Potential Problems for Users of Modernized GPS Signals in Mixed-Mode Operations

June 15, 2006  - By

PRN 17, the first IIR-M satellite launched in September 2005, began broadcasting the second GPS civil signal, L2C, in December 2005. PRN 17 is the first in the new generation of GPS satellites with a new feature called flex power. According to the U.S. Air Force, flex power adds the capability for the Department of Defense to increase power on both P- and M-code (both military) signals to defeat low-level enemy jamming.

When flex power was enabled for testing (for a very short period of time), a problem was observed by certain GPS users. This problem was associated with the definition of the phase relationship between L2C and legacy L2 P/Y. In this scenario, users who are operating L1/L2/L2C GPS equipment, in conjunction with legacy L1/L2 GPS equipment, could have a problem maintaining carrier-phase ambiguity resolution with any modernized satellite operating in modes where signal phase relationships are changing or are unknown.

This is not just a flex power issue, but a potential issue with any new modernized GPS signal if provisions are not included to inform users in real time of signal phase relationships. This is potentially a long-term problem because there will be a mixed set of modernized/legacy signals for an extended period of time, as well as a mixed set of modernized/legacy user equipment. The important thing is that these potential problems can be fixed by broadcasting appropriate data in the GPS navigation messages in a timely manner.

This fix to this potential problem would slightly change the GPS user interface specifications and add bits for defining the phase relationship between the modernized and legacy signals. This data would have to be added to both the L1 and L2C signals since, for the time being, there is no data on the L2C signals. For L1C, (in the draft L1C specification) the phase relationship between L1C and L1 C/A has been defined. For L2 and L2C interoperability during modernization, a similar parameter to provide the phase relationship between the L2 P/Y and L2C is needed for mixed equipment processing. (Refer to Section subframe 3, page 7 signal phase of the newly released Draft IS-GPS-800 L1C specification dated April 19, 2006.)

Another possible solution is for L2C-capable receivers in a network to track both L2C and L2 P/Y simultaneously, to directly measure the phase difference between the two phases. However, the drawback is that the more robust L2C signal will be tracked at times when the legacy L2 P/Y cannot &#151 the main reason for implementing L2C in the first place.

— Eric Gakstatter
Contributing editor of the Survey & Construction newsletter

Eric Gakstatter

About the Author:

Eric Gakstatter has been involved in the GPS/GNSS industry for more than 20 years. For 10 years, he held several product management positions in the GPS/GNSS industry, managing the development of several medium- and high-precision GNSS products along with associated data-collection and post-processing software. Since 2000, he's been a power user of GPS/GNSS technology as well as consulted with capital management companies; federal, state and local government agencies; and private companies on the application and/or development of GPS technology. Since 2006, he's been a contributor to GPS World magazine, serving as editor of the monthly Survey Scene newsletter until 2015, and as editor of Geospatial Solutions monthly newsletter for GPS World's sister site Geospatial Solutions, which focuses on GIS and geospatial technologies.

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