shield made of conductive tape, such as aluminum or copper can be wrapped around the dongle and properly grounded. The conductive tape must not contact the USB shield. Of course, a properly made metal shield for the dongle would work better, but more expensive. NOTE: I have used aluminum tape to shield several dongles and put them on top of my Dell computer and have no RF noise from the computer.

The dongle has plenty of gain without a LNA if used with a proper antenna. As posted by Youseff and Leif on the SDR# yahoo forums, the LNA will lower the noise floor and increase IP3 if the antenna is faulty or the feedline is faulty or too long. So with short lengths of coax - less than 0.1 wavelength- from the antenna to the dongle, a LNA is not normally needed. Note that 0.1 wavelength at 401Mhz is rather short, so the dongle will need to be mounted within a few inches of the feed point of the antenna.

Adam Alicajic's article at posted above is an excellent article. Adam designs very good LNA and I have been using the LNA4ALL for about a year. I can attest that placing the LNA within a wavelength of the dongle will deteriorate the signal and lowers IP3.

One major problem with the dongles is that the IP3 can be destroyed by strong commercial stations which are far outside the bandwidth we select in SDR#. This can also occur with a low power signal close to the antenna or dongle. The electronics in the ThumbPointer may overload the front-end of the dongle and cause strong artifacts that fall withing the bandwidth of SDR# or overload the analog/digital chip which will produce an unusable output of the dongle. This can be measured in the lab, which I am sure you are aware.

We must be careful in using a LNA as it can very easily destroy our intended signal. A bandpass filter may be required between the antenna and dongle to block strong commercial stations so as to not overload the A/D section of the dongle.

and lastly, ferrite beads work wonders on power lines, USB cables, CAT5/6 cables. Shielding is the name of the game after building the best antenna within our means.

Bobbie 19th Mar 2016
Thanks for taking the time to type so many words, Bobbie! All very useful, especially your own experiences of using the LNA4ALL.

As I wrote elsewhere, I hope that development of ThumbNet stations will be in two parts:

1. One where all you guys experiment to find out what works best (and we are certainly happy to donate hardware to enthusiastic experimenters). We expect that most people will be doing this with whatever comes to hand, and according to their own preferences, and that some good, solid, useful results will come out of it.

2. The more formal path that we will take to ensure that ThumbNet stations are as good as they can be for receiving ThumbNet data. We'll take the results from 1. and use best practices to continue to evolve the complete stations (that we donate to experimenters) in a very controlled manner.

So what's my point here?

My point is that in the background, we are working on assembling a kit of parts and techniques for ThumbNet, partially based on what we know already, and partially based on what you guys are telling us. One example is that we will experiment with an off-dongle LNA for *our particular arrangement* to quickly find out if it is worth having, and if so - where.

The second, more important example (still confidential) is our new design of "dongle" that can be specialised for ThumbNet, but also useful for other SDR experimenters. We will incorporate more narrow filtering and probably localised shielding rather than putting the whole device into a metal box or covering it in tape.

There. It took me a long time to get to that point, but that's it. For our ThumbNet-specific stations we will filter and shield locally. Which is exactly what you were saying anyway :-)

Shaun 19th Mar 2016
Thanks Shaun

One option for shielding the dongle is 'painting' the inside of the dongle shell with 'liquid' aluminum alloy or some mixture of sliver and copper paint. I'm sure there are better materials to use, but these are available to me at present. Placing a small stranded copper braid to the painted shield via a drop of the paint and soldered/mechanically pressed to the coax shield connector..

thanks for reading my long post. Bobbie 19th Mar 2016
Bobbie - no, thank *you* for writing the long and thoughtful post! I just wish that this was under a more useful heading than "Tracker Calibration Procedure"! As we improve the forum we'll start moving posts around to the most useful places.

Again, good thoughts about the conductive painting. We were planning to just go with conventional metal "cans" around the key components to avoid accidental short-circuits, but it's good to know about alternatives :-)

Shaun 19th Mar 2016