GPS System Segments - Understanding GPS: Principles and Applications - page 108

received RF CDMA satellite signals are usually filtered by a passive bandpass
prefilter to reduce out-of-band RF interference.
This is normally followed by a preamplifier. The RF signals are then
downconverted to an intermediate frequency (IF). The IF signals are sampled and
digitized by an analog to digital (A/D) converter. The A/D sampling rate is typically
2 to 20 times the PRN code chipping rate [1.023 MHz for L1 C/A code and 10.23
MHz for L1 and L2 P(Y) code]. The minimum sampling rate is twice the stopband
bandwidth of the codes to satisfy the Nyquist criterion. For L1 C/A code only sets,
the stopband bandwidth may be slightly greater than 1 MHz. Alternatively, the
stopband bandwidth is slightly more than 10 MHz for P(Y) code sets. Oversampling
reduces the receiver sensitivity to A/D quantization noise, thereby reducing the num-
ber of bits required in the A/D converter. The samples are forwarded to the digital
signal processor (DSP). The DSP contains N parallel channels to simultaneously
track the carriers and codes from up to N satellites. (N generally ranges from 8 to 12
in today's receivers.) Each channel contains code and carrier tracking loops to per-
form code and carrier-phase measurements, as well as navigation message data
demodulation. The channel may compute three different satellite-to-user measure-
ment types: pseudoranges, delta ranges (sometimes referred to as delta
pseudorange), and integrated Doppler, depending on the implementation. The
desired measurements and demodulated navigation message data are forwarded to
the processor.
Note that GPS receivers designed for use in handheld devices need to be power
efficient. Depending on the implementation, these receivers may trade off suscepti-
bility to high-power in-band interferers to achieve minimum power supply (e.g., bat-
tery) drain. High dynamic range receiver front ends are needed in interference-
resistant receivers, and the necessary components (e.g., amplifiers and mixers with
high intermodulation product levels) require high bias voltage levels.
3.4.1.3 Navigation/Receiver Processor
A processor is generally required to control and command the receiver through its
operational sequence, starting with channel signal acquisition and followed by sig-
nal tracking and data collection. (Some GPS sets have an integral processing capabil-
ity within the channel circuitry to perform these signal-processing functions.) In
addition, the processor may also form the PVT solution from the receiver measure-
ments. In some applications, a separate processor may be dedicated to the computa-
tion of both PVT and associated navigation functions. Most processors provide an
independent PVT solution on a 1-Hz basis. However, receivers designated for
autoland aircraft precision approach and other high-dynamic applications normally
require computation of independent PVT solutions at a minimum of 5 Hz. The for-
mulated PVT solution and other navigation-related data is forwarded to the I/O
device.
3.4.1.4 I/O Device
The I/O device is the interface between the GPS set and the user. I/O devices are of
two basic types: integral or external. For many applications, the I/O device is a