I’ve decided to spend a few days with some background and general tutorial subjects. Some of my usual rants will be interspersed if needed.
Here’s a nice historical write up courtesy of Kirt Blattenberger, webmaster of the RF Café (http://www.rfcafe.com). Well worth a look, I’ve posted some of my own comments at the end.
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The Navigation Satellite Timing and Ranging Global Positioning System (NAVSTAR GPS) has been operational, incredibly, for almost three decades. The first satellite in the originally planned 24-member constellation was launched on July 14, 1974. A total of 11 Block I satellites (built by Rockwell) were launched into 10,900 nautical mile orbits between 1978 and 1985 on the Atlas-Centaur booster rocket. In 1982, the DoD decided due to budget cuts to reduce the constellation number from 24 to 18, but by 1988, the number was back to 21 plus three orbiting spares. In 1989, the first of the Block II NAVSTAR satellites was launched. 28 were scheduled for construction and launch because of attrition from old technology and malfunction. By 1991, 24 satellites were in orbit and commissioned. Mission accomplished.
Until 1997, the most accurate GPS signal, the L2, was made available to the public only on a sporadic basis. The L2 signal was closely guarded by the military for use in their critical aircraft, spacecraft and munitions guidance systems. Advances in technology that most people outside the highly classified DoD community will never know about, and pressure brought on by civilian groups to make the L2 signal available full-time are credited for the policy change. Since that occasion, GPS devices and products that incorporate GPS have grown exponentially.
Commercial GPS really got its launch (no pun intended) during the first Gulf War, when concerned parents and spouses bought GPS units by the caseload to send to their husbands and children in the deserts of Iraq. In those days, the GPS receivers and computational engines were the size of a cigarette pack, often took minutes to acquire and compute signals, and drew large amounts of current. Since only the less accurate L1 signal was available for these units and many had only a couple receiver channels, the accuracy was limited to around 10-20 meters (good enough for a desert in a sand storm). The military was enjoying accuracies as good as 5 meters with the L2 signal and many channels. Block III satellites will generate a new L5 signal, a higher power (roughly 4x), modified version of the L2 intended for civilian use to provide better coverage with less sensitive receivers. Now, GPS receivers are integrated onto a single slab of silicon and routinely provide 12 to 16 channels and achieve positional accuracies unfathomable in the early 1990s. Their current draw is measured in tens of milliamps.
Today, GPS receivers can and are integrated into just about any kind of device that is not bolted down (and some that are): cell phones, automobiles, boats, watches, vending machines, shopping carts, full-size airplanes and model airplanes, and even the new Gizmondo, GameBoy-like controller (for location-based gaming). Map software can be had that, when combined with a solid state magnetic compass (ala the Nokia 5140 phone), provides the operator with directions that are detailed enough to allow navigation instructions like, “Go straight ahead for 200 feet and turn left at Main Street, then proceed 50 feet to the Starbucks on the right.” “Real” GPS devices like those available from Trimble, Garmin and Magellan provide even more amazing features.
GPS is now a technology that the folks at Aerospace Corporation, when beginning their study in 1963 on the development of a space system as the basis for a navigation system for vehicles moving rapidly in three dimensions (leading directly to the concept of GPS), could never have dreamed would be at such an advanced state of maturity forty years later. Those who are still around can take pride in the system to which they gave birth. Why, without their foresight, for instance, the people involved in the frontier-advancing concept of GPS art would never have been able to indulge in their craft. What is GPS art, you might ask? It is the process of using a GPS tracking program to record an operator’s path along the ground (or in the air or water) in a shape that results in an outline of a pre-planned, recognizable object. As you might expect, there are websites dedicated to chronicling the ample talent out there. One of such websites is GPSDrawing.com. There, you will find not only a large collection of GPS art that includes tic-tac-toe games, pictures of whales and text messages, but also instructions on how to generate such masterpieces yourself. Isn’t technology wonderful?
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There were quite a few at Aerospace (and MITRE) a similar ‘not for profit’ think tank contractor shop. Certainly there were some government employees with vision too. I had the distinct honor to work a short time with H. Beat Wakernagel, a founding father of the system. In 1995, Beat was working actively on devices that would replace aircraft attitude gyros with GPS sensors and dozens of other projects (most classified at the time as we worked for USAF Space Command) which are still in most cases unimplemented. GPS in the US is currently about where it was in Japan in 1996, we will see a tremendous increase in both uses and market penetration.
To wind up an already too long blog entry I want to note that the reason the general public hears so many different numbers regarding the GPS satellites in orbit is that _every_ GPS satellite that has ever reached orbit is still up there. The oldest ones are turned off, but many can still be activated for system backup or research work. There are on the order of 50 birds aloft .. and they will all be there for thousands of years.
