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AMATEUR RADIO BY GARRY CRATT, VK2YBX The Global Positioning System Amateur radio operators now accept satellite communications as a normal and regular mode of operation. But perhaps few realise how farreaching satellite communications are becoming. This article describes the revolutionary satellite navigation system which is now in use: the Global Positioning System (GPS). The Global Positioning System, when fully implemented, will consist of a constellation of 21 operational (plus three spare) navigation satellites, funded by the US Department of Defence, which will replace the now aging Transit satellite navigation system. It can be used by both military and civilian operators for the navigation of aircraft, boats, submarines and land-based vehicles. The service is free of charge and can provide civilian users with accuracy typically in the order of 30 metres. GPS can provide precise and continuous global, all-weather navigation data. It is capable of providing latitude, longitude, altitude and time, and hence (by computation) the speed and direction of motion in real time. Presently there are 16 operational satellites in orbit. The entire complement of 24 satellites should be in place by mid 1993. Known also as NAVSTAR, the GPS system will comprise a constellation of satellites in six orbital planes, with each group of four satellites circling the globe at about 20,200km. This arrangement will give an orbital period of 12 hours, or two orbits per satellite, per day. Each satellite transmits navigational signals with special coding, unique to each "bird", on two frequencies simultaneously. These signals are 11 (1575.42MHz) and 12 (1227.6MHz). The 11 carrier is modulated with a "coarse acquisition" (C/ A) code which has a frequency of 1.023MHz. Both the 11 and 12 frequencies are also modulated by a second code, called the P-code, which has a much faster switching rate of 10.23MHz. Both codes carry information at a data rate of 50 bits per second. This information gives the precise position of the satellite in space, plus clock data and other information, allowing a GPS receiver to determine the range to the satellite. The Micrologic Expl~rer GPS Designed specifically for use in boats, cars and aircraft, the Micrologic Explorer can accurately respond to changes in position and speed in less than two seconds. In addition to showing latitude, longitude and altitude, the unit can also display the local time (accurate to one second) and the distance and speed in either nautical miles and knots, statute miles and mph, or kilometres and km/h. All receiver functions and navigation functions are completely automatic and up to 100 waypoints can be entered into the unit's memory, either by entering the latitude and longitude or the present position. There are audible and visual alerts for waypoint arrival, crosstrack error and anchor watch, and the unit can display the direction to a waypoint at the press of a key. The claimed accuracy of the unit is 50 metres RMS, 0.1 knots RMS and 75 metres RMS for altitude U!lder the 74 SILICON CHIP current system of selective availability. For further information, contact Coursemaster Autopilots Pty Ltd, 7 Smith St, Chatswood 2067. Phone (02) 417 7097. Each spacecraft is equipped with two caesium-beam clocks, to ensure that each satellite transmits the code at exactly the same time. Hence, a GPS receiver on Earth can tell not only from which satellite the navigation data is being transmitted, but the distance to that particular satellite. This is done by measuring the time that the signal takes to travel from one satellite to the GPS receiver, using a second satellite as a time reference. Because the speed of light (and radio waves) travels at 300,000 kilometres per second, we can calculate that the signal travels 30cm per nanosecond. Hence, by measuring the time taken for the signal to arrive at the receiver, the range to the satellite can be computed. In order to calculate the precise position of the GPS receiver, some logic must be employed. If only one satellite were used, it would be true to say that the location of the receiver must be somewhere on a sphere centred around the satellite at a distance equal to the "range". If a second satellite is also used, and the range measured from it to the receiver, the location of the receiver must be somewhere along the line of intersection of the two overlapping spheres. If a third satellite is used, the probable location of the receiver can be determined even more accurately. This kind of computation is called a 3D "fix", and requires three satellites for navigation data plus a fourth for precise timing. The receiver must also be able to calculate the exact location of each satellite. Because only 16 satellites are currently available, this sometime gives rise to a situation where only two satellites are visible at one time, and hence no fix can be obtained. While these outages only last for a few minutes, they can occur at any time, day or night. In such instances, where the GPS receiver is being used for mobile navigation, a back-up system must be used, such as Omega, Loran C (where available), or manual navigation. This shortcoming is now being overcome, through the regular launching of additional satellites. Almanac data In order for the receiver to be "loaded" with sufficient data to enable it to calculate the exact location Called the "Pyxis", this portable GPS system from Sony packs the electronics and antenna into one compact hand-held unit. It allows you to check and store your latitude & longitude; check your direction, speed & altitude; check the distance & direction to your destination; plan & store up to nine routes & destinations; & track distance, direction & altitude of points passed. It also tells the time and can graphically compare your current direction with the most direct route to your destination. For further information, contact Sony Australia or Navigation Management Systems, 89 Cheryl Crescent, Avalon Beach 2107. Phone (02) 997 5117. of all satellites, a general "Almanac" is transmitted constantly, which gives the approximate location of each satellite. This data is transmitted slowly and it takes up to 13 minutes for the receiver to acquire it. This data is accurate for up to six months and gives the receiver sufficient information so that it knows roughly where to look for satellites. Information that pinpoints the exact location of the satellite is transmitted every 30 seconds and is referred to as "ephemeris" data. This data is a set of equations that produce the exact location of that particular satellite. This is quite a complex task and the gravitational effect of the Moon and the solar wind must be taken into account. Once the location of each satellite is known and the range to each satellite is known, a complete position fix is possible. Because the satellites are constantly moving, setting and rising, and because it requires four satellites to give a position fix, the ideal receiver should be able to receive and process five satellites. Four of these receiver channels will be tracking satellites, whilst the fifth will be searching for the next satellite, which will be needed when one of the others disappears over the radio horizon. Differential mode When used in the differential mode, a 1-3 metre accuracy can be achieved. Basically differential operation is achieved by placing reference receiving stations at fixed sites. Such stations can monitor GPS signals, calculate errors, and then broadcast correction factors to properly equipped maJANUARY 1992 75 Boasting a large liquid crystal display, the Magnavox MX100 GPS Navigator uses graphics to convey a wide range of information. A useful safety feature is the man overboard button (MOB) which instantly logs the exact latitude & longitude. The unit then provides steering data back to that point. Contact Coursemaster Autopilots Pty Ltd, 7 Smith St, Chatswood. bile receivers. This approach could be used to circumvent some of the deliberate errors introduced by the operators of the GPS system. Deliberate GPS errors Basically, the US Government offers two different levels of service. Military users have access to the "Precise Positioning Service" (PPS), which is generally capable of providing accuracies of up to 16 metres or better. Non-military commercial users, will have open and unlimited access to the "Standard Positioning Service" (SPS), which can provide accuracy to 100 metres for 95-98% of the time. The US Department of Defence introduces artificial errors into the GPS signals to degrade the accuracy for SPS users. This is done to deny any The Magnavox MX100 comes with a high-gain antenna that can be mounted in any convenient location & connected to the unit via a 15-metr.e cable. potential adversary access to the full system accuracy. Considering the recent "Gulf War" conflict, where GPS played a vital part in Allied military operations, this is understandable. There are two techniques used to introduce errors. They are Selective Availability (SA), and Anti-Spoofing (AS). Under SA, the orbital parameters are offset and the satellite clock signals are dithered slightly, thus giving the user an incorrect value for time and range. Anti-Spoofing involves encryption of the P code so that it becomes impossible to interfere with GPS operation by transmitting false signals. From a user's point of view, the affect of AS is to deny access to the LZ frequency. Authorised military users can be given decryption keys that cancel out [,tf.fC6ffE:"~:··i·~ Vo1...1r p,:,:::.i t.ic,n ~-~ 4oa52. 728 1.,.1 73a3,3, ?OE: ·~·. ; I..Jel,:11:.i t•-1 I ~ COG ' II SOl3 I'::._::, ~..::.I ·::.11,,:' _::, I'::. Ui. '':/ ...! ' ' .. The "Plotter" mode on the MX100 provides selectable gull's-eye views of the boat or waypoints, along with important navigational data (including current position, speed and course. 76 SILICON CHIP The "Navigate 2" screen displays the next waypoint, plus position, time & ETA. Other display modes include the "Position" mode & the "Sailplan" mode which lets you enter up to 200 waypoints over 20 routes. the artificial errors to give the maximum accuracy. Apart from artificially introduced errors, factors such as satellite clock error, ephemeris errors and ionospheric errors can also degrade GPS accuracy. Absorption of satellite signals can also cause propagation errors, introducing geographic errors of up to 75 metres. A GPS NAVIGATION ~NM S..-- v For the right advice at the best price ... PYXIS IPS-360 Private networks Main Features: Presently, there are several privately operated differential networks operated on a subscription basis, in locations such as the North Sea and the Gulf of Mexico. These applications are intended for offshore oil exploration. Some countries are looking at the possibility of providing public broadcast differential GPS services. Finally, there are surveying applications for GPS where , by measuring the phase difference on the satellite carrier signal, from one position to the other, over a long period of time, (typically one hour), an accuracy of 5mm or so can be achieved for stationary receivers. • Displays latitude, longitude & altitude. • Displays speed & direction, plus distance & direction to your . destination. • Let's you plan & store up to 9 different routes (50 waypoints, 50 destinations). Further reading: • Compares current direction with the most direct route to the destination. (1). "GPS Reference Guide". Navigation Management Systems, cl- Aviation Business and Commercial Operations Network Pty Ltd (suppliers of GPS receiving equipment). Phone (02) 997 5117; fax (02) 997 8729. (2). "Marine Navigation With The Global Positioning System, Questions and Answers". Available from Coursemaster Autopilots Pty Ltd. Phone (02) 417 7097; fax (02) 417 7557. (3). "Differential Operation of the Global Positioning System". IEEE Communications, July 1988, Vol. 26 No 7. • Tracks distance, direction & altitude of points passed en route & displays time to 1-second accuracy. • Accuracy: position 30-100 metres; velocity 0.3 knots (0.56km/h). NAVIGATION MANAGEMENT SYSTEMS 89 Cheryl Crescent, Avalon Beach 2107. Phone (02) 997 5117 Mobile (018) 22 2804 INTRODUCING THE NEW MXIOO GPS NAVIGATOR FROM MAGNAVOX. The unerring accuracy of six-channel continuous satellite tracking. Dazzling displays of navigational data. Turn it on and the double supertwist LCD screen fills with bright, high-resolution data. Position. Speed. Headihg. Cross- track error. Time and distance to the next waypoint. All updated at one-second intervals. Enter up to 200 waypoints with complete descriptions. Plus 20 routes with as many as 20 waypoints each . There's even a Man Overboard button that plots a return course to the exact spot. See a demonstration of the new MXlOO today. • • • • • • • • • 7 Smith Street, Chatswood NSW 2067 Australia Tel: (02) 417 7097 Fax: (02) 417 7557 Telex: AA89426 Unique Magnavox Features: • • • • Six-Channel continuous satellite tracking. • Advanced double super-twist LCD screen . • Features no other GPS Navigator offers. • Man Overboard feature plots a return course to the exact spot. • Legendary Magnavox reliability. • • • • • • • • COURSEMASTER AUTOPILOTS - Austral ia's world lead e r in auto pilot t e chn o lo gy