subframe 4 includes ionospheric correction parameters for single-frequency users (see Section 7.1.2.5) and parameters so that user equipment can relate UTC to GPS system time (see Section 2.6.3). Page 25 of subframes 4 and 5 provide configuration and health flags for SVs 1–32. The data payloads of the remaining pages of subframes 4 and 5 are currently reserved.
L2C signal is formed by the chip-by-chip multiplexing of the CM (with data) and CL codes. The fact that L2C devotes one-half its power to a component without data (CL) is an important design feature shared by the other modernized GPS signals.
This feature enables very robust tracking of the signal by a GPS receiver (see Section 5.3.1).
The L2C signal has an overall chip rate of 2 × 511.5-kchip/s rate = 1.023 Mchip/s, which accounts for its similar power spectrum to the C/A code. There are important differences between the L2C and C/A code signal power spectra, how- ever. Since both CM and CL are much longer than the length-1,023 C/A code, the maximum lines in the L2C power spectrum are far lower than the maximum lines in the C/A code power spectrum. As will be discussed in Chapter 6, the lower lines in the L2C power spectrum lead to greatly increased robustness in the presence of narrowband interference.
The CM and CL codes are generated using the same 27-stage linear feedback shift register shown in Figure 4.22. A shorthand notation is used in the diagram. The number that appears in each block in the figure represents the number of stages (each holding 1 bit) between feedback taps. CM and CL codes for different satellites
10,230 chip-code generator
767,250 chip-code generator
Navigation message (25 bps)
Chip-by-chip multiplexer
1.023-MHz clock 1/2
Rate 1/2 FEC
511.5 kHz clock
L2C signal
CL code CM code
Figure 4.21 Baseband L2C signal generator.
Shift direction Initial conditions
1 3
1 1
3 3 2 3 3 2 2 3
Output
Figure 4.22 CM and CL PRN code generation.
are generated by different initial loads of the register. The register is reset every 10,230 chips for CM and every 767,250 chips for CL. The CM code repeats 75 times for each repetition of the CL code. At the 511.5-kchip/s rate, the period of the CM code is 20 ms (one P(Y) code data bit period) and the period of the CL code is 1.5 seconds (one X1 epoch or Z-count).
The rate one-half constraint-length FEC scheme used to encode the 25-bps L2C navigation data into a 50-baud bit stream is shown in Figure 4.23.
The minimum specified received L2C power level for signals broadcast from the Block IIR-M and IIF satellites is−160 dBW [10].
4.5.2 L5
The GPS L5 signal is generated as shown in Figure 4.24. QPSK is used to combine an in-phase signal component (I5) and a quadraphase signal component (Q5). Dif- ferent length-10,230 PRN codes are used for I5 and Q5. I5 is modulated by 50-bps navigation data that, after the use of FEC using the same convolutional encoding as L2C, results in an overall symbol rate of 100 baud. A 10.23-MHz chipping rate is employed for both the I5 and Q5 PRN codes resulting in a 1-ms code repetition period.
G1 (171 octal) G2 (133 octal)
Data input (25 bps)
Symbol clock
Output symbols (50 sps ) (Alternating G1/G2)
Figure 4.23 L2C data convolution encoder.
L5 data
message Add
CRC
10–symbol Neuman- Hofman code
Code generator 10.23-MHz
Code Clock 1 ms epochs
XI(t) 1 kbaud
XQ(t) Encode
with FEC 100 sps
1 kbaud 276 bits 300 bits
50-Hz data clock QPSK
modulator L5 Signal
Carrier 100-Hz Symbol Clock
I5
Q5 20–symbol
Neuman- Hofman code Figure 4.24 L5 signal generation.
Neuman-Hofman (NH) synchronization codes [6] are modulated upon I5 and Q5 at a 1-kbaud rate. For I5, the 10-symbol NH code 0000110101 is generated over a 10-ms interval and repeated. For Q5, the 20-symbol NH code 00000100110101001110 is used. Every 1 ms, the current NH code bit is modulo-2 added to the PRN code chip. For example, on I5, the PRN code repeats 10 times over each 10-ms interval. During this interval, the PRN code is generated normally (upright) for repetitions 1–4, 7, and 9 (the zero bits in the I5 NH code 0000110101) and is inverted over repetitions 5, 6, 8, and 10 (corresponding to the set bits in the I5 NH code). The start of the I5 NH code is aligned with the start of each 10-ms data symbol that results from the FEC encoding. The Q5 NH code is synchronized with the 20-ms data bits.
The I5 and Q5 PRN codes are generated using the logic circuit shown in Figure 4.25, which is built around three 13-bit linear feedback shift registers. Every 1 ms, the XA coder is initialized to all 1s. Simultaneously, the XBI and XBQ coders are ini- tialized to different values, specified in [18], to yield the I5 and Q5 PRN codes.
The minimum specified received L5 power level for signals broadcast from the Block IIF satellites is –154.9 dBW [18].
4.5.3 M Code
The modernized military signal (M code) is designed exclusively for military use and is intended to eventually replace the P(Y) code [19]. During the transition period of
1 2 3 4 5 6 7 8 9 10 11 12 13
1 2 3 4 5 6 7 8 9 10 11 12 13 Exclusive OR
Initial XBI state
Exclusive OR All 1s
1-ms epoch Code clock
XA(t) XA coder
XBI coder XBI State for SV i
Reset
XBQ(t+niTc) XBI(t+n T )i c XI (t)i
XQ (t)i
1 2 3 4 5 6 7 8 9 10 11 12 13 Initial XBQ state
Exclusive OR XBQ coder
XBQ state for SV i
Decode 1111111111101 Reset to all 1 second
on next clock
Figure 4.25 I5 and Q5 PRN code generation.
replacing the GPS constellation with modernized SVs, the military user equipment will combine P(Y) code, M code, and C/A code operation in the so-called YMCA receiver. The primary military benefits that M code provides are improved security plus spectral isolation from the civil signals to permit noninterfering higher power M code modes that support antijam resistance. Other benefits include enhanced tracking and data demodulation performance, robust acquisition, and compatibil- ity with C/A code and P(Y) code. It accomplishes these objectives within the existing GPS L1 (1,575.42 MHz) and L2 (1,227.60 MHz) frequency bands.
To accomplish the spectral separation shown in Figure 4.20, the new M code employs BOC modulation [3]. Specifically, M code is a BOCs(10,5) signal. The first parameter denotes the frequency of an underlying squarewave subcarrier, which is 10×1.023 MHz, and the second parameter denotes the underlying M code genera- tor code chipping rate, which is 5×1.023 Mchip/s. Figure 4.26 depicts a very high level block diagram of the M code generator. It illustrates the BOC square wave modulation of the underlying M code generator that results in the split spectrum signals of Figure 4.20.
M code BPSK-R(5) generator
2fCO= 10.23 MHz fCO=
5.115 MHz
BOC (10,5) M-codes Square wave
Figure 4.26 M code signal generation.
Table 4.10 Summary of GPS Signal Characteristics
Signal
Center Frequency (MHz)
Modulation Type
Data Rate (bps)
Null-to-Null Bandwidth
(MHz)* PRN Code Length
L1 C/A code 1,575.42 BPSK-R(1) 50 2.046 1023
L1 P(Y) code 1,575.42 BPSK-R(10) 50 20.46 P: 6187104000000
Y: cryptographically generated L2 P(Y) code 1,227.6 BPSK-R(10) 50 20.46 P: 6187104000000
Y: cryptographically generated
L2C 1,227.6 BPSK-R(1) 25 2.046 CM: 10,230
CL: 767,250 (2 PRN sequences are chip-by-chip multiplexed)
L5 1,176.45 BPSK-R(10) 50 20.46 I5: 10,230
Q5: 10,230
(two components are in phase quadrature)
L1 M code 1,575.42 BOC(10,5) N/A 30.69* Cryptographically generated L2 M code 1,227.6 BOC(10,5) N/A 30.69* Cryptographically generated
L1C 1,575.42 BOC(1,1) N/A 4.092* N/A
* For binary offset carrier modulations, null-to-null bandwidth is defined here as bandwidth between the outer nulls of the largest spectral
*lobes.
The M code signal will be broadcast through the Earth-coverage L-band antenna on the Block IIR-M and later GPS satellites. The minimum anticipated Earth-coverage M code power level is−158 dBW on L1 [19]. For Block III and later GPS satellites, a higher power M code signal is also planned to be broadcast in lim- ited geographic regions. The minimum received power for this higher powered sig- nal, referred to asspot beamM code, is anticipated to be−138 dBW [19].
4.5.4 L1 Civil Signal
The United States is planning to add a modernized civil signal upon the L1 frequency within the Block III time frame [20]. The design of this new signal, referred to as L1C, was still underway at the time of this writing. The modulation will likely be BOCs(1,1), based upon the recommendations from [20].