Technical Features of GPS

GPS signals are transmitted in two frequencies, L1 (1575.42 MHz) and L2 (1227.60 MHz). The carrier signals are modulated with a unique Pseudo-Random Noise (PRN) sequence for each satellite. The signals from each satellite are separated by the GPS receiver using CDMA technique. Currently, there are ranges of PRN codes in use, which includes the Coarse/Acquisition (C/A) code, widely used for the civil applications with L1 frequency modulation; secondly, the Precise (P) code, served specifically for military applications, with L1 and L2 frequency modulations and last one is the Y-code, which has been used to replace of P-code if activation of anti-spoofing has taken place. The navigation messages are the modulated data onto these codes and are broadcasted from GPS satellite and the messages received are common to all satellites but unique to the transmitting satellite. The data from the navigation messages includes the time of message transmission, clock corrections, and data relevance to health for all satellites, coefficient for ionospheric delay model, coefficients to calculate coordinated universal time (UTC) and a Hand Over Word (HOW) for the transition from C/Y-code to P(Y). The satellite status could be known through the almanac, which describes the details such as location of orbital, and PRN numbers, which are valid for up to 180 days. The updated version of the almanac is the ephemeris where the information obtained through this could be valid for

only four hours but it allows the receiver to calculate the tracked satellite‟s current position (DoD, 2004).

The GPS receiver estimates the spaces to the tracked satellites, and this is explained as a pseudo-range- the range to the satellite and the receiver‟s clock offset. These pseudo-ranges are the basic GPS observable that is attained by using the C/A and/or P-codes transformed into the carrier signal. GPS receiver generates a signal similar to the received PRN from the satellite. The generated signal by the receiver keeps shift in time until a correlation is achieved between the two signals (from the satellite and the receiver), but, this time shift is the time taken for the signal to travel from satellite to receiver. As the signal of the satellite is akin to the speed of light, the pseudo-range is established by multiplying the time difference by the speed of light. At least four satellites are required in order to compute a position solution. Based on the pseudo-range measurements (_i ), the position calculations are described as the following:



 

 



i  x x ²  y y ²  z z ² ct

 (2.1)

Where:

 (x_u,y_u,z_u) are the unknown user receiver position coordinates.

 (x_i,y_i,z_i) are the known satellite coordinates.

 t is the offset of receiver clock from the system time. u

 c is the speed of light.

At least four pseudo-ranges are required to obtain the unknown receiver‟s coordinates:



 

 



 ₁ ² ₁ ² ₁ ²

1



 

 



There can be a solution to the above-mentioned equations with the use of iterative techniques or a solution can be arrived at with the use of least squares method.

The accuracy of the solution increases with a sophisticated solution and with the use of four satellites. The carrier phase measurements can also be used for computing distances to the satellites and while quantifying using this method, the carrier signal is modulated with the use of C/A code. The number of complete cycles can be decided which have occurred and also the distances of satellites can be measured from the wavelength in addition to the integer uncertainty. Also, satellites are locked while using this method of determination. The quality solution is given with this and also, this is of help while solving ambiguity in integrity.

The number of satellites tracked with a precise geometry is the main factor for deciding the performance of the GPS system. There are three possibilities for the occurrence of error as mentioned by Kaplan & Hegarty (2006). They are:

· Satellite-based

· Signal-based

· Receiver measurement errors

These errors mainly affect pseudo-range measurements and limited satellites visibility. These errors are also described as satellite orbital shifting and clock errors. Atmospheric delays and multipath effects cause errors in interpretation of signals. There are also additional sources of errors like receiver measurement errors are originated by the receiver noise, software resolution and stability.

Signal-based errors are the major contributor in the total measurement errors.

These errors escalate in urban canyons and areas with elevated surroundings.

These environments lead to signal blockage, causing deficient strong satellites being fruitfully trailed for position calculation. The role of each error source in the calculated position error can be explained in terms of User Equivalent Range Error (UERE). Tables 2.1 and 2.2 show estimates of typical contemporary UERE budgets. Table 2.1 describes a typical UERE budget for a single frequency C/A code receiver. Table 2.2 shows the UERE budget for a dual frequency P(Y) code receiver.

UERE Error Sources Error (m)

Broadcast Clock 1.1

Ephemeris (Orbital Errors) 0.8

Ionospheric 7

Tropospheric 0.2

Receiver noise and resolution 0.1

Multipath 0.2

Table 2.1: GPS Standard Positioning Service typical UERE Budget (Kaplan and Hegarty, 2006).

UERE Error Sources Error (m)

Broadcast Clock 1.1

Ephemeris (Orbital Errors) 0.8

Ionospheric 0.1

Tropospheric 0.2

Receiver noise and resolution 0.1

Multipath 0.2

Table 2.2: GPS Precise Positioning Service typical UERE Budget (Kaplan and Hegarty, 2006).

These values are not fixed and are dependent on the conducted measurements‟

scenario and conditions.

In recent decades, substantial consideration has been paid to developing Differential GPS (DGPS) (Section 2.2.2). The main aim of the research is to eliminate or reduce the GPS error sources and achieve greater performance positioning. The availability of DGPS systems is wide with different ranges of coverage, different structure, differential data formats and several augmentation data deliverability means. Conversely, Assisted GPS (A-GPS) is one of the DGPS system that has been developed to increase the speed of position fixing, where it helps to provide navigation information using GPS ephemeris from a station, which are remote assisted to GPS users through a carrier network such as mobile network (Hjelm, 2002). Beside, currently in the market, there are High Sensitivity GPS (HS-GPS) receivers available, which are being used in support of positioning accuracy of GPS.

This technology improves the positioning fixing rate and the overall GPS positioning in challenging navigation areas, by enabling the acquisition of weak GPS signals down to -190 dBW level. However, the problems of signals availability are not solved till now, particularly during conditions where the availability of satellites (<4 satellites) is insufficient such as in densely areas and indoor environments (Esmond, 2007; Lachapelle, 2007).