General GPS Antenna Information

APPLICATION NOTE

Global Positioning System andPrecise Time & Frequency

The Global Positioning System (GPS) is aworldwide radio-navigation system formedfrom a constellation of 24 satellites thatcontinuously orbit the earth. Each GPSsatellite has on board several atomic clocksthat are precisely synchronized to UniversalTime Coordinated (UTC) provided by the U.S.Naval Observatory (USNO). Coded signalsare broadcast by each of the satellites withthe exact time and position of the satellite.All GPS receivers use an antenna to receivethese signals. By using a GPS receiver opti-mized for time and not position it is possibleto get extremely precise time synchronizationwith the satellite’s atomic clocks.

GPS Antennas and Cables

The signals from the GPS satellites operatein the “semi-visible” spectrum of the L1band (1575.42 MHz) with a minimum signallevel of -162.0 dBW. With this very low signalstrength the GPS antenna must be able to“see” the sky to acquire the signals.Practically speaking the antenna must havea clear view of the sky and thus be mountedon a roof, or in some cases in a window. Theantennas are relatively small, coffee cup sizeor smaller, and are connected to the GPSreceiver typically via coaxial cable.

Since the GPS signal is very weak theantenna usually amplifies the signal to driveit through the cable to the receiver. Antennacable however offers some resistance andthe GPS signal strength will attenuate as ittravels down the cable. GPS receiver sensi-tivity is finite so if the cable length is too

long the signal will be too weak for thereceiver to detect it. Consequently it is veryimportant to know the distance in advancebetween the antenna and the receiver so thatthe proper cable solution can be installed.

Antenna Types

There are two basic antenna types used withGPS timing receivers; roof mounted andwindow mounted. The roof mounted antennais required for the more accurate GPS clockssince at least three satellites are required tobe in view at all times to maintain timingaccuracy, typically nanoseconds to UTC. Thewindow mounted antenna is applicable forthe network time servers which operate witha lesser degree of accuracy, typically microsec-onds or in some cases low milliseconds toUTC, and can function with as few as oneintermittent satellite in view. The roof mountedantenna is always preferable since by natureof its location has the best view of the sky.A variation on the roof antenna is the GPSDown/Up converter used for very long cableruns. This is a special GPS antenna thatreceives the GPS signal and down converts itto a lower frequency that is then sent downthe cable. Next to the GPS receiver is an upconverter that converts the signal back tothe original frequency and delivers it to theGPS receiver. This process is transparent tothe GPS receiver.

Antenna Placement and Mounting

Roof Antenna Placement: When selecting asite for the roof antenna, find an outdoorlocation that provides full 360-degree visibilityof the horizon. In most cases, this meanslocating the antenna as high as possible, suchas on the roof. Any obstructions may degradeunit performance by blocking the satellitesignals. Blocked signals can increase thetime for satellite acquisition, or preventacquisition altogether.

A short mounting mast and hose clamps areprovided with the roof antenna to mount theantenna to a pole or the peak of a building.The antenna mounting mast and clamps arewell suited to attach the antenna to a ventpipe or mast affixed to the roof. The pipe mustbe rigid and able to withstand high windswithout flexing (see Figure 1).

GPS Receivers can be susceptible to reflectedGPS signals called multipath. Multipathinterference is caused by reflected signalsthat arrive at the antenna out of phase withthe direct signal. This interference is mostpronounced at low elevation angles from 10to 20 degrees above the horizon. The heightof the mast/antenna may be extended upwardto prevent multipath interference. The antennashould also be at least three to six feet (1-2 m)from a reflecting surface.

General GPS Antenna Information

2

FIG.1 Typical roof antenna mounting

Window Antenna Placement: The windowmount antenna is suitable for use only withthe network time server products withappropriate versions of firmware. For windowmounted antenna installations it is best touse a window with the best view of the sky.For windows with equivalent views, orienta-tions that face the equator are preferred.Generally more satellites will be in viewtoward the equator than away from it, eastor west facing windows will also work. Polarfacing windows will also work but in generalare not preferred. Windows that have thebest view of the sky are always preferredregardless of orientation (see Figure 2).

Attach the antenna above the window sillversus at the top of the window. This willimprove the upward visibility from theantenna to the sky. Note that some windowglazing treatments may reduce or block theGPS signals, preventing the time serverfrom acquiring the time.

Antenna CableConfigurations/Options

Antenna cabling solutions typically varydepending on how far the antenna is installedfrom the GPS receiver. 150 feet (45 m) is theunaided cable length limit for many GPStiming receivers. Adding a GPS inline amplifierextends the cable length an additional 150feet (45 m). Beyond 300 feet (90 m) alternativemethods may be used. Figure 3 highlightsthe cable lengths and the antenna solutionsthat enable them.

In-line Amplifier: In-line amplifiers over-come signal attenuation in by amplifying theGPS signal, adding an additional 150 feet (45 m) in cablelength. The inline amplifierattaches directly in line with the antenna cableand uses the same power as the antenna;no extra wiring is required. Mounting the amplifier inside the mounting mast helpsprotect it from moisture and exposure to theelements. See Figure 4 for a typical mastmount application.

GPS Down/Up converter: The GPS Down/Upconverter makes cable runs of 250 to 1500feet (75 m to 457 m) possible. GPS signal down

3

FIG.2 Antenna is in the preferred window

FIG.3 Cable lengths and antenna solutions

L1 GPS Antenna and Fiber Optic Link

L1 GPS Antenna Down/Up Converter

L1 GPS Antenna(roof or window)

150 ft.unaided

1500 ft.

6500 ft.0 ft.

Log Scale

300 ft. withinline amplifier

conversion requires a special GPS antennaand corresponding signal up-converter. Theantenna module converts the signal down toa lower frequency that is less susceptible to attenuation, and transmits it the length of the cable to the up-converter. The up-converter restores the signal to the normalGPS signal frequency for the receiver (seeFigure 5). The down/up conversion processis transparent to the GPS receiver. As withany precision GPS timing receiver, only cabledelay and down conversion delays need to beentered into the receiver. Power is suppliedby the GPS receiver. In the case of Bus levelGPS receivers an external power supply isused. It is important to note that the cableused in GPS down/up conversion is differentthan the standard cable.

Fiber Optic Links: Fiber-optic connectionsfunction as a transparent link between theantenna and GPS receiver equipment. Theselinks eliminate the limitations of coppersystems by enabling longer transmissiondistances while retaining the highest level ofsignal quality. In addition, fiber optics provideseveral other significant network advantages,including simplified network design, ease ofinstallation, and immunity from EMI/RFI and lightning (see Figure 6).

Lightning Arrestor: In-line lightning arrestorsare mounted on a low impedance groundbetween the antenna and the point where thecable enters the building. They require no addi-tional power or wiring except the ground lead.

4

FIG.4 Typical mast mount of in-line amplifier

GPS ANTENNA

MOUNTING MAST

INLINE AMPLIFIER

GPS RECEIVER

FIG.5 GPS down/up converter

GPS ANTENNA DOWN/UP CONVERTER

GPS RECEIVER

UPCONVERTER

5

Cable Delay

GPS position as well as precise UTC time isdetermined at the point the GPS signals arereceived at the antenna. Since the antenna istypically attached to the GPS timing receivervia a cable, signal propagation delays throughthe cable cause the time calculated by thereceiver to be slightly behind UTC. In GPSclocks with nanosecond and microsecondaccuracies this is a critical factor. In productssuch as network time servers cable delay is not important because time transfer overIP networks degrades the time to themillisecond level.

Cable delay is a function of the cable type.RG-59 cable for example typically delays thesignal 1.24 ns/ft. For 50 feet of cable (15 m)the delay would be 62 nanoseconds. Cabledelay is removed by advancing the antennasignal inside the GPS receiver. In this example,advancing the signal +62 nanosecondsremoves all cable latency. Solutions such asthe GPS Down/up converter also introducesignal latency but this latency can also beremoved by adjusting the signal. All precisionGPS timing receivers with nanosecond ormicrosecond timing accuracy have the abilityto compensate for cable delay.

Advanced Planning

It is time well spent to estimate in advancethe cable length from the GPS antenna tothe receiver for any planned installation.Cable lengths that are too short or too longcan each introduce problems. In some casesadding an inline amplifier and some extracable may be a quick and economical solution.In other instances retrofitting for a GPSdown/up converter may be necessary whichwill require installing a different cable type.Keep in mind that some extra cable coiled ina ceiling leftover from over estimating thecable length is not necessarily negative.Provided you know the length of the totalcable, the cable delay can be accounted forand the timing accuracy maintained.

FIG.6 Fiber optic connections

GPS ANTENNA

FIBER OPTICTRANSMITTER

FIBER OPTICRECEIVER

GPS RECEIVERSINGLE MODE FIBER

©2002 Symmetricom. Symmetricom and the Symmetricom logo are registered trademarks of Symmetricom, Inc. All specificationssubject to change without notice. AN/GPSANT/D/1202/PDF

SYMMETRICOM, INC.

2300 Orchard ParkwaySan Jose, California 95131-1017tel : 408.433.0910fax : 408.428.7896info@symmetricom.comwww.symmetricom.com