A mathematician by education, Dr. West would churn numbers and process data from satellites to help determine their exact location. In December , Dr. At the induction, the Air Force recognized Dr. Today, there are at least 31 operational GPS satellites orbing the Earth and impacting every aspect of our life. Now see: How accurate is the altimeter in a GPS watch? Save my name, email, and website in this browser for the next time I comment.
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Do you like our blog? Wanna join the team? Contact us: info geoawesomeness. Sign in. Log into your account. Forgot your password? Password recovery. Focus on the columns of the one-way passive ranging techniques with the red outline. The A shows the user needs an atomic clock. The X shows the user needs only a crystal clock. The option later selected for GPS is designated as G.
The patent U. Figure 5. The most capable option, circled in green, became the basis for the White Sands prototyping and testing, and then evolved into GPS. More B Studies. From to , program B continued with trade-off studies including: signal modulation, user data processing techniques, orbital configuration, orbital prediction, receiver accuracy, error analysis, system cost, and comprehensive estimates of the tactical mission benefits.
Of these studies, the most important were those aimed at selecting the best passive ranging technique for the navigation signal. By , it appeared that the best technique was a variation of a new communications modulation known as CDMA.
Pioneering this signal were several outstanding scientists, Dr. Fran Natali and Dr. Jim Spilker both of Philco-Ford , and Dr. Charlie Cahn of Magnavox. This signal has many names. In addition to CDMA, it is sometimes called spread spectrum, since the energy of the signal was spread over a wide range of radio frequencies. The name code-division is used because each satellite is assigned its own coded signal. Each was a binary digital sequence selected to be uncorrelated with other signals and also uncorrelated with time shifts of the signal itself.
The expected, powerful advantage of this technique was that all satellites would broadcast on exactly the same frequency. It would clearly lend itself to digital signal processing. Furthermore, and very important, any time-shifts induced by the receiver for the various satellite signals would be effectively eliminated. Fortunately, in a technique for selecting orthogonal codes was invented by an accomplished applied mathematician, Dr.
Robert Gold of the Magnavox Corp. Naturally these are now known as the Gold codes. His solution resolved the third CDMA issue stated above. White Sands Tests. For these initial tests, B arranged four transmitters in a configuration known as the inverted range. Interestingly, the more capable receiver was the MX that was only on loan from Magnavox.
These transmitters broadcast CDMA signals from locations that were similar to a satellite configuration except that they were broadcast from the ground. For the simulation of satellite geometry, a balloon-based transmitter was also included for the aircraft-landing tests. Al Gillogly of Aerospace spent many hours installing and troubleshooting the test configuration.
By , program B had successfully proven the effectiveness and accuracy of the CDMA signal by demonstrating that such a configuration would achieve 5-meter, 3-dimensional navigation accuracy. Much credit for the painstaking analysis of these results should go to Bill Fees of Aerospace who wrote the final detailed test report.
These test results answered most of the remaining issues regarding the CDMA signal. The tests also confirmed the power of the modulated signal by showing that all satellite signals could, indeed, be received simultaneously on the same frequency.
These tests also corroborated the expectation that ranging to four satellites eliminated the need for a highly precise user atomic clock, while still supporting full, three-dimensional navigation. This became an extremely important feature of GPS. If each user had required an atomic-clock class frequency-standard, no inexpensive user equipment could have been produced within the technology horizon visible at that time.
This is still true today. The Air Force developed a plan to reduce both risks. This included a proposal in early to deploy a four-satellite demonstration system.
This proposal addressed both risks. It would reduce the technology readiness risk in the clocks by launching simple L-band transponders. In many circles, this proposal was erroneously thought of as B because it came from that office, but in fact, the operational concept for B never contemplated or advocated using transponders in the final operational system. Transponders had been rejected for the operational system because they could be easily jammed from the ground.
Such a jamming signal would overpower the transponder and steal all of the transmitted energy away from the transponded navigational signal. This enemy jamming would shut down the entire system, clearly an unacceptable risk. Proposed Initial Constellation. To demonstrate four-satellite, passive ranging capability, B had studied a number of orbital configurations, including geo-synchronous and lower inclined orbits. The program proposed to place a constellation of three or four synchronous satellites in orbits over the United States.
This array would allow extended periods of four-satellite testing without committing to a full global employment. If this demonstration were successful, the next step would have been to add three more longitudinal sectors, each with its own array.
Again, the principal redeeming feature of this approach was that there was some hope of it being funded. The Air Force in the Pentagon placed enormous pressure on the B program to come up with the absolutely cheapest way to demonstrate the four-satellite approach. This proposed constellation design was a reasonable compromise, given the boundary conditions of a four-satellite demonstration and absolutely minimal cost.
It is interesting that the Japanese, with a requirement to supplement GPS with satellite signals to improve coverage in urban areas where there are high shading angles , have designed a very similar constellation. The Japanese configuration is intended to improve coverage restricted to their longitudinal sector of the globe. In , the U. Navy initiated a second satellite program, named Timation, under the direction of Roger L. Easton, Sr. This project ran parallel to, and was in competition with, the Air Force Program.
It subsequently developed a number of experimental satellites, the first of which was called Timation 1. This small satellite, weighing 85 pounds and producing 6 watts of power, was launched on May 27, Timation 1, developed by NRL, was a miniaturized, innovative design.
The quartz clock was less stable than expected, apparently due to temperature and cosmic-ray effects. The key feature of Timation 1 was that it included a very stable quartz clock. By , NRL demonstrated single-satellite position fixes, accurate to about 0. Easton, p. To calibrate ionospheric group delay, the satellite broadcast on two frequencies very similar to the technique pioneered by the Transit program.
Its quartz oscillator was expected to be somewhat more stable, about one part in Again, a large frequency shift was observed in the clocks that was finally traced to a solar proton storm. NRL was able to demonstrate ranging accuracies of approximately feet to a fixed location. Timation NTS The last satellite in the original Timation series was launched in July Bill Huston , to the Program Director Col.
Bradford Parkinson. The gross weight had been increased to pounds with a power requirement of watts. This satellite, developed by Pete Wilhelm of NRL, was placed at an orbital altitude of 7, nautical miles. Timation NTS-1 carried two slightly modified commercial rubidium clocks. Unfortunately, attitude-stabilization problems induced temperature variations that masked any quantitative performance evalulation.
The atomic clocks were not useful as prototypes for GPS. The NTS satellites were strictly technology-testing satellites. For many reasons, they had no role in the development of the operational satellites by the JPO and Rockwell. They were the only ones used in the operational testing during phase I of GPS. NTS-1 included two small, lightweight rubidium oscillators as clocks. A German commercial company called Efratom had independently developed these models. Amazing at the time, they only consumed about 13 watts of power and weighed some four pounds each.
The Global Positioning System GPS is a positioning, navigation and timing service based on a constellation of satellites which are owned by the U. Reserved only for military use upon its inception, GPS is now available to everyone, and has been for some time. Due to its widespread use, GPS technology is now well incorporated into our daily lives. Used in your car to navigate traffic or by your smartphone to offer more accurate and customized web search results, GPS is a technology we have become so comfortable with, we often take it for granted.
In total, there are at least 24 operational satellites in the GPS constellation, with additional satellites in reserve that can be activated when needed. As of May , GPS. The satellites circle the Earth two times a day at 20, km 12, miles up. The U. What started off as a method of studying the Earth from space quickly grew into a universal technology utilized by nearly every country in the world.
One of the major milestones was the ending of Selective Availability SA in , which allowed civilians access to more precise GPS readings and opened the door to new technology advancements. The first portable GPS receiver developed for consumers was made by electronic navigation company Magellan. The inaugural device, the NAV , weighed in at 1. At the time, the high cost of satellite navigation meant that outside the military, only freight, delivery and select other companies could afford to use systems.
These days, portable GPS tracking is much more affordable. For example, GPS is essential to fleet management , especially for tracking vehicle location and driving behavior with telematics , as well as routing and dispatching. As the capabilities of technology continue to expand, one can only imagine what GPS technology will look like in the future.
Using physical maps or asking strangers for directions is now a thing of the past. New navigation systems will power businesses and government services around the world.
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