GPS Satellite Signal Characteristics - Understanding GPS: Principles and Applications - page 123

provide additional design parameters for waveform designers to use. The resulting
modulation designs can provide enhanced performance when bandwidth is limited
(due to implementation constraints at transmitter and receiver, or due to spectrum
allocations). Also, modulations can be designed to better share limited frequency
bands available for use by multiple GNSS. The spectra can be shaped in order to
limit interference and otherwise spectrally separate different signals. In order to
obtain adequate performance, such modulation design activities must carefully con-
sider a variety of signal characteristics in the time and frequency domains, and they
should not concentrate exclusively on spectrum shape.

4.3 Legacy GPS Signals

This section details the legacy GPS navigation signals—that is, those navigation sig-
nals that are broadcast by the GPS SVs up through the Block IIR class (see Chapter
3). The legacy GPS SVs transmit navigation signals on two carrier frequencies called
L1, the primary frequency, and L2, the secondary frequency. The carrier frequen-
cies are DSSS modulated by spread spectrum codes with unique PRN sequences
associated with each SV and by a common navigation data message. All SVs trans-
mit at the same carrier frequencies in a CDMA fashion. In order to track one SV in
common view with several other SVs by the CDMA technique, a GPS receiver must
replicate the PRN sequence for the desired SV along with the replica carrier signal,
including Doppler effects. Two carrier frequencies are required to measure the iono-
spheric delay, since this delay is related by a scale factor to the difference in signal
TOA for the two carrier frequencies. Single frequency users must estimate the iono-
spheric delay using modeling parameters that are broadcast to the user in the navi-
gation message. (Further information on ionospheric delay compensation is
contained in Section 7.2.4.1.) The characteristics of the legacy GPS signals are
further explained in the following sections.

4.3.1 Frequencies and Modulation Format
A block diagram that is representative of the SV signal structure for L1 (154 f 0 ) and
L2 (120 f 0 ) is shown in Figure 4.5 (where f 0 is the fundamental frequency: 10.23
MHz). As shown in Figure 4.5, the L1 frequency (154 f 0 ) is modulated by two PRN
codes (plus the navigation message data), the C/A code, and the P code. The L2 fre-
quency (120 f 0 ) is modulated by only one PRN code at a time. One of the P code
modes has no data modulation. The nominal reference frequency, f 0 , as it appears to
an observer on the ground, is 10.23 MHz. To compensate for relativistic effects, the
output of the SV's frequency standard (as it appears from the SV) is 10.23 MHz off-
set by a ∆ f / f of 4.467 × 10 −10 (see Section 7.2.3). This results in a ∆ f of 4.57 × 10 −3 Hz

10.22999999543 MHz [10]. To the GPS receiver on the ground, the C/A
code has a chipping rate of 1.023

and f 0 =

10 6 chips/s ( f 0 /10 × = 1.023 MHz) and the P code

has a chipping rate of 10.23 10.23 MHz). Using the notation

introduced in Section 4.2.3, the C/A code signal uses a BPSK-R(1) modulation and
the P code uses a BPSK-R(10) modulation. The P code is available to PPS users but
not to SPS users since the CS normally configures an AS mode in the SV. When AS is

× 10 6 chips/s ( f 0 =