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DIGITAL TRANSLATORS FOR GPS TSPI APPLICATIONS
Alison Brown and Mark Sturza
NAVSYSCorporation
18725MonumentHill Road
Monument,CO 80132
BIOGRAPHY
The
Alison
K.BrownPresident
isthe
of
Navy
presently
Trident
NAVSYS Corporation.
II
assessment.
systems, estimation and control theory
systems
engineering
and analysis. Her expertise includes GPS systems design.
simulation, integration and test, GPS receiver design, strapdown
and
UCLA,
an
and
MS
University,
from
Kalman
MIT
filter
and
a
BA
design.
in
She
engineering
has
a
from
PhD
of GPS experience, previously with Litton Systems. Magnavox
Advanced Systems and Products, and Teledyne Systems. His
expertise includes all aspects of navigation system development.
test
MSEE
from
and
application.
USC,
He
and
an
MBA
holds
a
from
as
the
with
aGP
r
Trident
The
from
Caltech,
Pepperdine.
{C4}
and
missiles
to
be
tracked
are
equipped
in Figure1. The TPS. aided by a GPS ReferenceReceiver,then
the
real-time
missile
TSPI
data.
The flight-hardware package developed for the Trident II (D5)
program is approximately 180 cubic inches in size and consists
Cambridge
England.
integration,
both
from
A.Sturzais a consultantto NAVSYS.Hehasover15years
Mark
In
translator which rebroadcasts the GPS signals at S-band to a
ground basedTranslatorProcessingSystem(TPS),as illustrated
computes
algorithms
GPS
for rangesafetyandmissileaccuracy
She
has over 12 years experiencein satelliteand inertialnavigation
system
uses
programs
{D5}
an
He
has
authored 18 GPS technical papers and has 5 GP$ patents.
GPStranslator which mixes.the L1 (1575.42
(1381.05MHz) GPSsignalsto $-band. Both the L1 and 13 GPS
signals are provided to allow dual-frequencycorrection of the
ionosphericgroup delay. The GPS RAJPOhas also developed
a GPS Translator T$PI System to provide accurate and cost­
effectivetracking of various types of high dynamic expendable
vehiclessuch as missilesand reentry vehicleInterceptors, The
of
a
MHz)
RAJPO
40
Abstract
CurrentlyanalogTranslatordesignsare used to instrument
missiletests in applications such as the Trident and ERIStest
programs.TheTranslatorinstalledon-boardthe missileconverts
the received L-band GPS signals to S-band and rebroadcasts
them to a ground-basedTranslator Processing System (TPS)
wherethe locationof the vehicleIs determined. The advantage
of this approach over using the GPS Range Pod Subsystemis
that the size, weight and cost of the flight hardware Is
substantially reduced. Also, by employing special purpose
trackingloops in the TPS,the GPSsignalsmay be trackedunder
very high dynamicsup to 50 g accelerationsand 60 g/sec jerk.
In addition,the custom signal processing in the TPS allowsthe
translatorsignalsto be acquiredvery rapidly,within a few
seconds.
Ballistic
cu.in.,
Missile
weighs
Translator
5
Ibs
(BMT)
and
Type
requires
56
W
A
of
is
and
L3
packaged
In
power.
NAVSYShavedevelopeda smallerBMTTranslatordesignunder
contractto the WesternSpaceand MissileCenter(WSMC).This
Is3Dcu.in.,weighsonly3 Ibsandrequires
only28W
Translator
of power.
The
BMT
limit
Translator
the
signal
S.band
signal
spectrum
Is
bandwidth.
only
Illustrated
In
the
C
/
A
Figure
code
2.
GPS
To
signals
are translated and broadcast to the TPS. The design allows
multiple translators to operate on different frequencies all sharing
the 2200-2290MHz S-band tele'!'etry frequencyallocation.
the current RAJPO Translator a 1.5 dB penalty is paid
an "ideal" design due to band limiting and power limiting. The
With
over
digital
Translator
received
the
operates
L-band
GPS
samples
on
by
signals
an
sampling
and
S-band
and
thermal
quantizing
noise
telemetry
and
link.
A
the
modulating
pilot
carrier
includedto aid in acquisitionand tracking ofthe telemetry
is
signal.
The RAJPO8MTTranslatoris packagedIn 40 cu. in. and weighs
5 Ibs. It is possible with existing technology to develop a
significantly smaller instrumentation package using a digital
Translatordesign. As shown in the paper, the digital approach
The Implementation loss of a digital Translator compared to an
"ideal"design is a function of the sampling rate and the number
of quantization levels. The paper includes a trade-off analysis of
permits
loss.
some
as
significant
improvements
applications
the
and
potential
for
also
signal
provides
encryption
in
tracking
additional
performance
capabilities
for
secure
for
digitalTranslatorcan be manufacturedusing currenttechnology
that can be packagedin a 5 cu. in. chip set volume. The paper
includes an overview of the design of such a device and a
discussion of the applications where the reduced size and
superior performance over the analog design would be of benefit.
INTRODUCTION
1.1
CURRENTANALOGTRANSLATORDESIGNS
Existing GPS Translator tracking systems have been developed
by the U.S.Navy and by the trl-serviceRangeApplicationsJoint
Program
Office
(RAJPO).
Both
translator
systems
are
telemetry
link
bandwidth
versus
the
implementation
By minimizing the Translator Implementation loss, it is
to Improve
the tracking
performance
of the ground-
data.
A
based
1.
required
possible
such
programs.
the
designed
to providereal-timeTime SpacePositionInformation(TSPI)data
for flighttest and tracking applications.
1.2
TPS,
improving
in
turn
the
accuracy
of
the
GPS
TSPI
DIGITAL GPS TRANSLATORS
Thesizeandweight of the GPSTranslatorcanbe furtherreduced
by developinga state-of-the-artdigital design. DigitalTranslators
take advantageof the recent developmentsin digital microwave
radios. A top-levelblock diagramfor a DigitalTranslatoris shown
In Figure3. As in a conventionalanalogTranslatorthe L-band
GPSsignalfirst passesthrough a L-bandpreamplifier.Thesignal
is then filtered to select either the P-code or the CIA-code
bandwidth. The filtered L-band signal Is next sampled and
quantizedby an AID converter. The AID converter sampling
clock is selectedto satisfy the Nyquist criteria (4 MSPSfor the
CIA-code).
The
AID
outputs
carrier.
Apilot
carrier
are
(PC)
is
used
then
added
to
modulate
an
to
the
S-Band
modulated
signal. The combined signal is the DigitalTranslatoroutput.
361
~
The digitalTranslator architectureeliminatesthe need for an IF
frequency and significantly reduces the filtering requirements,
The result is significant savings in size, weight, and power
dissipation.
2.
L-band spectrum.
The NAVSYS
design allowsthe PC to be placedat 1.92 MHzbelow
the center frequency, as specified for the BMT, or at other
frequencyoffsets if preferred.
power of the translated
translator
DIGITALTRANSLATORDESIGN TRADES
2.2
2.1
PERFORMANCE
DESIGN
Conventionalanalogtranslatorsbandlimitthe L-bandinputsignal
The variables that must be considered in the design of a digital
translator are bandwidth, input frequency, and output spectrum.
The nominal bandwidth required to accommodate the main lobe
of
the
code
spectra
is
2
MHz
for
the
CIA-code
and
20
MHz
and process
P-code.
The
bandwidth)
advantages
as
bandwidth)
are
of
compared
to
output
a
CIA-code
a
bandwidth
translator
P-code
and
(20
operating
MHz
A
the
20
MHz
range.
main
3
times
the
for
However,
limiting amplifiers.
the
of
the
the
code
spectra
P-code)
band
(2
results
limiting
the
In
noise
a
MHz
for
0.92
dB
results
in
a
the
CIA-code,
correlation
0.51
loss.
dB
reduction
in
C/A-
rangeof a p.code
operating
lobe
MHz
post
code Translator has
through
for
(2
translator
spectrum
translatoroutput signalis processed. Bandlimitingthe GPSsignal
to
the
the bandlimlted
Bothof theseoperationsresultin SNRdegradationwhen
correlation
noise.
degradation.
translator (the output power density is 10 dB higher). The
advantagesof the P-codetranslatorare improvedaccuracy,anti­
spoofing,and better jammingtolerance. It should be noted that
the wider 20 MHz bandwidthdoes not preclude use of the C/Acode. Thus a P-code translator can operate with a CIA-code
receiver.
Thus
The
GPS
signal
the
net
effect
SNR
in
is
the
band
a
0.41
limited
dB
SNR
spectrum
Is very small, so the limiting amplifier results in an additional 1.06
dB
SNR
degradation.
degradation
Thus
Is
1.47
the
total
analog
translator
SNR
dB.
Digitaltranslatorsbandlimitthe l-band inputsignal,sampleit, and
then quantizethe samples. Bandlimitingthe GPS signal to the
main lobe of the code spectra and samplingat the Nyquistrate
resultsin an SNRdegradationof 0.44 dB. Singlebit quantization
results in an additional 1.96 dB of loss. Thus the total digital
translatorSNRdegradation is 2.4 dB.
Theinputfrequencycanbe eitherthe L1 channel(1575.42
or the L2 channel(1227.6MHz). Both the CIA and p.code
MHz)
spectrum are availablein the L1 channel while the L2 channel
only providesthe p.code spectrum. Thus the L2 frequencyonly
makes sense for a P-code translator. Use of a dual L1/L2
translator or individual L1 and L2 translators would allow for
The analog translator is seen to have a 0.93 dB performance
edge. The significanceof this edge dependson the application.
Inthe majorityof casesth~ size,weight, power,andcost benefits
ionospheric compensation resulting in Improved TSPI accuracy.
of the digitaltranslatoroutweighthe performancepenalty. A
The digitaltranslatorapproachprovidesa varietyof output
comparison
spectrum options. In the digital translator the input spectrum is
band limited, sampled and quantized. The resulting digital bit
stream could be used to phase, frequency, amplitude. or pulse
modulate an S-band carrier to form the output signal. Phase
modulation is preferable in that it results in a constant envelope
3.
keying
BPSK
(OPSK)
spectrum.
of
(ATPS) which can be used to track the digital Translator signals
analog BMT Translator spectum Illustrated in Figure 2. The ATPS
has
been
developed
by NAVSYS to provide a low cost, portable
alternative
to the current
RAJPO TPS. This has been acheived by
Ouadraphase shift
a custom
developing
preamplifier
/downconverter
PSK modulationformats is that special translator receiversare
required.
In addition to the translated L1 or L2 input. spectrum, the
translator output must also contain a pilot carrier (PC) signal.
The PC is used by the translator receiverto removethe signal
dynamic effects of the S-band link. The BMT Type A spec
requiresthat the PC output power be 10 dB lessthan the output
Table 1.
TranslatorComparison
TYPE
WEIGHT
SIZE
POWER SNR DEGRADATION
1.47
40
BMT
NAVSVS
Analog
NAVSVSDigital
CU
30
5
IN.
CU
5CU
4
IN.
IN.
<1
module
which
interfacesto a conventionalGPS navigationset. This approach
substantiallyreducesthe developmentandproductioncost ofthe
ATPS by taking advantage of advances in current receiver
technology.
and otherM-ary
OPSK
DIGITALTRANSLATOR PROCESSINGSYSTEM
is illustratedin FIgure4. The spectrum of the digital Translator
appears on a spectrum analyzer to be identical to that of the
requiresonly 1/2 of the output bandwidthof the
The disadvantage
in Table 1.
A block diagramof the AdvancedTranslatorProcessingSystem
signalthat can be passedthrough a Class C poweramplifierwith
littledegradation.Thereare a varietyof phasemodulationformats
that can be utilized. Binary phase shift keying (BPSK)is the
simplestto implement,requiresthe same output bandwidthas an
analogTranslator.and resultsin an output spectrumcompatible
with analog translator, and results in an output spectrum
compatible with analog translator receivers.
is shown
LBS
dB
56W
1.47
28W
LBS
LB
362
8W
2.4
dB
dB
A rack mounted configuration of the current ATPS architecture is
illustrated in Figure 5. A single rack can host up to 15 Vehicle
Tracking Systems (VTS) and a Reference Receiver. Each VTS is
controlled through a PC interface which also provides a data
logging capability. One PC can operate up to four VTS In the
design. By comparison, the current RAJPO TPS requires
two racks of equipment to track a single Translator. The ATPS
design has resulted in reduction of size, weight and power by a
factor of 25 over the current RAJPO TPS equipment.
The SDI BrilliantPebble concept requires miniaturizedtracking
4.
miniaturized Translators would substantially simplify the traffic
control problem.
devices
such
as the DigitalTranslatorto provideguidanceand
mid-coursecorrection. Other SDI experimentscould also take
advantageof this technology as sizeand weight are criticalin any
space-basedsystem due to the launch costs.
current
APPLICATIONS
DigitalTranslatorshavea wide range of militaryand commercial
applications. The following -military projects will require
miniaturizedtracking systems. The small size (5 cu.ln.) and
weight( <11b.) of the DigitalTranslatordescribedin this paper,
will make It idealfor these applications.
The DigitalTranslatorset is sufficientlyrobust (gun-rugged)for
integrationintoartilleryprojectilessuch as registrationrounds. In
this application,the GPS Translator can be used to track the
trajectoryof the round and compute the ballistictrajectoryof the
projectile,as Illustratedin Figure6.
The small size
of the unit,alsomakesit idealfor Integration
into
radiosondes.ThesuperiorprecisionavailablewithGPSallowsfor
highly accurate wind speed determination. More accurate
windfindingsystemswould be of particularbenefitIn supporting
space and missilelaunchfacilities.
Applicationsexist for a lightweight, Inexpensivedevicesuch as
the DigitalTranslatorin tracking spacecraftsuch 8S SDItargets,
tracking satellites or other free-flying platforms. The Space
StationIs requiredto monitor hundreds of free-flyingplatforms,
vehiclesand personnelin its vicinity. Equipping all of these with
Commercial applications for miniaturizedTranslatorsalso exist.
Emergency services and vehicle fleet operators are becoming
aware of the
of vehicle tracking systems. A miniaturized
Translator is a candidate for locating emergency vehicles, trucks,
vans, stolen cars and even personnel. due to its small size and
benefit
weight.
In quantity production, the Translatorcould be a highly
cost effectivealternativefor these applications.
REFERENCES
"Designof a MiniaturizedGPSTime-Space-PositionInformation
SystemFor Tracking Applications".NAVSYS-Q189-107.
January
1989
in GPSTranslatorTechnology",J.B. McConnell,RH.
Greenberg, R.B. Pickett, PoCoWiJdhagen,AK. Brown. ION
"Advances
Satellite Division Conference Proceedings, September 1989
"GPS Predetection Data Recording and Post-Test
System",
NAVSYS
0989-114, September 1989
Processing
363
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