Innovation: The GPS L2C Signal

October 1, 2008 By: Liliána Sükeová, Marcelo C. Santos, Richard B. Langley, Rodrigo F. Leandro, Thatchineswaran Thirumurthi GPS World

A Preliminary Analysis of Data Quality

INNOVATION INSIGHTS with Richard Langley

FIFTY-SIX AND COUNTING. That's the number of GPS satellites that have been launched over the past thirty years beginning with the first prototype (Block I) satellite, space vehicle number 1, in February 1978. Ten Block I satellites were successfully launched between 1978 and 1985 to demonstrate the feasibility of GPS.

Richard Langley

The first satellite of the Block II operational constellation was launched in February 1989. The four year hiatus in launches was due, in part, to the Space Shuttle Challenger disaster as it had been planned to launch the operational satellites using the Shuttle. Following the accident, it was decided to continue with expendable rockets for GPS launches but to switch to the newly designed Delta II rocket.

The pace of Block II launches was rapid with five launches of the original Block II design in 1989 and four in 1990. A modified version of the Block II satellite — the IIA —was developed and between 1990 and 1997, 19 Block IIAs were launched. The Block II and IIA satellites established the operational GPS constellation. Full operational capability was declared on April 27, 1995.

A new satellite was developed for replenishing the constellation as the earlier satellites were retired. Following an initial launch failure, twelve of the Block IIR satellites were launched between 1997 and 2004. All of the satellites in the Block I, Block II, Block IIA, and Block IIR constellations transmitted what are now called the legacy signals: the C/A-code on the L1 frequency of 1575.42 MHz and the P-code on L1 and the L2 frequency of 1227.60 MHz. The P-code has been encrypted to yield the Y-code since January 1994, denying its direct access by most civil users.

Since the C/A-code was only transmitted on the L1 frequency, civil users have had to rely on suboptimal semicodeless techniques for the dual-frequency operation necessary for direct cancellation of ionospheric biases. In 1998, Vice-President Al Gore announced that a new civil signal on L2 would be transmitted by future GPS satellites. This new signal–L2C–joined the legacy signals beginning with the launch of modernized Block IIR satellites. Six of these Block IIR-M satellites have been launched to date.

In this month's column, we'll overview the characteristics of the new L2C signal and take a look at some of the analyses of received signals carried out by a team of researchers from the University of New Brunswick.

"Innovation" is a regular column that features discussions about recent advances in GPS technology and its applications as well as the fundamentals of GPS positioning. The column is coordinated by Richard Langley of the Department of Geodesy and Geomatics Engineering at the University of New Brunswick, who welcomes your comments and topic ideas. To contact him, see the "Contributing Editors" section.

Under the GPS modernization program, two new civil signals are being added to the legacy signals transmitted by the GPS satellites. The new signals are L2C, or L2 civil, and L5.

Table 1 In-orbit Block IIR-M satellites

The L2C signal is to be transmitted on the L2 carrier frequency (1227.60 MHz) by all Block IIR-M and Block IIF satellites along with the legacy P(Y)-code and the new military M-code. The L2C signal is already being transmitted by six Block IIR-M satellites now in orbit (see TABLE 1 and FIGURE 1). Two more Block IIR-M satellites are scheduled to be launched.