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NTP Server Static GPS Antenna Systems

By David Evans

The Global positioning System (GPS) is often used by computer equipment, such as NTP Server systems, to provide an accurate timing reference for time critical applications. This article provides an overview of GPS for timing applications and describes the equipment used to install a GPS antenna in a static location.

Overview - Using GPS for Accurate Time

The Global Positioning System is a US military system for worldwide navigation. The system consists of 24 orbiting satellites, each satellite has a highly accurate atomic clock on-board synchronised to UTC time. The satellites continuously broadcast time and position information. The time and position information can be obtained worldwide with a GPS receiver and antenna. GPS works continuously in any whether conditions, anywhere in the world. Additionally, there is no set up fee or subscription charges to utilise the GPS systems. Many computer timing systems and NTP Server systems utilise GPS as an accurate external timing reference.

The Accuracy of GPS Timing Systems

GPS receivers provide highly accurate position and timing information. Typically, a GPS receiver can provide positioning information to an accuracy of 15m. NTP Server systems can obtain timing information from GPS to a resolution of a few nanoseconds.

The GPS Signal

The transmitted GPS signal is very weak low-power radio signal, designated L1 and L2. L1 is the civilian GPS frequency transmitted at 1575.42 MHz. The signals travel by line of sight and can pass through clouds, glass and plastics but are blocked by objects such as metal and brickwork. Therefore, the ideal location for a GPS antenna is on rooftop with a full 360-degree view of the sky. However, often installation on the side of a building or in a window can provide adequate results. As a rule of thumb, the better the view of the sky, the greater the likelihood of a good consistent signal lock.

GPS Timing Antenna Types

The GPS antenna acts as an amplifier to boost the GPS signal for transmission along a cable, usually coax, to the GPS receiver. GPS Timing antennas provided with NTP server systems utilise a pole-mounting system. The antenna screws to a threaded pole for installation on rooftops. This arrangement provides the GPS antenna with a rigid mount easily able to withstand high winds without damage. Typically the GPS antenna is fairly small in size, measuring less than 90cm in diameter. Low-cost patch type antennas are also available, but these are generally better suited to vehicle applications.

GPS Antenna Cable Types and Cabling Distances

The cable distance that can be utilised by a GPS antenna depends mainly on the amplification of the GPS antenna and the quality of coax used in the installation. A typical GPS timing antenna may have a gain of 35 db. Relatively low-quality coax such as RG58 has an attenuation of 0.64 db/m at 1575 MHz. Therefore, a cable run of 55m can be obtained using RG58 cable. With very high quality coax cable, such as LMR400, an unaided cable run of 200m can be achieved. However, very high quality coax can be expensive. A good price-performance compromise is LMR200 cable, which can be run unaided to 80m.

Extending Cabling Distance with In-Line GPS Amplifiers

In-line GPS amplifiers provide further amplification of the GPS signal to increase the cable distance between the GPS antenna and receiver. GPS amplifiers are fitted in-line with the antenna cable and obtain power from the receiver via the coax cable. No external power-supplies are required. Typically, a GPS amplifier may add a further 20 dB of gain, adding 30m of low-quality RG58 coax, 40m of LMR200 coax or 100m of high quality LMR400 coax. Additionally, multiple in-line amplifiers may be utilised to further increase cable distance.

Sharing a Single Antenna Between Multiple Receivers - GPS Splitters

GPS splitters allow a single GPS antenna to be utilised by two or more NTP server systems. The GPS splitter splits the signal received from the GPS antenna into multiple outputs for synchronizing multiple NTP servers. GPS splitters are generally available with 2, 4 or 8 outputs.

Protecting GPS Systems - GPS Surge Suppressors

Surge suppressors protect expensive NTP server equipment from electro-static discharges, such as lightning, that may be picked up by an externally mounted GPS antenna. Surge suppressors are installed in-line on the coax cable between the antenna and receiver, ideally where the cable enters the building. Surge suppressors require a low-impedance ground, to discharge any received surge. The surge suppressor requires no power-supply or additional cabling.

About The Author

Dave Evans develops atomic clock time synchronisation systems to ensure accurate time on PC networks. Find out more about GPS NTP Server solutions at http://www.timetools.co.uk



Give Your GPS Device A Boost With Mapping Software. A program such as OziExplorer ($85; www.oziexplorer.com) for desktop Windows and its Windows CE-based sibling OziExplorerCE ($30) can help you by letting you import digital maps into your GPS device and set up a prearranged route. The software interacts with your device by downloading tracks and waypoints, and you can set your own waypoints and tracks on maps you create yourself. The software supports real-time tracking, reflecting the position of the GPS device as you physically move. Maptown's Memory Map ($111.10; www.maptown.com) is another mapping program with similar features.
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Qualcomm's Vision of the Wireless Future "When he finished, there were 20 brand-new, CDMA-based mobile handsets and devices arrayed on the table, the high-tech souvenirs of Belk's most recent trip to Hong Kong and Japan.... 'These are not PowerPoint slides,' Belk said. 'These are real devices that are already shipping in volume in Asia.' " at Business 2.0]

And there's the rub. When I do my presentations, that's all I have - pictures of prototypes or of devices not available in the U.S. Europe and Asia really are 18 months ahead of us in this area.

"Beale also discussed Qualcomm's gpsOne location technology.... It also opens the door for many new location-based services, such as traffic information and local weather forecasts. On Japan KDDI's network, there are already more than 20 location-based services available -- all of which are relatively inexpensive, costing less than $5 per month."

I'm looking forward to being able to travel and have information come to me automatically based on my location. For example, if I'm at a conference and I'm looking for a restaurant, I'd like my phone/PDA/whatever to know where I am and offer a list of nearby choices. Maybe it should tell me what movies or plays are on tonight and if there are still tickets available. You get the idea....

"Predictably, both Belk and Beale anticipate strong growth for the wireless industry as ever-more-compelling products and services become available and the industry settles on a single communication standard. How long will that take? Belk, whose career began in the personal computing business in 1983, likened the wireless industry today to the PC business of the 1980s. 'I wouldn't worry about the wireless industry yet, it's still relatively young,' he said. 'Just look how long it took the PC industry just to get to the point of standardized parallel cables.' "

Not to beat a dead horse, but the ebook industry is even younger than PCs, and PDAs have really only come into their own during the last few years. So don't count any of these technologies out just yet.

Lots of interesting articles from today's PC World. First up, Microsoft Takes on MapQuest: "MSN launches MapPoint online mapping service, offering maps, directions, and more."

Apparently this is going to be yet another component of MS' push for .Net services with hooks into other MSN services. MapPoint is XML-based, which makes it interesting in other ways, and there it launches with a phone-based direction service. The articles notes that this is another step towards location-based services. "For example, you might someday be able to click on a Windows Messenger buddy's name and retrieve a map showing the location of his or her home."

Good or bad? You make the call. I like parts of this, but I'm incredibly wary of .Net.

Write Here, Write Now: And you thought you were overloaded with information now, just wait. Hewlett Packard is working on a technology to let folks print messages in mid-air based on their location incorporating GPS technology. I find this stuff fascinating, even if no one seems to have thought of a good use for it yet. The first sentence of the article is right, though: "The kids are going to love this." in New Scientist via RCPL's Liblog]

When the ALA summer conference was in San Francisco in 1997, the SF Museum of Modern Art had a fascinating exhibit called Icons: Magnets of Meaning. I spent hours browsing through it, but one of the pieces that has always stuck in my mind was called @: Marking the Electrosphere . It talked about the meaning of that one little symbol. How it can define, place, and root you in the world, but at the same time let you be found anywhere. Integrated, widespread use of GPS is going to take this to a whole new level.

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