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You can buy excellent – and reasonably cheap – hand-held GPS units these days. So why would you want to add GPS to a PC? The applications, as they say in the classics, are limited only by your imagination! M ost people are familiar with GPS: the Global Positioning System run by the United States Government. It allows a GPS receiver to locate its position anywhere on the planet by analysing signals received from a series of orbiting satellites. (For a more detailed explanation of GPS, see the separate panel in this article). Quite a number of hand-held receivers are available at reasonable prices. These show your position as a latitude and longitude on an LCD readout. They’re ideal for bushwalkers, hikers, etc – and fishermen love them because they can get back to that secret spot – exactly! Some of the higher end models allow the position to be displayed on a map along with other numerous features. But what if you’d like to interface a GPS directly to your PC or laptop? Most of the cheaper hand-held units don’t support an external interface at all – or if they do, it is an expensive option. There is also the question of battery life (which is normally quite short anyway). If you’d like extended logging of data, you’re up for yet another add-on for external power, not to mention an external antenna if it’s not convenient to mount the entire unit in a spot that gives good coverage. As you can imagine the task of tracking and analysing signals received from the up-to-12 satellites that can be in view at a single time is quite a complex task. Fortunately quite a number of OEM (original equipment manufacturer) GPS modules are available that perform most of the real work and interfacing one of these modules to a By PETER JOHNSON 18  Silicon Chip PC or other serial device is quite easy. This article describes constructing such a unit at a cost considerably less than using a hand-held unit and gives some pointers on getting some usable data out of the GPS module once it’s built. Why do it? Linking a GPS unit to a personal computer is very much the doorway to countless other applications. We are not even going to try to list those applications but anyone who has ever needed to know where something/someone was at any particular time, where it/they went from there, how long it took it/them to get there and so on . . . they are the types of applications which immediately spring to mind. Having a GPS unit in your hand will tell you where you are (it’s great The GPS interface sitting on the keyboard of a notebook PC. It’s actually upside down so you can see the GPS module itself (right side of board). The active antenna lead connects on the right side, while RS232 data goes off to a suitable COM port via the socket on the left. if you’re lost!). Feeding that data into a PC then allows it to become really useful! Still not convinced? OK, here’s one application: rally driving. The GPS unit would know exactly where the vehicle is and, with the right software, a notebook PC could “tell” the driver (ie, in actual speech), what grade corner is coming up, what direction it takes, obstacles en route, etc. It would be far more accurate than any human navigator and wouldn’t make “ouch” mistakes! Another? How about delivery drivers, with all “drops” preprogrammed in to the notebook PC? Circuit description The circuit (Fig.1) is quite straightforward as the GPS module undertakes most of the “real work”. All we require to interface it is a suitable power supply and a circuit to convert the TTL serial levels used by the board to the RS-232 levels used by a PC. REG1 is a LM-2940CT-5 linear regular used in a standard configuration to provide the +5V required by the GPS module and active GPS antenna. Note that the GPS module consumes around 250mA of current when in operation so if the unit is to be operated from an input voltage much above the 6V recommended a heatsink will be required to dissipate the heat. IC1, a MAX232, performs conversion from the TTL levels used by the GPS module to RS-232 levels for the external interface. As well as containing a DC-DC converter to increase the voltage levels, the MAX232 also provides 15kV of ESD (electrostatic discharge) protection on the RS-232 interface to provide some Table 1 – Laipac TF10 GPS Receiver Module Connections Pin Function 1 +5V DC Active antenna power 2 +5V DC Power input 3 Battery backup power 4 +3.3V DC Power input 5 Push-button reset, active low 6 Reserved 7 Reserved 8 Reserved 9 Reserved 10 Ground Pin Function 11 Host serial data output A 12 Host serial data input A 13 Ground 14 Aux serial data output B 15 Aux serial data input B (DGPS)      16  Ground 17 Reserved 18 Ground 19 1 PPS time mark 20 Reserved April 2001  19 Fig.1: the interface consists mainly of the MAX-232 RS-232 level converter and a few power supply components. Battery backup is optional, especially if the unit is to be continuously powered. level of protection to the expensive GPS module. The GPS module itself plugs into connector J2 and performs forms the “brains” of the project. The board contains the RF front-end to receive the 1.575GHz signal from the GPS satellites and an on-board RISC processor running at around 50MIPS to calculate time differences between the received satellite signals and triangulate this into the latitude, longitude and altitude of the receiver. The position information and a very accurate time (each satellite contains four atomic clocks) are available through the serial port. The clock is also available as a series of 100ms TTL pulses at pin 19 with the time reference being the negative edge. The time is accurate to UTC within ±1µs. parity. Pin-outs for the module are shown in Table 1. Configuration commands may also be sent to the GPS module through pin 12, the host serial data input, although for most applications configuring the GPS module is not necessary as the default power-on “command set” instructs the GPS to send all the information necessary. Astute readers will pick up that several connections on the PC board have been made to the reserved pins on the module. Why? While the recommended module is the Laipac TF10 there is somewhat of an industry standard known as being “Rockwell Compatible” and these modules have similar pin configurations. This design is capable of also working with most of these modules, although because of the large number of variants it is recommended you carefully check the data sheets before using a different module with this project. For the same reason the project uses a double-sided PC board when single sided could have been used –some modules have the mounting connector reversed and require mounting on the top side of the PC board rather than the bottom. While modules with the connector pointing towards the bottom of the module are probably the most popular, modules with the connector pointing to towards the component side are quite popular with some portable equipment manufacturers The GPS module The processed data is available on pin 11 of the module and is sent as NMEA sentences (see below) at 4800bps with 8 data bits and no 20  Silicon Chip Fig.2: the component overlay of the double-sided PC board, from the component side. The blue tracks are on the component side. This board is more complicated than it needs to be to allow alternate GPS modules to be fitted. Parts list Looking straight down on the “normal” component side of this double-sided PC board: there’s not much to solder here so you shouldn’t have any problems. . . because they make the design slightly more compact. Construction Mount the TF10 module on the bottom of the PC board! I hope that’s got your attention but if you’re an advanced constructor you probably often skip the assembly instructions. It is the only assembly point that may be different to what you expect looking at the component overlay (Fig.2). As noted the PC board may be used with a variety of GPS modules, but the TF10 module supplied with the kit must be mounted so that the socket points towards the solder layer of the board, or back the front to what would normally be expected. With a TF10 module you may like to consider using a socket for IC2, but with some other modules that mount on top of the PC board that may not leave enough clearance, so check the physical requirements of the particular module first. Other than that normal construction methods apply. It will be easiest to start with the 20-pin GPS connector first, followed by the low-profile passive components such as the diode and five tantalums. Follow this with the voltage regulator, D-9 connector, MAX232 IC and finally the 1000µF electrolytic capacitor. The TF10 module may secured to the board using nylon spacers of 6mm length and 6mm diameter, along with four 15 x 3mm steel screws. It is recommended however that the GPS board not be inserted until the testing procedure below has been followed. Testing and final assembly For initial testing leave the GPS board disconnected and apply 6-9V 1 PC board, 108 x 80mm, double sided, code RCS PJGPS2K1 1 Laipac TF10 GPS module, SMA right angle, Type 4 OEM connector 1 20-pin (2x10) female straight header socket, 2mm centres 1 D-9 female connector, rightangle PC board mounting 1 3-way screw terminal, PC board mounting 1 TO220 mounting kit 1 3mm screw, 10mm long 4 3mm screws, 16mm long 5 3mm nuts 4 6mm Nylon spacers, 6mm long 4 PC board Nylon supports, 20mm long Semiconductors 1 LM2940T-5 low dropout regulator (REG1) 1 Maxim MAX232N RS-232 level converter (IC1) 1 1N4004 diode (D1) Capacitors 1 1000µF 16VW electrolytic (C1) 1 0.22µF 10VW tantalum (C2) 4 1µF 16VW tantalum (C3-6) . . . and there’s even less on the “underside” of the board – just the GPS module which plugs into the socket you previously soldered underneath. April 2001  21 Table 2 – Example data received from GPS module $GPRMC,040055.999,A,4250.5522,S,14718.4910,E,0.08,143.68,060101,,*11 $GPGGA,040055.999,4250.5522,S,14718.4910,E,1,08,1.3,58.9,M,,,,0000*25 $GPGLL,4250.5522,S,14718.4910,E,040055.999,A*20 $GPGSA,A,3,21,29,15,14,25,11,03,31,,,,,2.7,1.3,2.3*3B $GPGSV,3,1,09,29,85,066,47,21,57,118,48,14,52,126,44,15,37,041,47*73 $GPGSV,3,2,09,31,31,278,46,11,30,231,47,03,20,325,48,25,13,010,44*73 $GPGSV,3,3,09,23,12,097,*4C $GPVTG,143.68,T,,M,0.08,N,0.1,K*61 See Table 3 below for an example of interpreting the “GPRMC” sentence from the receiver that contains the time and position information. The example data is as per the first line shown above. to the power connector. You will notice on the component overlay there are two power connections, +6V DC and battery backup. The battery backup is optional and may be connected to a 3V battery to save the GPS almanac while the main power is off. This allows the unit to perform a quicker “warm start” when power is applied because the unit will have an idea where the satellites should be. Battery backup is not necessary if you plan to have the main power source available constantly. Use a multimeter to check that the voltage between pin 15 (GND) and pin 16 (Vcc) of IC1 is 5V (±0.25V), to confirm that the voltage regulator is operating correctly. Once this has been confirmed give the board a quick check for any shorted tracks, install the GPS module and attempt to use the module as described below. If at any stage you’re unsure if the GPS module is operating correctly you can perform a “loop-back” test by removing the GPS module and inserting a piece of wire between pins 11 & 12 on the socket. This will cause data received from the serial port to be sent back through the MAX232 chip to the serial port. You should be able to connect to the serial port with a communications program, such as HyperTerminal, and see that characters typed are received back. Characters being echoed back should cease once the link is removed, otherwise you either have a short on the PC board or in the serial cable. This will confirm that the RS232 converter is operating and the cable is connected to your PC correctly, although it will not help check 22  Silicon Chip the communications parameters are set correctly. The active antenna The recommended antenna is supplied with a 5m cable, making it more than long enough to reach, for example, a vehicle roof. Speaking of that, best performance will be achieved if the antenna is mounted on a horizontal metal surface (such as a vehicle roof) to act as a ground plane. In fact, the antenna has a magnetic base to make mounting on a vehicle very easy. Table 3 - Interpreting the GPRMC sentence DATA ELEMENT DESCRIPTION $GPRMC Defines this record as “recommended minimum GPS data”. 040055.999 UTC time in format hhmmss.sss. The example record was received at 04:00:55 UTC (+99ms). A “A” indicates valid position calculated, “V” indicates invalid position. 4250.5522 Latitude in format ddmm.mmmm. To convert to the more common degrees, minutes seconds (°, ', ") format multiply the decimal part (0.5522) by 60 to get the seconds component. The example is 42°, 50', 33". S S indicates south of the equator; N is north of the equator. 14718.4910 Longitude in format dddmm.mmmm.           Convert as for latitude giving 147°, 18' 29". E E indicates east of the meridian, W indicates west. 0.08 Speed over ground in knots.       Multiply by 1.852 to get kilometres per hour. 143.68 Course over ground in degrees. Only accurate when the receiver is moving so bearing can be calculated from previous position. 060101 UTC date in format DDMMYY, The example is 6-Jan-2001. (empty) Magnetic variation. Not provided by TF10. *11 Checksum of the message in hexadecimal. It is the 8-bit exclusive-OR of all characters between the “$” and “*” delimiters. CR/LF Line is terminated with a carriage return/line feed combination. Table 4 - Free/shareware GPS software on the Internet commlinx.com.au/gps_diag.htm Name: GPSDiag Software written by the author using Borland Delphi that shows position information received from the module along with other information such as speed, altitude, satellite positions and signal strength. It also displays the raw data received and is mainly intended as a diagnostic tool to get started. iliketheinternet.com/gps.html Name: NMEAgent Above is the recommended active GPS antenna, available from CommLink Solutions. Inset at right is the antenna from the opposite side. If you cannot mount it on a ground plane you will probably still get adequate signal but it will take longer to initialise and the chances of errors in the data are higher. Regardless of whether it is moun-ted on a ground plane or not, the antenna needs to be able to “see” a significant proportion of the sky with minimal obstruction from buildings, etc – if it cannot see the sky, it cannot see the satellites which it needs to receive data from. As a rule of thumb, for best performance at least a third of the sky should be visible from the location you mount the antenna. That’s not to say it won’t work indoors – it possibly will, as long as you don’t have a metal roof or metallised insulation blocking the incoming signals. And note that if you move the antenna from one place to another without it being turned on, it may take a few minutes to store a new almanac and therefore allow valid data to be received. During this time you will receive only a series of zeros for latitude and longitude. Connecting to the computer The female D-9 connector on the board is configured as a DTE (data terminal equipment). This means the unit can be connected directly to the serial port on a PC with a straight-through cable. Operation of the board with a modem will require a male-male null modem cable. Data is sent at 4800bps with 8 data bits and no parity and can be received with any terminal emulator program such as HyperTerminal supplied with Windows. Once the board is connected, powered up and the COM port selected you should see data being received. For a while it looks like gobbledegook then, as the antenna almanac builds, invalid data (0’s for latitude and longitude) Screen grab from the author’s “GPSDiag” software showing data coming in from eleven satellites but only nine are used (data quality of others is too low). As the module needs only three data for an initial fix and one thereafter, the positional data (151°18'13.938" E, 33°40'28.56" S) and other information here would be regarded as extremely accurate. Somewhat similar to the above but also allows gathering and averaging of positions over a long period to obtain a very accurate position of a fixed location. maptrax.com.au/freestuff/ Name: Trax 2.2 Australian-produced GPS mapping software that is easy to use and provides a map of Australia as part of the installation. The map provided doesn’t contain a lot of detail but more detailed maps may be purchased. The software is fairly limited but it is an ideal starting point, being very easy to install and use. gpsu.co.uk/index.html Name: GPS Utility When you want to get into the real stuff and start plotting your positions on a map, this seems to be the best package around. The free download has some limitations but at $US40 for the registered version it is excellent value for money and this package is much easier to setup and use than others that provide as many features. It also works with a wide variety of maps and you can “register” your own bitmap, TIF and JPEG files by providing the known latitude and longitude of a few points on the map. diku.dk/users/elgaard/eps/index.html Name: EPS – The Elgaard Positioning System A Java-based GPS and mapping system. For those into Java this will provide an excellent starting point for other projects but probably won’t be easy to follow for Java novices. April 2001  23 These two screen grabs are from the “TRAX 2.2” software (see overleaf). First screen actually shows the whole of Australia with Sydney targeted as our location (gee, just as well it got that right!). The black crosshairs and red concentric circles mark our position, while the blue arrow shows our “course” (obviously invalid ’cos our office isn’t moving – we hope!) The second screen shows the only level of zoom possible with this demo software (yep, we’re still in Sydney) but if we wanted purchased more maps, it could go down to street level. The camera images on the map below, by the way, show red light and fixed speed camera locations – your PC can even give you a voice warning as you approach these when mobile! and finally, after perhaps a minute or so, (maybe more if it the first time the unit has been turned on) data that looks something similar to that in Table 2 (and the screen grab overleaf). The NMEA standard Confused? Those familiar with GPS will immediately recognise the data in Table 2 as NMEA sentences. This is the standard for GPS communications devised by the National Marine Electronics Association and is the universal standard describing how GPS devices should send data to a host, such as your PC. The complete NMEA specification actually covers quite a range of marine related devices and as the document is copyright it must be purchased from the NMEA. Fortunately many freely available sources describe the sentences that relate to GPS and such information is widely available on the Internet. Try using your favourite Internet search page for the terms “NMEA” and “GPS” or alternatively the Internet site http://commlinx.com.au/NMEA_GPS. htm contains a good overview and examples of the sentences you’ll most likely want to interpret. Those who don’t feel confident reading the NMEA specification and writing code to communicate with the module needn’t feel intimidated. The ’net provides a plentiful source of “ready-to-go” solutions for GPS. A few pointers to get you started are shown in Table 4 and in the references. 24  Silicon Chip References The NMEA 0183 Standard for Interfacing Marine Electronics Devices. Published by NMEA, PO Box 3435, New Bern, NC 28564-3435, USA. http://www.nmea.org TF10 Reference Guide. Available from Laipac Technology Inc, 105 West Beaver Creek Road Unit 207, Richmond Hill, Ontario L4B 1C6, Canada. http://www.laipac.com Peter Bennett’s GPS and NMEA Site. http://www.vancouver-webpages. com/peter/ Wheredyageddit? CommLinx Solutions is the Australian distributor for Laipac GPS & Telephony products. The TF10 OEM GPS module is priced at $176 including GST. A suitable active GPS antenna with 2-metre lead and SMA connector is $49.95 including GST. A complete kit of components (not including antenna) is available for $247.50 including GST. See http://commlinx.com.au/products.htm for more details, fax orders to (03) 6273-5227 or write to CommLinx Solutions, 9 Wattle Avenue, Lutana, Tas, 7009. The 20-pin OEM connector is also available from CommLinx Solutions for $5.00 including GST or can be obtained from Farnell electronics, part number 511407. Farnell orders can be placed at http://www.farnell.com.au or by calling 1300 361 005. The PC board is produced by RCS Radio and is available as PCB 4981s. All other components are available from retail electronics distributors. Global Positioning System L ike many of today’s technology breakthroughs, GPS was originally a military system. Initially four NAVSTAR satellites, the first launched in 1978, formed the backbone of the system. As satellites go, they aren’t very big: about 1.5m wide and 5m long. In orbit (17,450km out), they weigh only 850kg. Each satellite contains ­four extremely accurate atomic clocks (one second in three million years!). This time information and satellite identification is transmitted on two L-band carriers around 1.575GHz. Today there are 24 of these satellites which provide coverage to every point on the planet. At least three satellites would normally be “visible” from anywhere; more important areas have up to twelve satellites available from featureless desert and often in blinding sandstorms. In fact, which to obtain data. GPS has been credited with having a decisive role in the Because the exact position of each satellite is known UN forces’ success. at any instant in time, a GPS receiver on the ground (or Most of today’s GPS receivers require an initial “fix” from in the air, or at sea) ­can work out precisely how far away no more than three satellites to establish their position – the that satellite is by comparing the time-stamped transmitted Laipac TF10 module used in this project is one such device. signal to the time it actually received that signal. Once the signal is received and position determined, it can Doing the same thing with the signal from a second keep accurate readings using only one satellite. Therefore satellite enables the GPS receiver to determine its position it is ideal in very poor signal areas. between the two. Adding a third signal enables a location It can take almost a minute to receive and analyse to be established; ie, a three-dimensional “fix”. enough signals to determine position from a “cold start”. And adding a fourth signal (or more) enables errors to Once the receiver knows where it is, a “hot start” gives a be virtually eliminated, giving even more accuracy. position in about eight seconds. While operating, Design accuracy is within 30 metres of true pothe information is updated about every 100ms. sition. Until last year, accuracy for “normal” users The output from the module is data in the was only 100m because of “selective availability” form of NMEA-0183 sentences. NMEA or SA errors, deliberately introduced into the stands for the National Marine Electronsystem to make it more difficult for non-friendly ics Association and has become the armed forces to use. standard for all GPS data output. But former US President ClinAn NMEA sentence contains an ton ordered SA be removed address field, a data field and a on 1st May 2000, to allow checksum. all users access to the miliWithin the data field can be tary-precision signal. such information as latitude Achieved accuracy is usuand longitude, north or south of ally better than 30m – many equator, east or west of 0° meridvehicle identification and ian, speed over ground in knots, tracking systems claim to be course over ground in degrees Basic Positioning (simplified to one plane only): able to show on which side of true, the date and time, and whethif the GPS receiver (at point A) knows it is a a road a vehicle is travelling certain time away from the red satellite, it must er the data is vaild or not.­ or parked – an accuracy of By the way, the reason that be somewhere on the red circle. Similarly, if at least 10m or even better. the exact positions of the GPS it also knows it is a certain time away from That’s not too bad from satellites is always known is that the blue satellite, it can only be where the red and blue circles intersect (points A & C). If a 17,450km away! they themselves use signals from third (green) satellite is added, it can only be at the other satellites to exactly deThe GPS system is fairly point A. Once it knows it is at point A, even if unaffected by weather; rain termine their own position. the GPS receiver temporarily loses data from and cloud generally have And it’s not only the US which one or two satellites it knows it cannot be at little impact but wet foliage has GPS satellites – in 1982, points B, C or D so it takes its data from one can cause problems. During the Russians launched their own satellite and works with that data until another the “Desert Storm” war in the system called GLONASS. Some comes into view. In the real GPS world, all of Middle East ten years ago, newer GPS receivers can operthe circles are actually spheres, so the system GPS was used extensively to ate using both NAVSTAR and operates in all three dimensions and can SC obtain positions in completely GLONASS. therefore give height. April 2001  25