[time-nuts] The Demise of LORAN (was Re: Reference oscillator accuracy)

[Mike Monett]

  "J. Forster" <jfor at quik.com> wrote:

  >> I've read and heard from this forum as well as a number  of other
  >> sources that  GPS  can  be   easily  jammed.  What  makes  GPS so
  >> vulnerable? How can it be jammed?

  > Signal strength.

  > LORAN transmitters  put  out multi-hundred  KW  to  MegaWatt class
  > pulses. Wiki  has a list. I would think a GPS bird  puts  out less
  > than 100 Watts CW.

  > Also, GPS  birds  are a LOT farther away,  especially  measured in
  > wavelengths (much higher path loss)

  > Those factors combine to make a huge difference in received power.

  > It could well be over 100 dB.

  >> From what I've heard a GPS jammer smaller than a deck  of playing
  >> cards can  easily wipe out GPS w/in a mile or more for a  week or
  >> longer.

  > John

  It should  be easy to locate a jammer. Go to the area where  the GPS
  signal is being jammed. Drive in some direction until the  signal is
  regained. Repeat to find three locations where the signal is lost.

  Three points define a circle. The diameter tells the strength of the
  jamming signal. The center defines the location.

  Once you are near the center, ordinary DF techniques  should quickly
  identify the source.

  For faster  response,  have  a number of  GPS  receivers  report the
  status of the GPS signal to a central location. This  would identify
  a moving jammer.

  It should also be possible to develop a GPS antenna with one or more
  nulls in the horizontal direction. Rotate the antenna until  the GPS
  signal is  regained.  The   null   points  to  the  jammer. Multiple
  receivers would  remove the ambiguity from antennas  with  more than
  one null.

  These techniques  should identify a jammer very quickly,  perhaps in
  hours or  minutes instead of weeks. I'm sure the  military  has some
  more advanced methods, as well as effective methods of  dealing with
  the threat.

  Mike Monett