GPS System Segments - Understanding GPS: Principles and Applications - page 107

Legacy PPS users generally employ sets that track P(Y) code on both L1 and L2.
These sets initiate operation with receivers tracking C/A code on L1 and then transi-
tion to tracking P(Y) code on both L1 and L2. Y-code tracking occurs only with the
aid of cryptographic equipment. (If the satellite signal is encrypted and the receiver
does not have the proper cryptographic equipment, the receiver generally defaults
to tracking C/A code on L1.) It is anticipated that the forthcoming YMCA receivers
will perform a direct acquisition of the M code signal. Following M code acquisi-
tion, the receivers will then track M code on both L1 and L2 if the receiver is capable
of dual-frequency operation. Otherwise, it will operate on either L1 or L2.
Alternatively, legacy SPS users employ sets that track the C/A code exclusively
on L1, since that is the only frequency on which the C/A code is generally broad-
cast. Forthcoming L1C, L2C, and L5 receivers will track signals on these respective
frequencies.
In addition to the receiver types mentioned earlier, there are other variations,
such as civilian semicodeless tracking receivers, which track the C/A code on L1 and
carrier phase of both the L1 and L2 frequencies. These receivers employ signal-pro-
cessing techniques that to do not require cryptographic access to the P(Y) code. Uti-
lizing the carrier phase as a measurement observable enables centimeter-level (or
even millimeter-level) measurement accuracy. (Carrier-phase measurements are
described extensively in Section 8.4.) Most receivers have multiple channels
whereby each channel tracks the transmission from a single satellite. A simplified
block diagram of a multichannel generic SPS receiver is shown in Figure 3.25. The

Digital signal
processor (DSP)

Antenna

Battery-powered
date/time
clock

Channel 1

Channel 2

Navigation/
receiver
processor

RF/IF
downconverter
A/D conversion

Prefilter/
preamp

Channel N

Local
oscillator
inputs

To DSP channels

Control display
unit

Frequency
synthesizer

Regulated DC power

Reference
oscillator

Power
supply

Figure 3.25 Generic SPS receiver.