07 The Global Positioning System

APRS Article 7

July-August 2000

The Global Positioning System

By Jeff Robbins

VE3JTR (Voice Call)

VA3JTR (APRS Call)

Since the inception of this column I have referred to this ‘thing’ called a GPS, and while I refer to it commonly, I have yet to really talk about what it is. The Global Positioning System (GPS) is a United States Department of Defense developed, worldwide, satellite-based radio navigation system. The GPS is a collection of 24 precise earth-orbiting satellites. These satellites allow any person who owns a GPS receiver to determine his or her latitude, longitude and altitude anywhere on our planet. The U.S. Air Force Space Command formally declared the GPS satellite constellation as having met the requirement for Full Operational Capability as of April 27, 1995. Requirements included 24 operational satellites functioning in their assigned orbits and successful testing completed for operational military functionality. GPS had previously been functional with all 24 satellites since December 8, 1993, but had not yet achieved full military specifications. Even earlier than that, when less than 24 satellites were in orbit, GPS was available, just not on a continuous basis. GPS was used during Operation Desert Storm in 1991.

GPS consists of two levels of service, Standard Positioning Service and Precise Positioning Service. The Standard Positioning Service is available to all GPS users on a continuous, worldwide basis. SPS is provided on a frequency that contains a coarse acquisition code and a navigation data message. SPS provides positioning accuracy within 100 meters, 95 percent of the time, horizontally and 156 meters, 95 percent of the time, vertically and also provides time transfer accuracy to UTC within 340 nanoseconds, 95 percent of the time.

The Precise Positioning Service is a highly accurate positioning, velocity and timing service that is available on a continuous, worldwide basis to military authorized users. Capable user equipment provides positioning accuracy within 22 meters, 95 percent of the time, horizontally and 27.7 meters, 95 percent of the time, vertically and time transfer accuracy to UTC within 200 nanoseconds, 95 percent of the time. PPS was designed primarily for U.S. military use, and their partners. The use of cryptography denies access to unauthorized users, primarily to prevent terrorist abuses of the system.

BUT HOW DOES IT WORK?

The GPS system requires two things to make it work: First, each satellite has an on-board atomic clock that gives it an extremely precise time base. The satellites send radio signals to your GPS receiver, and the extremely precise time bases make it possible for the receiver to determine exactly how far away each satellite is. By knowing the speed of the radio signals, which is the same as the speed of light, your GPS receiver can multiply the speed of the radio signal, by the time it took to reach the receiver, and come up with the distance from the satellite to the receiver. Item one: distance from satellite to GPS receiver.

Second, each receiver has stored in memory a table that indicates exactly where each satellite is in its orbit at any given moment in time. This table’s existence is possible because the DoD placed the satellites in such extremely precise orbits. Item two: the exact position in space of the satellite is known.

So know we have the capability of knowing exactly the distance from our GPS receiver to a precisely known position in space (the satellite). If we do this two more times, with different visible satellites, we can triangulate our position on the earth. If we take a fourth measurement, our GPS receiver can also determine its altitude. Now the next challenge is getting our GPS receiver clock exactly synchronized with the atomic clocks on the satellites. Lets back up a step, and look at our three measurements that can pinpoint our position on earth. Now if our clock is off by just a small fraction of a second, our position report could be off by hundreds of kilometers! How can we overcome this without installing an atomic clock in our GPS receiver? The concept is actually quite simple. If we take a fourth measurement, we would find that by comparing triangulations using several combinations of three satellites, the receiver would produce different results from each calculation. The GPS receiver can now modify its clock slightly until the different combinations of triangulation come up with the same position result! Most late model GPS receivers are capable of working with twelve different satellites at once, so you can imagine that the more different measurements that are taken, the more times the GPS receiver can check its work, the more precise the timing and location will be. Pretty amazing feat for a little handheld device, don’t you think? As you can imagine, it is actually a lot more complicated than I have made it sound, but you now know the theory behind the Global Positioning System.

Late Breaking News

On May 1^st of this year, President Clinton announced a decision to stop the intentional degradation of the Global Positioning System signals. This decision went into effect at midnight on May 2^nd. Essentially, this means that civilian users of GPS will be able to pinpoint locations up to ten times more accurately than they do now. So you can now take the previous paragraphs on the two levels of service as a history lesson, and be sure that the reported position in APRS is much more accurate. The entire Press Release has been included in this issue for your information.

Ottawa Amateur Radio Club APRS Summit 2000

The Ottawa Amateur Radio Club sponsored the first Eastern Ontario APRS Summit at the Rideau Campus of Algonquin College on April 15 this year. Well known, APRS equipped 18 wheeler, Marc VA3DRV, organized the summit. The APRS summit was combined with CanWarn training, making it a full day for those attending. The summit was well attended by 69 APRS hams, and the agenda was full. The list of speakers included: Eric Meth, VE3EI, who spoke on APRS digis; Richard Hagemeyer, VE3UNW, who spoke on internet gateways and APRS servers, and Mark Sproul, KB2ICI, author of WinAPRS, who spoke on a variety of topics. Many hams were appreciative for the opportunity to put a face to the little moving icons on the screen! Due to previous commitments, I was unable to attend, however, rumour has it that another summit is being planned for Labour Day weekend! I am sure that an equally interesting agenda is in the planning stages. Unfortunately, I have learned that the YL and I have a wedding to attend that weekend, so I guess traveling to an APRS Summit is not in the cards for us until Spring 2001!

Canadian APRS Email List

Want to keep up with the latest in Canadian APRS news and views? Sign up for the email reflector dedicated to APRS in Canada. Eric, VE3GSI, in Napanee has suggested promotion of this email list for discussion of Canadian APRS issues, so here goes! Sign up now by sending a blank email to aprs-list-subscribe@ontario.aprs.net, or visit my webpage and click on the link in the Mailing List section. That’s all for now, see you ON THE MAP!