GPS System Segments - Understanding GPS: Principles and Applications - page 106

New civil signals L2C and L5 have null-to-null bandwidths of 2.046 MHz and
20.46 MHz, respectively. The military M code can be processed within the exist-
ing L1 and L2 24-MHz bandwidths. Since M code signal power is defined
within a 30.69-MHz band around the center frequency, approximately 92% of
this power is within the 24-MHz band. (GPS signal characteristics are contained in
Chapter 4.)
The addition of new signals (M code, L1C, L2C, and L5) will require new anten-
nas for some users. For example, those utilizing L1 C/A code and L2C will need a
dual-band antenna. (Dual frequency measurements enable determination of the ion-
ospheric delay and provide robustness to interference. Ionospheric delay determina-
tion and compensation are discussed in Chapter 7.) SOL signal users that require
operation in the ARNS bands will need antennas to receive C/A code on L1 and the
L5 signal on L5. At the time of this writing, RTCA was developing aviation stan-
dards for a dual-band L1/L5 antenna. Some receivers may be tri-band. That is, they
will receive and process the signals broadcast on all three GPS frequencies, L1, L2,
and L5, which will require a tri-band antenna. Reference [33] provides details on
one approach for a tri-band (L1/L2 M code and L5) antenna design.
Antenna designs vary from helical coils to thin microstrip (i.e., patch) antennas.
High-dynamic aircraft prefer low-profile, low-air resistance patch antennas,
whereas land vehicles can tolerate a larger antenna. Antenna selection requires eval-
uation of such parameters as antenna gain pattern, available mounting area, aerody-
namic performance, multipath performance, and stability of the electrical phase
center of the antenna [34].
Another issue regarding antenna selection is the need for resistance to interfer-
ence. (In the context of this discussion, any electronic emission, whether friendly or
hostile, that interferes with the reception and processing of GPS signals is considered
an interferer.) Some military aircraft employ antenna arrays to form a null in the
direction of the interferer. Another technique to mitigate the effects of interference is
to employ a beam-steering array. Beam-steering techniques electronically concen-
trate the antenna gain in the direction of the satellites to maximize link margin.
Finally, beam forming combines both nulling and beam steering for interferer miti-
gation. (References [35-37] provide detailed descriptions of the theory and practical
applications of nulling, beam steering, and beam forming.)
3.4.1.2 Receiver
Chapter 5 provides a detailed description of receiver signal acquisition and tracking
operation; however, some high-level aspects are described herein to aid our discus-
sion. Two basic receiver types exist today: (1) those that track L1 C/A code and P(Y)
code on L1 and L2 and (2) those that only track C/A code. In light of the GPS mod-
ernization effort, these are referred to as legacy receivers. Forthcoming military
receivers are being referred to as YMCA. That is, they will track L1 C/A, L1 and L2
P(Y), and L1 and L2 M code. The forthcoming civil signals, L1C, L2C, and L5, will
require new receivers to be built. It is envisioned that a number of receiver types will
be available. Most likely, these will be dual band to achieve ionospheric compensa-
tion and increased interference immunity. As mentioned earlier, ARNS band users
will require dual band (L1 and L5) receivers and antennas.