Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1998-03-26
2001-05-22
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
Reexamination Certificate
active
06236673
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a receiver for receiving signals of a satellite navigation system.
REVIEW OF RELATED TECHNOLOGY
In performing high-precision positioning tasks with the aid of the GPS (Global Positioning System) satellite navigation system, it has been found that multi-path propagation of a satellite signal is the most common source of errors in measurements of signal travel time and carrier phase. Due to scattering and reflection of electromagnetic waves by obstacles, the waves are disturbed at the receiving site (e.g., at the antenna of a GPS locator device), which can result in amplification or attenuation of the receiving-field intensities. If either the transmitter or receiver moves, or even if both move, the changes in receiving-field intensities vary over time, leading to time-varying interferences. Controlling the effects of these interferences is crucial in positioning tasks accurate to the centimeter range, for example in an instrument takeoff and landing system for aircraft or a vehicle's steering system.
In principle, the relative time shift of two arbitrary signals can be calculated or measured with a cross-correlation function. Satellite navigation systems employ so-called pseudo-noise signals, which appear at first glance to be purely random but are actually strongly deterministic. If a cross-correlation is formed between a received satellite navigation signal and an identical pseudo-noise signal generated in the receiver, the so-called pseudo-signal travel time t
m
can be measured.
The pseudo-signal travel time differs from the real signal travel time between the satellite and the receiver by the offset of the satellite time from the receiver time. If the two clocks (e.g., clocks in the GPS satellite and in the ground locating device receiver) were coherently coupled, the pseudo-signal travel time would be identical to the real signal travel time. If a last pseudo-travel time measurement t
m−1
is known, a tendency (or, trend) of the time shift between the received signal and the signal generated in the receiver can be identified.
This tendency toward change is necessary for taking into account the Doppler-frequency shift in the pseudo-noise signal C
i
(CA.e)
(t) generated in the receiver, with (CA.e) (Coarse/Acquisition) representing the GPS pseudo-noise code generated in the receiver and accessible to the civilian user. Otherwise, the decorrelation of the two pseudo-noise signals would increase and, in the worst case, a correlation would no longer be identified, necessitating a new, time-consuming search.
Therefore, to take into consideration the Doppler-frequency shift, a variably-actuatable oscillator must be used in generating the internal pseudo-noise signal C
i
(CA.e)
(t). This job can be performed, for example, with a voltage-controlled oscillator that actuates the internal signal generation.
For adjusting the Doppler-frequency shift, GPS receivers and other spread-band multiplex receivers use only a time-shift control loop (DLL or Delay Lock Loop) from analog technology (FIG.
1
). The control loop (DLL) suppresses or regulates disturbances in the receiving signal, for example due to movement of the satellite or the receiver.
The DLL includes a characteristic property, the “discriminator characteristic”, which determines its functioning.
In the DLL of
FIG. 1
, the controller characteristic is generated by a unit 10, which generates the C/A code and modulates three signals with this internal pseudo-noise. One is the on-time pseudo-noise signal C
i
(CA.e)
(t). In addition, two further pseudo-noise signals are present that only differ from C
i
(CA.e)
(t) in their symmetrical, temporal shift t
v
. Here C
i
(CA.e)
(t−t
v
) precedes the on-time pseudo-noise signal by t
v
, and C
i
(CA.e)
(t+t
v
) follows by t
v
. The pseudo-noise signals that are modulated and sampled with the intermediate frequency ZF are indicated by S
i
a(CA.ZF.e)
(t−t
v
) and S
i
a(CA.ZF.e)
(t+t
v
) In a first correlator (
11
), the cross-correlation function
χ
f
(
CA
.
ZF
.
se
)
(
τ
)
=
1
T
∑
m
=
0
T
Δ
t
S
i
a
(
L
1
.
CA
.
ZF
.
s
)
(
m
Δ
t
)
S
i
a
(
CA
.
ZF
.
e
)
(
m
Δ
t
-
t
v
-
τ
)
(
1
)
is formed for the early pseudo-noise signal, and the cross-correlation function
χ
s
(
CA
.
ZF
.
se
)
(
τ
)
=
1
T
∑
m
=
0
T
Δ
t
S
i
a
(
L
1
.
CA
.
ZF
.
s
)
(
m
Δ
t
)
S
i
a
(
CA
.
ZF
.
e
)
(
m
Δ
t
+
t
v
-
τ
)
(
2
)
for the late pseudo-noise signal is formed in a second correlator (
12
). T represents the chip duration, &Dgr;t represents the time offset, s the received signal and e the signal generated in the receiver, and L
1
a first GPS carrier frequency in the L band, while m represents multiples of the sampling interval, and a indicates that the signal is discrete.
Correlator
11
further includes a first multiplier
111
, a filter
112
, and a second multiplier
113
; correlator
12
includes analogous elements
121
,
122
, and
123
. The illustrated correlators are exemplary, and any means for correlating the signals is contemplated by the present invention.
The inversion of one of the two cross-correlation functions in an inverter
13
and subsequent addition in an adding element 14
&khgr;d
(CA.ZF.se)
(&tgr;)=&khgr;f
(CA.ZF.se)
(&tgr;)−&khgr;s
(CA.ZF.se)
(&tgr;) ((3)
generates a summation signal which is the raw discriminator characteristic that is illustrated in
FIG. 2
for an ideal case. In a real case, higher frequency components occur in the formation of the cross-correlation function due to correlation at the intermediate frequency and the shape of
FIG. 2
is the envelope of a correlation fine structure whose amplitude limits form the envelope. The cross-correlation according to Equation (3) must therefore be smoothed with a low-pass filter function before it can be used for control the phase shift. This is done in low-pass filter
15
.
The low-pass filter
15
is also called a “loop filter”; its design, as well as the time shift t
v
between the early and the late pseudo-noise signals, essentially determines the dynamic properties of the entire DLL.
A voltage-controlled oscillator
16
, which supplies the frequency for the internal pseudo-noise signal generator
10
, can be actuated with the smoothed difference &khgr;
(CA.ZF.se)
between the early and the late cross-correlation function &khgr;
(CA.ZF.se)
(&tgr;). If the DLL is locked in, the internally-generated pseudo-noise signal is maintained exactly synchronous with the received pseudo-noise signal.
In the locking process, the Doppler-frequency shift is determined with a search method. Without a priori knowledge, this search process usually lasts 15 to 20 minutes. If the approximate receiver and satellite coordinates are known, the search process can be limited, and lasts only about two to three minutes.
If only the receiver or transmitter moves, this results in a change in the Doppler frequency of the received signal. This also changed the temporal relationship of the received pseudo-noise signal and the internally-generated pseudo-noise signal. If the received signal precedes the internally-generated signal, the clock rate of the internal signal is increased until the two codes again match exactly. If the received signal follows the internal signal in time, however, the clock rate is correspondingly reduced until exact synchronization is attained.
The synchronicity of the internal pseudo-noise signal and the received signal is used to measure the pseudo-signal travel time t
m
and to decode data. For measuring the pseudo-signal travel time t
m
, corresponding measuring pulses must be derived from the internal pseudo-noise signal and the receiver oscillator so that the temporal difference between them can b
Browdy and Neimark
Corrielus Jean B.
Deutsches Zentrum fur Luft-und Raumfahrt e.V.
Pham Chi
LandOfFree
Receiver for receiving signals of a statellite nagivation... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Receiver for receiving signals of a statellite nagivation..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Receiver for receiving signals of a statellite nagivation... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2535217