Transit - US Navy Navigation Satellite System (NNSS)
Transit is a US Navy first generation satellite radio navigation system (the world's first), designed and built by JHU/APL - the entire development program (S/C hardware, ground
Transit - US Navy Navigation Satellite System (NNSS)
Transit is a US Navy first generation satellite radio navigation system (the world's first), designed and built by JHU/APL - the entire development program (S/C hardware, ground support network, user equipment, etc.) was conducted within APL at Laurel, MD, USA; RCA (later named: GE Astro Space, now Lockheed Martin) was the satellite integrator starting with Oscar-18 (Transit-O-18). The program was initiated by Frank T. McClure in the spring of 1958, based on JHU/APL's Doppler tracking discoveries by William H. Guier and George C. Weiffenbach (the Sputnik orbit could be determined from RF Doppler data) and the realization of the “navigation problem” (it states: if the position of the satellite were accurately known, then Doppler data could tell an observer on the ground his unknown position). The Transit demonstration program was sponsored by DARPA (Defense Advanced Research Projects Agency) and officially begun in 1959. 1) 2) 3) 4) 5) 6)
Figure 1: William H. Guier, Frank T. McClure, and George C. Weiffenbach (l to r) discuss the principles of the Transit Navigation System (image credit: JHU/APL)
The initial navigational accuracy requirement was set at 0.5 nmi (926 m) with an ultimate goal of 0.1 nmi (185 m). It was recognized from the beginning that significant improvement in geodesy would be needed to achieve these accuracies and that the Doppler shift measurements from orbiting near-Earth spacecraft were an excellent source of data to make this improvement. Thus, a worldwide network of tracking stations was established, and a constellation of geodetic satellites was proposed to obtain the necessary Doppler data.
The Space Segment:
The first satellites (up to and including Transit-3B) were spherical in shape. Transit-1A was completed in seven months after initial program funding. Transit-4A and -4B were drumshaped (almost a cylindrical body with four solar panels) to provide more space for solar cells. In addition, operational frequencies of 150 and 400 MHz were used for the first time. The dual frequency corrected for ionospheric errors. On-board time keeping was done with cesium oscillators. Throughout the Transit system service period (1959 - 1996) four generations of cesium frequency standards and three generations of hydrogen masers were experienced, starting with a stability of 5 parts in 1010 to about 2 parts in 1015 for the best tuned hydrogen maser.
Figure 2: Illustration of the Transit-1B experimental satellite (image credit: JHU/APL)
Transit-1A to -4B were regarded experimental satellites. The mass of each experimental satellite was about 136 kg. The TRAAC (Transit Research And Attitude Control) satellite, also built by APL and launched together with Transit-4B on an Ablestar vehicle (launch Nov. 15, 1961), flew the first gravity-gradient boom in history. However, stabilization was not demonstrated since only about 1 m of the 18 m boom (total length) deployed. The TRAAC S/C lost signal on Aug. 12, 1963 due to the degradation of solar cells. 7) 8)
The next series of Transit satellites (starting with -5A-1) were regarded as early operational prototype spacecraft. They were launched on a solid-fuel Scout vehicle from VAFB, CA, into po