The GPS Observer
Satellite positioning with GPS is the dominant method of surveying throughout the world. For high-accuracy surveying, relative positioning prevails; two receivers gather data at the same time and the difference in position between the receivers is calculated.
For many GIS applications, Differential GPS (DGPS) is the observation method of choice; this method can determine accuracies of points in the submeter range. Today it's possible to do satellite positioning with receivers that use a combination of GPS and Russian GLONASS satellites, and in a few years the European Union will have its Galileo satellite system operational.
As a result of these advancements, a new acronym is being flaunted today-GNSS, short for Global Navigation Satellite System. In January 2006 a new magazine arrived in my mailbox. The title on the front cover was Inside GNSS with a subtitle of "GPS | Galileo | Glonass." Not long after, I received a flyer from the Institute of Navigation (ION) advertising its next conference as "ION GNSS 2006." Inside GNSS is not a trade magazine for the surveying profession; the only advertiser that even approaches our area of interest is Novatel. Likewise, ION has concentrated on hardware systems and tutorial seminars by Navtech Seminars. However, POB advertisers are also starting to use the term "GNSS."
GPS is not the only show in town now, but it's the only one out there that can stand alone. The following is a brief description of the status of the three orbit systems.
GPSThe Global Positioning System is fully operational, with between 27 and 29 satellites in six orbital planes. In December 2005 the first of the Block IIRM (military) satellites was launched. This is a Block II replenishment satellite with the L2C second civil frequency. However, the U.S. Air Force has emphasized it would not guarantee the availability or quality of L2C signals until initial operational capability (IOC), which usually requires 21 operational satellites, is complete. With seven Block IIR satellites remaining to be launched, IOC may not take place for quite some time. However, many of us old-timers remember when the Block II constellation had not attained IOC, but the system was operational for surveying.
The first Block IIF satellite launch is scheduled for 2008, and 12 satellites are planned. All Block IIF satellites will have L2C and an additional frequency, L5, at 1176.45 MHz, for a total of three civil frequencies.
In October 2005, the U.S. Department of Defense published an interesting report called "The Future of the Global Positioning System" (available online at www.acq.osd.mil/
dsb/reports/2005-10-GPS_Report_Final.pdf). The report is 109 pages and gives information on what is being planned for GPS for the next 15 years. The main development coming up is GPS III, which is still under design. This will be a 30-satellite constellation in three orbital planes; the first launch was scheduled for 2013, but it appears this will be delayed because of design problems. GPS III satellites will be lighter in weight, allowing two satellites to be launched with one rocket.
The reason the Department of Defense is delaying some programs is that the expected life of existing on-orbit satellites has greatly exceeded design specifications. For example, the satellite in slot D5, PRN 15, launched on Oct. 1, 1990 and put into operation on Oct. 15, 1990, is still operational. Based on the U.S. Naval Observatory's listing of the GPS constellation status (available at ftp://tycho.usno.navy.mil/pub/gps/gpstd.txt), I believe 12 GPS satellites have been operational for more than 10 years.
GLONASSThe GLONASS system is approaching full operational status. There are 16 satellites in the first and third orbital planes; the second orbital plane has no satellites.
The GLONASS system has developed significantly since the fall of the Soviet Union. In addition to the original GLONASS satellites, Russia now has the GLONASS-M satellite with two civil frequencies, and will soon launch the GLONASS-K satellite, which has three civil frequencies. Of the 16 satellites on orbit, four are GLONASS-M satellites. Russia makes multiple satellite launches; as many as three GLONASS-M satellites and six GLONASS-K satellites can be launched at one time. Eighteen satellites are planned to be in operation before the end of 2006, with the complete constellation of 24 satellites operational by 2011.
At the present time, GLONASS satellites are used by at least three GPS manufacturers to complement the GPS satellites. I don't know of any receivers in North America that receive signals from only the GLONASS constellation.
GalileoThe European Space Agency (ESA) launched the first Galileo satellite, GIOVE A, in December 2005. This is the first satellite of the Galileo constellation, and it has three primary objectives:
- To secure use of the frequencies allocated by the International Telecommunications Union for the system.
- To demonstrate critical technologies for the navigation payload of future Galileo satellites.
- To assess the radiation environment of the orbits planned for the Galileo constellation.
The next major launch will be four satellites, which should be completed by 2008; full operational capability is to be completed by 2010. In all, there will be 30 satellites in three orbital planes.
Future System CoordinationWill the three satellite positioning systems have the same coordinate system? The answer is no. Both GLONASS and Galileo position in their own coordinate systems; the coordinate system for GPS, the World Geodetic System of 1984 (WGS 84), is a specific reference system for just GPS.
Referring again to the defense department's report on the future of GPS, there is an agreement between the United States and the European Union to make coordinates compatible. A transformation will be made from International Terrestrial Reference System (ITRF) coordinates to the Galileo Terrestrial Reference Frame (GTRF) and WGS 84. There have been three WGS 84 updates; the latest is the 2001 update WGS 84(G1150).
It would be more technically correct to define coordinates in the International Terrestrial Reference Frame (ITRF). In the United States and throughout the world, Continuously Operating Reference Stations (CORS) are positioned in ITRF. The NAD83 coordinates for those stations in the U.S. CORS network are also given because we use NAD83 for civil surveying applications. The National Geodetic Survey uses a 14-parameter transformation to convert from ITRF to NAD83.
Considering this information, should I change the name of my column to "The GNSS Observer"? Not in the foreseeable future. As my summary of the status of each system shows, GLONASS works as a complement to GPS, and Galileo is a few years away from being operational.