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SEMINAR REPORT 2008-09 SATRACK

INTRODUCTION

According to the dictionary guidance is the

‘process of guiding the path of an object towards a given

point, which in general may be moving’. The process of

guidance is based on the position and velocity if the target

relative to the guided object. The present day ballistic missiles

are all guided using the global positioning system or GPS.GPS

uses satellites as instruments for sending signals to the missile

during flight and to guide it to the target. SATRACK is a system

that was developed to provide an evaluation methodology for

the guidance system of the ballistic missiles. This was

developed as a comprehensive test and evaluation program to

validate the integrated weapons system design for nuclear

powered submarines launched ballistic missiles. This is based

on the tracking signals received at the missile from the GPS

satellites. SATRACK has the ability to receive record,

rebroadcast and track the satellite signals. SATRACK facility

also has the great advantage that the whole data obtained

from the test flights can be used to obtain a guidance error

model. The recorded data along with the simulation data from

the models can produce a comprehensive guidance error

DEPARTMENT OF COMPUTER ENGG GOVT.W .P.T.C.NEDUPUZHA-1-


model. This will result in the solution that is the best flight

path for the missile.



GPS SIGNALS

The signals for the GPS satellite navigation are two L-

band frequency signals. They can be called L1 and L2.L1 is at

1575.42 MHz and L2 at 1227.60 MHz .The modulations used

for these GPS signals are

1. Narrow band clear/acquisition code with 2MHz

bandwidth.

2. Wide band encrypted P code with 20MHz bandwidth.

L1 is modulated using the narrow band C/A code only.

This signal will give an accuracy of close to a 100m only. L2 is

modulated using the P code. This code gives a higher accuracy

close to 10m that is why they are encrypted. The parameters

that a GPS signal carries are latitude, longitude, altitude and

time. The modulations applied to each frequency provide the

basis for epoch measurements used to determine the

distances to each satellite. Tracking of the dual frequency GPS

signals provides a way to correct measurements from the

effect of refraction through the ionosphere. An alternate



frequency L3 at 1381.05MHz was also used to compensate for

the IONOSPHERIC effects.



Fig:1 Satrack concept

SATRACK CONCEPT

Guidance system evaluation concept of very early

weapons systems depended on the impact scoring techniques.

This means that the missile was shot and the accuracy was



formulated on the scoring or the target destruction. This

evaluation method was unacceptable for evaluating the more

precise requirements of the latest systems. A new methodology

was needed that provided insights into the major error

contributors within the flight-test environment. The existing

range instrumentation was largely provided by radar systems.

They however did not provide the needed accuracy or range in

the broad ocean test ranges. The accuracy projections needed to

be based on the high confidence understanding of the

underlying system parameters. SATRACK was developed with

the necessary hardware and telemetry stations.

The figure shows the SATRACK measurement concept.

The main parts are the GPS satellites, the missile translator and

ground telemetry stations. The missile receives the signals from

the GPS satellites. They are translated to another frequency and

relayed to the ground telemetry stations. The telemetry station

records the data for playback and for post processing.

The satellite signals received at the missile are

translated to S-band frequencies for the telemetry station using

the missile hardware called translators. The ground based

telemetry station record the data after reception through the



antenna after digitising the signals. Some ground sites uses L1

C/A signals to provide real time tracking solutions.



GPS TRANSLATOR

This flight hardware is fixed in the missile. The

translator receives the GPS signals and they are amplified,

shifted to an intermediate frequency, filtered to cover the

satellite signal modulation bandwidth, shifted to an output

frequency. Then they are amplified for transmission to one

or more ground stations.



Fig. 2 GPS Translator



The translator does the following

1. Received the satellite signal

2. Translated it to a missile telemetry frequency (S-band)

3. Rebroadcast the received signal

GPS translator are of both Analog and digital types The

Analog translators heterodyne the L-band signal to S-band

adds a pilot carrier to allow the monitoring of the reference

oscillator variations. Both wide and narrow band type of

Analog translators are used. Digital translators down-convert

the received L-band GPS signal to near base band and digitises

it. This digitised data is modulated into an S-band carrier and

transmitted to the ground stations.

FIELD SUPPORT EQUIPMENT

SATRACK is the most useful tool because of its post

flight processing facility .The ground equipment consists of

receiving antenna, data recorder and auxiliary reference

timing systems. The equipment receives the translated GPS

signal along with other telemetry signals and distributes it to

the data recorder. Most ground stations are capable of

generating a precise atomic timing standard. The earlier



equipments were narrowband recorders that relied on high-

speed tape recorders. These gave up to 14 tracks of recording

channels with four mega samples per second. The translator

processing system was developed for the national missile

defence exoatmospheric re-entry intercept subsystem where it

served as a real-time GPS processor for range safety as well as

data recorder. Some later versions were capable of processing

data from both analogue and digital translators.

PORTABLE GROUND EQUIPMENT

This hardware is used for the post flight processing and

tracking of the satellite signals. The SATRACK facility

processes the raw data into a time series of range and Doppler

measurements for each satellite, and the Kalman filter, which

incorporates various corrections and generates a navigation

solution for the missile. The system has undergone a lot of

redesign and development as the requirements evolved with

new type of translators and receivers. The latest system

processes the wideband L1/L2 signals dual frequency P-code

as required by wide band translators. The system hardware is

based on Analog Device SHARC processor. Most of the custom

GPS processing hardware is based on field programmable gate



arrays [FPGA]. Each board has the ability to track up to eight

channels. The user interface is done using windows based PC

workstations.

POST FLIGHT TRACKING AND DATA PROCESSING

This is the most important part of the SATRACK technology

FIG 3 Basic SATRACK configurations.

For a number of days surrounding the missile flight, GPS

signals are received, tracked, and recorded at the GPS

tracking sites.



During the missile flight, GPS signals are received by

missile, translated in frequency, and transmitted to the

surface station(s).

A tracking antenna at the station receives the missile

signals, separates the various components and records the

data.

The post-flight process uses the recorded data to give

satellite ephemeredes clock estimates tracked signal-data

from the post-flight receiver, and missile guidance sensor

data.

After the signal tracking data are corrected, the entire data

element and the system models are used by the missile

processor to produce the flight test data products.



The figure shows how the post flight tracking

facility accomplishes precision tracking of the GPS signals

through the playback of the recorded translator signals. High

accuracy satellite ephemeredes and the clock estimate

covering their span of test flight is obtained. These data along

with the processed telemetry data help provide the tracking

aids for the post flight receiver and measurement estimates

for the missile processor. The translator passes signal for all

the satellite in view of the missile antenna and the post flight

receiver provides all in view satellite signal tracking. During

play back satellite signals are tracked through delay locked

loops.

For range code modulation and phase locked loops

for carrier phase tracking.

The post flight processing of the recorded data is used

to test the accuracy of the measurements that is to evaluate

the guidance system. The concept can be explained based on

the block diagram given below.



Fig: 4 strategic weapons systems accuracy evaluation concept

The procedure was developed by whish the

uncertainties with whish we observe a performance as well as

the finitude of test programs was translated in to specified

confidence in the accuracy parameters being estimated.

Information theory provided the basis for developing the

algorithms that could quantify the confidence with which

accuracy could be estimated. Next performance needed to be

known, not just the system level but at the subsystem level

also. The accuracy evaluation program had to be able to

isolate faults and estimate performance of the subsystems or

the various phases of the system. Since the allowable number

of test used for the determination of estimates were limited to

10to 20 the instrumentation had to be of high quality to

provide the high confidence measurements hence to get good

confidence estimates. In addition to this, we also needed to



extrapolate the untested condition that is to predict tactical

performance with high-quantified confidence from test data.

Data from each accuracy test was analysed using some

variant of the Kalman filter. Within these filters are the detailed

models of both the system and the instrumentation for each

system. The figure depicts how this analysis is accomplished.

Given a particular test or scenario measurement, data are

collected on the various subsystems. Using rigorous methods,

these data are collected with prior information generally

developed and maintained by builders of the various parts of the

system under test. This prior information is necessary for the

single test processing, given the incomplete observability of the

error sources. The outputs of the filter provide the basis for

understanding particular realizations of system and subsystem

behaviour. Analysis results provide insight in to the sources and

causes of the inaccuracy. The results of the multiple tests –the

outputs of the Kalman filter –serve as the inputs to the

cumulative parameter estimation process. All prior information

regarding the relative error models is removed so that the

estimate accuracy is derived solely from the test data.



Fig: 5 reconstructions of sources of missile impact miss distance

error

The graph shows a hypothetical diagram used to allocate

contributions to the impact miss. This method is based on

projecting each error contributor and its uncertainty into impact

domain.

1. first level allocation is at the subsystem level: initial

conditions, guidance, and deployment and re-entry

2. second-level allocation provides data for major error groups

within each subsystem eg: accelerometers

3. Third-level allocation (not given in figure) produces

estimates of fundamental error terms of guidance model

eg: an accelerometer scale factor error.



This process solves the highly non linear equations for

the means, variances, and Markov parameters that characterize

the overall

system accuracy performance. In addition uncertainties in the

parameter estimates are calculated so that we have a

quantitative measure of our confidence in the solution .The

ultimate desired product is system performance under tactical

not test conditions. Here we rely heavily on the tactical gravity

and weather conditions developed from data and

instrumentation. These models along with deterministic

simulations of the system are then used to propagate the

fundamental model parameter estimates and the uncertainties

to the domain of interest-system accuracy at the target.

The carrier phase tracking of the signals provide the

critical measurements .The measurements of the GPS signal;

phase sense range changes along the line of sight for each

signal to a small fraction of the wavelength usually a few

millimetres. These measurements which when compared to

their values computed from guidance sensor data and satellite

position and velocity estimates, provide most of the

information. Noise in the measurement of the recovered GPS

range code signals is of secondary importance. In essence, the

inertial sensors provide high frequency motion information



better than the signal processes, the Doppler information

senses the systematic errors associated with the inertial

sensors and the range data provide an initial condition for all

the dynamic measurements. The range noise remaining after

the process of smoothing of the noise is smaller than

the other bias like uncertainties that set the limit on absolute

position accuracy e.g.: the satellite position.

The missile and satellite trajectories including

stimulated errors for satellite position and clocks were used

dot drive the satellite signal generators to produce the

simulated GPS signals. These are then passed through digitally

controlled phase shifters and time multiplexing switch to

emulate the missile GPS antenna network. This is connected to

a missile translator hardware simulator that produced the GPS

signals at S-band. An S-band antenna hardware simulator

produced the outputs, which were recorded by the prototype

telemetry station receiver, and the recording equipment .The

hardware simulator drivers were conditioned to encompass all

anticipated effects including signal refraction through the

ionosphere and troposphere. The recorded data were

equivalent to the data that would be received from telemetry

site.

The post flight processing facility now has all the inputs,

GPS ephemeredes, clock files, and telemetry data and



translated signal data tape. These data are then processed

and an estimate of the underlying model errors is produced. In

addition, the testing of the post processing system is done by

this method.

EVOLUTION OF SATRACK

SATRACK has evolved over a quarter century. The

original development for the Trident I missile (C4) began in

earn -est. in1974.4 The C4 version (SATRACKI) was a

technology develop -pment program aimed at(1) gaining

insight into what was needed for improved accuracy and (2)

developing an adequate accuracy evaluation n system.

SATRACK, as it evolved, is the fulfilment of the second major

objective.SATRACK I proved that it could provide adequate

estimates of guidance subsystem errors for individual flights. It

was a pioneering effort in that both the tracking methods and

the large Kalman filter processing techniques were pushing the

state-of the-art. The second phase (SATRACK II) was a major

upgrade in response to the stringent measurement

requirements set by the Accuracy Evaluation System study.5

The study established the total weapon system instrumentation

requirements for the Trident II (D5) missile in accordance with



specified accuracy evaluation objectives, Including SATRACK as

well as other prelaunch, in-flight, and Re-entry area

instrumentation. SATRACK II has been operational since 1987. A

general upgrade has been initiated to replace aging

components of the D5 test system.

The new SATRACK ground recording equipment is

currently in the final stages of checkout, upgrading at the post

flight facility is well along, and preliminary design of

replacement missile test components is beginning.

Furthermore, efforts have been focused for the last several

years on a new GPS translator system to support Trident re-

entry body testing. The upgraded system will not only

modernize the facility hardware and software functions, it will

also substantially extend SATRACK Performance capabilities.

and its influence on the Peacekeeper guidance model. The

Navy will continue to test and evaluate the Trident Weapon

System with the primary goals of detecting changes to system

performance caused by aging components and assessing

system modifications needed to extend its lifetime.

In this regard, we recognize that SATRACK

evaluation is only one part of system accuracy assessment,

and accuracy assessment is only a part of the total weapon

system evaluation. Equal diligence is needed in all aspects of

monitoring and maintaining the Trident system. Continued D5



accuracy evaluation support will remain the primary objective

for SATRACK. However, in parallel with this activity, we have

identified a natural extension of SATRACK capabilities that can

support precision intercept evaluations for national and

theatre ballistic missile defence flight tests.

This realization grew out of our ERIS flight

test experience and our support to the Strategic Defence

Initiative Organization for the development of a precision

intercept test capability for the Brilliant Pebbles Program. Both

of these projects and the continued support of NMD test

objectives have, with Navy concurrence, taken advantage of

the unique APL facilities developed for Trident. As noted in a

companion article (Thompson, this issue) on a high-precision

sled test, we successfully completed an IR&D project devoted

to demonstrating the measurement capability needed for

precision intercept test evaluations.

An earlier IR&D project developed a

translator design for this purpose that was the basis for a new

Trident translator system used for supporting special reentry

body tests. The upgraded SATRACK postflight tracking facility

will support existing C4 and D5 translators and reentry body

translators as well as the replacement translator to be

selected for the new D5 test missile kit. When completed, we



will refer to this configuration as SATRACK III, which will take

full advantage of technology growth in processing hardware

and software to produce a workstation-based facility that is

easily reconfigured to support a wide range of tests.The

capabilities of the new configuration and the expected

reduced test tempo of Trident flight tests naturally produce a

processing capacity that can be extended to support critical

ballistic intercept testing of other high-priority defense

programs. Our studies indicate that this capability is required

to adequately support model validation of precision missile

intercept systems, and we have configured the postflight

tracking subsystem architecture to be easily expanded to

eventually support such tests.

The office responsible for general range

applications, RAJPO, is developing a translator-based GPS

range system (TGRS) that includes a capability for intercept

support missions. This system is intended to serve both range

safety and postflight evaluation objectives for a variety of

range users. Many new applications are expected to use TGRS

digital translators and their ground station recording

equipment. A newupgrade to the APL postflight tracking

facility will provide an interface for TGRS data processing in

the

near future.



MAJOR BREAKTHROUGHS

1. EVALUATION CAPABILITY FOR CUMULATIVE FLIGHT

The limitations of the test geometry prohibit

observations of all the errors in any single flight test. Since

each test flight provides observations of the underlying system

missile guidance error models, the data can be combined from

may flight tests. The final cumulative analysis of flight test

data produces a guidance error model of the weapons system.

It combines observations from each flight to derive a missile

guidance model that is both tactically representative and

based completely on the flight test data. This model combined



with other similarly derived sub system models helps develop

planning factors used to assign weapons system targets

2. FULL DIGITAL IMPLEMENTATION.

The full digital implementation is of the Portable ground

equipment and processing facility. So, the results are

expected to be repeatable. This is a very big improvement

over the Analog circuitry

such as the Analog PLLs used for carrier- phase tracking loop.

In addition, the digital implementation removes the need for

periodic hardware calibration that accompanies the analog

circuits

3. BATCH MODE PROCESSING

This type of processing allows hardware to operate

with software like flexibility. As the pure software system was

too slow, hardware that is fully configurable under software

control implemented the most computing intensive portions of

the process such as signal correlation, generation of local code

and carrier signal mixing. It is possible to acquire the signal



with virtually no acquisition delay by conducting extensive

searches with initial batch of data until all the signals are

found.

4. FLEXIBLE ARCHITECTURE RECEIVER

The batch mode processing has been applied to

stand alone real time capable receiver called FAR. It retains

the essence of batch mode architecture. While maintaining the

capability to process the data in real time. FAR is a single

channel L1 C/A only receiver with a front-end data storage

memory that buffers unto one s of data. It can track up to 16

satellites in real time without any loss from channel

multiplexing

CONCLUSION

SATRACK is a significant contributor to the successful

development of and operational success of the trident

weapons system. It provides a unique monitoring function that

is critical to the maintenance of strategic weapons systems.

The development and research leading up to this technology

has been instrumental in bringing out the latest in GPS

receiver, translators, data recorders etc. Several special tests

have been conducted with various combinations of inertial



systems, GPS receivers, translators as well as RF/antenna

designs. Special tests have demonstrated that accuracy may

be achieved to support potential new and extremely

demanding tactical strike scenarios. The development of

SATRACK looks forward to the implementation of the Low Cost

Missile Test Kit. [LCTMK]. One other main development from

this technology was the development of sophisticated tools for

optimal target patterning. Instrumentation, analytic methods,

and modelling and the use of limited and expensive flight tests

assets were also born out of the SATRACK research.

REFERENCES

Marc Camacho and Sung Lim:”SATRACK tests missile

accuracy,” IEEE Instrumentation and Measurement pp: 37-

45 June 2003

D. R. Coleman and L.S. Simkins,”The fleet ballistic missile

accuracy evaluation program,” Johns Hopkins APL Tech



Digest Vol.19 No.4

pp 393-397 1998

T.Thompson, L.J.Levy, and E.Westerfield,” The SATRACK

system: Development and applications,” Johns Hopkins APL

Tech Digest Vol.19 No.4 pp 436-446 1998

David .E. Mosher,” Ballistic missile defence,”IEEE Spectrum

pp29-39 September 1997

Shneydor N. A ,”Missile guidance and Pursuit,”Herwood

Publishers pp 1-3,47-48 1998

ACKNOWLEDGEMENT

I extend my sincere gratitude towards Mrs.Seena.I.T,

Head of Department, Govt.Women’s Polytechnic, Nedupuzha,



Thrissur for giving us her invaluable knowledge and wonderful

technical guidance.

I express my thanks to Mrs.Seena.I.T, our seminar co-

ordinator and other lecturers of the Computer Department,

Govt.Women’s Polytechnic, Nedupuzha, Thrissur for their kind

co-operation guidance for preparing and presenting this

seminar.

I also thank all the other faculty members of Computer

Department and my friends for their help and support.

RESMI.K



CONTENTS

INTRODUCTION

GPS SIGNALS

SATRACK CONCEPT

GPS TRANSLATOR

FIELD SUPPORT EQUIPMENT

PORTABLE GROUND EQUIPMENT

DATA RECORDING AND POSTFLIGHT PROCESSING

EVOLUTION OF SATRACK

MAJOR BREAKTHROUGHS

ADVANTANGES

DISADVANTAGES

FUTURE DIRECTIONS

CONCLUSION

REFERENCES



ABSTRACT

The origin of the missile can be traced back to the

roman war machine the catapult. The guided missile was born

when Werner Von Siemens suggested a guide torpedo for

submarines in the late 19th century. From these beginnings the

present day trident and tomahawk are guided from the skies

using the GPS signals. This seminar deals with the

measurement concept that tests the missile accuracy.

SATRACK receives, rebroadcast, records and tracks the

satellite signals sent by the GPS signals. The reception and

rebroadcast of the signals is done by a missile hardware called

the GPS translator. The ground telemetry stations consist of

the RF antenna and recorders for the data. Post-flight

processing and modelling are done later at the SATRACK

Facility. Also the major error contributors to the missile flight

are determined by the modelling done. There is extensive use

of simulated signals in this method. This seminar also throws

light on the major breakthrough technologies that were

developed during the research leading up to the final form of

this technology. The major advantages, disadvantages and

future applications of this method are also discussed. This

guidance system evaluation concept is the best in the current

test and evaluation technology for guided weapons systems.


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