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Any DME experts out there? — Parallax Forums

Any DME experts out there?

rmcbrmcb Posts: 39
edited 2012-05-02 07:14 in General Discussion
Hi,

I'm looking to build my own, lower powered, version of the Distance Measuring Equipment that used be a standard piece of your basic avionics package. I understand the principles fairly well but don't have an electronics background or much experience for that matter.

From what I understand based on the research that I've done, I need a transmitter/receiver that can send out an inquiry pulse on a particular frequency and a transponder that can receive the inquiry pulse and respond with a pulse of it's own, on a separate frequency. The transceiver that sent the original pulse would then receive the response pulse and calculate the time interval between when the inquiry was sent and the response was received. Using the known speed of the radio wave, the distance to the transponder can be determined.

Does anyone on this forum have any experience with this type of project and would be willing to point me in the right direction regarding where to acquire the necessary components, etc.?

Thanks much,

Ryan

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-04-24 13:15
    Ryan,

    Over what distances do you need to operate? If it's for indoor robotics, say, the time delay will be too short for any but very exotic electronics to measure.

    -Phil
  • rmcbrmcb Posts: 39
    edited 2012-04-24 13:56
    Hi Phil,

    Thanks for the response. It would be outdoors, typically a quarter mile or less. I'd like to be able to measure distances as short as 150 feet and up to 1500 feet.

    Ryan
  • xanaduxanadu Posts: 3,347
    edited 2012-04-24 17:02
    You can read a lot online about DME in airplanes. It is very simple but the transmitter is overkill for what you're doing.

    There's a way to put an XBee in loop-back mode then measure its signal strength. Do you need a bearing to the station, or just distance to the station?

    Are the distance you mentioned line of sight?
  • RickBRickB Posts: 395
    edited 2012-04-24 19:43
    rmcb:

    These chips will do the hard part.

    http://www.acam-usa.com/Time-To-Digital-Converters.html
  • rmcbrmcb Posts: 39
    edited 2012-04-24 19:54
    Hi Jon,

    Thanks for the information. I have found most of my DME information online and as you said it does appear to be pretty simple. Can you explain what you mean by overkill as far as the transmitter goes?

    I don't need a bearing but it would be nice to have, but I'm primarily looking for distance. Mostly line of sight but not always.

    I'm not familiar with the Xbee but I'll look it up.

    Thanks,

    Ryan
  • xanaduxanadu Posts: 3,347
    edited 2012-04-24 20:31
    You did say that you looked online haha, sorry. What I meant by overkill is the aviation DME system is a long range signal with low resolution (for ground stuff). When you track a DME it can be as much as 1/2 mile off. I guess you probably were not planning on building an aviation style DME so that probably doesn't matter.

    What does matter is the range and how sensitive you want it to be. So if you needed to measure 1500', would 1' increments be okay or would 50' increments be ok?

    The reason I ask, some homing beacons work on the principle of the signal getting stronger or weaker meaning you're heading in the right direction or not. A DME actually tells you that distance.

    I'm not entirely positive about using an XBee to measure distance, but you should be able to use it as a basic homing device. I started out with the XBee kit and the first exercise was to make one XBee (the station in your case) loop back to another (the robot, etc). The XBee outputs a PWM signal on a pin that can be read by your microcontroller. So in a very loose sense, after you move a certain distance you can measure the signal and see how much stronger it got. It would require some initial calibration to see those values and turn them into anything useful. Then once it was calibrated the environmental factors are going to play a major role.

    Another XBee problem is the robot would need to move and then check the signal, if the signal got weaker it would need to move again and again and compare those signals to know which one was the best route. It would also need to move a lot, because the signal strength wouldn't change much without. A magnetic compass would probably be needed too.

    Okay now I'm just thinking out loud hehe. This sounds like a fun project I think I might try it out with a couple XBees and see how it goes.

    If you need something accurate maybe find a cheap homing beacon and hack it.
  • idbruceidbruce Posts: 6,197
    edited 2012-04-25 03:57
    RickB

    Thanks for that link.

    Bruce
  • prof_brainoprof_braino Posts: 4,313
    edited 2012-04-25 05:37
    RickB wrote: »
    rmcb:

    These chips will do the hard part.

    http://www.acam-usa.com/Time-To-Digital-Converters.html

    Do you what they cost?
  • Spiral_72Spiral_72 Posts: 791
    edited 2012-04-25 06:11
    prof_braino wrote: »
    Do you what they cost?

    Surely not more than the 2k linear pixel Fairchild Imaging CCD I planned to use for the short distance DME.
  • rmcbrmcb Posts: 39
    edited 2012-04-25 07:00
    RickB:

    Thanks so much, those chips will do exactly what I am looking to do. And down to picoseconds too!

    Very cool!

    Thanks much
  • rmcbrmcb Posts: 39
    edited 2012-04-25 07:16
    Jon,

    From what I understand the 'sensitivity' of the current airborne DME is +/- 1 nautical mile, NM. But a lot of that can be attributed to the fact that the vehicle carrying the device, in this case, an airplane, is in motion at the time the slant range measurement is taken. The direction of motion can be toward the beacon, parallel to, at an angle to, etc. so the precision of the measurement is affected. I understood that if the airplane was on the ground and stationary when it took the measurement, that it would be far more precise, assuming that the ground clutter wasn't too bad.

    Ideally I'd like to be able measure down to the one foot level of sensitivity, the 1' increments that you mention above.

    Thank you,

    Ryan
  • prof_brainoprof_braino Posts: 4,313
    edited 2012-04-25 07:28
    Anybody know how much these parts cost? As in a dollar amount?
  • Spiral_72Spiral_72 Posts: 791
    edited 2012-04-25 12:35
    rmcb wrote: »
    Ideally I'd like to be able measure down to the one foot level of sensitivity, the 1' increments that you mention above.
    Ryan

    I couldn't find (in less than 5 minutes) a price for the part. I was interested as well. I'll look again later.

    For a bit of rough calculations which may or may not mean anything:

    Light travels at 186282.4 miles per second, or 983571072 foot per second

    divided that by 250ps (0.00000000025s), the supposed resolution of the device is: 0.245892768 foot or just short of 3 inches.


    Uh, surely not!? I need to check that again.
  • RickBRickB Posts: 395
    edited 2012-04-25 20:57
    From an old Sparkfun forum post...

    Postby NleahciM » Mon Nov 26, 2007 6:58 pm

    David - regarding Acam - they do have distribution. I believe "Transducers Direct" is their US based distribution. IIRC, the TDC-GPX costs ~$180 in low quantities, and the TDC-GP2 costs ~$35 in low quantities. That was about a year ago when I talked with them, so those prices may have changed.

    NleahciM
  • rmcbrmcb Posts: 39
    edited 2012-05-01 05:46
    Gents,

    Thanks for the comments and information. I'm not familiar with how to read the specs on those chips but from what I can tell the would be capable of registering/measuring timing differentials in the picosecond range. Am I reading that correctly?

    Based on the light speed calculation above:

    Light travels at 186282.4 miles per second, or 983571072 foot per second

    A device that can measure picosecond differences could be accurate enough to measure distances in feet, right?

    R
  • RS_JimRS_Jim Posts: 1,768
    edited 2012-05-01 06:38
    If my math is correct, you could measure down to about 4inches with the gp1. I would love to collaborate on a DME's project. However I have no funds for hardware but lots of ideas! I once studied the manual from an old analog Collins DME and have a pretty good idea as to how they work. I believe that a ground station can be built with a prop and data transceiver with no need for the picosec resolution. Let me know if there is any way we can work together on this.
    Jim
  • prof_brainoprof_braino Posts: 4,313
    edited 2012-05-02 07:14
    I'm interested in how this compares to the DS00VQ100 timing chip from the TOF laser thread. The ACAM chip was though to be harder to use, I'll be interested to see it compares.

    If you have continuous radio signals, perhaps you could use down-conversion and measure the distance with a slower clock?
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