1920s
Origins of radionavigation Early WW II LORAN, the first navigation system to employ timedifference-of-arrival of radio signals, is developed by the MIT Radiation Laboratory. LORAN was also the first true all-weather position-finding system, but is only two-dimensional (latitude and longitude).

1959
TRANSIT, the first operational satellite-based navigation system, is developed by the Johns Hopkins Applied Physics Laboratory (APL) under Dr. Richard Kirschner. Although Transit was originally intended to support the U.S. Navy’s submarine fleet, the technologies developed for it proved useful to the Global Positioning System (GPS). The first Transit satellite is launched in 1959.

1960
The first three-dimensional (longitude, latitude, altitude) time-difference-of-arrival navigation system is suggested by Raytheon Corporation in response to an Air Force requirement for a guidance system to be used with a proposed ICBM that would achieve mobility by traveling on a railroad system. The
navigation system presented is called MOSAIC (Mobile System for Accurate ICBM Control). The idea
is dropped when the Mobile Minuteman program is canceled in 1961.

1963
The Aerospace Corporation launches a study on using a space system as the basis for a navigation system for vehicles moving rapidly in three dimensions; this led directly to the concept of GPS. The concept involves measuring the times of arrival of radio signals transmitted from satellites whose positions are precisely known. This gives the distances to the known satellite positions—which, in turn, establishes the user’s position.

1963
The Air Force begins its support of the Aerospace study, designating it System 621B. By 1972, the
program has already demonstrated operation of a new type of satellite-ranging signal based on pseudorandom noise (PRN).

1964
Timation, a Navy satellite system, is developed under Roger Easton at the Naval Research Lab (NRL) for
advancing the development of high-stability clocks, time-transfer capability, and 3-D navigation.
Timation’s work on space-qualified time standards provided an important foundation for GPS. The first
Timation satellite is launched in May 1967.

1968
DoD establishes a tri-service steering committee called NAVSEG (Navigation Satellite Executive Committee) to coordinate the efforts of the various satellite navigation groups (Navy’s Transit and Timation programs, the Army’s SECOR or Sequential Correlation of Range system). NAVSEG contracted a number of studies to fine-tune the basic satellite navigation concept. The studies dealt with some of the major issues surrounding the concept, including the choice of carrier frequency (L-Band versus CBand), the design of the signal structure, and the selection of the satellite orbital configuration (a 24- hour figure 8s constellation versus “Rotating Y” and “Rotating X” constellation).

1969–1972
NAVSEG manages concept debates between the various satellite navigation groups. The Navy APL
supported an expanded Transit while the Navy NRL pushed for an expanded Timation and the Air Force
pushed for an expanded synchronous constellation “System 621B.”

1971
L2 frequency is added to the 621B concept to accommodate corrections for ionospheric changes.

1971–1972
User equipment for the Air Force 621B is tested at White Sands Proving Ground in New Mexico. Ground
and balloon-carried transmitters simulating satellites were used, and accuracies of a hundredth of a mile
demonstrated.

April 1973
The Deputy Secretary of Defense determines that a joint tri-service program be established to consolidate the various proposed positioning/navigation concepts into a single comprehensive DoD system known as the Defense Navigation Satellite System (DNSS). The Air Force is designated the program manager. The new system is to be developed by a joint program office (JPO), with participation by all military services. Colonel Brad Parkinson is named program director of the JPO and is put in charge of jointly developing the initial concept for a space-based navigation system.

August 1973
The first system presented to the Defense System Acquisition and Review Council (DSARC) is denied
approval. The system presented to DSARC was packaged as the Air Force’s 621B system and therefore
not representative of a joint program. Although there is support for the idea of a new satellite-based
navigation system, the JPO is urged to broaden the concept to include the views and requirements of all the services.

December 17, 1973
A new concept is presented to DSARC and approval to proceed with what is now known as the NAVSTAR GPS is granted, marking the start of concept validation (Phase I of the GPS program). The new concept was really a compromise system negotiated by Col. Parkinson that incorporated the best of all available satellite navigation system concepts and technology. The approved system configuration consists of 24 satellites placed in 12-hour inclined orbits.

June 1974
Rockwell International is chosen as the satellite contractor for GPS.

July 14, 1974
The very first NAVSTAR satellite is launched. Designated as Navigation Technology Satellite (NTS)
number 1, it is basically a refurbished Timation satellite built by the NRL. The second (and last) of the
NTS series was launched in 1977. These satellites were used for concept validation purposes and carried the first atomic clocks ever launched into space.

1977
Testing of user equipment is carried out at Yuma, Arizona.

February 22, 1978
The first Block I satellite is launched. A total of 11 Block I satellites were launched between 1978 and
1985 on the Atlas-Centaur. Built by Rockwell International as developmental prototypes, the Block Is were used for system testing purposes. One satellite was lost as a result of a launch failure.