Satrack-evaluation methodology2012-13 CHAPTER 1 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 model. This will result in the solution that is the best flight path for the missile. SATRACK was developed to validate and monitor. The Trident missile guidance error model in the System Flight Test Program. It is the primary instrumentation and processing system responsible for accuracy evaluation of the Navy‟s Strategic Weapon System. Instrumentation and processing systems available when the Trident Development Program began could not meet this need. APL conceived and led the development of the SATRACK system to fulfill this requirement. Prototype instrumentation required for the missile and ground station data collection functions were developed at APL to validate the concept, and we generated specifications controlling the development of the operating missile and ground station hardware. We also developed and operate the SATRACK processing facility, which includes a unique preprocessing hardware and software configuration and an extensive Dept of ECE, VKIT, Bangalore Page 1 Each satellite transmits a unique PRN code. and information about the time and status of the entire satellite constellation. which is freely available to the public. usually reserved for military applications.Satrack-evaluation methodology 2012-13 CHAPTER 2 GPS SIGNALS Global Positioning System (GPS) satellites broadcast radio signals to enable GPS receivers on or near the Earth's surface to determine location and synchronized time. These sequences only match up. when transmitted at 1. Department of Defense (DOD) for both military use and use by the general public. when they are exactly aligned. The navigation messages include ephemeris data. or strongly correlate. Bangalore Page 2 . repeats every millisecond.S.000 bits. used to measure the distance to the satellite. 2. 2. and the restricted Precision (P) code. each satellite's P-code PRN code is 6.1871 × 1012 bits long (6.187. 2. the PRN codes are highly orthogonal to one another.023 bit deterministic sequence called pseudorandom noise (also pseudorandom binary sequence) (PN or PRN code) which.23 Mbit/s). called the almanac. ~720. This is a form of code division multiple access (CDMA). GPS signals include ranging signals. which allows the receiver to recognize multiple satellites on the same frequency.213 gigabytes) and only repeats once a week (it is transmitted at 10.100. and navigation messages.1.2 Precision code The P-code is also a PRN however. The GPS system itself is operated by the U.023 megabits per second (Mbit/s).1 BASIC GPS SIGNAL The original GPS design contains two ranging codes: the Coarse/Acquisition (C/A) code. which does not correlate well with any other satellite's PRN code.1. In other words. VKIT.1 Coarse/Acquisition code The C/A code is a 1.000. The extreme length of the P-code increases its Dept of ECE. used to calculate the position of each satellite in orbit. Narrow band clear/acquisition code with 2MHz bandwidth. This has allowed companies to develop semi-codeless approaches for tracking the P(Y) signal. However.000. but it is known that it is applied to the P-code at approximately 500 kHz. To prevent unauthorized users from using or potentially interfering with the military signal through a process called spoofing.000. Wide band encrypted P code with 20MHz bandwidth.000 bits. Tracking of the dual frequency GPS signals provides a way to correct measurements from the effect of refraction through the ionosphere. without knowledge of the W-code itself.42 MHz and L2 at 1227. the P-code PRN is actually a small segment of a master P-code approximately 2. The Y-code is what the satellites have been transmitting since the anti-spoofing module was set to the "on" state. synchronize with the P-code. The encrypted signal is referred to as the P(Y)-code. They can be called L1 and L2.60 MHz. a special encryption sequence. To that end the P-code was modulated with the W-code.000. Bangalore Page 3 . The parameters that a GPS signal carries are latitude.05MHz was also used to compensate for the ionospheric effect. An alternate frequency L3 at 1381. after obtaining the current time and approximate position.L1 is at 1575. 2. it was decided to encrypt the P-code. L2 is modulated using the P code.35 × 1014 bits in length (235. The signals for the GPS satellite navigation are two L-band frequency signals. Whereas the C/A PRNs are unique for each satellite. altitude and time. ~26.The modulations used for these GPS signals are 1. The details of the W-code are kept secret. the code is so long and complex it was believed that a receiver could not directly acquire and synchronize with this signal alone.[1] which is a slower rate than that of the P-code itself by a factor of approximately 20. This signal will give an accuracy of close to a 100m only. to generate the Y-code. The modulations applied to each frequency provide the basis for epoch measurements used to determine the distances to each satellite.Satrack-evaluation methodology 2012-13 correlation gain and eliminates any range ambiguity within the Solar System. VKIT.This Dept of ECE. longitude. It was expected that the receiver would first lock onto the relatively simple C/A code and then.716 terabytes) and each satellite repeatedly transmits its assigned segment of the master code. L1 is modulated using the narrow band C/A code only. This code gives a higher accuracy close to 10m that is why they are encrypted. only recording . Translated it to missile telementry frequency (S-band) 3. Received the satellite signal 2. Shifts the carrier frequency to near zero 3.only signal frequency. Then digitally records the full signal data The receiving equipment does not provide a SATRACK signal tracking function. shifted to an intermediate frequency filtered to cover the satellite signal modulation bandwidth. Receives the translated GPS signal 2. Coherently samples the signal to capture the desired spectrum 4. shifted to the desired output frequency and amplified for transmission to one or more ground stations. L3 at 1381. Dept of ECE. spectrum of the L2 signal due to system constraints on the signal bandwidth. Bangalore Page 4 . L1.And band width matter. to compensate for ioospheric effects.05MHz. On the ground signal recording equipments is a simple extension of the normal telementry support function required for all flight tests.The translator output signal uses the missile tlementry band (S-band) and the same station-tracking antenna used for all the missile telementry GPS signal.Because the Trident 11translator was not able to capture the full 20MHz. the US Navy and US Air force agreed to use an alternate frequency.Although not part of the accuracy function.Satrack-evaluation methodology 2012-13 is a significant effect at the GPS prime frequency. VKIT.The station 1.Some ground sites uses the L1 C/A GPS signals to provide a real-time tracking solution for range saftey. Then rebroadcast the received signal (ie no signal processing or tracking function were provided by the missile hardware) The translator name also indicated that this signal relay device was for missile tracking and not for communications from translator perspective.The device that a accomplishes this function was called a “translator” to emphasize that the missile hardware 1. Satellite signals received at the missile are amplified.The additions frequency transmits for the duration of a Trident Test flight and is modulated with the narrowband ranging code which allows the translator to use dual-frequency signals (L1and L3) even with the bandwidth limitations. 21 Satrack concepts 2. VKIT. The existing range instrumentation was largely provided by radar systems.Satrack-evaluation methodology 2012-13 Fig: 2.2 SATRACK CONCEPT Guidance system evaluation concept of very early weapons systems depended on the impact scoring techniques. A new methodology was needed that provided insights into the major error contributors within the flight-test environment. This evaluation method was unacceptable for evaluating the more precise requirements of the latest systems. Bangalore Page 5 . The accuracy projections needed to be based on the high confidence Dept of ECE. This means that the missile was shot and the accuracy was formulated on the scoring or the target destruction. They however did not provide the needed accuracy or range in the broad ocean test ranges. 3 GPS TRANSLATOR This flight hardware is fixed in the missile. the missile translator and ground telemetry stations. The missile receives the signals from the GPS satellites.31 GPS Translator Dept of ECE. Bangalore Page 6 . The main parts are the GPS satellites. Then they are amplified for transmission to one or more ground stations. They are translated to another frequency and relayed to the ground telemetry stations. The figure 2.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. VKIT.Satrack-evaluation methodology 2012-13 understanding of the underlying system parameters. Some ground sites uses L1 C/A signals to provide real time tracking solutions. As shown in the fig 2. shifted to an intermediate frequency. The translator receives the GPS signals and they are amplified.31 Fig.21 shows the SATRACK measurement concept. SATRACK was developed with the necessary hardware and telemetry stations. 2. shifted to an output frequency. The telemetry station records the data for playback and for post processing. filtered to cover the satellite signal modulation bandwidth.2. These gave up to 14 tracks of recording channels with four mega samples per second. Some later versions were capable of processing data from both analogue and digital translators. 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 Dept of ECE. Received the satellite signal 2. This digitised data is modulated into an S-band carrier and transmitted to the ground stations. Digital translators down-convert the received L-band GPS signal to near base band and digitises it. Bangalore Page 7 .Satrack-evaluation methodology 2012-13 The translator does the following 1.The ground equipment consists of receiving antenna. and the Kalman filter. 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. 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. The earlier equipments were narrowband recorders that relied on high-speed tape recorders. data recorder and auxiliary reference timing systems. The SATRACK facility processes the raw data into a time series of range and Doppler measurements for each satellite. which incorporates various corrections and generates a navigation solution for the missile. Most ground stations are capable of generating a precise atomic timing standard.4 FIELD SUPPORT EQUIPMENT SATRACK is the most useful tool because of its post flight processing facility . 2. 2. Translated it to a missile telemetry frequency (S-band) 3. The equipment receives the translated GPS signal along with other telemetry signals and distributes it to the data recorder. Both wide and narrow band type of Analog translators are used. VKIT.5 PORTABLE GROUND EQUIPMENT This hardware is used for the post flight processing and tracking of the satellite signals. VKIT. Bangalore Page 8 . 2.6 POST FLIGHT TRACKING AND DATA PROCESSING This is the most important part of the SATRACK technology FIG: 2. Dept of ECE. Most of the custom GPS processing hardware is based on field programmable gate arrays [FPGA].Satrack-evaluation methodology 2012-13 frequency P-code as required by wide band translators. Each board has the ability to track up to eight channels. The user interface is done using windows based PC workstations.61 Basic SATRACK configurations. The system hardware is based on Analog Device SHARC processor. A tracking antenna at the station receives the missile signals.61 shows how the post flight tracking facility accomplishes precision tracking of the GPS signals through the playback of the recorded translator signals. and transmitted to the surface station(s). High accuracy satellite ephemeredes and the clock estimate covering their span of test flight is obtained. GPS signals are received. During the missile flight. The figure 2. and recorded at the GPS tracking sites. Bangalore Page 9 . 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.For range code modulation and phase locked loops for carrier phase tracking. 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. separates the various components and records the data. VKIT.Satrack-evaluation methodology 2012-13 For a number of days surrounding the missile flight.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. and missile guidance sensor data. The post-flight process uses the recorded data to give satellite ephemeredes clock estimates tracked signal-data from the post-flight receiver. After the signal tracking data are corrected. GPS signals are received by missile. tracked. Dept of ECE. the entire data element and the system models are used by the missile processor to produce the flight test data products. translated in frequency. The fig 2. This prior information is necessary for the single test processing.Satrack-evaluation methodology 2012-13 Fig: 2. Information theory provided the basis for developing the algorithms that could quantify the confidence with which accuracy could be estimated. not just the system level but at the subsystem level also. these data are collected with prior information generally developed and maintained by builders of the various parts of the system under test. Using rigorous methods. given the incomplete observability of the error sources. VKIT. Bangalore Page 10 . 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. Given a particular test or scenario measurement. data are collected on the various subsystems.62 depicts how this analysis is accomplished. Data from each accuracy test was analysed using some variant of the Kalman filter. 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.62 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. Within these filters are the detailed models of both the system and the instrumentation for each system. Next performance needed to be known. The outputs of the filter provide the basis Dept of ECE. The accuracy evaluation program had to be able to isolate faults and estimate performance of the subsystems or the various phases of the system. The results of the multiple tests –the outputs of the Kalman filter –serve as the inputs to the cumulative parameter estimation process. Bangalore Page 11 . In addition uncertainties in the parameter estimates are calculated so that we have a quantitative measure of our confidence in the solution. and deployment and re-entry 2. variances.63 Reconstructions of sources of missile impact miss distance error The graph shows in the above fig 2. All prior information regarding the relative error models is removed so that the estimate accuracy is derived solely from the test data. VKIT. Fig: 2. guidance. First level allocation is at the subsystem level: initial conditions. This method is based on projecting each error contributor and its uncertainty into impact domain.The ultimate desired product is system performance under tactical not test conditions. and Markov parameters that characterize the overall system accuracy performance. Analysis results provide insight in to the sources and causes of the inaccuracy. 1.Satrack-evaluation methodology 2012-13 for understanding particular realizations of system and subsystem behaviour. Here we rely heavily on the tactical Dept of ECE. 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. Second-level allocation provides data for major error groups within each subsystem eg: accelerometers 3.63 a hypothetical diagram used to allocate contributions to the impact miss. Satrack-evaluation methodology 2012-13 gravity and weather conditions developed from data and instrumentation. the inertial sensors provide high frequency motion information better than the signal processes.: the satellite position. Bangalore Page 12 . These measurements which when compared to their values computed from guidance sensor data and satellite position and velocity estimates. VKIT. In essence. 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. These are then passed through digitally controlled phase shifters and time multiplexing switch to emulate the missile GPS antenna network. the testing of the post processing system is done by this method. 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 hardware simulator drivers were conditioned to encompass all anticipated effects including signal refraction through the ionosphere and troposphere. Dept of ECE. Noise in the measurement of the recovered GPS range code signals is of secondary importance.The measurements of the GPS signal. The recorded data were equivalent to the data that would be received from telemetry site. An S-band antenna hardware simulator produced the outputs. 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 interestsystem accuracy at the target. The post flight processing facility now has all the inputs. The carrier phase tracking of the signals provide the critical measurements . These data are then processed and an estimate of the underlying model errors is produced. GPS ephemeredes. This is connected to a missile translator hardware simulator that produced the GPS signals at S-band. phase sense range changes along the line of sight for each signal to a small fraction of the wavelength usually a few millimetres. provide most of the information. which were recorded by the prototype telemetry station receiver. 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.g. In addition. clock files. and telemetry data and translated signal data tape. and the recording equipment . So. the digital implementation removes the need for periodic hardware calibration that accompanies the analog circuits 3. the data can be combined from may flight tests. Dept of ECE. 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.Satrack-evaluation methodology 2012-13 CHAPTER 3 MAJOR BREAKTHROUGHS 3. This model combined with other similarly derived sub system models helps develop planning factors used to assign weapons system targets 3. the results are expected to be repeatable. VKIT.phase tracking loop. The final cumulative analysis of flight test data produces a guidance error model of the weapons system. Bangalore Page 13 . In addition. generation of local code and carrier signal mixing.1 EVALUATION CAPABILITY FOR CUMULATIVE FLIGHT TEST ACCURACY The limitations of the test geometry prohibit observations of all the errors in any single flight test. As the pure software system was too slow. Since each test flight provides observations of the underlying system missile guidance error models.3 BATCH MODE PROCESSING This type of processing allows hardware to operate with software like flexibility. hardware that is fully configurable under software control implemented the most computing intensive portions of the process such as signal correlation. This is a very big improvement over the Analog circuitry such as the Analog PLLs used for carrier. 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.2 FULL DIGITAL IMPLEMENTATION. The full digital implementation is of the Portable ground equipment and processing facility. It can track up to 16 satellites in real time without any loss from channel multiplexing Dept of ECE. It retains the essence of batch mode architecture. FAR is a single channel L1 C/A only receiver with a front-end data storage memory that buffers unto one s of data.Satrack-evaluation methodology 2012-13 3. Bangalore Page 14 . VKIT. While maintaining the capability to process the data in real time.4 FLEXIBLE ARCHITECTURE RECEIVER The batch mode processing has been applied to stand alone real time capable receiver called FAR. It acquires the signal with no delay. Dept of ECE. Bangalore Page 15 . 2. 2. 4. It is used in Army and Air Force missile test applications. 3. The full digital implementation of processing facility leads to big improvement over analog circuitry. It provides the best flight path for the missile DISADVANTAGES 1. It provides a unique monitoring function that is critical to the maintenance of the Navy‟s strategic weapon system. It can support potential new and extremely demanding tactical strike scenarios.Satrack-evaluation methodology 2012-13 CHAPTER 4 ADVANTAGES 1. Large amount of infrastructure is needed to support it. 2. Implementation of large Kalman filter to process technique APPLICATIONS 1. VKIT. GPS receivers. [LCTMK]. The development and research leading up to this technology has been instrumental in bringing out the latest in GPS receiver. VKIT. The development of SATRACK looks forward to the implementation of the Low Cost Missile Test Kit. Special tests have demonstrated that accuracy has been achieved to support potential new and extremely demanding tactical strike scenarios. translators as well as RF/antenna designs. analytic methods. translators.Satrack-evaluation methodology 2012-13 CHATER 5 CONCLUSION SATRACK is a significant contributor to the successful development of and operational success of the trident weapons system. data recorders etc. Dept of ECE. and modelling and the use of limited and expensive flight tests assets were also born out of the SATRACK research. Instrumentation. Bangalore Page 16 .several special test have been conducted with various combinations of inertial systems. One other main development from this technology was the development of sophisticated tools for optimal target patterning. It provides a unique monitoring function that is critical to the maintenance of strategic weapons systems. ” IEEE Instrumentation and Measurement pp: 37-45 June 2010 [2] D.Satrack-evaluation methodology 2012-13 CHAPTER 6 REFERENCES [1] Marc Camacho and Sung Lim:”SATRACK tests missile accuracy. L. R.19 No.”The fleet ballistic missile accuracy evaluation program. Bangalore Page 17 .”Missile guidance and Pursuit.”IEEE Spectrum pp29-39 September 1997 [5] Shneydor N.” Johns Hopkins APL Tech Digest Vol. Mosher.Westerfield.19 No.” Ballistic missile defence.4 pp 436-446 1998 [4] David . and E.J.E. A. Simkins.” Johns Hopkins APL Tech Digest Vol.Levy.4 pp 393-397 1998 [3] T.S.Thompson.47-48 1998. Coleman and L.”Herwood Publishers pp 1-3. VKIT.” The SATRACK system: Development and applications. Dept of ECE.