**3. The GPS structure**

While each of these systems has unique characteristics, all have major aspects in common. Each has a space segment, control segment, and user segment. What is more, all are based on transmitting radio frequency (RF) signals in a one-way fashion from satellites to receivers on and near the Earth's surface. Using measurements obtained from these signals, a GNSS receiver can find its position, velocity, and time (PVT) solution. Moreover, all GNSS systems use the notion of time-of-arrival (TOA) ranging. This requires measuring the signal transit time and the time interval the signal takes to travel between the satellite and the receiver to calculate the receiver-to-satellite range [1]. The transmitter-to-receiver distance can then be

This chapter provides an overview of Global Positioning System (GPS) and GLONASS and their signals. First, it describes the system architecture in terms of the three main segments: control, space, and user. Then, it addresses the new civilian and military GPS signal characteristics, highlighting their significance. Following that, it briefly discusses the GPS measurement error sources. The chapter also covers essential aspects of the GLONASS system, including GLONASS signal characteristics, the GLONASS modernization program, the GLONASS Radio Frequency (RF) plan, pseudorandom (PR) ranging codes, and the intra-system interference navigation message. Finally, advantages of combining both GPS and GLONASS are

GPS provides three-dimensional positioning and navigation services for both civilian and military users [2]. The GPS receivers use the TOA ranging to generate code pseudorange to determine the user's position. They also monitor changes in signal frequency to produce a rate of change of range measurements to determine velocity [3]. The time between the transmission of the signal and its arrival at the receiver is measured. The transmitter-to-receiver distance can then be obtained by scaling the signal transit time by the speed of light. Using the concept of trilateration, a GPS receiver can determine its position using the measured travel time along with the satellites' locations that are obtained from the navigation message carried by the signal. Though three satellites can be used to determine the user's position, at least four

**Figure 1** illustrates the concept of position fixing by trilateration by using the range to three satellites. Using four satellites to find the position improves the accuracy of the solution by eliminating the receiver clock offset. The first and second user-to-satellite range measurements define two spheres on two different satellites, and the intersection of these two spheres defines a circle of possible receiver positions. A third range measurement, intersecting with the first two, narrows those receiver positions to an ambiguous pair, while the fourth measurement resolves this ambiguity and determines the clock bias. The GPS positioning equations are found in [1–6]. Military GPS signals are more robust against interference and spoofing than civilian signals [3]; hence, the position determined by military signals is more precise

obtained by multiplying the signal transit time by the speed of light.

listed to give the reader insight into the benefits of such integration.

are required owing to an additional estimation of the receiver clock offset.

than the position determined using civilian signals.

**2. Overview of GPS**

120 Multifunctional Operation and Application of GPS

As mentioned earlier, the GPS is composite of three segments [7]: the space segment, a constellation of satellites orbiting the Earth at very high altitudes; the control segment, made up of a group of ground control stations; and the user segment, a user's equipment or simply the variety of military and civilian receivers. **Figure 2** illustrates the three segments, which are discussed in greater detail in this section.

#### **3.1. The space segment**

The GPS space segment is made up of a constellation of satellites that continuously broadcasts RF signals to users. In recent years, the US Air Force has operated 32 GPS satellites, of which 24 are available 95% of the time [4]. GPS satellites travel in medium Earth orbit (MEO) at an altitude of approximately 20,200 km, and each circles the Earth twice a day, meaning that the orbital period is approximately 12 h [7]. These satellites are distributed among six equally-spaced orbital planes, each having a target inclination of 55° [6], a satellite distribution that improves the visibility of satellites to GPS users across the globe, thereby enhancing navigation accuracy. GPS satellites broadcast RF signals containing coded information and navigation data, enabling a receiver to calculate pseudoranges and Doppler measurements to estimate position, velocity, and time.

In June 2011, the US Air Force successfully expanded its GPS constellation, known as the "Expandable 24" configuration [9]. Three of the 24 slots were upgraded, and six satellites were repositioned; thus, three additional satellites were added to the constellation. With a 27-slot constellation, GPS improved satellite visibility across the globe. **Table 1** summarizes the features of the current and future generations of GPS satellites, including Block IIA (second generation, "Advanced"), Block IIR ("Replenishment"), Block IIR (M) ('Modernized"), Block IIF ("Follow-on"), and GPS III [10].

**Figure 2.** The GPS segments [8].

#### **3.2. The control segment**

Made up of a global network of ground facilities that track GPS satellites, the GPS control segment's main tasks are the control and maintenance of the system through monitoring and analyzing signal transmissions and sending commands and data updates to the GPS constellation.

Referring to [7], the current operational control segment includes a Master Control Station (MCS), an alternate master control station, 12 command and control antennas, and 16 monitoring sites. The locations of these facilities are shown in **Figure 3**.

#### **3.3. The user segment**

The user segment is represented by the wide array of types of GPS receivers. These capture and track satellite signals and process signals transmitted by GPS satellites, estimate the user-to-satellite ranges and range rates, and compute a PVT solution [12]. A GPS receiver had cost more than \$100,000 in the mid-1980s; nowadays, an on-chip receiver is available in the market for less than \$20, and it is estimated that more than 1 million receivers have been produced each year since 1997 [1]. As GPS is available at no direct charge to users, they can use receivers at any time and any place across the globe to determine their position [6].

**Legacy satellites**

Block IIA 6 Operational

•

Coarse acquisition (C/A)

• • •

On-board clock monitoring

P(Y) code on L1 and L2

• (L2C)

•

New military M code

• •

Improved accuracy, signal

strength, and quality

• • • •

Begins launching in 2016

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15-year design lifespan

Satellites 9+: laser reflectors;

search and rescue payload

No Selective Availability

Advanced atomic clocks

•

Enhanced signal reliability,

accuracy, and integrity

signals for enhanced jam

resistance

•

Flexible power levels for

• •

Launched since 2010

12-year design lifespan

military signals

• •

Launched in 2005–2009

7.5-year design lifespan

Second civil signal on L2

•

Third civil signal on L5

frequency (L5)

C/A code on L1

•

All legacy signals

•

All Block IIR (M) signals

• • (L1C)

Fourth civil signal on L1

All Block IIF signals

code on L1 frequency for

civil users

•

Precise P(Y) code on L1 and

L2 frequencies for military

• •

Launched in 1997–2004

7.5-year design lifespan

users

• • **Table 1.**

The features of the current and future generations of GPS satellites [10].

Launched in 1990–1997

7.5-year design lifespan

Block IIR 12 Operational

Block IIR (M) 7 Operational

Block IIF 6 Operational

GPS III

Now in production

**Modernized satellites**


**3.2. The control segment**

**Figure 2.** The GPS segments [

122 Multifunctional Operation and Application of GPS

8].

**3.3. The user segment**

6].

constellation. Referring to [

position [

Made up of a global network of ground facilities that track GPS satellites, the GPS control segment's main tasks are the control and maintenance of the system through monitoring and analyzing signal transmissions and sending commands and data updates to the GPS

(MCS), an alternate master control station, 12 command and control antennas, and 16 moni

The user segment is represented by the wide array of types of GPS receivers. These capture and track satellite signals and process signals transmitted by GPS satellites, estimate the user-to-satellite ranges and range rates, and compute a PVT solution [12]. A GPS receiver had cost more than \$100,000 in the mid-1980s; nowadays, an on-chip receiver is avail

able in the market for less than \$20, and it is estimated that more than 1 million receivers

users, they can use receivers at any time and any place across the globe to determine their

toring sites. The locations of these facilities are shown in **Figure 3**

have been produced each year since 1997 [

7], the current operational control segment includes a Master Control Station

.

1]. As GPS is available at no direct charge to



To prepare the GPS signal for transmission by the satellite, first, an XOR operation is applied to combine the binary navigation message with the code. If the message bit and the code chip are the same, the result is 0; if they are different, the result is 1. Second, the combined signal is merged with the carrier using binary phase shift keying (BPSK) modulation: a "0" bit leaves the carrier signal intact, whereas a "1" bit causes the signal to be multiplied by −1 and shifts

As mentioned above, the PRN code patterns are nearly orthogonal, an important property that makes the satellite identification process much easier [2]. Two codes are orthogonal when the sum of their term products shifted arbitrarily against each other is nearly zero. The cross

Another important property of PRN codes is that each PRN pattern is almost uncorrelated

and *C*(l)

(*i* + *n*) ≈ 0, *for all k* ≠ *l* (1)

(*i* + *n*) ≈ 0, *for all* |*n*| ≥ 1 (2)

, is expressed as

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the carrier by 180°. **Figure 4** illustrates this process.

∑<sup>1</sup>

∑<sup>1</sup>

**Figure 4.** GPS signal structure [15].

with itself:

correlation function for satellites m and n, with PRN codes *C*(*k*)

<sup>1023</sup> *C*(*k*)

This orthogonality makes the cross satellite interference small [14].

<sup>1023</sup> *C*(*k*)

(*i*) ⋅ *C*(*l*)

(*i*) ⋅ *C*(*k*)

**Figure 3.** The locations of the GPS Master Control Station, an alternate Master Control Station, 12 command and control antennas, and 16 monitoring sites [11].
