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ARTYKUŁY Elektronika Complex tracking circuit design (Rozbudowany układ śledzenia obiektów) |
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The wireless market continues to evolve at an ever growing pace. Increased consumer demand for smart phones, together with the widespread adoption of new form factor devices such as tablet PCs, is providing a dynamic landscape. Application stores, together with new, feature-rich mobile devices and enhanced network capabilities, are delivering new business models and compelling user experiences for consumers [1].
Wireless navigation is also a good example of intuitive product development. Turn it on and it tells you where you are. Put in the destination and it tells you how to get there. In our opinion presented project, called CTCD, is probably most complete and affordable GPS unit available today. Our proposition combines GPS monitoring with GPRS transmission to tracking application.
This tracker provides tracking, reporting and monitoring, capable of updates every friction of seconds. Our unit works worldwide with both GPRS/SMS standards. In this paper we present the most compatible working modes to present vast possibilities of this project.
Popular solution for this kind of device is to combine GSM or GPRS module with GPS chip and Intel 8051 compatible microcontroller [2]. There are also modules providing both of these technologies – GPS/GPRS+GPS. One of the biggest problems with those solutions are: poor support from developers side, too simple software applications and slow working databases. Those features should enable using this device by many users and giving them feedback information online. Main advantage of this kind of construction is that only a few implementations of on-market GSM modules are using it to its full potential.
Another challenge is to construct device small, energy-saving and stand-alone, which can work very long on one fully-loaded battery cell. Therefore, when hardware device is used only as standalone sender, some functions, like headphones or LCD display, are disabled. If device is powered from battery, it is recommended to turn them off, or just avoid installing them on device if it’s not necessary to use them.
CTCD device is designed around GSM/GPRS+GPS module and simple microcontroller. First two modules are connected with microcontroller via USART interface. It is recommended to use separate serial transition ports integrated into microcontroller for GSM/GPRS and GPS parts, if both are used in the same time (for example, if AGPS is active).
Simultaneous using many devices on one USART will produce errors. The baud rate supported by tested device is between 4800 and 115200 bps. All data are transmitted in unsigned char type. Data buffer is also used.
Beside USART interface, GSM turn_on and status pins are also connected to and controlled by microcontroller. GPS might be also operated by microcontroller, if necessity occurs.
Device requires two separate power sources. Main one is mandatory to turn device on. Maintenance power allows to save ephemerides and data in memory, also to keep real time clock synchronized with satellites, when modules are switched off. It can be provided by alkaline button cell battery or high capacity capacitor.
First step is to measure position. GPS part receives signals from satellites and saves ephemerides in memory [3]. Meantime, The Real Time Clock is also being synchronized. In the future, there might be possibility to connect with any other global navigation positioning system, like ГЛОНАСС (GLONASS), 北斗 (Beidou) or Galileo, or even with all of these systems via all-in-one solution [4]. Unfortunately, currently only GPS is fully operational.
For some purposes, like tracking packages, it is enough to check position only once per two hours. Taking less measurements expands operating time of single data upload. Its because overdue ephemerides and Real Time Clock desynchronized in compassion with satellites. Hot start could be impossible and all data from each satellite would have to be downloaded once again, before object gets localized. In presented solution, passive antennas are used, because of their low power consumption.
After reading measurements, GPS starts to work in sleep mode. It operates in low power consumption state. GPS part is waken-up after configurable propagation time (no longer than two hours). During sleep mode, chipset is turned off and no data is transmitted from device. Only RT C keeping voltage is needed to keep clock synchronized with satellites and wake up GPS after propagation time.
When uploading operation is done, all information about localization is sent to microcontroller. Data transmitted from GPS are encoded in NMEA standard. Transmitting takes moderate amount of time. Few first processes are always empty or bugged, they are not important and not included into further calculations. Only one line is needed to read full information about analyzed object localization.
If measurements and data transmitting processes last for fifteen minutes and propagation time is set to two hours, then GPS would change its mode to sleep state in one hour and forty five minutes.
During preliminary tests, hot start took less than ten seconds.
Microcontroller reads only one $GPGGA line [5]. To choose the best one, dedicated algorithm seeks for single command, selected from fully transmitted and quality checked chunk of data. Next operation is handled by the parser. It cuts all information, that are useless for end-user. Then it wakes up GSM/GPRS module by sending high signal on turn_on input and awaits for status signal. It takes few seconds before GSM part is waked up and serial port is ready to communicate with microcontroller. If GSM and GPS parts are both powered on, AGPS could be used to take measurements faster, and with better precision.
After turning on or waking up the GSM/GPRS module, it automatically tries to connect with the wireless network. It is possible without any commands sent from microcontroller, when safe-lock PIN is turned off. GSM always asks for SIM card produced in standard ISO7816 to login into designated network [6].
Preliminary tests showed, that current had about 2A in the peak, while device was trying to identify wireless signals. Without it, GSM/GPRS part could have worked correctly, but wouldn’t been able to login into network. Those limitations make it very hard to implement solar technology as main power source for this solution.
When GSM/GPRS part is connected to the network, the signal is passed onto status output. Microcontroller uses commands in AT standard to send parsed data to the database. If actual position is the same as the previous one, then it is unnecessary to send it once again. If sent despite this fact, it is possible to observe within database, that device is still working.
More than one position could be saved in SIM card memory [7]. GSM could be turned on only once per many measurements. It is very energy-saving solution (important especially if device works in very cold weather conditions) and makes this hardware hard to detect, because it doesn’t transmit data all the time. To send information about position it is possible to use SMS or GPRS (optional) standards via GSM network [8]. After sending the data, microcontroller turns off the GSM module.
During tests with device stored within the building, all operations usually taken less than 40 seconds to generate final results.
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