Compressing Time & Distance

By: George Spafford

December 26, 2006

A lot has been said lately of the “flat earth” and how technology has brought groups together in unparalleled ways. Part of this has been achieved by leveraging information about location through the use of the Global Positioning Satellite (GPS) system. Through the use GPS technologies organizations, and even individuals, can track the positions of items of interest. One questions that inevitably arises is “what is GPS anyways” and that is what we will explore in this article.

The military has long acknowledged the need to understand location and have steadily pushed navigational aids. In the 1960s the US Department of Defense began a series of initiatives that were forerunners to the modern GPS system with. The first phase of the Navigation Satellite Timing And Ranging (NAVSTAR) contract was awarded in 1973 and on June 26, 1993, the last of the 24 NAVSTAR satellites were put into orbit finally completing a full constellation of the system we have come to know as “GPS”.

Essentially, there are 29 GPS satellites that orbit the earth twice per day at a distance of 12,000 miles. There are 24 in active use and three backups. The satellites weigh approximately 3,500 pounds and use solar panels for power along with battery backups and have a life expectancy of approximately 10 years. Their locations and synchronizations are controlled by ground stations and the goal is that there should be four satellites “visible” from any spot on earth.

Surprisingly, each satellite’s transmitter is only 50 watts. Given their frequency and distance, this explains why GPS receivers are relatively sensitive to obstructions and line of sight. The GPS satellites actually send out two sets of signals – L1 for civilian use and L2 for military use.

The actual GPS transmissions are continuously streaming from each satellite and a given transmission contains a pseudorandom identification code, “ephemeris” and “almanac” data. The ephemeris data communicate the current health of the satellite as well as the onboard date and time of transmission which are precisely controlled by atomic clocks. It is this precise timing information that is so very critical. If a receiver knows when the data was sent and that it travels at the constant speed of light, then it can derive the distance the signal traveled. Lastly, the almanac data contains the location of the transmitting satellite as well as that location of other satellites.

The other part of the GPS equation are the receivers. Today, we see them built into everything from vehicles to PDAs to phones to specialized GPS receivers. These units do exactly what the name implies – they :receive the signals. Information is not transmitted to the satellites.

The units use a process known as “trilateration”to pinpoint location and altitude from three satellites. With only two, it can approximate longitude and latitude. On the other hand, as the number of satellites increases, the more data the receiver can correlate and improve its location estimation. Many receivers can report the satellites they have locked onto and the estimated accuracy of the location.

When the satellites were first launched, their civilian signals were purposefully degraded to approximately 100 meter resolution due to a US government policy of “Selective Availability” out of concerns of national security. The GPS technology has always been viewed as “dual use” meaning it has value to both the military and commercial groups. The civilian use of GPS has skyrocketed and by executive decree, President Clinton turned off Selective Availability on May 1 st, 2000 allowing for civilian receivers to be accurate to approximately 10 meters. Through the use of ground stations, resolution is constantly improving.

Receivers have come a long way in terms of accuracy. What is acceptable is, of course, up to the user and the need at hand. The current system does have sources of error that will affect resolution and the following are examples:

• Receiver Clock Errors – because they typically do not contain atomic clocks as accurate as the satellites’.
• Signal Multipath Errors – occur when signals bounce off objects and arrive at the receiver at different times.
• Atmospheric Errors – are delays caused as signals pass through the ionosphere and troposphere of the Earth’s atmosphere

The US Federal Aviation Administration (FAA) and Department of Transportation (DOT) could see the merits of GPS but understandably wanted better accuracy. The Wide Area Augmentation System (WAAS) is limited to North America and uses 25 ground reference stations to improve accuracy to three meters provided the receivers can lock on to the signal. This type of correction using another signal source is known as “Differential GPS.”

For IT groups, GPS can bring a new dimension of near-real time location information for diverse applications in logistics, sales, personnel movements and so on. Giving more specific examples, organizations are buying GPS units for consultants and sales staffs to cut down time wasted moving between client sites. Businesses are using GPSes in delivery trucks to not only speed delivery but also reduce distance traveled and thus minimize vehicle and labor costs. The list goes on and on with the common thread being that time spent moving between two planned points must be reduced and/or the location of an item, or even person of importance, must be known at all times by a monitoring group.

We have the capacity to determine location like never before. Whether we are installing receivers in cars to assist people navigating to a destination, tracking the progress of a shipment or monitoring the movement of tectonic plates, GPS can assist. The business applications of this tried and true technology are really limited only by economics and our imagination.

The list of current satellites and their status is available at: http://gge.unb.ca/Resources/GPSConstellationStatus.txt

Note, This article is focused on the US system. There are Russian GPS satellites and a European GPS known as “Galileo”is planned to be in operation by 2010 and compatible with the US system.

Currently other governments are working on projects similar to WAAS. Japan is working on the Multi-Functional Satellite Augmentation System (MSAS and Europe is pursuing the Euro Geostationary Navigation Overlay Service (EGNOS).