Dr. George Ludwig Oral
 “Cosmic-Ray Instrumentation in the First U.S. Earth Satellite”, George H. Ludwig., The Review of Scientific Instruments, Vol. 30,
No. 4, April, 1959.
[This is a “Must-Have” article for those interested
in early transistor history. Excellent coverage of the state of transistor
technology available for the first satellites].
 U.S. Army
Historical Webpage - Explorer Info
[This site contains a wealth of historical
information relating to Explorer and Jupiter-C. Be sure to watch the Army
movie “The Big Picture” which captures all the excitement of the January
31, 1958 Explorer I launch].
 “Instrumenting the Explorer I Satellite”, Henry
Richter et al, Electronics, Feb 6, 1959.
[An excellent companion article to  above. This
article covers the work done at the Naval Research Lab and JPL to develop
the transistor transmitters for Explorer].
 University of Iowa
– James Van Allen Day 90th Birthday Recognition, 2004.
[This is the premier site for Explorer information,
and detailed discussion of Van Allen’s work. Dr. Ludwig has contributed
several articles and multiple photos to this site].
Early History of Spacecraft Electronics”, Mark Williamson, IEEE
Engineering Science and Education Journal, Vol. 10, Issue 2, April 2001.
[Useful technical information and photos of
Vanguard, Explorer and Sputnik. Well written].
Conditioning for Satellite Borne Energetic Charged Particle Experiments”,
George H. Ludwig, NASA Technical Note D-1080, August 1961.
[This paper by Dr. Ludwig documents additional
transistor circuitry for use in satellite instrumentation intended to
measure cosmic rays. It is interesting to note the progress in transistor
technology evident in this 1961 paper, compared with Ludwig’s classic 1959
paper , referenced above].