Hierarchical delay lock loop code tracking system with...

Details Hierarchical delay lock loop code tracking system with... Hierarchical delay lock loop code tracking system with...

2000-02-16

2002-02-05

Chin, Stephen (Department: 2634)

Pulse or digital communications

Spread spectrum

Direct sequence

C375S150000, C375S367000

Reexamination Certificate

active

06345068

ABSTRACT:

FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to receivers of signals modulated by pseudorandom noise, such as the receivers used in navigation systems, and, more particularly, to a receiver based on a hierarchy of delay lock loops to maintain code lock.
Radio navigation systems are used for providing geographic location and time information. Examples of these systems include the United States' Global Positioning System (GPS) and the Russian Global Navigation System (GLONASS). These systems rely on satellites in orbit around the Earth. They allow the derivation of precise navigation information including three-dimensional position, velocity and time. Normally, reception of signals from at least four satellites is required for precise location determination on four dimensions (latitude, longitude, altitude and time). Once the receiver has measured the respective signal propagation delays, the range to each satellite is calculated by multiplying each delay by the speed of light. Then the location and time are found by solving a set of four equations that incorporate the measured ranges and the known locations of the satellites. The highly precise capabilities of the system are maintained by means of atomic clocks on board the satellites and by ground tracking stations which continuously monitor and correct satellite clock and orbit parameters.
In the GPS system, each satellite transmits two direct-sequence-coded spread spectrum signals: an L
1
signal at a carrier frequency of 1.57542 GHz and an L
2
signal at a carrier frequency of 1.2276 GHz. The L
1
signal consists of two phase-shift keyed (PSK) spread-spectrum signals modulated in phase quadrature: the P-code signal (“P” stands for “precise”) and the C/A-code signal (C/A stands for “Coarse/Acquisition”). The L
2
signal contains only the P-code signal. The P and C/A codes are repetitive pseudorandom bit sequences which are modulated onto the carriers. These bits are called “chips” in spread spectrum parlance. The clocklike nature of these codes is used by the receiver in making time delay measurements. The codes of each satellite are unique, allowing the receiver to distinguish between signals, from the various satellites, that share a common carrier frequency. Also modulated onto each carrier is a 50 bit-per-second data stream which, for each satellite, contains information about system status and satellite orbit parameters which are needed for the navigation calculations. The P-code signals are encrypted, and are intended to be decrypted only by classified users. The C/A signals are available to all users.
The operations performed in a GPS receiver are for the most part typical of those performed in any direct-sequency spread spectrum receiver. The spreading effect of the pseudorandom code modulation must be removed from each signal by multiplying by a time-aligned, locally generated copy of its code, in a process known as despreading. Because the appropriate time alignment, or code phase, is not known at receiver startup, it must be sought during the initial acquisition stage. Once found, proper code time-alignment, also called “code lock”, must be maintained during the tracking phase of receiver operation, as the satellites move relative to the user.
Once despread, each signal consists of a 50 bit-per-second PSK signal at some low frequency. This frequency is uncertain because of the Doppler shift caused by relative motion between the satellite and the user, and also because of receiver local clock error. During initial signal acquisition, the signal must be sought in a frequency range which allows for this uncertainty. Once the Doppler frequency offset is determined approximately, carrier demodulation can compensate for it by digital processing means.
Most of the functions described so far are performed by digital means. After high speed A/D conversion, despreading is performed using special hardware controlled by a microcontroller. The microcontroller also performs additional digital signal processing tasks, such as data detection, timing recovery and navigation.
One mechanism commonly used for maintaining code time-alignment is the so-called “delay-lock loop” (DLL). A DLL tracking system which correlates early, current and late versions of the locally generated pseudorandom noise code signal with the received composite signal typically is used to maintain code lock in each channel. This code lock must be maintained despite multipath propagation and despite sudden motion of the receiver.
The DLL, first introduced by Spilker (J. J. Spilker Jr., “GPS Structure and Performance Characteristics”, Navigation Vol. 25 No. 2 pp. 121-146 (1978)), is based on correlating the incoming signal with two time-shifted versions of the pseudorandom noise code generated at the receiver, an early version and a late version.
FIG. 1
shows the ideal normalized correlation function between the incoming signal and the pseudorandom noise code. In
FIG. 1
, the abscissa is the time lag between the incoming signal and the receiver's code generator, in units of T
C
, the code chip time. When the receiver's code generator is exactly synchronized with the incoming signal, the correlation function is almost unity. When the receiver's code generator leads or lags the incoming signal by more than T
c
, the correlation function is almost zero. (In the GPS C/A-code signal, for example, chip “epochs” are 1023 chips long, so the correlation function is −1/1023 at leads and lags greater than T
c
and 1 at perfect synchrony.) In between, the correlation function is linear. In most prior art receiver architectures, the early (E) and late (L) correlation timings differ initially by 2&Dgr;=T
c
, as shown in FIG.
1
. In particular,
FIG. 1
shows the timing of the early correlation and the late correlation relative to the incoming signal when the current correlation, which is actually used to despread the incoming signal, and which is performed at a time exactly halfway between the early correlation and the late correlation, is exactly synchronized with the incoming signal, i.e., at time zero. At perfect synchrony, the difference between the early correlation and the late correlation is zero. When the current correlation leads or lags perfect synchrony, the difference between the early correlation and the late correlation is as shown by the curve labeled “k=1” in FIG.
2
. This curve is produced by sliding the vertical lines labeled “E” and “L” in
FIG. 1
leftward and rightward while maintaining the 2&Dgr;=T
c
spacing between the two lines, and is a plot of the difference between the length of the “E” line and the length of the “L” line as a function of the time half way between the two lines. For obvious reasons, this curve is called an “S-curve”. It provides a measure of the timing mismatch between the incoming signal and the receiver's code generator.
FIG. 3
is a block diagram of a coherent delay lock loop tracking system
10
of the prior art. The arrows show the direction of data flow. Pseudorandom noise code is generated by a code generator
12
. An incoming signal C(t) is multiplied by this code in a current multiplier
14
, an early multiplier
16
and a late multiplier
18
. The code input to early multiplier
16
is advanced by &Dgr; (block
24
) relative to the code input to current multiplier
14
. The code input to late multiplier
18
is delayed by &Dgr; (block
26
) relative to the code input to current multiplier
14
. The outputs of early multiplier
16
, current multiplier
14
and late multiplier
18
are low pass filtered (blocks
20
,
21
and
22
, respectively). The outputs of low pass filters
20
and
22
are correlation signals that are subtracted (block
28
) to produce the corresponding S-curve value. Thus, early multiplier
16
and low pass filter
20
together constitute an early correlator; similarly, late multiplier
18
and low pass filter
22
together constitute a late correlator. The S-curve value is a control signal which is transformed by a loop

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