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Description  |
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BACKGROUND AND SUMMARY OF THE INVENTION
This invention relates generally to navigation systems and more
specifically to a system for positioning radiosondes, sonobuoys, aircraft,
ships, land vehicles, and other objects on or near the earth's surface
using satellites of the Global Positioning System (GPS). The GPS is a
multiple-satellite based radio positioning system in which each GPS
satellite transmits data that allows a user to precisely measure the
distance from selected ones of the GPS satellites to his antenna and to
thereafter compute position, velocity, and time parameters to a high
degree of accuracy, using known triangulation techniques. The signals
provided by the GPS can be received both globally and continuously. The
GPS comprises three major segments, known as the space, control, and user
segments.
The space segment, when fully operational, will consist of twenty-one
operational satellites and three spares. These satellites will be
positioned in a constellation such that typically seven, but a minimum of
four, satellites will be observable by a user anywhere on or near the
earth's surface. Each satellite transmits signals on two frequencies known
as L1 (1575.42 MHz) and L2 (1227.6 MHz), using spread spectrum techniques
that employ two types of spreading functions. C/A and P pseudo random
noise (PRN) codes are transmitted on frequency L1, and P code only is
tranmitted on frequency L2. The C/A or coarse/acquisition code, is
available to any user, military or civilian, but the P code is only
available to authorized military and civilian users. Both P and C/A codes
contain data that enable a receiver to determine the range between a
satellite and the user. Superimposed on both the P and C/A codes is the
navigation (Nav) message. The Nav message contains 1) GPS system time; 2)
a handover word used in connection with the transition from C/A code to P
code tracking; 3) ephemeris data for the particular satellites being
tracked; 4) almanac data for all of the satellites in the constellation,
including information regarding satellite health, coefficients for the
ionospheric delay model for C/A code users, and coefficients used to
calculate universal coordinated time (UTC).
The control segment comprises a master control station (MCS) and a number
of monitor stations. The monitor stations passively track all GPS
satellites in view, collecting ranging data and satellite clock data from
each satellite. This information is passed on to the MCS where the
satellites' future ephemeris and clock drift are predicted. Updated
ephemeris and clock data are uploaded to each satellite for
re-transmission in each satellite's navigation message. The purpose of the
control segment is to ensure that the information transmitted from the
satellites is as accurate as possible.
GPS is intended to be used in a wide variety of applications, including
space, air, sea, and land object navigation, precise positioning, time
transfer, attitude reference, surveying, etc. GPS will be used by a
variety of civilian and military organizations all over the world. A
number of prior art GPS receivers have been developed to meet the needs of
the diverse group of users. These prior art GPS receivers are of a number
of different types, including sequential tracking, continuous reception,
multiplex, all in view, time transfer, and surveying receivers.
A GPS receiver comprises a number of subsystems, including an antenna
assembly, an RF assembly, and a GPS processor assembly. The antenna
assembly receives the L-band GPS signal and amplifies it prior to
insertion into the RF assembly.
The RF assembly mixes the L-band GPS signal down to a convenient IF
frequency. Using various known techniques, the PRN code modulating the
L-band signal is tracked through code-correlation to measure the time of
transmission of the signals from the saellite. The doppler shift of the
received L-band signal is also measured through a carrier tracking loop.
The code correlation and carrier tracking function can be performed using
either analog or digital processing.
The control of the code and carrier tracking loops is provided by the GPS
processor assembly. By differencing this measurement with the time of
reception, as determined by the receiver's clock, the pseudo range between
the receiver and the satellite being tracked may be determined. This
pseudo range includes both the range to the satellite and the offset of
the receiver's clock from the GPS master time reference. The pseudo range
measurements and navigation data from four satellites are used to compute
a three dimensional position and velocity fix, to calibrate the receiver's
clock offset, and to provide an indication of GPS time.
The receiver processor controller (RPC) processing and memory functions
performed by a typical GPS receiver include monitoring channel status arid
control, signal acquisition and reacquisition, code and carrier tracking
loops, computing pseudo range (PR) and delta range (DR) measurements,
determining data edge timing, acquisition and storage of almanac and
ephemeris data broadcast by the satellites, processor control and timing,
address and command decoding, timed interrupt generation, interrupt
acknowledgment control, and GPS timing, for example. These functions are
fixed point operations and do not require a floating point coprocessor.
The navigation processing and memory functions performed by a typical GPS
receiver include satellite orbit calculations and satellite selection,
atmospheric delay correction calculations, navigation solution
computation, clock bias and rate estimates, computation of output
information, and preprocessing and coordinate conversion of aiding
information, for example. These functions require significant amounts of
processing and memory and are generally performed using a floating point
coprocessor.
The GPS standard positioning service provides a navigation accuracy of 100
m 2 dRMS. A number of applications of the GPS require higher levels of
accuracy. Accuracy can be improved using a technique known as differential
GPS (DGPS). This technique involves operating a GPS receiver in a known
location. The receiver is used to compute satellite pseudo range
correction data using prior knowledge of the correct satellite pseudo
ranges, which are then broadcast to users in the same geographic area. The
pseudo range corrections are incorporated into the navigation solution of
another GPS receiver to correct the observed satellite pseudo range
measurements, thereby improving the accuracy of the position
determination. Correlation of the errors experienced at the reference
station and at the user location is dependent on the distance between
them, but they are normally highly correlated for a user within 350
kilometers of the reference station.
An alternative to the GPS receiver known in the prior art is the GPS
translator or transdigitizer, as described in U.S. Pat. No. 4,622,557, for
example. These translators or transdigitizers typically include only the
antenna assembly and RF assembly portions of a GPS receiver. Translators
are typically employed in missile tracking applications where small,
lightweight, expendable sensors are required. The GPS C/A code spread
spectrum signals received by the translator are combined with a pilot
carrier and transmitted at S-band frequencies (2200 to 2400 MHz). A GPS
translator processor located at the telemetry tracking site receives these
translated GPS C/A code signals and estimates the position and velocity of
the object. The transdigitizer retransmits the digitally sampled GPS
signal at 2 Msps using quadraphase modulation at 149 to 170 MHz.
Known variants of the GPS translator are the digital translator and the
transdigitizer. An object-borne GPS digital translator or transdigitizer
operates to convert the GPS C/A code spread spectrum signals to base band
and perform in-phase and quadrature phase sampling at a rate of about 2
MHz. Transdigitized or translated GPS signals are processed in a ground
based translator processing system in a similar manner to GPS signals.
A third variant of the GPS translator is the codeless GPS receiver, as
typified by the teachings of U.S. Pat. No. 4,754,283. This receiver
ignores the bi-phase code and recovers the carrier frequency of all
satellites in view of the receiving antenna. A telemetry transmitter
transmits a signal that contains the GPS carrier frequency information to
a ground-based telemetry receiver. This data is used to derive the speed
of the sonde. Since the GPS code is not tracked, the position of the sonde
cannot be computed using this method. This system uses a telemetry link at
403 MHz with a bandwidth of 20 KHz and has the advantage of requiring less
bandwidth than the transdigitizer but the disadvantage of only providing
velocity data instead of both position and velocity data.
In summary, prior art GPS receivers may be one of three types. In the first
type, all navigation processing activities occur at the receiver, which
outputs the position and velocity of the tracked object using either a
single computer or an RPC and navigation computer, in which there is
substantial interconnection between the RPC functions and the navigation
functions for satellite selection and acquisition. In the second type of
GPS receiver, the GPS signal is remoted by translation or variations
thereof and the signal is tracked at a ground processing facility where
the object position and velocity are derived. In accordance with this
latter approach, significant bandwidth is required to transmit the
translated signal. In the third type, the carrier frequency of the GPS
signals is measured and retransmitted to the ground processing facility
where only the velocity of the object can be derived.
It is therefore the principal object of the present invention to provide a
low cost tracking system for radiosondes, sonobuoys, aircraft, ships, land
vehicles, and other objects, using GPS satellites, that is capable of
providing the position and velocity of multiple objects without requiring
a 2 MHz bandwidth data link.
This and other objects are accomplished in accordance with the illustrated
preferred embodiment of the present invention by providing a GPS sensor
module that supplies the data required to locate a particular object, a
one-way telemetry link, and a data processing workstation to process the
data and display the object position and velocity. The GPS sensor module
comprises an antenna and a sensor. The sensor operates autonomously
following application of operating power. The sensor digitally samples the
signals from visible GPS satellites and stores this data in a digital
buffer. No processing functions are performed by the sensor, thereby
permitting significant reductions in the cost thereof. The raw satellite
data stored in the buffer, interleaved with other telemetry data from the
sonde or other object, are transmitted back to the data processing
workstation. Using this set of raw satellite data, the position and
velocity of the sensor can be determined at the time the data was recorded
by the sensor to a precision of 100 meters. If differential corrections
are also provided at the data processing workstation, the accuracy of the
position fix can be improved to better than 10 meters. If a 20 kHz data
link is used and the GPS signals are sampled at 2 Mbps, a 1-second set of
GPS data can be provided every 100 seconds, or a 0.5-second set of GPS
data every 50 seconds, or a 0.1-second set of data every 10 seconds. The
principal advantage afforded by the present invention is its ability to
provide extremely accurate position, velocity, and time information for
radiosondes, sonobuoys, and other objects using a low cost sensor and a
conventional data telemetry link. By eliminating all processing functions
performed in prior art GPS sensors, significant cost reductions are
achieved over existing GPS receiver designs. By reducing the data link
bandwidth from the 2 MHz required of prior art transdigitizers,
conventional telemetry links may be employed to retransmit the data. For
low cost data applications, such as sonobuoys or radiosondes, a position
and velocity fix is only required at a low rate (e.g. every 10 seconds), a
requirement that is accomodated by the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram illustrating the primary components of the GPS
object tracking system of the present invention.
FIG. 2 is block diagram of the sensor employed in the GPS object tracking
system of FIG. 1.
FIG. 3A is a block diagram of one embodiment of the RF/IF section of the
sensor of FIG. 2.
FIG. 3B is a block diagram of another embodiment of the RF/IF section of
the sensor of FIG. 2.
FIG. 4 is a block diagram of the digital data buffer employed in the sensor
of FIG. 2.
FIG. 5 is a flow chart of the process employed to determine the GPS
measurement from telemetry data.
FIG. 6 is a flow chart of a high speed correlation and complex
multiplication algorithm employed in two of the functional blocks of the
flow chart of FIG. 5.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to an apparatus and method for computing
the position and velocity of multiple objects equipped with low cost
sensors using a data processing workstation. The GPS satellite signals are
digitally sampled in accordance with techniques employed in conventional
digital GPS receivers, and the data is periodically recorded in a digital
data buffer. This data is then interleaved with other telemetry data from
the object being tracked and transmitted using a conventional telemetry
data link having a typical bandwidth of 20 KHz. The GPS data block is
recorded and processed by the data processing workstation to compute the
position and velocity of the sensor, at the time the data was sampled.
Differential GPS corrections are also provided at the data processing
workstation to improve the accuracy of the position computation.
Referring now to the block diagram of FIG. 1, the object tracking system of
the present invention primarily comprises a sensor 10, a data buffer 20, a
telemetry link 30 that includes a telemetry transmitter 25 and a telemetry
receiver 40, a GPS data processor 50, a GPS reference receiver 60, and a
data processing workstation 70.
A representative implementation of sensor 10 employs a simplified method of
frequency synthesis, a temperature compensated crystal oscillator (TCXO)
21 or other low cost oscillator, and a digital data buffer (DDB) 20.
Detailed block diagrams of sensor 10 are shown in FIGS. 2 and 3A.
Frequency synthesis within sensor 10 is based on an oscillator frequency
(FO) of 6.1539 MHz that is produced by temperature compensated crystal
oscillator 21. A local oscillator frequency (LO), produced by local
oscillator 23, is 256.times.FO=1575.3984 MHz. The power of 2 multiplier
(256) simplifies the design of a phase lock loop (PLL) 25 that is employed
to lock the frequency of the local oscillator 23 to that of the crystal
oscillator 21. The resulting intermediate frequency (IF) is 1575.42
MHz-256 FO=21.6 KHz. This IF is far enough above DC to allow distinction
between positive and negative doppler shifts. It is also close enough to
DC to result in minimal post correlation noise increase due to spectral
noise folding. The sampling clock used by the digital data buffer 20 is
FO/3=2.01513 MHz, a frequency that is ideal in that it is not related to
the 1.023 MBS chip rate. The time sample of the raw GPS data is stored in
the digital data buffer 20 and is transmitted at a lower rate to the GPS
data processor 50. An alternative embodiment of the RF/IF section of
sensor 10 is illustrated in FIG. 3B. The antenna output is bandpass
filtered and amplified by a preamplifier. The preamplifier output is
bandlimited to the C/A spectrum of 2 MHz. The bandlimited signal is
digitized by a 1-bit analog to digital converter.
Data is transferred between sensor 10 and GPS data processor 50 via a
conventional data telemetry link 30. Exemplary of the conventional
telemetry links that may be employed for this purpose are those operating
in the 403 MHz meteorology frequency band. The GPS data processor 50 may
comprise, for example, a high speed digital signal processing card
embedded in the data processing workstation 70. The data processing
workstation 70 processes the block of GPS data recorded by the sensor 10
and provides pseudo-range and delta-range measurements for all visible GPS
satellites to the data processing workstation 70.
Data processing workstation 70 uses the data provided by GPS reference
receiver 60 and GPS data processor 50 to compute the object (sensor 10)
position and velocity.
Referring now to FIG. 4, a pair of GPS data buffers 200, 202 store a block
of the 1-bit analog-to-digital samples at 2 MHz in a memory buffer. GPS
data buffers 200, 202 comprise 256K.times.8-bit memory devices that are
configured in a double-buffering arrangement to allow one of them to be
receiving GPS data while the other one is transmitting GPS data.
A clock generation logic unit 204 serves to generate timing signals used
for synchronization. Signals that specify the operating clock rates are
applied as inputs to clock generation logic unit 204. One of these input
signals indicates the rate at which GPS data will be read from an A/D
sampler and stored into a memory buffer. The other one of these signals
indicates the rate at which the GPS data is taken from the memory buffer
and transmitted to the receiving station over telemetry link 30. Clock
generation logic unit generates a signal used to drive a system timer.
A master control logic unit 206 provides control signals for the remainder
of the circuit comprising data buffer 20. It utilizes the clock signals
generated by clock generation logic unit 204 to drive its timing and
sequencing functions. The functions provided by master control logic unit
206 are 1) selection of the device to load a parallel/serial transmit
register; 2) switching of the device to unload the serial/parallel receive
register; 3) switching of the appropriate clock rate to GPS data buffer
address counters; and 4) selection of the memory buffers for acquisition
and transmission.
Shift registers 208 provide for the reception of the serial GPS data and
the packing of it into an 8-bit data packet. The master control logic unit
206 then places the data packet into the current selected received data
memory buffer. Shift registers 208 also takes an 8-bit data packet from
the currently selected device (header, system time, analog data, or GPS
data) and places it in the transmit register to be sent out serially.
A system timer 210, driven by clock generation logic unit 204, is employed
to time-stamp the current data being transmitted so that the time it was
acquired can be identified.
A header generator 212 serves to generate a unique binary pattern which is
used to identify the beginning of a new data record. This allows the
receiving station to recognize when a new data record has been sent.
An analog data converter 214 facilitates the inclusion of analog data which
is gathered from various ones of the remote sensors 10. This data may be
from just one or from several of sensors 10 that are time multiplexed.
The GPS data received by sensor 10 is interleaved with other digital
telemetry data, such as pressure, temperature, and humidity, and is then
transmitted as a block of data with an identifying header and time tag
through the telemetry link 30. In the preferred embodiment of the present
invention, a 25-millisecond block of GPS data is recorded (50 kilobits).
This GPS data can then be relayed to the ground in 1 second using a
50-kbps data link, or in 10 seconds using a 5-kbps data link, or in 100
seconds using a 500-kbps data link. In alternative embodiments of the
invention, smaller blocks of data may be selected, interspersed at regular
interval s. This is equivalent to a multiplexed GPS receiver approach
where 5-millisecond segments of data are collected, separated at
20-millisecond intervals. This approach would provide improved performance
in a high acceleration environment. A variety of block sizes and
frequencies for data transmission can be implemented, depending on the
circumstances and the available data bandwidth.
Referring now to FIG. 5, the telemetry data is received by the telemetry
receiver 40 of FIG. 1 and is passed to the GPS data processor 50 for
processing. In the preferred embodiment of the present invention, GPS data
processor 50 comprises a digital signal processing microcomputer card
installed in an IBM personal computer. The GPS data block is first
separated from the telemetry data and is then processed to derive the GPS
pseudo-range and delta-range measurements. The first step of this process
is to acquire the GPS signals. The list of visible satellite IDs and
estimates of their expected frequency shift and code phase is provided by
the data processing workstation 70 from the GPS reference receiver 60.
This information is used to search for the GPS satellite signals. The
search through the recorded GPS data block is repeated at different code
phases and frequencies until the signal is acquired. The software then
switches to the track mode to measure the code phase and frequency using
the complete GPS data block, initialized with the coarse estimate of phase
and frequency from the search algorithm.
A key feature of the present invention is the high speed code correlation
and complex multiplication algorithm illustrated in FIG. 6. To perform a
high speed search, it is necessary to perform multiple code correlations
at different code phases to detect the GPS signal. The algorithm
illustrated performs these functions in parallel in software. Using a
TMS320C40 chip, nine correlators can be operated in parallel in real time.
Using a 10-millisecond dwell period, the full 2046 possible half-chip C/A
code phases can be searched in 2.27 seconds. Once the signal has been
initially acquired at start-up, reacquisition requires significantly
smaller search windows and can be performed in a fraction of a second on
each new data block.
The same high speed code correlation and complex multiplication algorithm
is used to perform code and carrier tracking. One-millisecond accumulated
in-phase and quadrature signals are provided for early, late, and prompt
code phases using a look-up table technique. The first step is to compute
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