56 GPS Signals Structure, Receivers, and Errors

Introduction: GPS Signals

We have already discussed the concept of a global navigation satellite system in our previous article in full detail (Global Position System: Different Segments of GPS, its working Principle, Popular Substitute of GPS). In this article, we are going to gain knowledge regarding the concept of GPS signals, receivers, and their errors.

A GPS satellite continuously transmits two carrier waves in the L-band of the spectrum (L-band frequency is also used for radio). These transmitted signals travel the earth with the speed of light. The fundamental frequency is used to create these carrier waves, which are generated using an accurate atomic clock. The two carrier frequencies are emitted at 1,575.42 MHz (the L1 carrier) and 1,227.60 MHz (referred to as the L2 carrier).

The corresponding carrier wavelengths are approximately 19 cm and 24.4 cm, respectively, which result from the relation between the carrier frequency and the speed of light in space. The presence of two carrier frequencies enables the correction of a significant GPS inaccuracy known as an ionospheric delay. The GPS satellites all use similar L1 and L2 carrier frequencies. The code modulation, on the other hand, is distinct for each satellite, which considerably reduces GPS signal interference. GPS signals include ranging signals, used to measure the distance to the satellite, and navigation messages.

Note: – Carrier phase measurements are based on the EDM (electronic distance measurement) concept, where the phase measurement is performed. The difference between the phase of the internal receiver oscillator and the received satellite carrier phase (as sensed by the receiving antenna) is the quantity monitored in GPS.

GPS Codes

The two GPS codes are known as coarse acquisition (or C/A-code) and precision (or P-code). Each code is made up of a series of binary numbers, zeros, and ones referred to as bits or chips. The codes are commonly known as PRN codes because they look like random signals (i.e., they are noise-like signals). But in reality, the codes are generated using a mathematical algorithm.

At present, the C/A-code is only modulated on the L1 carrier, but the P-code is modulated on both the L1 and L2 carriers. Because the carrier phase shifts by 180° when the code value changes from zero to one or one to zero, this modulation is known as bi-phase modulation.

Characteristics of C/A Code

• The C/A-code is a stream of 1,023 binary digits (ones and zeros) that repeats itself every millisecond.
• The chipping rate of the C/A-code is 1.023 Mbps.
• The duration of one bit is approximately 1ms, or equivalently 300m
• Each satellite is given a unique C/A-code, which allows GPS receivers to determine which satellite is sending a certain code.
• When compared to the P-code, the C/A-code range measurement is less exact.
• It is less complex and is available to all users.

Characteristics of P-Code

• The P-code is a very long sequence of binary digits that repeats itself after 266 days
• 10 times faster than C/A code
• Its rate is 10.23 Mbps
• Multiplying the time it takes the P-code to repeat itself, 266 days, by its rate, 10.23 Mbps
• The P-code is a stream of about 2.35 × 1014 chips! The 266-day-long code is divided into 38 segments; each is 1 week long.
• These, 32 segments are assigned to the various GPS satellites. That is, each satellite transmits a unique 1-week segment of the P-code, which is initialized every Saturday/Sunday midnight crossing. The remaining 6 segments are reserved for other applications
• This code is primarily designed for the military perspective. It was available to all users until January 31, 1994
• At the time, the P-code was encrypted by adding an unknown W-code to it. The resultant encrypted code (encryption process is called the antispoofing (AS)) is known as the Y-code, and it operates at the same chipping rate as the P-code.

The navigation signal from GPS is a data stream added to both the carrier frequencies as binary biphase modulation at a low rate of 50kbps. It is made up of 25 frames of 1,500 bits each, for a total of 37,500 bits. This indicates that the entire navigation message takes 750 seconds, or 12.5 minutes, to transmit. GPS navigation message contains the following information:

1. the coordinates of the GPS satellites as a function of time
2. satellite health regarding information
3. clock correction
4. satellite almanac
5. Atmospheric data etc.

GPS Receivers

In simple words we may say about a GPS receiver is an L-band radio processor. It interprets GPS satellite signals and solves navigation equations so that users may simply determine their position, accurate time, and velocity. We can classify GPS receivers on the following basis: –

Based on Features

• Not self-contained receiver

It is also known as ‘GPS mice’. These lack a screen and must be linked to a computer to display the GPS receiver’s current location. Bluetooth (wireless) can be used to connect the GPS to the PC. This type is most commonly found in car navigation systems.

• Self-contained receiver

It has a screen and is incorporated into the computer. Sometimes, additional features e.g. electronic compass, barometer, etc. are found in this type. It is primarily used in boating and aviation.

• Sophisticated receiver

Is used by professionals e.g for mapping, GIS, agriculture, transport, etc. Though the operating concept is the same as with others, the only significant difference is its storage capacity and improved precision, which allows for the storing of a larger quantity of data that can be addressed later in the office.

Based on Level of Accuracy

• C/A Code receiver

These receivers have a location and differential correction accuracy of 1 to 5 m, resulting in a 5-second occupancy time. Because of recent advancements in GPS receiver architecture, it can now deliver sub-meter accuracy down to 30 cm.

• Carrier Phase receiver

With differential correction, these receivers give GPS position accuracy of 10-30 cm. The distance between the receiver and the satellite is calculated by counting the total number of waves that support the C/A code signal, which is more precise and takes around 5 minutes of the occupancy time.

• Dual frequency receiver

It offers GPS location accuracy based on differential correction within sub centimeters and survey-grade accuracies.  The signals from satellites are given to these receivers based on two frequencies at the same time. The employment of two frequencies improves the removal of atmospheric and other defects, improving accuracy.

GPS Errors

Both GPS pseudo-range and carrier-phase estimates are influenced by a variety of random defects or errors and biases (systematic errors). These errors may be divided into three types: those caused by the satellites, those caused by the receiver, and those caused by signal propagation.

• Satellite errors include ephemeris, or orbital, errors, satellite clock issues, and the impact of selective availability. The latter was purposefully deployed by the United States Department of Defense to reduce autonomous GPS accuracy for security concerns. It was, however, canceled on May 1, 2000).
• Receiver clock errors, multipath errors, receiver noise, and antenna phase center variations are examples of errors that originate at the receiver.
• The signal propagation errors include the delays of the GPS signal as it passes through the ionospheric and tropospheric layers of the atmosphere.

In addition to the influence of these errors, the geometric placements of the GPS satellites as perceived by the receiver alter the precision of the estimated GPS position.

GPS Error Classification Based on Observation: –

• Errors in GPS satellite orbit, clock, and position geometry:

These errors are characterized primarily in terms of satellite broadcast signals with transmission time from the satellite to the receiver. It contains satellite clock, orbit, and location errors, as well as Selective Availability.

• Receiver clock-dependent error source:

These errors are characterized mostly in terms of receivers. It comprises clock, cycle slip, noise, and antenna phase problems. These errors are caused by receivers.

• Propagation errors and atmosphere-dependent errors:

Propagation errors are described in terms of signal propagation from satellite to receiver. It contains errors pertaining to the ionosphere and troposphere. Errors are produced as a result of signal delays in the upper (ionosphere) and lower atmospheres (troposphere).

• Errors caused by signal multipath and atmospheric interference:

GPS Signal multipath errors are caused by reflected signals from geographically based buildings, high rocks, automobiles, power lines, or water. Atmospheric interference mistakes are caused by atmospheric factors such as humidity, temperature, and pressure.

• User mistakes:

These errors are generated by incorrect information input in the GPS receiver, relaying 2D position in place of 3D position, distance error, and human body defects.

• Random-based nature errors:

These errors are caused by unexpected and unpredictable changes in nature or environmental factors, such as electronic or instrumental noise errors.

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