GPS Modernization Steps Forward

  • Published
  • Global Positioning Systems Wing
In the fall of 2009, the U.S. Air Force will achieve another milestone on the path toward GPS modernization. Operators at the 50th Space Wing and the 2nd Space Operations Squadron will upload new software to the IIR-M satellites enabling the first broadcast of a transitional Civil Navigation (CNAV) message on the already transmitted second civil signal (L2C). Once the go-ahead is given, the new message will broadcast on the L2C signal emanating from the IIR-M satellites. With the upcoming launches of the remaining two IIR-Ms, there will be a total of eight modernized GPS satellites broadcasting L2C with the new CNAV message. Additionally, the IIF satellites will be launched with the built-in capability to broadcast the new message. 

The first steps toward GPS modernization began with a White House press release on March 30, 1998, announcing that the United States intended to modernize its GPS Position, Navigation, and Timing (PNT) service with the addition of a second civil signal on the L2 frequency (1227.60 MHz). In 2001, the then GPS Joint Program Office at the Space and Missile Systems Center, Los Angeles AFB, CA, and the civil community began work defining the implementation of the new signal. Many expected a replication of the L1 C/A signal, however, a modernized L2C signal was designed with several significant advantages over L1 C/A. The L2C signal will provide a lower tracking threshold and better cross-correlation protection than L1 C/A. The data portion of the L2C signal is also different; instead of the current "legacy" navigation (LNAV) structure with subframes of data repeating in a fixed pattern as on L1 C/A, the CNAV structure which will be used on L2C has individual messages which can be broadcast in a flexible order with variable repeat cycles. The CNAV structure, as defined in Interface Specification (IS)-GPS-200D, 7 Dec 2004, (available online at allows up to 63 different message types, of which 15 types have already been defined. The 15 CNAV message types will be incrementally phased in over time, with the first CNAV message to broadcast being the "default message," also known as Message Type 0. Message Type 0 consists of a twelve second, 300 bit long message including the preamble, satellite pseudorandom noise (PRN) number, message type ID (=0), GPS Time of Week, a sequence of alternating 1s and 0s, and a Cyclic Redundancy Check (CRC) parity block. The GPS Time of Week will change every 12 seconds, as will the CRC bits. 

The addition of the L2C signal, with the modernized CNAV messages, is only part of the larger GPS modernization program. In addition to L2C, a third civil signal will be added centered at the L5 frequency (1176.45 MHz) as well as M-Code signals on L1 and L2. L5 will also carry CNAV messages and will broadcast from the IIF satellites and the GPS III satellites. 

The United States continues to modernize the GPS constellation and ground segment to bring added capability to civil and military users alike. Adding the first type of CNAV message to L2C is one more step in the roll-out of the overall GPS modernization plan.