Transit (NNSS)

Quick facts

Overview

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)}

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.

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.

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 polar orbits. The spacecraft mass of the first satellite, Transit-5A-1, was 55 kg. Three-5B series S/C were launched by Thor Ablestar vehicles, each with a piggyback 5E series S/C. The 5E S/C series had a solar power supply, while the 5B series S/C had nuclear power supplies. The Transit-5A series evolved into the Transit-5C-1 satellite with solar power supply.

Orbits: Transit satellite series orbits were generally at altitudes (apogees) of about 1100 km. The initial satellite launches (up to Transit-4B) were from Cape Canaveral, FL. All further launches took place from VAFB, CA, with polar orbit inclinations (Table 1).

The Transit Oscar series

The operational series of Transit satellites, more commonly referred to as “Oscar” series [or SOOS (Stacked Oscars on Scout)], closely followed the design of Transit-5C-1 with an important change: Hysteresis rods were installed on the solar panels to dampen the residual motion after the S/C despin maneuver in early orbit. The method of magnetic core storage (30 kbit) was used. Oscar-11 (Transit-O-11) was not used, it was later modified and launched as Transat on Oct. 27, 1977 from VAFB (Scout-D1 vehicle). The design life of the Oscar series was 3 years. Early Oscar series satellites had only short lifetimes. However, beginning with Transit-O-12 (Oscar-12), the satellites demonstrated an average orbital lifetime of more than 14 years. In fact, two S/C, Transit-O-13 and Transit-O-20, operated more than 21 years.

Note: the Transit Oscar series should not be confused with the AMSAT Oscar series.

TIP (Transit Improvement Program)

TIP was initiated in 1969 with the objective to provide a radiation-hardened satellite. Each TIP satellite was equipped with a minicomputer providing a memory of 64 kbyte. In addition the 150 and 400 MHz transmitters were redesigned, providing an output power of 3 W (150 MHz) and 5 W (400 MHz). A hydrazine thruster system was used to correct for orbital precession. - Each TIP satellite had the requirement to broadcast ephemerides for five days autonomously without input from the ground. This in turn required the development of a drag-free satellite series, referred to as “Triad” (Transit-improved DISCOS). ^{9) 10)}

The DISCOS (Disturbance Compensating System) technology was spearheaded by Stanford University (the “ball-in-the-box” concept). Triad uses a three-body system [the actual satellite at bottom facing nadir, DISCOS at the center position, and a nuclear power supply (radioisotope thermal generator) at the top] connected by deployable booms. A major improvement in the TIP-satellite design was the introduction of the quadrifilar helix antenna. All major electronics subsystems were made redundant, radiation-hardened integrated circuits were interconnected on a ceramic substrate to obtain high-density packaging.

Triad-1 (launched Sept. 2, 1972) was the first satellite to fly a completely gravitational orbit, free from all surface forces such as drag and radiation pressure. The orbit could in fact be predicted for up to 60 days.

Triad-2 (TIP-2) and Triad-3 (TIP-3) satellites were each equipped with a redundant pulsed-plasma electric propulsion system (Isp = 225 kg s) used for drag compensation. The 1 kg Teflon, used for both thrusters, provided a fuel supply for 10 years.

Nova satellites. Three more spacecraft, nearly identical to the TIP series design, were built by RCA on Navy request. Improvements included: the addition of magnetic damping to the DISCOS and a stiffening to the boom assembly, reference clock, computer capacity, 6 m gravity boom, S/C mass was 166 kg. The NOVA satellites used the electric propulsion system (PPT system, Isp = 543 seconds, total impulse = 2450 Ns) of the TIP series satellites to compensate for drag with thruster firings. The NOVA satellites had operational lifetimes of 8 to 9 years. ^{11) 12)}

Control Segment

Transit system operations and control are conducted at NAVSOC (Naval Satellite Operations Center, since 1962), Point Mugu, CA - working through tracking stations at Laguna Peak (near Point Mugu, CA), Prospect Harbor, ME, Rosemont, MN, Wahiawa, HI, Finegayan, Guam (since 1993) and at Falcon AFB, Colorado Springs, CO (since 1988). The tracking stations collected Doppler data and transmitted in the uplink the predicted ephemeris to each satellite. This data was stored on-board for continuous rebroadcasts to the user community. ^{13) 14)}

When Transit navigation services were terminated/decommissioned on Dec. 31, 1996, NAVSOC retained other mission assignments, among them operations of GFO-1 and NIMS (Navy Ionospheric Monitoring System). On Jan. 1, 1997, the remaining Transit system constellation (of six Oscars in three orbital planes) became NIMS with a new application, namely to utilize the Transit system resources for computerized ionospheric tomography (CIT). In this setup, the NIMS satellites are being used as dual-frequency beacons by ground collection sites to determine the free electron profile of the ionosphere.

User Segment

The Transit navigation operates on the principle that a receiver's position can be determined on a single satellite pass by measuring the Doppler shift for a period of 10 to 15 minutes. A user's receiver on the ground measured the time history of the refraction-corrected Doppler data and recorded the orbital ephemeris as the satellite passed overhead. The user was able to calculate position initially within a few hundred meters, later within 15 to 25 meters. Some limitations: The user had to know his altitude and the satellite ephemeris; an error was introduced for an unknown vessel speed. A position fix with the Transit constellation was only available every 35 to 100 minutes. Navigation information could only be obtained by instruments of slow-moving users (like ships). For stationary users, such as surveying and oil platform location, integration of measurements from several passes yielded rms accuracies in the order of 5 m. Toward the end of Transit system operations in 1995, commercial receiver prices were as low as $1000. The 2-D system did not permit velocity determination.

In 1964, the Transit system became operational. In 1967 Transit navigation services were made available to the general public (commercial and private users). Commercial companies were permitted to manufacture and to sell low-cost receiving equipment. This action resulted in the use of more than 80,000 privately-owned Transit receivers (in particular in the field of commercial shipping). Also, oil-drilling platforms were among the first to use Transit to determine the boundaries of oil deposits. ¹⁵⁾

To reduce the size of instrumentation that had to be carried to remote places for site survey operations, a special-purpose receiver called the Geoceiver was developed by the Magnavox Corporation. This self-contained instrument was slightly larger than a briefcase and combined single-sideband receivers for the Transit operating frequencies, a standard frequency oscillator, and a data reduction system to provide a data record for each satellite pass. APL used this data record along with tracked orbital data to determine a site survey.

During its 32 years of operation, the Transit Navigations System (or NNSS) provided accurate and reliable global navigation for the US Navy and for the civilian community. The system was continuously improved, in addition it contributed to many technology advances (see Table 2).

Legacy of Transit: It all began with the Transit all-weather global navigation system. That system helped propel America into the space age, sparking a large number of firsts in space engineering, technology, and science, and led to the eventual development of today’s Global Positioning System. Transit formed the foundation of what we are and what we do in the Space Department today. The remarkably short time from initial concept to operational availability was achieved by a group of professionals who were able to design, build, launch, and operate some 20 spacecraft (7 of which did not reach orbit) in approximately 4 years. ¹⁸⁾

References

1) Robert J. Danchik, “An Overview of Transit Development,” Johns Hopkins APL Technical Digest, Vol. 19, No. 1, 1998, pp. 18-26, URL: http://www.jhuapl.edu/techdigest/TD/td1901/danchik.pdf

2) William H. Guier, George C. Weiffenbach, “Genesis of Satellite Navigation,” Johns Hopkins APL Technical Digest, Vol. 18, No. 1, 1998, pp. 14-17, URL: http://www.jhuapl.edu/techdigest/TD/td1901/guier.pdf

3) H. D. Black, R. E. Jenkins, L. L. Pryor, “The TRANSIT System, 1975,” APL/JHU Technical Memorandum, Dec. 1976, URL: http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA037398&Location=U2&doc=GetTRDoc.pdf

4) Steve M. Yionoulis, “The Transit Satellite Geodesy Program,” Johns Hopkins APL Technical Digest, Vol. 19, No. 1, 1998, pp. 36-42, URL: http://www.jhuapl.edu/techdigest/TD/td1901/yionoulis.pdf

5) Carl O. Bostrom, Donald J. Williams, “The Space Environment,” Johns Hopkins APL Technical Digest, Vol. 18, No. 1, 1998, pp. 43-52, URL: http://www.jhuapl.edu/techdigest/TD/td1901/williams.pdf

6) “The Legacy of Transit,” URL: http://www.jhuapl.edu/techdigest/td/td1901/index.htm

7) Note: The very first Transit satellites transmitted signals at four frequencies: 54, 162, 216, and 324 MHz. The signals provided experimental data to evaluate ionospheric effects as a function of frequency. The final design is based on a two-frequency method for correcting ionospheric error.

8) “An Overview of the Navy Navigation Satellite System,” URL: http://sd-www.jhuapl.edu/Transit/

9) J. Dassoulas, “The TRIAD Spacecraft,” Johns Hopkins APL Technical Digest, Vol. 12, No. 2, pp. 2-13, June 1973

10) Daniel DeBra, “A Satellite Freed of all but Gravitational Forces: TRIAD I" AIAA 12th Aerospace Sciences Meeting, Washington, D.C., January 30-February 1, 1974, AIAA Journal Vol. 11, No 9, Sept. 1974, pp. 637-644, URL: http://einstein.stanford.edu/content/sci_papers/papers/DeBra_AIAA-TRIAD-I_1974.pdf

11) W. L. Ebert, S. J. Kowal, R. F. Sloan, “Operational NOVA Spacecraft Teflon Pulsed Plasma Thruster System,” AIAA-89-2497, AIAA/ASME/SAE/ASEE 25th Joint Propulsion Conference, Monterey, CA, July 10-12, 1989

12) Y. Brill, et al., “The Flight Application of a Pulsed Plasma Microthruster: the NOVA Satellite,” AIAA-82-1956, 16th International Electric Propulsion Conference, Nov. 1982

13) Gary C. Kennedy, Michael J. Crawford, “Innovations Derived from the Transit Program,” Johns Hopkins APL Technical Digest, Volume 19, No. 1, 1998, pp. 27-35, URL: http://www.jhuapl.edu/techdigest/TD/td1901/kennedy.pdf

14) A. J. Tucker, “Computerized Ionospheric Tomography,” John Hopkins APL Technical Digest, Vol. 19, No. 1, 1998, pp. 66-71, URL: http://www.jhuapl.edu/techdigest/TD/td1901/tucker.pdf

15) Lauren J. Rueger, “Development of Receivers to Characterize Transit Time and Frequency Signals, John Hopkins APL Technical Digest, Vol. 19, No. 1, 1998, pp. 53-59, URL: http://www.jhuapl.edu/techdigest/TD/td1901/rueger.pdf

16) E. J. Hoffman, “Spacecraft Design Innovations in the APL Space Department,” Johns Hopkins APL Technical Digest, Vol. 13, No. 1, 1992, pp. 167-181

17) R. B. Kershner, “Technical Innovations in the APL Space Department,” Johns Hopkins APL Technical Digest, Vol. 1, No. 4, 1980, pp. 264-278

18) Stamatios M. Krimigis, “APL’s Space Department After 40 Years: An Overview,” Johns Hopkins APL Technical Digest, Vol. 20, No 4, 1999, pp. 467-476, URL: http://www.jhuapl.edu/techdigest/td/td2004/krimigis.pdf

The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (eoportal@symbios.space).