WILLIAM H. GUIER and GEORGE C. WEIFFENBACH
When Sputnik I was launched on Friday, October 4, 1957,
we followed the news of the orbiting Sputnik with high interest
but with little thought of personal involvement. The following
Monday, at APL, former staff member Bob Bogle reminded us that
APL was a laboratory with the capability of receiving the satellite
signals and understanding the telemetry code to obtain useful
information. He also pointed out that, surprisingly, there seemed
to have been no serious effort at APL involving Sputnik over the
weekend. Presumably everyone, like us, assumed somebody else would
be "at it."
That first Monday evening there was hardly a receiver/antenna
combination at the Laboratory that did not have a group trying
to receive the Sputnik signals. We had one distinct advantage
over other groups: one of us was completing a Ph.D. dissertation
in microwave spectroscopy and had a high-quality communication
receiver and the necessary understanding of how to make precision
frequency measurements. Because of the fortunate happenstance
that the Beltsville, Md. WWV 20-MHz-standard hroadcast was at
nearly the Sputnik frequency, the output from the receiver could
be made to be the audible difference between the WWV standard
and the Sputnik signal. Superimposed was the WWV timing signal.
That first evening was fun! Hams were listening and talking "all
over the bands" about the launch and exchanging information
about when it would be over various locations. At one point our
search picked up a broadcast by the Russians (in English on one
of the ham bands) that listed times of passage over major cities
around the world, including Washington, D.C. We now knew when
to listen! Also, during that period we had sufficiently refreshed
our memories on the subject of orbital mechanics to have a good
idea of what the satellite's period (95 minutes) and inclination
(approximately equal to 64 degrees) must be and to estimate the
number and grouping of successive passes available at our latitude.
Our first reception of the Sputnik seemed only a momentary triumph
when we realized that there was no telemetry on the 20 MHz signal,
just a pure tone that at first appeared to wander in frequency
by a surprisingly large amount. After about 5 minutes, it was
unmistakably clear that the wandering was the moving Sputnik's
Doppler shift, which we were by then recording with precise frequency
and time information. Without a telemetry signal to play with,
we turned to considering the Doppler shift and the information
that might be gained from it.
We "borrowed" a General Radio wave analyzer from Ed
Cochran's laboratory, and by playing the recorded signals back
at different frequency settings (approximately equal to 2 Hz bandwidth),
we obtained accurate (digital) frequency versus time data. Using
those data, it was a natural step to refine the estimates of time
of closest approach of the satellite to our antenna. After some
laborious hand calculations on a Frieden mechanical calculator,
we refined the period of the satellite orbit. Within a few days
we were making the most accurate predictions of the next times
of signal appearance and were phoning such "alert times to
Vanguard Headquarters in Washington, which had set up an information
clearing house for Sputnik fact and folklore.
To our disappointment, the signals stopped after a few days because
of the limited storage battery capacity in the Sputnik I, and
we were left with only the data we had taken and the typical post-adventure
letdown one feels after an intensive effort. Now we could use
our professional thinking, the outstanding Laboratory resources,
and the support of the Director and Management to place our "stunt"
on an accurate quantitative basis. Without yet having a clear
idea of an application for such a capability, we joined with the
Laboratory's new Digital Computing Center, in particular with
Charley Bitterli and his colleagues who helped greatly in computer
processing the Doppler data to obtain better orbital parameters
and to generate alert times. Laboratory specialists in receivers
and antennas (Harry Zink in particular) helped to improve the
accuracy and reduce the labor of Doppler data reduction.
To our great joy, the Russians launched Sputnik II on November
3, 1957, so we had a renewed interest in tracking "live"
satellites. With the processing power available with the digital
computer, we were able to try relatively complex experiments with
the Doppler data, including determinations of the Sputnik transmitter
frequency, a correction for ionospheric refraction (Sputnik II
broadcast on both 20 and 40 MHz), and several different methods
for parameterizing the satellite's motion that were more suitable
for near- earth satellites than for planetary motion.
The following step was the giant one: Dr. Frank T. McClure conceived
both the inverse process of using Doppler information for navigation
and the major components of an operational navigation system.
Together, he and Dr. R. B. Kershner completed the conceptual design
of the Transit Navigation Satellite System, which remains basically
unchanged today.