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DGPS BROADCAST SITE GPS ANTENNA INSTALLATION GUIDELINES

AND CERTIFICATION STANDARD OPERATING PROCEDURE

5 JULY 1995

DIFFERENTIAL GPS BRANCH ELECTRONICS ENGINEERING CENTER

UNITED STATES COAST GUARD

TABLE OF CONTENTS

1.0 INTRODUCTION
1.1 SCOPE
1.2 BACKGROUND
1.3 ANTENNA DESCRIPTIONS
2.0 GPS ANTENNA INSTALLATION GUIDELINES
2.1 INSTALLATION WITHIN THE RADIOBEACON GROUND PLANE
2.2 INSTALLATION OUTSIDE THE RADIOBEACON GROUND PLANE
2.3 ANTENNA SHADING
2.4 MULTIPATH ABATEMENT
2.5 DISTANCE BETWEEN GPS ANTENNA MASTS
2.6 UNDERGROUND OBSTACLES
2.7 RADIO FREQUENCY INTERFERENCE
2.8 ANTENNA SITE SELECTION RECORD
3.0 GEODETIC MONUMENTS
3.1 DISTANCE BETWEEN MONUMENTS
3.2 SHADING AND MULTIPATH
3.3 UNDERGROUND OBSTACLES
4.0 SITE CERTIFICATION
4.1 GEODETIC SURVEY
4.2 MULTIPATH ANALYSIS
4.3 SITE RECORD

1.0 INTRODUCTION

The United States Coast Guard (USCG) is implementing Local Area Differential Global Positioning System (DGPS) services at over 50 sites. These broadcast sites will include equipment that complies with the international standards developed by the Radio Technical Commission for Maritime Services (RTCM) and the Broadcast Standard developed by the USCG. The applicable RTCM standards were developed by Special Committee 104 (SC-104) and a SC-104 working group,which focused on DGPS Reference Station (RS) and Integrity Monitor (IM)performance. Installation will include pre-site surveys to determine the state of existing radiobeacon equipment and determine the locations of the antennas that will receive signals from the Global Positioning System (GPS) satellites. The antenna mounting requirements at each site will be unique depending on the local terrain, existing buildings, and other structures. Once the approximate location for the GPS antennas is determined, an optimum mount must be selected. Antenna mounts will either consist of hardware for attaching the antennas to existing buildings or towers, or require new masts unguyed towers). Once a mount design is finalized and approved, the hardware will be ordered and shipped to the site. When the installation crew returns to the site, the antenna mount will be constructed. The GPS antennas will be mounted when they are available (during installation of RS's and IM's).

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1.1 SCOPE

This Standard Operating Procedure provides guidance to the installation teams for selection of GPS antenna locations. In addition it describes the data gathering and processing required for site certification.

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1.2 BACKGROUND

Due to the nature of the GPS satellite orbits and signal structure, there are two main concerns related to antenna placement. Since the satellites appear in all areas of the sky, shading due to hills, buildings, trees, and other obstructions should be minimized as far as practical. Sky blockage below 7.5 degrees of elevation is not a major concern, because corrections will only be broadcast for satellites above this mask angle. However, it should be noted that satellites below the mask angle will be tracked so that corrections will be ready when they rise above 7.5 degrees. In addition, steps must be taken to reduce the effects of multipath. Multipath effects are caused by reflected signals arriving at the antenna. These reflections are referred to as indirect signals. The signal arriving straight from the satellite is called the direct signal. The best case is when the direct signal is much stronger than all indirect signals combined.

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1.3 ANTENNA DESCRIPTIONS

The RS will include an omni-directional L1/L2 GPS receiving antenna. The IM will include an omni-directional L1 GPS receiving antenna and a radiobeacon receiving antenna. A gain pattern has been specified for the GPS antennas which minimizes multipath effects by attenuating the signals received below 7.5 degrees. The gain roll-off from 7.5 degrees to -20 degrees in elevation will be greater than 10 dB. The RS and IM will supply any power or signals needed by the antennas. Both the RS and the IM will function properly with a continuous antenna cable between 5 and 30 meters long without in-line amplification. The antenna assembly will have an internal thread mount of contractor-selected standard size. The mounted antenna assembly will be capable of withstanding sustained winds of 100 mph, and gusts of 135 mph without failing, yielding, loosening, or otherwise degrading or permitting a change of mounted antenna orientation.

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2.0 GPS ANTENNA INSTALLATION GUIDELINES

The following information is provided to assist the installation team in selecting the most reasonable locations for GPS antennas. Four GPS antennas will be located at each broadcast site. Two each for the redundant RS's and IM's. These four antennas will be mounted in pairs at two locations per site. Each pair will consist of one RS antenna and one IM antenna. Under normal operating conditions, the RS antenna at one location will be used with the IM antenna at the other location. This will be done to reduce the similarity in the multipath received at the RS and the IM. The greater the separation between the antennas, the less similarity there will be; the maximum distance practicable should be used. As a general rule, there must be at least a 22 meter distance between the locations of the RS/IM antenna pairs. There may be sites where extraordinary considerations will override this desired separation. An offset in the height of the antennas, even of a few feet, will provide some additional protection. Typically, GPS antennas will be mounted on 10' or 20' (3 or 6 meter)masts. In some cases, higher masts may be required to reduce horizon blockage and multipath effects. These masts must be sufficiently sturdy to keep sway within +/- 8 cm, expected worse case (three sigma).

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2.1 INSTALLATION WITHIN THE RADIOBEACON GROUND PLANE

For installation within the radiobeacon ground plane, one GPS antenna mast will be next to the hut, near a corner away from the radiobeacon transmitting antenna. The second mast site should be selected to avoid digging cable conduit runs across ground plane radials. It should be located as far from the transmitting antenna as the GPS antenna cable will allow. The cable will run down the mast (3 or 6 meters), through the conduit, into the hut, and up to the rack-mounted equipment. The cable run within the hut may require up to 4 meters. With two antennas per mast, an additional meter will be required at the top of the mast, to run out on the yardarm (about 2 meters long). With a 30 meter cable, and a 6 meter mast, this would limit the distance between masts to 19 meters. Therefore, 3 meter masts should be used for the second site ( away from the hut).

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2.2 INSTALLATION OUTSIDE THE RADIOBEACON GROUND PLANE

When the equipment hut is outside the radiobeacon ground plane, the GPS antenna masts should both be sited away from the hut in opposite directions. Allowance must be made for running the cable out of the hut (4 meters), up the mast (3 or 6 meters), and out on the yardarm (1 meter). So the antenna masts should be sited 19 or 22 meters from the hut, depending on the mast height. The masts can be about the same distance from the radiobeacon transmitting antenna.

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2.3 ANTENNA SHADING GPS

Antenna sky blockage should be minimized above 5o. Sometimes it is impossible to mount antennas above all obstructions. The antenna must be mounted high enough so that obstructing projections more than 20o above the horizon are rare and over small sections of azimuth. This includes trees, bushes, hills, buildings, and towers for antennas and power lines. If obstructions are in the vicinity and can't be avoided, then the GPS antennas should be located as far as possible from them. A sky plot, consisting of the direction and elevation of obstructions, will be required at each site. The sky blockage from one antenna on the others must be minimized if masts are of different heights. For antennas 25 meters apart, an offset of 3 meters will not violate the 7.5 degree mask angle. Three meters is about 10 feet, which is the standard height of antenna mast sections (1 meter = 3.28 feet). For a distance, d, between antennas, the vertical offset, y, should be less than the distance times the tangent of 7.5 degrees: y < d x (tan 7.5o) = d x (0.132).

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2.4 MULTIPATH ABATEMENT

The GPS antennas should be located as low as possible, yet still above nearby structures. The obstructions listed in Paragraph 2.1 provide reflecting surfaces that will introduce multipath signals. Mounting the GPS antennas above or away from such obstructions will minimize the effects of multipath. Indirect signals will also be reflected from the earth's surface. The strength of the indirect signals is dependent on the properties of the earth's surface. Wet soil reflects more than dry soil, and water surfaces are very good reflectors. The total energy reflected from the sea's surface is dependent on the sea state. Indirect signals arriving from large distances can still effect the GPS measurements.

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2.5 DISTANCE BETWEEN GPS ANTENNA MASTS

The distance between GPS antenna masts is a trade-off between cable length and reducing the similarity of the multipath environments of the RS and the IM. Due to the limited cable length, the antennas will be mounted relatively close to each other. The IM will notice if the multipath errors are large enough to throw the overall system performance out of tolerance. The IM will detect a problem for a single satellite whenever more than two satellites are being tracked. The site certification process provides reasonable assurance that multipath errors will not typically throw the system out of tolerance.

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2.6 UNDERGROUND OBSTACLES

For selection of GPS antenna sites, one must consider information about underground objects, such as radiobeacon ground radials, power and telephone cables, other utilities, pipes, etc. Antenna masts will be set in concrete footings that will be about six feet deep, and cables will be run in conduit buried one to two feet down. Each mast will be strapped to a ten foot(three meter) grounding rod for lightning protection.

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2.7 RADIO FREQUENCY INTERFERENCE

The GPS antennas must be located to avoid interference from radio frequency transmitters in the vicinity. GPS antennas must be away from the main beam of any communication satellite antennas, outside the cone of any radars, and at least 12 meters away from the radiobeacon antenna (if the site uses a 1000 Watt transmitter).

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2.8 ANTENNA SITE SELECTION RECORD

Once the GPS antenna sites are selected, data will be recorded to justify the site selection. The Site Selection Record will include a sky plot and a site layout sketch. Both plots can be sketched on maneuvering boards. The sky plot will be based on measurements taken every five degrees of azimuth with the compass and clinometer. Measurements should be taken as close to the actual location of the antenna as possible. Record directions using degrees magnetic. Even if a mast is proposed, the measurements should be taken from eye level in a standing position. Calculations can be made back at EECEN to extrapolate to the necessary mast height. The site layout will facilitate this extrapolation. The range to any objects within 50 meters should be measured to the nearest meter. The site layout sketch should include the direction and distance to objects within about 200 meters. Outside of 50 meters, estimates are acceptable. Outside of 200 meters, only objects blocking the sky should be mentioned (these can be noted on the sky blockage plot). Nearby objects of special interest include bodies of water (oceans, bays, lakes, rivers, puddles, pools, retention ponds; ask local point-of-contact if rain water accumulates nearby), beaches, cliffs, buildings, antenna and power line towers, telephone poles, hills, mountains, railroads, fences(note chain link, wood, brick, barbed wire, etc.), roads, guard rails, parking lots, utility boxes, etc. Also note soil type (sand, soil, rocky) and ground cover (mowed lawn, weeds, bushes, trees).

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3.0 GEODETIC MONUMENTS

The National Geodetic Survey (NGS) will install geodetic monuments at the DGPS broadcast sites to support the surveying of GPS antenna positions. These monuments are permanent survey markers. The pre-installation process includes recommending monument locations for NGS to consider. GPS survey antennas will be used from two meters above the monument, so many of the principles described in Section 2 apply. The following paragraphs describe the criteria for selecting survey monument locations. After locating two reasonable locations for monuments, mark these proposed locations on the site layout sketch in accordance with Paragraph 2.8.

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3.1 DISTANCE BETWEEN MONUMENTS

Two monuments are typically selected with significant separation, this reduces the chance both might be accidently destroyed by construction equipment or erosion. It is also desirable that one be in a publicly accessible location, and the other in a secure location where equipment can be left unattended. It is also desirable, but not required, that the monuments be visible from each other so that surveyors may use traditional optical methods to determine azimuths by using both. Distance from the RS/IM antenna masts is best kept under a few hundred meters. One kilometer is acceptable, and 2 km is worst case.

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3.2 SHADING AND MULTIPATH

Horizon obstructions and multiple reflectors should be minimized from the point of view two meters above the monument. This means the view should have open sky more than 10o above the horizon, with few projections above 20o, worst case. Under difficult conditions this can be violated, as NGS surveys can be satisfactorily performed with complete azimuth obstruction up to 25o. The shading by future GPS antenna masts should be considered.

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3.3 UNDERGROUND OBSTACLES

Underground obstacles should be considered. For stability, a 40' how many meters) rod will typically be set beneath the location, and care must be taken not to destroy utilities or radiobeacon ground radials.

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4.0 SITE CERTIFICATION

Once the RS's and IM's are installed, the broadcast site can begin operating. When broadcasting begins, the station health in the RTCM message should indicate unmonitored until the site is certified. Three conditions must be met before site certification is complete, they are:

  • antenna position data available (see Paragraph 4.1)

  • multipath analysis completed (see Paragraph 4.2)

  • IM installed and operational

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4.1 GEODETIC SURVEY

The position of each GPS antenna will be surveyed to determine its exact location within 10 cm. The position survey will use the North American Datum of 1983 (NAD-83). The installation crew must survey the RS antennas' geodetic positions (+/- 10 cm) in order to begin DGPS broadcasting. The positioning survey will be conducted in accordance with the overall installation Standard Operating Procedure. After the installation, NGS will analyze data in order to verify and refine the initial position survey to +/- 1 cm or better.

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4.2 MULTIPATH ANALYSIS

After the RS's and IM's are installed, the RS will be commanded to begin broadcasting and the IM will monitor the broadcast to verify that multipath effects are not corrupting the GPS measurements to the extent that overall system accuracy is outside of tolerance.

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4.3 SITE RECORD

The Antenna Site Selection Record for each broadcast site will be updated to include the site certification data reports (surveyed antenna position and multipath analysis). In addition, sky blockage plots and site layout sketches will be updated after antenna masts are installed. Thus, the final Site Record will have sky blockage plots taken from the point of view of the GPS antennas, and site layouts that include the antenna masts and the survey monuments.

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