The GPS L1 band Indeed most of the applications in the world nowadays are based on the signals transmitted at this frequency. We describe all of these signals modulated in the L1 RF carrier frequency in the next lines:. Finally, the technical characteristics of the existing GPS signals in the L1 band are summarized in the following Table 1. For a long time different signal structures for the M-Code were under consideration [J.
Both solutions result from the modulation of a non-return to zero NRZ pseudo random noise spreading code by a square-wave sub-carrier. While the Manchester code has a spreading code of rate equal to that of the square-wave, the BOC signal does not necessarily have to be so, being the only constraint that the rate of the spreading code must be less than the sub-carrier frequency. The interesting aspect about these signals is that, like the conventional sub-carrier modulation, the waveform presents a zero at the carrier frequency due to the square-wave sub-carrier.
Finally, it is important to note that for all the figures next the commonly used expressions for bandwidths in MHz must be understood as multiplied by the factor 1. This remains valid for all the bandwidths in this thesis, unless different stated. GPS is transmitting in the L2 band The P Y Code and M-Code were already described shortly in the previous chapter and the properties and parameters are thus similar to those in the L1 band.
These two signals are time multiplexed so that the resulting chipping rate is double as high as that of each individual signal. Errors are introduced into the Broadcast Ephemerides by truncating the orbital information in the navigation message. The Precision P code, sometimes called the Precise Positioning Service PPS , is modulated onto the L1 and L2 carriers allowing for the removal of the first order effects of the ionosphere.
The P code is referred to as the Y code if encrypted. Y code is actually the combination of the P code and a W encryption code and requires a DoD authorized receiver to use it. Originally the encryption was intended as a means to safe-guard the signal from being corrupted by interference, jamming, or falsified signals with the GPS signature.
Because of the intent to protect against "spoofing," the encryption is referred to as "Anti-spoofing" A-S. The pseudorange is the "distance" between the GPS satellite at some transmit time and the receiver at some receive time. Because the transmit time and the receive time are different, it is impossible to measure the true range between the satellite and the receiver.
The basic definition of the pseudorange observable is:. Another observable, based on the carrier phase of the signal, does not require the actual information being transmitted.
The basic definition of the phase observable is:. The integer number of cycles, N, is typically not known and varies for every receiver-satellite combination. As long as the connection between the receiver and the satellite is not broken, N remains constant while the fractional beat phase changes over time. Because of the ambiguous nature of N, it is referred to as the ambiguity and can either be solved for by using the code pseudoranges, or estimated.
The loss of signal lock between a GPS satellite and the receiver is referred to as "cycle slip. The GPS signals passing through the atmosphere encounter refraction effects including ray bending and propagation delays. These include the atmospheric effects of the troposphere and ionosphere. The largest effects of the troposphere can be avoided by prescribing an elevation mask for your receiver, thereby avoiding signals from low elevation satellites.
By processing signals received from the satellites, a GPS receiver can determine its own position with an uncertainty of less than 10 m. All GPS satellites broadcast on at least two carrier frequencies: L1, at Each satellite broadcasts a spread-spectrum waveform, called a pseudo-random noise PRN code on L1 and L2, and each satellite is identified by the PRN code it transmits.
There are two types of PRN codes. The second type is a precision P code with a chip rate of chips per millisecond.
GPS reception is line-of-sight, which means that the antenna must have a clear view of the sky. The signals can be received nearly anywhere on Earth where a clear sky view is available. The primary purpose of GPS is to serve as a radionavigation system, but it has also become the dominant system for the distribution of time and frequency signals. Navigation solutions have become part of our daily life due to their widespread use in a range of applications including agriculture, navigation by land vehicles, and pedestrian navigation.
A key navigation technology used in such applications is Global Navigation Satellite Systems GNSSs , and several such systems currently provide this service. Similarly, the European Union satellite navigation system, Galileo, is scheduled to be fully operational in While each of these systems has unique characteristics, all have major aspects in common.
Each has a space segment, control segment, and user segment. 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 obtained by multiplying the signal transit time by the speed of light.
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. GPS provides three-dimensional positioning and navigation services for both civilian and military users [ 2 ]. 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.
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 , 2 , 3 , 4 , 5 , 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 than the position determined using civilian signals. The concept of position fixing by trilateration using signals from three satellites. Figure 2 illustrates the three segments, which are discussed in greater detail in this section.
The GPS segments [ 8 ]. The GPS space segment is made up of a constellation of satellites that continuously broadcasts RF signals to users. GPS satellites travel in medium Earth orbit MEO at an altitude of approximately 20, km, and each circles the Earth twice a day, meaning that the orbital period is approximately 12 h [ 7 ]. 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.
Three of the 24 slots were upgraded, and six satellites were repositioned; thus, three additional satellites were added to the constellation. With a slot constellation, GPS improved satellite visibility across the globe. The features of the current and future generations of GPS satellites [ 10 ].
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. 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 ].
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 ]. GPS satellites produce a central L-band frequency of GPS signals consist of a carrier signal with frequency L1 or L2, a unique code assigned to each satellite, and a data message conveying information about satellite position, velocity, and clock bias.
The two carrier frequencies are modulated by a combination of the data message and the unique code to carry required information to the user. The duration of the P code is about 7 days, and it modulates both L1 and L2. Used only by the military, this code has a rate of The last key part of the GPS signal is the navigation message.
It takes Its most important parts are the ephemeris, almanac data, and satellite clock bias parameters.
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