Flight Electronics supportTIFR Balloon Facility provides flight support electronics systems, such as the Telemetry, Telecommand, Tracking and other control/support instrumentation that are required for balloon flights and for the experiments. Electronics engineers are also available to assist the user with equipment interfacing and to provide information on the capabilities of various systems. It is necessary for the experimenters to interact with the electronic engineers during the planning and preparation of the interface between the balloon flight support instruments and the experimental systems. TelemetryThe Telemetry system transmits data from the airborne scientific payload. The data is received by the ground station, decoded, displayed and stored (along with the time code). Various analog and digital information from the payload can be communicated through PCM-PM link in the S-Band. The operating telemetry range is about 500 km. TelecommandThe Telecommand system enables the remote control of balloon borne scientific and control instruments during the flight. It is also used for balloon flight control (balloon ascent/descent and flight termination). The Command system uses a PCM-FSK-FM modulation on an S-Band link. An error detection scheme is employed here to reduce the possibility of the command being decoded incorrectly due to transmission errors. The operation range is about 500 km.
TrackingThe S band telemetry receiving system itself does the auto tracking of the balloon. The GPS used in the on-board telemetry gives the exact location of the balloon in terms of latitude and longitude, ascent rate and height above mean sea level and also the prevailing wind speed and direction. A software program utilizes the GPS parameters to lock on to the balloon in case there is a break in S band auto tracking. An ATC Transponder is used on-board with which the ATC at Hyderabad (Shamshabad) Airport and other airports can locate and identify the balloon using their secondary surveillance radar. Ground StationThe Ground Station is located at the TIFR Balloon Facility, Hyderabad, and is capable of sending signals to, and decoding signals from, the airborne Telecommand and Telemetry units. It is equipped with Telemetry Receivers, PCM Decoders, Telecommand Encoders, Telecommand Transmitters, a Time code generator, Chart recorders and Data Display Stations. Online analysis, display, and storage of experimental data is provided by the electronics support group. Airborne ElectronicsTelemetry EncoderTIFR Balloon Facility provides telemetry encoders that can be configured as required for the experiment. The scientists should intimate the electronics engineers, in advance, regarding their telemetry requirements. The telemetry encoder can accept analog inputs in the range 0 to 6V and -4.5V to + 4.5V, and digital inputs with logic levels of 0 and 9V. Analog input signals are sampled and digitized by a 12 bit ADC. Digital input signals are sampled as per telemetry rate in accordance with experimenter's requirement. The encoder output is in Bi-phase-L format at rates up to 200 kbps. There are 8 bits per word (7 information bits and one parity bit). 32 words form a frame and 8 frames form a super frame. Each frame contains four words for frame synchronization, and one word as frame counter for the super frame synchronization. Experimenters can use their own PCM encoders with specifications conforming to IRIG standards. Telemetry TransmitterThe S band Telemetry Transmitter carrier frequency 2259 MHz. is phase modulated by the Bi-phase(L) signal from the PCM encoder. The RF power output of the transmitter is nominally 1 watt. The transmitting antenna used is a quadri-filar (helical) omni-directional circular polarized antenna. The maximum phase deviation allowed is one radian. Telecommand ReceiverOutput from the receiver is given to the command decoder. The RSSI voltage of the receiver is also provided as an indication of the received signal strength (monitored through telemetry). The sensitivity of the receiver is about -110 dBm and the dynamic range is 100 dB. Telecommand Decoder and CCUThe telecommand system provides 63 ON_OFF commands and 3 data commands of 8 bits. Each command is a 16 bit code (numbered in hexadecimal code.) An error detection scheme is employed in the telecommand system, based on cyclic coding. For this, 15 parity bits generated from the basic command word, are added to it, to from a 31 bit command frame. The Telecommand Decoder decides the command signals sent from the ground station, and issues commands to various on -board subsystems. This receives the baseband telecommand signal(PCM-FSK) from the command receiver. The digital data derived from this is checked for its correctness as a valid command code. For the 'ON/OFF' type command, the decoder outputs a 50 ms pulse on one of the 63 output lines depending on the command received (this can be used to activate a corresponding on-board subsystem). For the second type of command called 'DATA' command, the decoder outputs an 8 bit digital data sent from ground station along with an enable pulse on one of the three address lines depending on the command, so that the data can be transferred to one of the three on -board sub-systems. A command control unit (CCU) is interfaced with the decoder to execute the commands sent from the ground station. This contains latching circuitry to extend the operational time of the commands available at the decoder output and relays are operated accordingly. This unit provides both SET/RESET contacts and ON/OFF 10 second contacts. Some of these are used for balloon flight control operations, and the remaining are available to user scientists to control the airborne scientific equipments as required by the experiment. Telecommand is used for various flight control operations such as detaching the small control balloons, dropping the ballast, apex valve opening and closing, timer extension and flight termination. Telecommands for experiments includes orientation of telescopes, operation of various motors, valves etc., and deployment or reeling down of sensors or tubes. The scientists should intimate their requirements for telecommand support well in advance.
Integrated Control Instrumentation Package GPS ReceiverAt TIFR Balloon Facility, Chapman 12 channel GPS receiver (Fig.8) is used in the payload for accurate positioning of the balloon during the flight. This receiver provides data (latitude, longitude, altitude and time) in NMEA format. This is fed to an interface unit which provides parallel data that can be given as input to the telemetry encoder.
On board GPS Unit BallastThis unit uses tiny steel shots for in-flight releasing of a controlled load from the balloon train. This is required to accelerate the ascent rate of the balloon, in case it slows down due to change in atmospheric conditions. The steel shots are held in a container with an energized electromagnet blocking the outlet at the bottom. The steel shots are released by telecommand which disconnects the supply to the electro magnet for the required time. The ballast operations are monitored through telemetry, the dropping of steel shots being optically sensed by a phototransistor. Apex ValveApex valve is fitted on the top fitting of the balloon. This is used for controlled release of Hydrogen gas from the balloon in order to bring it down to lower altitude. The apex valve is operated by telecommand and operation is monitored through telemetry radio-sonde. TimerThis unit (Fig.9) is used as a preset timer for flight termination. It is a back up unit in addition to telecommand for flight termination. Cut-off time is programmed in the timer just before the flight as per the flight duration requirement. There is a provision for extending or advancing the cut-off time through telecommand during the course of the flight.
New Timer (Micro controller based) BeaconA MF beacon (Fig.10) is used in every balloon flight as a safety measure as stipulated by civil aviation authorities. The beacon transmits the signal 'TFR' in Morse Code to forewarn approaching aircraft of the descending payload train in space. An AM transmitter at 1.665 MHz. along with a half wavelength wire antenna is used for this purpose. The power output is 10 Watts. Beacon operation is monitored using a radio receiver.
Fig 10 MF Beacon ATC Radar TransponderTransponders have become a modern necessity for all vehicles flying in today's airspace environment, The ATC transponder is one of the key safety navigation devices on the load line of the balloon train. The transponder functions in the environment of air traffic control radar beacon system of Hyderabad airport. As a result, the detection and location of the balloon by the secondary surveillance radar at the airport is visible on screen to the on-duty controllers. Coded Mode A/Mode C transmissions that are received are processed at the ATC and the balloon position relative to the airway route is displayed on the radarscope with a permanently assigned identification code `2736' of the balloon transponder. Since our transponders are equipped with altitude encoder, the altitude data is also displayed on the radarscope. Due to critical nature of the transponder for navigational safety, test and calibration of the units is essential after each recovery and recertification prior to launch has to be done.
ATC Radar Transponder Radio Direction FinderThe payload separation for the flights launched from Hyderabad is operated by remote control from the central control room in the balloon facility, based on GPS location and the terrain map, unlike aircraft command system used by CSBF supported flights. Therefore, the payload recovery after balloon cut-off is a vital part of the flight operations. GPS position coordinates received during the parachute descent generally correspond to 2 to 3 km in altitude before touchdown of the payload on the ground. Therefore, the search team has to generally explore about 5 km radius around the extrapolated touch down point. However, the payload may drift farther away, depending upon the wind at that place, to distances as far as 10 km. The Payload finder unit has therefore been incorporated to help in pinpointing the location of the payload within a range of 5000 meters. The onboard unit consists of a 500 mW transmitter operating at 1680 MHz which is switched on at an altitude of 5 km using a one-way pressure switch. The operator uses a helical hand-held antenna along with a tuned receiver and head phones, which detects the carrier frequency and gives a tone on the ear piece attached to a 1680 MHz receiver. The tenor of the tone increases as one approaches the payload thereby helping in homing on to the payload. A signal meter attached to the antenna gives the amplitude of the received signal which increases on approaching the source.
Radio Direction Finder (Transmitter and Receiver) GPS SondeThe altitude and the balloon position obtained from redundant sub-systems is normally sent on the S-band telemetry. In the case of S-band link failure, the situation is not tenable. In order to have independent check on the balloon altitude and position, a GPS sonde is launched on the load line and which uses an independent telemetry channel at 403 MHz. On board data recorderUnder normal conditions, the experimenter data and house keeping data from the balloon is transmitted to the ground station using a S-band link which has an operational range of about 400 Km. As a redundant measure, in case of any transmission loss or noisy data, an on-board telemetry data storage system has been designed and fabricated using a single mother board computer card and a 2 GB flash memory along with the interface circuits. The unit is small, low cost, low power, solid state, high performance and rugged solution to the in-situ data recording needs of balloon borne payloads. The single board computer from Lippart has a 300 MHz Intel processor with 256 MB RAM and can handle 8 GB of flash memory. It uses 8W of power and can operate in the temperature range of -20°C to +70° C.
On board data recorder Software routines required to store the telemetry data using parallel port are written in C language. The recorder has been tested to operate in the low pressure environment of the stratospheric balloon altitudes. The unit is designed to faithfully store digital data in such a way that the data can be analyzed, reproduced, and replayed similar to the telemetered data. The data acquisition at present is via parallel port and we have tested the unit up to 100 Kbps. For most of the experiments, the parallel stream of data output to the transmitter from the encoder is available and is used to store the data Ground Station FacilitiesThe ground station is located at the TIFR Balloon Facility and is capable of sending signals to and decoding signals from the airborne telecommand and telemetry units. For bit rates up to 500 Kbps, the telemetry works satisfactorily . However, the range of the telemetry is degraded by the receiver sensitivity of the tracking system in case the bit rate increases above 500 Kbps. The major units in the ground facilities are (i) The tracking system (ii) data decoding (iii) data storage (iv) display stations and (v) telecommand. Tracking, Telemetry & Command (TT&C) and Data StorageThe ground station data receiving and TT&C system include a steerable 3.8 m dish antenna with an 11 dB gain and the preamplifier with 14dB gain and a receiver. The detailed parameters of the antenna are listed in Table 1. The functional diagram of entire S-Band Tracking, Telemetry and Command system is shown in figure 11. The same dish antenna also acts as the command uplink via diplexer operation. Telemetry signals transmitted from the on-board telemetry transmitter are received by the dish antenna. The feed in turn generates 3 types of signals i.e. sum, azimuth and elevation error signals. These signals are fed to low noise amplifiers (LNA). All three LNAs have a gain of about 40dBm. The amplified signals are then fed to frequency down converter, which is configured as a three channel input and 3 channel output device. The input frequency is thus down converted to the standard 70MHz Intermediate Frequency and the signals are further passed on to the main 70MHz demodulator and tracking receiver to generate signals for the auto tracking system. The auto tracking system drives the azimuth and elevation motors of the antenna with respect to the error signals and remain locked to the exact target position within the beam width. The main 70 MHz demodulator demodulates the base signal which is fed to the bit synchronizer for signal conditioning and extracting data and clock signals. The output is a bi-phase L bit stream. The data and the clock signals are fed to the de-commutation system to be decoded as per the user defined program and this data is made available to the end user on the Ethernet. The decoded information is displayed on several display stations and is also recorded in real time in permanent storage media for further off-line analysis.
Table 1: Operating parameters of the TT&C antenna at Hyderabad ; ; Each Display station provides information which is specified for different users like GPS data and the derived parameters such as ascent and wind parameters for the balloon control team, Balloon Trajectory will be displayed for the recovery team and the experimenter information for the experimenters. The uplink used to send commands for payload and balloon control operations uses a PC based command Encoder, which generates the format compatible with the onboard decoder and sends it serially bit by bit to the transmitter for modulation. The modulated signal from the transmitter is amplified by a 50W Solid State Power Amplifier (SSPA before being fed to the dish antenna.
Figure : Functional diagram of the Ground station Telemetry receiverRadio transmissions from the air-borne telemetry system are fed through the 3.8 m. dish antenna to a S band receiver. The signal output from the tracking dish is used as a feedback for orienting the antenna towards the balloon. The demodulated (PM) output from the receiver is fed to the Telemetry Decoder and is also stored in two PCs. PCM decoderThe PARK Control Decoder-decommutator is used to decode process and display the data output from the telemetry receiver and also stores the data. This PCM decommutator decodes the standard IRIG format up to 32 bits per word. It is keyboard programmable and the programmes can be saved on a hard disk. |
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