Muzzle Velocity Doppler Radars

Discussion in 'General Airgun Chat' started by Ballisticboy, Sep 1, 2019.

  1. Ballisticboy

    Ballisticboy Active Member

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    With the increase in interest in Doppler radar chronographs I thought it may be useful to some for me to pass on some of the things I have learnt using these devices. Now I am not a radar expert or anything like an expert, I am just a bloke who has had to use them in dozens of different applications to track everything from 155mm shells down to 4.6mm bullets and explosive formed slugs from warheads. During this time I was lucky enough to work with someone who is almost certainly the most experienced operator of Doppler radars of all kinds for tracking gun launched projectiles in the UK and whose expertise is acknowledged in many other countries. I learnt a lot from him in a close working relationship based on mutual respect and a lot of traded insults on each other’s expertise.

    All Doppler radars work by measuring the frequency change in the signal being reflected from the moving projectile. This gives an accurate measurement of the projectiles velocity relative to the radar. The radar does not know where the gun is, it does not care. The radar does not know where the projectile is either, it has no way of measuring the distance from itself to the projectile, all it knows is the time since it started measuring, the time the projectile entered its beam and the velocity of the projectile relative to the radar at any time since entering the beam. Any other data has to be derived from what has been measured. It does not measure the velocity of the projectile at the gun muzzle.

    The expensive Doppler radars I was using were basically the same as the FX and LabRadar systems available today, just a lot more powerful, able to calculate more from their data and, in the case of tracking radars, able to work out where the projectile is in the sky due to having a very narrow beam and a moving radar head which enables the beam to follow the trajectory of the projectile. The fixed head radars like the ones you can buy use a wide beam to try to capture the projectile as soon as possible and keep it in the beam. Ignoring the tracking radars, the fixed head radars we used had the advantage of software which could calculate the velocity of the projectiles relative to the gun through being told where the gun muzzle was relative to the radar. So the best the sets you can buy can do is to try to work out the velocity of the projectile as if it had started at the radar.

    Suppose we had an ideal set up of radar and gun. The raw velocity data from the radar would look something like this. The actual muzzle velocity was 775ft/sec.

    dp1.jpg

    Each dot represents a data point. This would be regarded as an excellent set of data most of the ones I have seen contain far less than this with a much larger spread of the random points. At the start of the data close to zero time you can see the points measure a much lower velocity. This is when the projectile is first entering the radar beam and the angles between the projectile direction of travel and the direction of the beam are highest. The data soon climbs to the velocities we want, in the case above after about one foot of projectile travel. To calculate the velocity at time zero the software in the radar unit has to curve fit the data and extend the resulting curve back to zero. The question then is what data does the software use?

    In the case above suppose the radar uses all the data and fits a straight line to it we get one result at time zero, but, if the software ignores the initial low reading data points and fits a line to the rest of the data we get a different result as shown below.

    dp2.jpg

    The black line is the curve fit obtained if we use all the data and predicts a velocity of 773ft/sec at time zero. The red line is the curve fit if we ignore the initial data and in this case we get a zero time velocity of 774.5ft/sec. Now those two figures are not that far apart probably an acceptable error for most purposes but this is an ideal case so what can go wrong?

    The point at which the radar detects the projectile will depend on where you have positioned the radar and your gun and the directions in which they are pointing. The distance to the start of the data can vary hugely, I have seen LabRadar data which did not start until the projectile was six yards away. In our trials we also found that the weather can affect the amount of data retrieved with some days when no data could be obtained. So let’s assume you are having a really bad day, you could easily end up with data like this (I have seen worse!).

    dp3.jpg

    Now the zero time velocities are 763ft/sec for all the data and 766.5ft/sec ignoring the first data point, both significantly below the 775ft/sec true value. The problem for the user is that you won’t know if your unit is working on all the data, selected data or how much data. The amount of data will vary with each shot and each setup. I have assumed a straight line fit to the data in all cases. Some designs may use other curve fits such as polynomial. This will introduce more changes to the predicted zero time velocity and the accuracy of the prediction in different circumstances.

    Based on many years of use of these devices there seem to be a number of vital actions you have to take to ensure consistent accurate use. The setup is all important. Get the unit as close to the muzzle of the gun as possible. Carefully align the direction of fire with the direction of the radar beam. In the example above with little data I built in a misalignment of 5 degrees which reduced measured values by about 2ft/sec. Be aware that the weather may affect the amount of data you are getting, the worst results we got would be when a weather front was approaching. Finally, be consistent in your positioning and line of fire relative to the radar for each shot. Good results can be obtained from this type of chronograph if you take all the precautions.
     
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  2. PA410goodshot

    PA410goodshot Member

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    Excellent many thanks for that write up. I have a better understanding now.
     
  3. Brian.Samson

    Brian.Samson Allowed in Sales Staff Member

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    Thanks for that Miles, very interesting reading.

    If your goal is to obtain an accurate muzzle velocity reading, do these types of device offer any advantage in terms of accuracy and reliability?.

    Given that the MV must be predicted using software and the accuracy of that prediction depends on some variable factors - it seems to me that these devices possibly aren't the best choice if your goal is to obtain an MV figure for the purposes of power testing.
     
  4. gdavison

    gdavison Active Member

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    I see them as a just another tool in the tool box, I do have on of the FX ones, however for my own testing it will never replace my Air Chrony or even my Combro. My main interest was if it was it close enough to the other two & easy enough to use would it be of use at the club when we chrono members rifles and NOT get shot (again) by yet another club member :eek:
     
  5. Jesim1

    Jesim1 Active Member

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    Great write up Miles, don't do yourself the injustice of covering your expertise, your knowledge on this forum will not be surpassed, and it's great you share this with us in such an easy way to read - and with pictures for the hard of hearing :D

    I think for airguns these are a good tool, especially for down range info which you cannot otherwise easily get, but I personally will stick to a more conventional crono for my own testing;)

    James
     
  6. Ballisticboy

    Ballisticboy Active Member

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    The problem is you can never be sure how much data the reading is based on. The range where we were testing had only ever used them singly until we went there and insisted on using two devices. They had always assumed the data they were getting was accurate but when we used two devices they found there could be significant differences between the two answers. It was only when we started digging into the software that we could see the difference in the amount of data. It got so bad one day that we went to Welsh Willy's shop and bought a Cauldwell chrono to see what that said.


    As I said the secret seems to lie in accurate, careful and consistent setup. Then you only have the weather and the signal being interfered with by something else to worry about. Just don't use it in extreme changeable weather or near to metal objects such as sheds (one caused us to demolish a sea wall once). Assuming you are not operating too close to the legal limit you should not be too bad using one of these if you fire a good number of rounds.



    The only problem with using things like the LabRadar is that the down range information seems to be based on data curve fits. This means that if you try to produce down range BCs or drag coefficients the shape of the drag curve for instance will be dictated by the curve fit which has been used on the data, not the true drag variation. If a straight line fit has been used because it is the simplest it will definitely not give the correct drag variation with speed. I doubt if the manufacturers will tell you what curve fit they are using.
     
  7. Brian.Samson

    Brian.Samson Allowed in Sales Staff Member

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    So.... no real advantage over what we already have then?
     
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  8. PA410goodshot

    PA410goodshot Member

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    So more of a convenient gimmick, than a real improvement, over what is already available on the market already.
     
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  9. gdavison

    gdavison Active Member

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    Agreed .. My only thing I can see is less potential of being shot by an inexperienced club shooter .. Of course some sort of simple "rig" added to a traditional Chrono, that positions the rifle / pistol under test to direct the shot would be a cheaper solution .. Of course it would need to cater for every shape pistol / Air rifle a club sees

    But we do all love tech gadgets ;)
     
  10. Brian.Samson

    Brian.Samson Allowed in Sales Staff Member

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    I just made a box for the chrono I use at the range, it has a sheet of clear Lexan polycarbonate sheet with a large circular hole cut in it. It's impossible to shoot my chrono now and lexan sheet is much cheaper to replace when it gets shot.

    I did like the idea of a labradar for giving me downrange results but now I know how it gets those results I'd be better off with two chrono's
     
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  11. RobF

    RobF Administrator Staff Member

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    I wonder if another few would do the trick. Muzzle, 11, 22, 33, 44, 55 yd’s.
     
  12. EV2UK

    EV2UK Active Member

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    These may be a bit of a gimmick but that can read out load your fps so they may be useful for blind shooters?
     
  13. Ballisticboy

    Ballisticboy Active Member

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    The advantage of the FX chrono is its compactness but it needs to be set up carefully to be used successfully.

    At the ranges the main advantage of the muzzle velocity radars as opposed to sky screens (very large chronos effectively) is that they can be used in multiples and with the guns at high elevations. However, even when using 155mm shells, they still need to be set up carefully. Multiple radars cannot be used with your air guns as, apart from the cost, they will interfere with each other or both just read the same returning signal.

    The LabRadar does not have the compactness advantage but is being sold on the ability to give values down range but here you have the problem of how much data is being gathered and how is the range and velocity at range being calculated. Unfortunately people in the US are using the LabRadar for pseudo-scientific research and presenting their "findings" as scientific papers. They are using the radar way beyond its true capabilities and don't forget the units the US gat are far more powerful than the units supplied here due to legislation so ours are even less capable. The fixed head units we were using were way more powerful than any FX or LabRadar and even on those usable data was seldom available for more than 100 metres. The large Doppler tracking radars were a different matter of course tracking 5.56mm bullets out to two or three kilometres but they cost millions and are not healthy if you happen to get in front of one, but it was one of those I managed to use to track a couple of pellets but only at low speed.


    Triggering down range radars is always the problem and your shots would have to go very close to the FX. I would also be worried about them interfering with each other.


    That is an advantage but obviously possible from any other type of chrono with the appropriate software if it was made available.
     

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