**10. GLONASS signal characteristics**

The GLONASS Interface Control Document (ICD) held by the Russian Institute of Space Device Engineering provides the detailed information about the structure of the GLONASS radio signals [22]. In contrast to GPS, GLONASS uses frequency division multiple access (FDMA) for signal modulation. This technique uses the same pseudorandom noise (PRN) code for all satellites to produce a spread spectrum signal. GPS, on the other hand, uses codedivision multiple access (CDMA) to identify each individual satellite. FDMA provides better interference rejection for narrow-band interference signals compared to CDMA. In CDMA a single source of narrow-band interference source can disrupt all GPS satellite signals simultaneously, such interference only affects one FDMA GLONASS signal at a time. A shortcoming of FDMA, however, is that it requires more spectrum than CDMA systems. GLONASS uses L1 in the range of 1602.0–1615.5 MHz and L2 in the range of 1246.0–1256.5 MHz to transmit C/A code and P code.

#### **10.1. GLONASS RF frequency plan**

The nominal values of L1 and L2 carrier frequencies are expressed as [22]

$$f\_{\rm t1} = f\_{\rm ou} + K\Delta f\_{\rm l1} \tag{3}$$

$$f\_{t2} = f\_{t2} + K\Delta f\_2 \tag{4}$$

seventh bit of a nine-bit shift register. The code is described by the irreducible polynomial

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The GLONASS P code is a 5.11-million-bit-long binary sequence. It is modulated onto the car-

The intra-system interference in GLONASS is due to the intercorrelation properties of the ranging codes and the used FDMA technique [22]. The interference, indeed, happens inside the receiver between the signals transmitted on frequency channel K = n and signals transmitted on neighbor channels K = n + 1 and K = n − 1. In other words, this interference occurs when

. The initial state is defined as each bit containing the value "1" [22].

1 + x<sup>5</sup> + x<sup>7</sup>

**10.4. High accuracy ranging code (P code)**

**Figure 7.** GLONASS antipodal satellites [3].

**10.5. Intra-system interference**

rier signal at a rate of 5.11 MHz and, hence, repeats every 1 s [3].

satellites with adjacent frequencies are visible at the same time.

where K is frequency channel number of the signals transmitted by GLONASS satellites in the L1 and L2 sub-bands:

$$\begin{aligned} \text{f}\_{\text{o1}} &= 1602 \text{ MHz}; \Delta \text{f}\_1 = 562.5 \text{ kHz}, \text{for sub-band L1} \\\\ \text{f}\_{\text{o2}} &= 1246 \text{ MHz}; \Delta \text{f}\_2 = 437.5 \text{ kHz}, \text{for sub-band L2} \end{aligned}$$

Each satellite has a standard nominal frequency, with a value of 5.0 MHz that generates the carrier frequencies L1 and L2. The system uses 12 channels to switch among its 24 operational satellites. Antipodal satellites in the same orbit plane are separated by an argument of latitude of 180°, as illustrated in **Figure 7** [26].

#### **10.2. GLONASS signal structure**

GLONASS satellites, too, transmit two PRN codes: a coarse acquisition (C/A) code and a precise (P) code. The C/A code is transmitted only on the L1 frequency, while the P code is transmitted on both L1 and L2 frequencies. GLONASS uses bi-phase modulation to merge the carrier signal with a modulo-2 summation of the PRN code at a rate of 511 kHz, the navigation message at a rate of 50 bps, and a 100 Hz auxiliary meander sequence [21].

GLONASS-K satellites also broadcast new CDMA signals in the L3-band at a carrier frequency of 1202.025 MHz [23]. The chipping rate for the ranging code is 10.23 Mcps, and it repeats every 1 ms. The new signal, however, uses a quadrature phase shift keying (QPSK) technique with an in-phase channel dedicated for data and a quadrature channel for pilot information. This signal spectrum is depicted in **Figure 8**.

#### **10.3. Standard accuracy ranging code (C/A code)**

The C/A code is a 511-bit binary sequence that is modulated onto the carrier frequency at a chipping rate of 0.511 MHz and thus repeats every millisecond [3]. It is derived from the

**Figure 7.** GLONASS antipodal satellites [3].

of FDMA, however, is that it requires more spectrum than CDMA systems. GLONASS uses L1 in the range of 1602.0–1615.5 MHz and L2 in the range of 1246.0–1256.5 MHz to transmit

The nominal values of L1 and L2 carrier frequencies are expressed as [22]

f01 = 1602 MHz; Δf<sup>1</sup> = 562.5 kHz, for sub − band L1

f02 = 1246 MHz; Δf<sup>2</sup> = 437.5 kHz, for sub − band L2

message at a rate of 50 bps, and a 100 Hz auxiliary meander sequence [21].

information. This signal spectrum is depicted in **Figure 8**.

**10.3. Standard accuracy ranging code (C/A code)**

*<sup>k</sup>*<sup>1</sup> = *f*

*<sup>k</sup>*<sup>2</sup> = *f*

<sup>01</sup> + *K*Δ *f*

<sup>02</sup> + *K*Δ *f*

where K is frequency channel number of the signals transmitted by GLONASS satellites in

Each satellite has a standard nominal frequency, with a value of 5.0 MHz that generates the carrier frequencies L1 and L2. The system uses 12 channels to switch among its 24 operational satellites. Antipodal satellites in the same orbit plane are separated by an argument of latitude

GLONASS satellites, too, transmit two PRN codes: a coarse acquisition (C/A) code and a precise (P) code. The C/A code is transmitted only on the L1 frequency, while the P code is transmitted on both L1 and L2 frequencies. GLONASS uses bi-phase modulation to merge the carrier signal with a modulo-2 summation of the PRN code at a rate of 511 kHz, the navigation

GLONASS-K satellites also broadcast new CDMA signals in the L3-band at a carrier frequency of 1202.025 MHz [23]. The chipping rate for the ranging code is 10.23 Mcps, and it repeats every 1 ms. The new signal, however, uses a quadrature phase shift keying (QPSK) technique with an in-phase channel dedicated for data and a quadrature channel for pilot

The C/A code is a 511-bit binary sequence that is modulated onto the carrier frequency at a chipping rate of 0.511 MHz and thus repeats every millisecond [3]. It is derived from the

<sup>1</sup> (3)

<sup>2</sup> (4)

C/A code and P code.

the L1 and L2 sub-bands:

**10.1. GLONASS RF frequency plan**

132 Multifunctional Operation and Application of GPS

*f*

*f*

of 180°, as illustrated in **Figure 7** [26].

**10.2. GLONASS signal structure**

seventh bit of a nine-bit shift register. The code is described by the irreducible polynomial 1 + x<sup>5</sup> + x<sup>7</sup> . The initial state is defined as each bit containing the value "1" [22].

#### **10.4. High accuracy ranging code (P code)**

The GLONASS P code is a 5.11-million-bit-long binary sequence. It is modulated onto the carrier signal at a rate of 5.11 MHz and, hence, repeats every 1 s [3].

#### **10.5. Intra-system interference**

The intra-system interference in GLONASS is due to the intercorrelation properties of the ranging codes and the used FDMA technique [22]. The interference, indeed, happens inside the receiver between the signals transmitted on frequency channel K = n and signals transmitted on neighbor channels K = n + 1 and K = n − 1. In other words, this interference occurs when satellites with adjacent frequencies are visible at the same time.

**11.1. Time reference systems**

**11.2. GLONASS time system**

*t*

measurements given in UTC(SU):

*t*

where

where

t

τn(tb

γn(tb

difference between the two time scales.

Both GPS and GLONASS have their own time systems; thus, it is not straight forward to make time transformation from GLONASS time into GPS time or vice versa. The most important factor one must account for when processing data from a combined GPS and GLONASS is the

As can be seen in **Table 2**, the daily satellite clock stability for GLONASS, GLONASS-M, and GLONASS-K is better than 5 × 10−13, 1 × 10−13, and 5 × 10−14, respectively. The time shift between

The following expression is used to align GLONASS satellite ephemeris at one instance with

t time of transmission of the navigation signal in the onboard time scale,

) correction to nth satellite time relative to GLONASS time at time tb

GLONASS-M satellites transmit the difference between the GPS and GLONASS time scale

GLONASS time could be transformed into GPS time using the following formula [27]:

*GPS* = *t*

*<sup>b</sup>*) − *γn*(*t*

) relative deviation of the predicted carrier frequency value of n-satellite from

*<sup>b</sup>*)(*t* − *t*

*UTC*(SU) + 03h 00mins (5)

*GLONASS* + *τ<sup>c</sup>* + *τ<sup>u</sup>* + *τ<sup>g</sup>* (7)

*GLONASS* (8)

*<sup>b</sup>*) (6)

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,

GLONASS time and the National Reference Time UTC (SU) is 3 h ICD (2008):

*GLONASS* = *t* + *τ<sup>c</sup>* + *τn*(*t*

τ<sup>c</sup> GLONASS time scale correction to UTC (SU) time,

.

<sup>b</sup> index of a time interval within current day,

nominal value at time tb

*τ<sup>c</sup>* = *τUTC*(*SU*) − *t*

(which is never more than 30 ns) [22].

*t*

**11.3. Time transformation**

*GLONASS* = *t*

**Figure 8.** L3 CDMA signal spectrum [23].

#### **10.6. GLONASS navigation message**

The navigation message contains immediate and non-immediate data. It is broadcast from GLONASS satellites at a rate of 50 bps to provide users with necessary data for positioning, timing, and planning observations [22].

The immediate data contains information about GLONASS satellites. It is broadcast of a navigation signal which includes mainly the enumeration of the satellite time and the difference between the onboard time scale of the satellite and GLONASS time. The difference between the carrier frequency of the satellite signal and its nominal value is also included in this data along with ephemeris and other parameters.

The non-immediate data, on the other hand, contain information about almanac of the system. Almanac data provides information about the status of all satellites in the current constellation, coarse corrections of the onboard timescale for each satellite with respect to GLONASS time. Almanac data also have information about the orbital parameters of all satellites (orbit almanac) and correction to GLONASS time with respect to UTC (SU) and some other parameters [22].
