Note: Descriptions are shown in the official language in which they were submitted.
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l~ADIO NAVIGATION SYST~:M USING OUT-OF-BAND PSEUDOLITFS
1~
The present invention relates to a radio navigation system using out-of-
band pseudolites in an environment of the Global Position System (GPS)
navigation system.
In the GPS navigation system a plurality of satellite vehicles orbiting the
earth broadcast direct sequence spread spectrum signals at a ~requency L1 and byreceiving a plurality of GPS satellite vehicles very accurate location or navigation
infolmation can be derived at almost any position on earth that is visible (in aradio sense) to the satellites. Stationary ground reference stations are used toprovide differential correction signals to mobile navigation receivers so that the
position signals are extremely accurate and can be used for land surveys, for
example. However there can be locations and times when there are insufficient
GPS satellite vehicles visible.
The obJective of the present invention is to provide a navigation system
that is available at all times in a given area and to provide a navigation system
wherein if no GPS satellites are visible, or an insufficient number of GPS
satellites are available, a system of pseudolites, including a reference station and a
master pseudolite station, are aYailable for providing pseudolite navigation
signals for extremely accurate navigation signals to the remote mobile receiversn a glven area.
SUMM~RY OF THF INVENTION
The invention overlays a system of stationary pseudolite navigation
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transmitters for broadcasting a GPS like signal at a frequency Fl, a non-GPS
carrier fi-equency, in tlle environment of GPS spread spectrum navigation signals
at a radio frequency L1. A plurality of pseudolite stations broadcast a plurality of
spread spectrum pseudolite navigation signals at a radio frequency Fl which are
at a different frequency than the frequency Ll. According to the invention, at
least one reference station is provided for receiving the GPS navigation and thepseudolite navigation signals and deriving navigation correction data (differential
GPS, kinematic observations data) signals. At least one of the pseudolite stations
serves as a master station in association with each reference station. A
communication link provides differential GPS and observation data signals from
each reference receiver to its master pseudolite stations which modulate the
reference station observations and integrity data for broadcasting to a plurality of
mobile receivers which receive the pseudolite and GPS navigation signals
including the navigation correction signals from the master pseudolite stations
and produce accurate navigation information therefrom in the presence or absenceof useful GPS navigation signals.
The present invention differs with respect to in-band pseudolites in the
following particulars:
1. Non-GPS Fl frequencies are available to wider range of potential
customers without danger of interfering with existing GPS facilities.
2. Prior art includes use of near GPS frequencies- e.g., within about 1-10
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C/A code nulls (approximately l0 MHZ) of Ll (See B. D. Elrod and A. J. Van
Dierendonck, "Testing and Evaluation of GPS Augrne~nted with Pseudolites for
Precision Landing Applications," Proceedings of DSNS '93, Amsterdam, The
Netherlands, 31 March 1993.), WAAS testing within about 2.5% of nominal GPS
L1 fi~equencies, and the technique up to 3.5~o different from nominal GPS
described in Brown US patent number 5,311,194.
3. In these cases, only trivial modifications are required to receive the
signals- the signals are "in-band" as far as the receiver architectures are
concerned. In addition, tl1e detailed signal paths within the receiver can be made
to be identical, further simplifying the modeling task.
4 The present invention relaxes the range of Fl to be almost a~ ,y.
n~ e frequencies in the USA at this time appear to be 1.9 GHz and 2.4 GHz,
for example.
5. Non-GPS frequencies Fl observations can be combined with GPS
observations to jointly form both carrier-smoothed DGPS and/or kinematic
solutions as shown herein
6~ The present invention does not assume pseudolite broadcasts are
synchronize(l, neither with one another, nor with GPS.
The present invention differs with respect to conventional GPS in the
following particulars:
1. The present invention employs pseudolites to augment GPS range
observations. (Conventional DGPS provides no additional range observations).
2. Pseudolites may employ pulsing to mitigate near/far limitations.
3. At least one pseudolite broadcasts DGPS reference information, in
additiol1 to range observations. (DGPS uses separate commnnication resource).
4. GPS augmentalion may radically collapse time to achieve On-the-Fly
-
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(OTF) kinelllatic solutioll.
l)ESCl~II'TION OF TIIE l)R~WINGS
The above and othcl- objects, advantages and features of the invention will
bccome mole appalcllt ~vhell con.sideled with the following specificat;on and
accompanyillg drawillg~s whcreill:
Fig. 1 is a schelnatic illustration of a GPS-pseudolite navigation system
incorporatillg the invelllioll,
Fig. 2 is a block diagram Or a master pseudolite incorporateLI in the
invcntion,
Fig. 3 is a block diagram of a receiver incorporated in the invention, (the
REF anLI MRX receivers being preferably, identical except with respect to the
uses of their outputs), and
Fig. 4 is a block diagr;~ of all auxilialy pseudolite refelence station as
used in the invelltioll.
DE~TAILED DESCRIr'l'lON OF THE INVENTION
Referrillg to Fig. I, a constellatioll Or GPS satellite.s GPS- 1, GPS-
2.. GPS-N proviLIe basic service to most areas including the area to be served by
lhe ~syslem of pseudolilc~ l'L- 1, I'L-2....PL-N. The area to be served may haveany numbel or plurality of mobile receivers MRX-1, MRX-2.. MRX-N. One or
mole of the referellce receivcl-s REF~ are provided and are capable of receivillg the
GPS sigll.lls .-l frcclucl)cy Ll .IIld al.so lhe GPS-like ~signals rroln tlle pseuLlolilc
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statiolls PL- 1, PL-2....PL-N at a frequency F1 which can be chosen over a wide
ral)ge "n~l nee~ n~L be llc,ll lllc GPS frequency Ll. A communication link CL,
wllich may be lan(l lhle, Iniclowave, fiber oplic, a short set of wires, etc. from the
rercrence rcceiver REF to one or more of llle pseudolites PL-I, PL-2...PL-N
supplies Differelllial GPS (DGPS), kinelllatic observations and integlity data to
the pseu~lolites. The pseudolites transmil GPS-like spread spectrum signals at
frequency I~l lo refcrencc ~lalioll REF and Ihe mobile receivers MRX-I, MRX-
2.. MRX-N in order to augment the GPS signals. One of the pseudolite stations
in this system (PL-I) has been ~lesignateLI a master pseudolite station. The master
p.seudolite nlod-llatcs thc obsclvation an~l integrity data received fiom the
reference station REF over communication link CL, thereby broadcasting this
inrol nla~ioll lo thc MRX population.
The mobile r eceivers MRX are capable of receiving GPS signals at
fiequency Ll and the GPS-like signals from the pseudolite stations PL-I, PL-
2...PL-N at fi-equency Fl. Receiver elements best realized as exact copy of
elemellt at tlle relel-ellce statioll REF in order for best opportunity for
implementation depelldenl errors lo cancel out. Since these are true receivers,
(i.e., they need not transmit signals), there may be any number of them in the
service area.
If no GPS satellites are vi.sible or in lille-of-sight within the service area
(e.g., al lhc referellce receivel- REE~, the system may operate willlout GPS. In that
case, it replaces GPS, rathel thall augmenting it, and the GPS time is not
available. If one GPS salellite is available within the service area, e.g., at the
refcrence rcceiver REF, tlle syslem can lransrer GPS time to lhe mobile receivers
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MRX as limited by observation errors and conventional limits of the Navstar GPS
Standard Positioning Service. Carrier-smoothed code (pseudo-range, Pl~)
observations are possible with only one GPS satellite visible. The pseudolites can
auglnent this PR, but must fully replace GPS if true kinematic solutions are to be
derived. If two or more GPS satellites are available, GPS time can be transferred
and GPS observations can participate in the kinematic solution.
The reference receiver is preferably adapted to handle the observations of
the non-master pseudolite stations. The invention contemplates reducing these
PRs modulo a preset value (set at above twice the expected maximum
measurement range). The reference station observations may be coll,prt;ssed to
be suitable for broadcast. For example, the most significant bits (MSB) of rangeand rate may be infrequently broadcast, with frequent updates supplied relative to
these values, thus reducing the number of bits required. Note that the non-master
pseudolites need not be synchronized to GPS or any other pseudolite. This may
present special problems at the receivers, since the resulting PR values are farmore arbitrary than true GPS. The master pseudolite may be synchronized to
GPS (if any) by mcans of observing its own DGPS terms in the commllnic~tton
link CL between the reference station R~F and the master pseudolite station.
This is not required but may be useful if pseudolites PL can replace GPS within
part of tlle service area and when GPS time is useful.
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Like the reference receiver REF, the MRX will need to handle
observations of the non-master pseudolites.
In general, there must be at least one reference station, and at least one
master pseudolite per reference station. However, there can be multiple reference
stations, and multiple master pseudolites can be connected to modulate the
observation and integrity data from any reference station. Systems may
incorporate multiple reference stations to provide the benefits of improved system
reliability and to extend the service area. Reliability improves when several
reference signals are available to the typical MRX. A larger area may be served
while m~intzl;nin~ nomhlal accuracy by spreading reference stations to cover it
evenly; i.e., so that there is a fixed upper limit to the distance from an MRX to its
closest available reference station. Accuracy at each MRX is limited by the
distance from that MRX to the reference station it is using.
This incorporation of multiple master pseudolites may be required in the
following situations. In some cases it is seen that a single master pseudolite
source may not illllmin~te the desired coverage area. These areas could include
regions near blockages, such as near bridges, large bllilflings and docks; regions
near moving blockages such as ships; or regions with highly irregular terrain,
such as canyons, mines and factories.
Referring to Fig.2 a reference frequency source such as crystal oscillator
~0 or other stab}e frequency source SFS is selected by selection switch SSw as asource for multi-synthesizer MS which generates and outputs a control signal
fconlrDI7 a carrier cos wfl(t), and fcode from one of the selected input reference
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frequencies. Oscillator CO is a crystal oscillator or better. The control unit CU
receives fcontrol and selectively uses an external interface EXT/lNT for controland/or is interfaced to a reference receiver which provides advantageously both
GPS reference observations and PL broadcast data and outputs a control signal tocode generator CG. Code generator CG generates a spread spectrum code at a
rate selected by use of tlle frequency fcode and has the ability to modulate a data
pattern on the spread spectrum code. The cos wf,(t) signal is binary phase shiftkeyed (E3PSK) in modulator or mixer M. Optionally, a pulser P controlled by an
output of control block CU C~ll be used to provide greater effective dynamic
range at the receiver and the thus forrnul~tefi signal is amplified and broadcast to
all of the mobile receivers MRX.
I~eferring to the receiver block diagram illustrated in Fig. 3, antenna and
front end A receives the L1 and F1 signals and splits the output, supplying dualGPS- like receivers Rl and R2, respectively. Receiver R1 is a traditional GPS
receiver for processing the Ll signal frequencies and receiver R2 has slight
modifications for processing thc - signal frequencies. Both receivers are lockedto frequency and timing signal inputs from frequency and timing synthesizer FTS
which is supplied with a base rrequency from source CO. Frequency and timing
synthesizer FTS generates reference and timing signals to coordinate receivers Rl
ancl ~2 and to drive control microprocessor CMP. Control microprocessor CMP
provides high level control and integrates the observations and data of receivers
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R I and R2. Reference interface RI outputs the reference link data to
communication link CL which is connected to the master pseudolite. The
solution output interface SO provides the navigation in~ormation to the user via a
utilization device, which may be a display, a recorder or perfo~n a control or
guidance function. The following table sets out some of the parameters:
Input code data data format Examples
Freq. rate rate andcontent
1575.42 1.023 50 bps ICD-GPS-200 NovaTel RT-20
Fl fcode = fcOdc variant of GSV 1012
1.023 ~ 21000 WAAS/LAAS (modif. forFl)
(typical, fd;
others
possible)
An exemplary l~lock diagram of an auxiliary pseudolite reference REF
(Fig. l) is illustrated in Fig. 4, which parallels Fig. 2. This example assumes
BPSK modulation of frequency Fl The pulser function block P' again is optional.
It provides a way of pulsing or chopping the output signal for greater effectivedynamic range at tlle receiver. Oscillator OSC-2 is also of the quartz crystal
oscillator family or better. The auxiliary pseudolite does not require an external
time base input. As indicated the pseudolite has provision for external interface
EXT INT and control. The pseudolite optionally broadcasts its location if known.The multi-synthesizer MS' and code generator CG' are the same as described in
connection with Fig.2. The control element or microprocessor CMP' selects the
spread spectrum code and any data to be modulated on it, and generates the
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pulsing element control signal and provides external interface signals. Its timing
is derived from the multi-synthesizer. The control element or microprocessor
controls the selection of the SS code, the pulsing control and services the external
interface.
While preferred embodiments of the invention have been shown and
described, it will be appreciated that various modifications and adaptations of the
invention will be obvious to those skilled in the art and it is intended that the
claims encompass such modification~ and adaptations.
WHAT IS CLAIMED IS:
