Amateur Radio operators normally operate at power levels much below commercial broadcasters, even with our 1.5 kW maximum power level.  Many hams like to operate QRP (5 watts or less) and still get very good signals when band conditions are right.  How far can we go to communicate with low power or poor propagation?  Several projects use advanced circuit and software techniques to allow you to dig signals out that seem to be well below the noise level.

• Coherent CW is a technique that employs stable and precisely calibrated transmitters keyed at a known "clock rate".  This allows receivers to use extremely narrowband filtering that increases the signal to noise ratio to a usable point, even if the sending rate is very slow.
• QRSS is a very slow speed (QRS) CW mode, designed to be used with narrowband filtering, but it is not based on signal coherence. 
• WSJT and WSPR are special digital weak signal modes developed by ham radio's own Nobel laureate, Joe Taylor, K1JT.  WSJT (Weak Signal - Joe Taylor) mode provides several modes optimized for meteor scatter, troposcatter, or EME (Earth - Moon - Earth, moonbounce) communication.  WSPR (Weak Signal Propagation Reporter, "whisper") is designed to monitor propagation paths on the MF and HF bands.

• ARRL's VHF Digital Handbook, 2008, Steve Ford, WB8IMY
  Includes WSJT for meteor scatter and moonbounce
• The ARRL Operating Manual, 9th Ed., 2007
  Information on using WSJT.

General

• VHF/UHF/Microwave/Weak Signal (Operating Specialties - ARRL Web)
• Weak Signal -- (VHF-DX, Meteor Scatter, EME-Moonbounce) (Radio Technology Topics, ARRL Web)

Coherent CW

• Coherent CW
  QST September 1975, pp. 26-27
  Amateur radio's new state of the art?
• Coherent CW -- The Concept -- Part 1
  QST May 1981, pp. 11-14
  Would you think that you could decrease your transmitter output power by a factor of 10 and increase signal readability by the same amount -- simultaneously? It's being done now.
• Coherent CW -- The Practical Aspects -- Part 2
  QST June 1981, pp. 18-23
  You'll now see how you can put the concept into practice.
• A DSP Version of Coherent-CW (CCW)
  QEX February 1994, pp. 25-30
  Obtain the advantages of coherent detection of Morse signals using DSP.

QRSS

• A DDS Based QRSS (and CW) Beacon, Matteo Campanella, IZ2EEQ
  QEX, Sep. 2007 p 24
  Letter: QEX, Nov 2007, p 60

WSJT and WSPR

• WSJT: New Software for VHF Meteor-Scatter Communication
  Joe Taylor, K1JT, QST December, 2001, pp. 36-41
  With this revolutionary software you can communicate over distances up to 1400 miles by bouncing signals off the ionized trails of ... tiny meteors.
• The Accidental EME QSO
  Dave Scroggins, W7OJT, and Mike Hasselbeck, WB2FKO, QST November 2005, pp 52-53
  While trying to set up a tropo scatter contact, the authors instead discover EME!

Coherent CW

• Coherent CW: Some Theory, Ernst F. Schroeder, DJ7HS
  CCW is not magic, it is just the application of well-known pieces of communication theory
  
• Coherent CW, Peter Lamb, G3IRM
  . . . a mode whose time has come?
• Coherent CW ..., George Heron, N2APB
  "The More You Know About a Signal, the Easier it is to Copy"
  
• Coherant CW beacon, Mike Masterson has beeen running a 10M beacon since 1997.

QRSS

• What is QRSS?  RSGB, M0AYF
• QRSS W0CH

WSJT and WSPR

• WSJT Home Page
  Current information and downloads for WSJT, WSPR, and related software.
• WSPRnet
  Blog and real-time monitor of WSPR propagation reports.