SATELLITE SYSTEMS - GPS Satellite Surveying - page 85

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45

and IIF will have two additional civil ranging codes, L2CM (civil moderate length)
and L2CL (civil long). As is the case for L1, the new L2 carrier will consist of two
BPSK modulated carrier components that are in phase quadrature with each other.
The inphase carrier will continue to be BPSK modulated by the bit train that is the
modulo-2 sum of the military P(Y)-code and the legacy navigation data D p (t) . There
will be three options available for BPSK modulating the quadrature carrier (also
called the L2C carrier or the new L2 civil signal):

1. Chip-by-chip time multiplex combinations of bit trains consisting of the
modulo-2 sum of the L2CM code and a new navigation message structure
D C (t) . The resultant bit trains are then combined with the L2CL code and used
to modulate the L2 quadrature carrier. The IIR-M space vehicles will have the
option of using the old navigation message D p (t) instead of D C (t) .
2. Modulo-2 sum of the legacy C/A-code and legacy navigation data D p (t) .
3. C/A-code with no navigation data.

[85

Lin
—
7.5
——
Lon
PgE

Th e chipping rate for L2CM and L2CL is 511.5 Kbps. L2CM is 10,230 chips long and
las ts 20 ms, whereas L2CL has 767,250 chips and lasts 1.5 s. L2CL is 75 times longer
th an L2CM. D C (t) is the new navigation data message and has the same structure as
th e one adopted for the new L5 civil signal. It is both more compact and more flexible
th an the legacy message.

3. 2.3.2 Civil L5 Code The carrier frequency of L5 is 1176.45 MHz. As is the
ca se for L1, two L5 carriers are in phase quadrature and each is BPSK modulated
se parately by bit trains. The bit train of the inphase component is a modulo-2 sum of
PR N codes and navigation data. The quadraphase code is a separate PRN code but
ha s no navigation data. The chipping rate of the codes is 10.23 MHz. Each code is
a modulo-2 sum of two subsequences, whose lengths are 8,190 and 8,191 chips that
re cycle to generate 10,230 chip codes. Therefore the length of these codes is 1 ms.

[85

3.2.3.3 M-Code For conventional rectangular spreading codes, which are the
basis of the P(Y)-codes, the C/A-code heritage signals, and the new L2C and L5
codes, the frequency bandwidth is inversely proportional to the length of the chip.
Modulating with faster chipping rates to improve or add additional signals might be
impractical because of frequency bandwidth limitations. More advanced modulations
have been studied recently that better share existing frequency allocations with each
other and with heritage signals by increasing spectral separation, and thus preserve the
spectrum. Betz (2002) describes binary-valued modulations, also referred to as binary
offset carrier (BOC). Block IIR-M and IIF satellites will transmit a new military M-
code signal on L1 and L2 that uses BOC.
If f c denotes again the chipping (code) rate and if we denote the subcarrier fre-
quency by f s then (Betz, 2002)

1
nT s =

2
n f s

f c = (3.89)