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Notes and Sources, page 253.

A textbook consulted on the geoid (page 110) is Theory of the Earth, by Don L. Anderson (Blackwell Scientific Publications, 1989).

Information about how APL's understanding of the geoid developed comes from papers and from interviews with Bill Guier and Harold Black (page 112).

Dave Smith, director of the division of terrestrial physics at the Goddard Space Flight Center gave me some very basic understanding of satellite geodesy.

Information about developing subsequent orbital determination programs and about the computers comes from reports in APL's archives and from interviews with Harold Black and Lee Pryor (pages 113 and 114).

Information about the problems with the solar cells (page 114) came from Bob Danchik.

General comments on satellite navigation.

1. At different places in this section, I have alluded to alternative ideas for navigation satellites. One, explained in "Navigation by Satellite" in Missiles and Rockets in October 1956, even talks of utilizing the Doppler shift for a navigation satellite. But this paper envisages almanacs and tables of position and calculations of the distance at closest approach. It implicitly assumes that the orbit would be known and, inevitably because it was written in 1956, does not account for the impact on orbits of Earth's complex gravitational field, nor for the impact of the ionosphere on the received signal. The paper does envisage the use of computers, but not the sophisticated curve-fitting techniques of Guier and Weiffenbach.

2."Possible Use of Syncom as a Navigation System--Microwave Loran." Memo for files, From L.M. Field cc L.A. Hyland (HAC archives 1990-09 box 6 folder 22).

This memo argues that Syncom would make a better navigation satellite than Transit if the station keeping were adequate. It expands on a memo

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