GPS Antennas Selection and Installation

Application Note – 24

GPS Antennas Selection and Installation

Image courtesy of Trimble Navigation Ltd.

Introduction

One of the most cost effective and hence popular ways of attaining high precision time and frequency reference signals is to receive the transmissions from the GPS (Global Positioning Satellite) system in a purpose designed time and frequency GPS receiver, such as the ptf 3203A. While this approach provides extremely accurate and stable time, timing and frequency references when implemented correctly, one area that cannot be over-stressed is the importance of attention to the initial installation of the GPS antenna including cabling to the receiver itself.

This paper outlines the key issues to address when undertaking GPS antenna installation including some of the most commonly encountered problems that result in unsatisfactory results.

GPS Signals

The GPS system currently consists of 32 satellites, orbiting the Earth at an altitude of just over 12,000 miles. The period for one orbit is just under 12 hours, i.e. each satellite orbits the Earth twice per day. Up to date status on the constellation is available from:  ftp://tycho.usno.navy.mil/pub/gps/gpstd.txt.

Due to their distance from Earth and the obvious necessity to travel through the Earth’s atmosphere (total thickness around 300 miles) , the satellite transmitted signals are quite small by the time they arrive at the Earth’s surface (approximately -130dBm) .  This low signal level requires careful handling in order to correctly detect and extract the information necessary to ultimately provide precision references.

GPS Antenna

The first element in the receiving  “chain” is the GPS antenna.  GPS antennas come in a variety of shapes, sizes and capabilities, many tailored for use in a specific application, e.g. positioning or timing. GPS World publishes an annual survey of over 400 different antennas, the 2012 survey can be downloaded here:  http://www.gpsworld.com/wp-content/uploads/2012/10/GPSWorld_2012AntennaSurvey1.pdf.

 The ideal antenna mounting position is in an open area, with a clear view of the sky, and away from any other interfering signal sources (e.g. radio, television, cellular transmitters/antennas), often on a rooftop provided the antenna to receiver distance does not become prohibitive. Unfortunately, due to the main application requirements for precision frequency and time references, few real life applications lend themselves to such an ideal situation, many being in industrial or densely populated areas.  

The most common causes of poor antenna performance are:

  • Poor satellite visibility
  • Multipath signal errors
  • Interfering signals

Poor Satellite Visibility

As previously mentioned, each satellite is orbiting the Earth once every (approximately) 12 hours. If a satellite were passing directly overhead, this would equate to a movement across the sky in degrees per minute of :

            360/(12 x 60) = 0.5 degrees per minute.

There are six orbital planes and in a position with a clear view, typically 9 satellites will be simultaneously in view, however if the antenna has an obstructed view of the sky the number of visible satellites can be greatly reduced, and in addition, the time an individual satellite is in view can be quite limited, see the diagram below:

Multipath Signal Errors

Another fairly common problem is errors caused by multiple paths of the same signal reaching the antenna. This is typically due to single or multiple reflections from nearby objects such as buildings, rock faces or other obstructions, see the diagram below. Whilst most GPS antennas are designed to minimize the effects of this (e.g. Choke Ring Antennas), nevertheless complete elimination of these effects is very difficult and it is preferable to avoid the potential for such errors at source whenever possible.

Interfering Signals

Due to the very low signal levels involved, the presence of other interfering signals can be a significant issue. This is especially true when the antenna is mounted in an area where there are other transmitting antennas present (e.g. radio/TV broadcasting, telecommunications cell towers etc.)

The commercial GPS signal is carried on an L1 carrier, at a frequency of 1575.42 MHz (there is also a military signal broadcast at 1227.6MHz)and most commercial GPS antennas have out of band rejection of approximately 20dB at offsets of 50MHz from the carrier frequency which helps to reduce interference from other signal sources.

In some cases, interference from other sources can be minimized by simple shielding of the antenna from the direction which the interference is emanating, again taking into account the impact of reflections of those signals. While this obviously reduces visibility of the sky, it is preferable to having unwanted interference that can cause overload of the receiver front end amplifier, see below:

Antenna Amplifier

The next link in the receive chain is typically an antenna amplifier. In general the amplifier will be integrated into the antenna package, utilizing the center conductor of the coaxial cable connection to the receiver to carry a DC power supply of 3 to 5VDC.  Gain of the integrated amplifier can vary from around 25dB to 50dB dependent upon make and model,  and as we will see below, selecting an antenna with the correct gain for the installation is crucial to optimizing performance of the system.

Receiver Requirements

Most time and frequency GPS receivers include a front end “receiver engine” designed to extract the necessary information from the received GPS signals. Due to the very low signal levels involved, and the necessity to extract this information from the inherent noise in the received signals, some fairly sophisticated correlation techniques are used, the detail of which is outside the scope of this paper. In addition, the front end receiver engine includes it’s own amplification, and keeping within the dynamic range of this front end amplifier is a critical aspect of the installation.

Usually the signal level at the receiver engine front end is described in terms of effective gain of the received GPS signal as seen from the receiver. For example, if the antenna and integrated amplifier have a gain of 30dB, and the interconnecting cable has a loss of 15dB (at the L1 carrier frequency of 1575.42 MHz) then the effective gain at the receiver will be 15dB.

A typical front end receiver engine will have a dynamic effective gain range from around 10dB to 25dB, and this number is what is used to calculate the required antenna (and integrated amplifier) gain, and the acceptable loss in  the interconnecting cable.

Different cable types exhibit different levels of signal attenuation, dependent upon the specific cable properties. Cable attenuation is usually quoted by the manufacturer in terms of dB per 100ft at specific frequencies. A list of some of the more common cable types and attenuations is shown below;

Cable Type ImpedanceLoss / 100ft
 (Ohms)at 1500MHz (dB)
   
RG585022.0
RG597511.0
RG 213/U5010.0
LMR-400505.0
Andrew Heliax503.0

In addition, the cable impedance matching becomes important for longer cable lengths, most receivers being a 50 ohm input impedance.

Clicking any of the hyperlinks above will take you to specifications for all the listed cables.

Once antenna position has been established, the distance from the antenna to the receiver, and in turn the necessary cable length can be determined. For longer cable lengths (200 ft and up) a decision must be made whether to use higher quality (lower loss) cable, or a higher gain antenna. Although a higher gain (50dB) antenna tends to be more expensive, the cost of installing long lengths of high quality cable can quickly outweigh this. For the longest distances, both a high gain antenna and low loss cable may become necessary. The table below shows a number of different lengths versus cable types/antennas;

Cable LengthAntenna TypeAntenna GainCable TypeCable Loss/100ftTotal LossEquivalent
(ft) (dB) (dB)(dB)RX dB gain
       
30Std30RG58226.623.4
50Std30RG59115.524.5
100Std30RG591111.019.0
250Hi Gain50RG591127.522.5
500Hi Gain50LMR-400525.025.0
650Hi Gain50LMR-400532.517.5

Although as a rule of thumb higher signal levels are preferable, as can be seen from the table, a couple of the lengths are quite close to the limit (receiver amplifier dynamic range of 10dB to 25dB). If the signal level is too close to the upper limit, it is advisable to add some additional attenuation with either a purpose designed attenuator (e.g. 3dB or 5dB pad) or by increasing the cable length for additional loss, thus insuring there is no risk of amplifier overload which will prevent correct operation.

Cable Length Compensation

One final point of consideration is the timing error that can be introduced due to long cable lengths. The calculated position (necessary for computing accurate time) is the position of the antenna, and a long cable introduces a timing error proportional to the length of the cable. The longer the cable from the receiver to the antenna,  the larger the resulting error. Cable delay is calculated using the speed of light, the cable length, and the ‘K’ coefficient for the particular cable.

Speed of light is 3 x 108 meters /second which equates to approximately 1 foot per nano second. For RG58 cable K is approximately 0.6 therefore cable delay is calculated from;

                L (feet)

     delay = ————   nano seconds

                 0.6

so for a 100ft length cable, a delay of approximately 166 nano seconds would be introduced. Most GPS based time and frequency receivers provide a facility for entering the antenna cable length and provide an automatic compensation for this delay.

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