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This paper deals with the VLSI Approach for design of Emergency Locator Transmitter of Aircraft with Global Positioning System Receiver. Emergency Locator Transmitter (ELT) is an emergency equipment fitted in the air craft which will locate the position during the event of a crash. The COSPAS-SARSAT (C/S) satellite system has been providing emergency alerting from system compatible ELT for a number of years and thus, has been instrumental in saving many lives around the world. However, locating the site of the crash using this satellite system is less accurate, ambiguous and sometimes involves abnormal delay. But, nowadays, technologies such as the Global Positioning System (GPS) can provide good location accuracy. The integration of a GPS receiver in the existing ELT and re designing the ELT using VLSI technology, would combine very accurate location determination and near instantaneous distress alert. These enhancements would reduce the overall time required to complete the rescue operation.
Keywords- ELT, GPS, Band gap, C/S satellite, G switch
The VLSI design of this integrated unit will be compact, efficient and more reliable. The primary objective of this paper is to describe the principles, VLSI design and analysis of an integrated GPS receiver and the ELT so that the GPS receiver could provide location information to be incorporated in the transmitted message of ELT. This new equipment ELT GPS could be incorporated with a lot of added extra features so the equipment can be used for non emergency operations also. The proposed VLSI approach for ELT based GPS design is done considering low power with local supply which turns on during air craft crash, high accuracy, low silicon area, integration of all blocks in single die. The sub blocks used in the proposed GPS based ELT system and their integration with simulation results are described below.
Power Supply and voltage regulator
Two 5V batteries are used in the proposed design, one 5V battery with regulated power supply of 2.5V is for dual port RAM and GPS and other 5V battery with regulated power supply of 2.5V for all the other blocks like crystal, PLL, DAC, modulator, reference circuit, timer, MUX, power amplifier etc. Dual port RAM and GPS 5V battery will be continuously charged through internal air craft power supply because GPS should continuously track latitude / longitude position and store the data in dual port RAM. Power supply for all the blocks used in mixed signal chip will be provided by other 5V battery which will turn on during aircraft crash through G switch. Two Internal regulators which converter 5V to 2.5V supply used for powering all the blocks for better accuracy. Figure 1 illustrates the Block diagram of Linear Voltage Regulator Circuit . The basic building blocks for linear voltage regulator are operational Tran conductance amplifier (OTA), feedback elements (Resistors) and pass transistor to source current from supply.
Figure 1. Block diagram of Linear Voltage Regulator Circuit
In mixed signals chips reference circuit helps in providing constant reference voltage irrespective of process, voltage and temperature variations. The most popular architecture is band gap  reference circuit generating low temperature coefficient constant output voltage. The band gap principle is adding a negative temperature coefficient with scalar multiplication of positive temperature coefficient to generate temperature independent output voltage. A diode connected bipolar transistor (VBE) voltage is -ve temperature coefficient cancels with k*âˆ†VBE provides +ve temperature coefficient to generate 1.23V constant output reference voltage used by voltage regulators, PLL, DAC and power amplifier. Figure 2 illustrates block diagram of Band gap Reference circuit.
Figure 2. Block diagram of Band gap Reference Circuit
If two BJTs operate with different emitter current densities (by a ratio of n), then the difference between their base-emitter voltages is given in below equation which is proportional to absolute temperature.
By taking the sum of VBE + Î”VBE we get constant output voltage of 1.23 which is approximately equal to energy band gap of silicon in particular technology.
Dual Port SRAM
In GPS based ELT system a dual port RAM circuit  is used to continuously store the latitude and longitude locations in normal mode and provide the last updated information to transmitter through DAC, Modulator and power amplifier during aircraft crash. Dual port SRAM is R/W memory circuit that permits the modification (writing) of data bits to be stored in a memory array, as well as their retrieval (reading). The SRAM design consists of SRAM cells, pre-charge, sense amplifiers, MUX, NAND gates, AND gates, NOR gates and row decoder. The popular, full CMOS 6-transistor cell configuration was used to design the SRAM memory array. The full CMOS configuration is shown in figure 3.
Figure 3. Block diagram of Band gap Reference Circuit
The full CMOS 6-transistor cell configuration was chosen for the cell array. The 6-T cell consists of two cross coupled inverters connected with the two nmos transistors on both the ends. Each nmos transistor is connected to an inverter on one side and bit line on the other side. The data value is stored in the net connected to the left side of the M4 nmos in figure 3. The inverse data value is stored in the net connected to the right side of the M3 nmos. The input signal i.e., address line comes from the row decoder, allows the cell to be connected to the complementary bit lines during reading and writing and disconnects otherwise .A sense amplifier circuit is used to read the data from the cell. In addition, it helps reduce the delay times and minimizes power consumption in the overall SRAM by sensing a small difference in voltage on the bit lines . A low-voltage sense amplifier was used in the SRAM design to support high performance. A row decoder is used to decode the given input address and select the word line. There are 8 rows and row contained 8 cells each. The decoder selects 1 of 8 word lines, with respect to the input address. The output of the decoder is fed to a 2-input AND. This AND is the word line driver. This AND supports a large capacitance on the word line. Each cell loads the word line with two transistors. Other input to this AND is the Clock. Only when both Clock and decoder output signals are enabled, the AND enables a word line to the rows of SRAM cell arrays.
Global Positioning System Receiver
In GPS based ELT system Global Positioning System Receiver  is used to continuous update the latitude and longitude data of the air craft into dual port RAM. The input signal to the GPS receiver is analog which is converted to parallel digital data and stores in dual port RAM. The block diagram of GPS receiver is illustrated in figure 4, in this application the Global Positioning System (GPS) Receiver is a 10 channel receiver system which is decided based upon bandwidth. It has hardware built for fast acquisition, fully integrated Radio Frequency filtering, Analog to digital converter and a fully integrated LNA that allows operation with either active or passive antennas. It also has reset circuits, real-time clock built with on-board crystal, in the hardware design. The other aspects required to be provided for the operation is the Power (DC) and the input signals from GPS. As the GPS is the multi-channel one (Typically 10-channel), Position jumps caused by one or more satellite signal lose will not be affecting the system. The fast-acquisition hardware design greatly reduces the time for signal acquisition when the receiver is initially powered up. The GPS receiver system operates from a single battery supply 2.5 VDC for low power consumption.
Figure 4. Block diagram of GPS receiver
The GPS receiver is designed for high-performance and low power consumption, with the benefit of using the system's existing clock reference. This receiver is ideal for integration into ELT system. The only external components required are the GPS RF filter, an IF filter (typically designed from inexpensive discrete), a three-component PLL loop filter, and a few other resistors and capacitors. The designed GPS receiver system integrates the reference oscillator core, the VCO and its tank, the synthesizer, a 1- to 3-bit ADC, and all signal path blocks except for the 1st IF filter. The following are the specifications for the Global Position System Receiver
Acquisition Time < 1 Second
Tracking < 2.5meters
Temperature -20 to 85Â° C
Altitude -1000 feet to 60,000 feet
Signal Level : -160dBm to -125dBm
Frequency : 1575.42 MHz
Noise Figure : 1.5 dB
Impedance : 50 ohms
G Switch is sensor element activates ELT system with a change of velocity of typically 3.5 fps Â± 0.5fps. The switch operates while being subjected to 30 G of cross axis forces. The ELT 110-406HM has an additional five G-switches providing for six activation coverage. The additional five G-switches activate at a G force of 12 G's. The G switch is designed with MOSFET driving a battery (i.e., battery turns it on) during aircraft crash which is detected by the velocity of aircraft.
Timer and Counter
The timer and counter  are essential blocks in ELT design because when The ELT activates automatically during a crash, the system transmits the standard swept tone (an international standard) on 121.5 MHz and 243.0 MHz. The 406.025 MHz transmitter turns on every 50 seconds for the duration of 440 milliseconds (for transmitting standard short message) or 520 milliseconds (for transmitting optional long message). When long message is sent, an encoded digital message with additional information is sent to the satellite. Typically, the information encoded in this message is serial number of the transmitter, country code, position co ordinates etc. The 406.025 MHz transmitter is programmed to operate for 24 hours and then shuts down automatically as per the international legislative requirements. The 121.5/243.0 MHz transmitter may continue to operate till the battery has exhausted its power which will be at least 48 hours. In most of the cases, the duration will be more.
In GPS based ELT system a frequency modulator is required to modulate the analog data provided from DAC which is at low frequency to 406MHz or 121.5MHz/243 data by using carrier signal. Frequency modulation uses the information signal from DAC, Vm(t) can vary the carrier frequency produced within a small range about its original value. The three signals in mathematical form are
Carrier: Vc(t) = Vco sin ( 2Ï€fc t + Ð¤)
FM: VFM (t) = Vco sin (2Ï€[fc + (âˆ†f/Vmo) Vm (t)]ï€ t + ï€© Ð¤)
The carrier frequency is replaced with a new time-varying frequency. A new term is also introduced: âˆ†f, the peak frequency deviation. In this new form, it is able to see that the carrier frequency term: fc + (âˆ†f /Vmo) Vm (t) now varies between the extremes of fc-âˆ†f and fc+âˆ†fÎ² = âˆ†f/fm , where fm is the maximum modulating frequency used.
The simplest interpretation of the modulation index, Î² ï€ is as a measure of the peak frequency deviation, âˆ†f. In other words, Î² represents a way to express the peak deviation frequency as a multiple of the maximum modulating frequency, fm, i.e. âˆ†f = Î²fm. Figure 5 illustrates the example for FM wave and its spectrum.
Figure 5. FM Signal and Spectrum
The bandwidth of a FM signal may be BW = 2 (Î² + 1 ) fm ; where Î² is the modulation index and fm is the modulating frequency used. Normally an FM radio has a significantly larger bandwidth than AM radio. In FM, both the modulation index and the modulating frequency can affect the bandwidth of operation. The efficiency of the transmitting system is generally improved by making the modulation index larger as possible. But larger the modulation index, makes the bandwidth larger which has its own disadvantages . In the design, the modulation index is limited to a value between 1 and 5, depending on the transmitter application. Another advantage of FM is, FM systems are very much immune to random noise.
The crystal oscillators are most popular to generate clock up to 10MHz-20MHz frequency with less than 2% accuracy. The crystal oscillator  provides input frequency to phase lock loop and obtain 121.5 MHz / 243 MHz / 406 MHz output frequencies. The advantage of a crystal oscillator in this application is its wide range of positive reactance values over a narrow range of frequencies. However, there are several ranges of frequencies where the reactance is positive; these are the fundamental, and the third and fifth mechanical overtones. Since the desired frequency range in this application is always the fundamental, the overtones must be suppressed. This is done by reducing the loop gain at these frequencies. Usually, the amplifier's gain roll off, in combination with the crystal parasitic and load capacitors, is sufficient to reduce gain and prevent oscillation at the overtone frequencies. The block diagram of the crystal oscillator is illustrated in figure 6 which consists of amplifier, Crystal, feedback resistors and capacitors.
Figure 6. Block Diagram of Crystal Oscillator
Phase Lock Loop
Phase lock loop circuit  operates as frequency synthesizer in this application by taking input signal of 20MHz from crystal oscillator and providing output frequencies 121.5MHz / 243MHz / 406MHz. The block diagram of the phase lock loop in illustrated below Figure 7.
Figure 7. Block Diagram of Phase Lock Loop
A phase-locked loop is a feedback system that operates on the excess phase of nominally periodic signals. The major blocks in phase lock loop are phase frequency detector, charge pump, loop filter, voltage controlled oscillator and divider. Output of the phase-detector is a voltage proportional to the phase difference between its two inputs. The loop is considered "locked" if the phase difference is constant with time. When the loop is locked, the control voltage is such that the frequency of the VCO is exactly equal to the average frequency of the input signal; however, there may be a static phase error present. This error tends to be small in a well-designed loop. In GPS based ELT system an input of 20MHz is used by PLL from crystal oscillator to generate output frequencies of 406MHz, 243MHz and 121.5MHz, this frequency clocks are used by modulator to modulate the data with respect to clock signal.
Digital to Analog Converter:
A current steering Digital to Analog Converter  is used in this application which will converter SRAM digital data to analog value and transmit through modulator and power amplifier. The current-steering DAC architecture is illustrated in figure 8. There is a number of current sources and switches. Depending on the input code, the current from the corresponding sources is directed by the switches to the output and terminated by an resistor. The matching errors will strongly influence the performance.
Figure 8. Digital to Analog Converter
In RF transceivers, power amplifiers play a major role and it takes a small-amplitude signal as its input and drives a very high power representation of the input signal into a very low impedance load. In GPS based ELT system, the analog output of the modulator is not capable of driving the antenna and hence power amplifier circuit is used to boost the power level of the signal. The power amplifier output power level is of the order of hundreds of mill watts or more, the power that the power amplifier needs to deliver to its load in it is a large quantity of the total power is consumed by the transmitter. A class C power amplifier is used in this application because it provides more than 90% efficiency.
A Buzzer driver is used to blow the horn output during aircraft crash. When an air craft is crashed then G-switch will turn on which activate GPS based ELT system and parallel drives buzzer to blow horn or it can also drive an LED system.
A Manual reset option is provided for GPS based ELT system along with G-switch to test the application on board along with creating an environment of aircraft system.
ELT is one of the simple circuits as far as the operation is concerned. As long as ELT is installed into its own mounting tray, it will activate only in the case of a crash, neither the cockpit switch nor the ELT unit G switch can be used to prevent automatic activation once the unit is installed properly in the tray. ELT is also designed against any inadvertent human error for automatic activation. The ELT cannot be accidently get activated by any other means such as, dropping, rough handling or shipping. When the ELT is in an activated, the emergency swept tone generated and a flashing panel LED light indicates a normally functioning unit. During normal operation, the switch configuration on front panel is the down ARM position. If ELT is activated accidentally, then resetting can be done by moving the front panel switch to "ON" then back to "ARM" position. We can also reset the ELT at the unit itself by positioning the switch on the ELT up to "ON" then immediately back down to "OFF".
Functionality of ELT: The Figure 9. Illustrates the integration of all above blocks to determine the functionality for GPS based ELT system. The functionality of the GPS based ELT system is described as follows, GPS based ELT system has two modes:
(a) Normal mode [Aircraft is fine] (b) Aircraft crash mode
Figure 9. VLSI Approach for Emergency Locator Transmitter (ELT) of Aircraft with Global Positioning System (GPS) Receiver
Normal Mode: In normal operation aircraft is running fine and the global position system receiving antenna will continuously track the latitude and longitude locations at a speed of 1575MHz and stores the data in dual port RAM. In normal operation band gap reference circuit(1) will generate 1.23V reference voltage to voltage regulator and voltage regulator will generate 2.5V output with respect to 5V input from battery for low power operation. The voltage regulator 2.5V output powers global position system receiver and dual port RAM.
Aircraft crash mode: When air craft is crashed i.e., aircraft crash mode then G-Switch will turn on the band gap reference circuit (2) and voltage regulator (2) which will power up all the other blocks [Crystal oscillator, PLL, modulators, digital to analog converter, timer, counter and power amplifier] parallely A Buzzer driver is used to blow the horn or it can also drive an LED system. Initially crystal oscillator will start functioning once voltage regulator (2) provides power and crystal oscillator will provide 15MHz low jitter clock to PLL. PLL will generate lock signal when the desired output frequencies 406MHz, 121.5MHz/243MHz clocks/carrier signals are obtained for modulators and digital to analog converter, the lock signal activates the read mode of dual port RAM which will latch the data on to the digital to analog converter. GPS based ELT system in aircraft crash mode will transmit 121.5MHz/243MHz clock signals continuously and 406.025 MHz data transmitter turns on every 50 seconds for the duration of 440 milliseconds (for transmitting standard short message) or 520 milliseconds (for transmitting optional long message). During activation, an encoded digital message is transmitted to the satellite. The information contained in this message is serial number of the transmitter, country code, position co ordinates etc. The digital to analog converter output to given to modulator, after modulation [data and carrier signal] the data is given to power amplifier to boost output power level and drive transmitting antenna.GPS based ELT system has standard code format of transmission and there are three coding options of this protocol that can be used with ELTs.
Band gap reference circuit is designed to generate 1.23V which is illustrated in Figure 10, band gap reference output is used by all analog blocks. DC-Temperature sweep analysis is done from -40 to 125C.
Figure 10. Band gap Reference circuit temperature analysis
The voltage Regulator Circuit is designed to generate 2.5V regulated output for low power from 5V battery supply. Figure 11 illustrates [Transient Analysis] 2.5V stable regulator output used to power all blocks in GPS based ELT system.
Figure 11. Voltage Regulator Transient Analysis
Figure 12 illustrates Phase Locked Loop output frequency 406MHz clock and lock signals. Lock signal is used to activate dual port RAM and data can be accessed.
Figure 12. PLL Output and Lock Signals
406MHz Modulator Output is illustrated in figure 16 generating modulated output to drive an antenna though power amplifier.
Figure 13. 406MHz Modulator Output
121.5MHz Modulator Output is illustrated in figure 14 generating modulated output to drive an antenna though power amplifier.
Figure 17. 121.5MHz Modulator Output
By integrating the above functionalities with the help of enhanced hardware and software design, the flight
safety can be further improved. The simulation results shows that by using VLSI technology, very accurate location determination and near instantaneous distress alert can be achieved. These enhancements would reduce the overall time required to complete the rescue operation