Data Processing for Positioning - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 116

Channel 1

Channel 2

Channel 3

Channel 4

68 ms

Time [s]

409,807

FIGURE 8.8. The transmit time and start of the subframe for four channels.

nel 1 has a travel time of 68 ms. The travel time of the remaining channels is then
computed with respect to channel 1. In the example this leads to travel times and
pseudoranges for the four channels as listed in Table 8.5.
In the present example, the time resolution is 1 ms, which corresponds to a
pseudorange of 300,000 m. To make the pseudorange more useful, the tracking
loop needs to find the start of the C/A code in the specific frame. This means that
the time resolution is the sampling time, and in this case the sampling frequency
is 38.192 MHz. This sampling frequency leads to a pseudorange accuracy of 8 m.
With the initial pseudoranges the receiver position can be computed. The output
of that computation is a receiver position and a receiver clock offset dt .

The clock offset can be used to adjust the travel time of the reference satellite. In
this case it was channel 1 with the travel time of 65 ms. And with this procedure
the receiver can estimate the actual pseudoranges after two computations.

( X , Y , Z )

8.4.2 Estimation of Subsequent Pseudoranges
Section 8.4.1 describes how the initial pseudoranges are estimated. This subsec-
tion describes how the subsequent pseudoranges are computed.

TABLE 8.5. Initial pseudoranges for all tracked satellites

Channel Travel time Pseudorange

[ms] [m]

1 65 19,486,509.8

2 74 22,184,641.9

3 81 24,283,189.1

4 69 20,685,679.6