See “Potential Problems for Users of Modernized GPS Signals in Mixed-Mode Operations” by Eric Gakstatter in The System news section, page 15, June issue.
Dear Mr. Gakstatter,
The Navstar GPS Joint Program Office (JPO) appreciates receiving inquiries regarding GPS signals, because it often helps us improve the signal documentation. We are particularly glad to field questions from GPS World because of your ability to “spread the word.”
We have entered a new era. During the 27 years since the first GPS satellite was launched in 1978, there have been only three navigation signals. When the first Block IIR-M satellite was launched on 25 September 2005, the number of navigation signals doubled with the addition of L2C and M codes on L1 and L2. Block IIF satellites will add L5, and Block III satellites will provide L1C. From the original three signals the number will grow at least to eight, not including the fact that L1C, L2C, L5, and the M codes each have two separate components, a pilot carrier and data. Further complicating the mix is the need to multiplex more than the two original signals on L1 and L2 while maintaining a constant total amplitude but permitting power to be shifted from one signal to another, that is, flex power. Also, instead of only two fixed message structures, now there are many, and the new ones are more flexible than before.
All of these changes are designed to improve performance, and they will. However, it is inevitable that in the process some confusion will result. For example, IS-GPS-200D addresses the phase relationship between L2P(Y) and L2C in one of the notes in Table 3-III (and similarly in paragraph 22.214.171.124) by stating: “The two carrier components on L2 [L2P(Y) and L2C] may not have the phase quadrature relationship. They may be broadcast on the same phase.” It was expected that this would inform users of the two possible phase relationships. However, only when receivers actually observed a phase change during on-orbit testing of the first IIR-M did the full implication of these few words sink in.
The purpose of such phase flexibility is to optimize a satellite’s power efficiency throughout its life and for each of its signal configurations. Unlike the past where the C/A and P(Y) phase relationship was defined and fixed, the multitude of new signals and their components may take on different phase relationships from time to time and from satellite generation to generation. This flexibility is needed to achieve the best overall results from signal modernization, but it may impose new tasks on user equipment and on differential messaging services.
For example, the JPO was not aware that L2C user equipment would employ cross-mode phase measurements between legacy (code-aided cross correlation) phase measurements and direct L2C phase measurements. In a common mode system, a very rare phase shift should cause little or no problem. (One question to commercial users is whether the satellite should be taken out of service briefly during such a phase change.) If cross-mode measurements are used, then the phase relationships must be known. This can be done by messaging, such as having the information in an almanac. However, it probably is better and certainly faster for the user equipment to monitor these relationships.
The JPO has had a long-standing process for dealing with such issues. It is called the Interface Control Working Group (ICWG). Information about the ICWG, including how to join, is available at: http://gps.losangeles.af.mil/engineering/icwg/. The JPO has long wanted more commercial participation so these types of questions can be discussed and resolved and the specifications improved. We invite you and your readers to participate, especially during this time of rapid signal modernization. Thank you.Respectfully, best regards,
— Mark C. Crews
U.S. Air Force, Navstar GPS Chief Engineer