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SHARED HISTORY AND STATUS OF GPS AND COMPASS

Mr. David Turner, deputy director of space and advanced technology, U.S. Department of State, provided an update on GPS constellation status and current U.S. government GPS policy, programs, and international activities. His comments are now summarized.

At the time of the workshop, the GPS constellation consisted of 31 operational satellites, although the baseline is a constellation of 24 satellites. These operational satellites included: 11 GPS Block IIA satellites, 12 GPS Block IIR satellites, 7 GPS Block IIR-M satellites, and 1 GPS Block IIF satellite. The GPS IIA satellites initiated the Block II series. Nineteen GPS IIA satellites were launched—the first on November 26, 1990, and the last on November 6, 1997. The Block IIR series are “replenishment” satellites developed by Lockheed Martin. Each satellite weighs 4,480 pounds (2,030 kg) at launch and 2,370 pounds (1,080 kg) once on orbit. The first successful IIR launch was in July 1997. The Block IIR-M satellites are modernized IIR satellites that include a new military signal and a second civil signal, known as L2C. The first IIR-M satellite was launched in September 2005. The Block IIF series are “follow-on” satellites developed by Boeing. Boeing is under contract to build a total of 12 Block IIF satellites. The Block IIF provides a third frequency for civil use (L5), allowing position determinations with even higher precision. The first Block IIF satellite was launched in May 2010.

Looking forward, GPS modernization will result in a variety of new signals, including civil signals L2C, L5, and L1C, all at different frequencies. L2C contains two distinct pseudo random noise (PRN) code sequences to provide ranging information—the Civilian Moderate length code (CM) and the Civilian Long length code (CL). The CM code is 10,230 bits long, repeating every 20 ms. The CL code is 767,250 bits long, repeating every 1,500 ms. Each signal is transmitted at 511,500 bits per second (bits/s); however, they are multiplexed together to form a 1,023,000 bits/s signal. CM is modulated with the navigation messages, whereas CL does not contain any modulated data and is called a dataless sequence. The L5 signal also contains two PRN ranging codes: the in-phase code (denoted as the I5-code); and the quadrature-phase code (denoted as the Q5-code). Both codes are 10,230 bits long and transmitted at 10.23 MHz.

The L1C civil signal is a more robust signal broadcast on the legacy L1 frequency (1575.42 MHz), which contains the coarse/acquisition (C/A) signal used by all current GPS users. The PRN codes are 10,230 bits long and transmit at 1.023 Mbps. It uses both pilot and data carriers like L2C. The modulation techniques used are BOC(1,1) for the data signal and TMBOC (time multiplexed binary offset carrier) for the pilot. Of the total L1C signal power, 25 percent is allocated to the data and 75 percent to the pilot.

GPS performance is better than ever and will continue to improve. U.S policy encourages worldwide use of civil GPS and augmentation systems. International



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