GPS Receiver-Related Subjects - Fundamentals of Global Positioning System Receivers: A Software Approach - page 337

can be used to predict the required number of ADC bits. From this curve one
can write an empirical equation to predicate the number of bits ( N b ) required as

J/S(db)
6

N b ≈ − 1 r

( 12 . 19 )

J/N(dB)
6

N b ≈ + 2

For example, if the J/S = 66 dB, then N b = 9 bits is the required value.

12.10 REAL TIME OPERATION OF A SOFTWARE RECEIVER ( 8,9 )

With advancements in computer power, a software receiver could already operate
in real time on a personal computer ( 8 ) . The computer operates at 2 GHz and
actually tracks 8 satellites in real time. To show how this would work, a real
time software receiver tracking only 4 satellites is demonstrated. The receiver
performs acquisition, then tracking, and finally finding the user location.
The acquisition of the receiver takes some time. The acquisition covers 20 kHz
bandwidth with frequency separation of 1 kHz. It takes about 0.15 second to find
one satellite and about 0.6 second to find 4 satellites. Because the tracking speed
is faster than the data input rate, the tracking will catch up with the input data.
The transition from acquisition to tracking problem discussed in Section 9.15
will thus no longer be of concern.
If special attention is given to improving the operating speed of the software,
a receiver should be able to track 12 satellites in real time ( 9 ) . For example, in this
receiver the local frequency used for acquisition covers − 10 to 10 kHz. Instead
of generating the frequency every time, it generates all the required frequencies,
separated by 175 Hz, and the results are stored. The receiver also uses fixed point
computations. This operation will speed up the acquisition operation.
If a field programmable gate array (FPGA) is used instead of a personal
computer, the receiver should be able to process wider bandwidth signals. For
example, in order to study the multipath problem, a wider bandwidth, such as
20 MHz, is desirable to obtain a finer correlation peak. The sampling rate should
increase to about 50 MHz. For such a receiver to operate in real time, a FPGA
would be used.
It is anticipated that in the near future as software GPS receiver will become
a reality because it uses a very simple front end. The performance should sur-
pass the hardware receiver in higher sensitivity areas because more powerful
acquisition can be implemented.

12.11 PASSIVE ALTIMETER ( 10,11 )

This section will illustrate the flexibility of a software GPS receiver. A two-
channel software GPS receiver with one antenna looking up and one looking