GALILEO - Understanding GPS: Principles and Applications - page 577

cast to the dedicated receiving ground stations in the 1,544-1,545-MHz band,
called L6 (below the E2 navigation band and reserved for the emergency services).
The SAR data, from SAR operators to distress-emitting beacons, will be used for
alert acknowledgment and coordination of rescue teams, and it will be embedded in
the OS data of the signal transmitted in the E2-L1-E1 carrier frequency.

10.4 Interoperability Between GPS and GALILEO

Interoperability simply means combining information (e.g., pseudorange measure-
ments, navigation data) from both the GPS and GALILEO systems at the user
receiver to achieve better performance than employing either system separately.
The level of interoperability is a result of an optimization process. Factors con-
sidered in this process were radio frequency compatibility (RFC), complexity of the
user equipment (receiver design), market prospects, vulnerability (common mode of
failures), independence of the systems, and national security compatibility issues.
Looking closer to the interoperability of GPS and GALILEO, three topics are of
primary importance:

• Signal in space;
•

Geodetic coordinate reference frame;

Time reference frame.

•

10.4.1 Signal in Space
The software implementation of the navigation message data processing in a user
receiver allows de facto interoperability between GALILEO and GPS for the mes-
sage data layer, at reduced receiver implementation cost and complexity. However,
more consideration was needed for the physical layer for the choice of GALILEO
signals carrier frequencies with respect to the GPS signals carrier frequencies. Pro-
cessing two signals with different carrier frequencies requires increased receiver
front-end complexity, as two separate down-conversion channels would be
required. Furthermore, processing two signals with different carrier frequencies
may cause frequency biases in the navigation solution. In particular, the high-preci-
sion position solutions that use carrier phase data may be degraded in terms of accu-
racy. GALILEO/GPS interoperability will be realized by a partial frequency overlap
with sometimes different signal structures or different code sequences. At E5a and
E2-L1-E1, GALILEO and GPS signals will be broadcast using identical carrier fre-
quencies to L5 and L1, respectively. This will drastically simplify user receiver RF
front-end design.
This simplification in RF front-end design comes, however, at the cost of
intersystem interference between both systems due to the overlapping of signal spec-
trum accounting for their respective modulations. In the L1 band, the level of
intersystem interference between secured or restricted-access signals and
nonsecured or open-access signals has been limited by ensuring a sufficient spectral
separation between them. This will allow, for instance, the jamming of civil signals
without affecting GPS M code or the GALILEO PRS signal. By contrast, the choice