Potential Problems for Users of Modernized GPS Signals inMixed-Mode Operations

When the new flex power feature aboard PRN 17, the first IIR-M GPS satellite,
was enabled for testing (for a very short period of time), a problem was observed
by certain GPS users.

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 3.5.4.6 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 — the main reason
for implementing L2C in the first place.