Monumentation, Instrumentation and Calibration Guidelines

The selection of a type of geodetic monument is guided by requirements for durability and long-term stability. Monuments are the physical objects to which geodetic measurements are made and the resulting coordinate values assigned. Usually they are designed to support physical access to the geodetic reference with either classical instruments or GNSS antennas. Geodetic monuments are preferably anchored to bedrock to minimize the impact of local soil instabilities. Tablets (or forced centering plates) are often mounted on the monuments to facilitate repeated centering of measuring instruments for high-accuracy applications. The cost of geodetic-quality monuments can vary greatly, from a few hundred to a few thousand dollars, depending on local geology and the need for deep drilling.

As seen in Table 1, guidelines for survey control monumentation have been developed by a number of CGRSC agencies. Recommendations for GNSS instrumentation and ancillary equipment have also been made available by a number of scientific organizations. The University NAVSTAR Consortium (UNAVCO) and the International GNSS Service (IGS), who monitor thousands of GNSS reference stations distributed globally, have published detailed information on monumentation. These can be found at the UNAVCO Knowledge Base on GNSS Station Monumentation and IGS Monumentation Design and Implementation Recommendations websites.

Instrument selection for a control survey depends mainly on precision requirements and the extent of the project area. For legal or engineering surveys within a municipal area, instruments such as total stations and spirit levels may be adequate. Technical specifications about such instruments are readily available from manufacturers and recommended observing methodologies described in land survey standards, such as those found in Table 1.

For geodetic-quality surveys over larger areas, GNSS technology is usually preferred. In this instance, instrument selection is critical to attain high precision. As a differential positioning methodology is usually employed, reliable information on the GNSS tracking equipment at the reference site is critical. This is particularly important when using GNSS corrections from an active control point operated by another agency. For real-time navigation, the reliability of the communication link to the reference station or network can also have significant impact on productivity.

Active control stations consist of one or multiple GNSS receivers and antennas along with ancillary power, computing and communication equipment for continuous operations. While the type of hardware installed is more or less standard, redundancy and configuration can vary significantly. The reliability and integrity of the measurements are the main criteria that drive reference station design. In general, RTK reference stations host a single GNSS receiver as user-corrections are usually derived from a multi-station network solution. Hosting multiple antennas and receivers is more common at stations that require autonomous operations and integrity monitoring for safety critical applications, such as Coast Guard DGPS and WAAS reference stations.

Table 2 lists the various components that may be part of continuous and autonomous GNSS tracking stations. Details about the various makes and models of geodetic-quality GNSS receivers can be found at UNAVCO or manufacturers websites.

Table 2 Components of GNSS active control stations

Component	UNAVCO Knowledge Base link
Receiver	http://kb.unavco.org/category/gnss-and-related-equipment/gnss-receivers/84/
Antenna	http://kb.unavco.org/category/gnss-and-related-equipment/gnss-antennas/14/
Antenna mount	http://kb.unavco.org/category/gnss-and-related-equipment/gnss-antenna-mounts/23/
Meteorological sensor	http://kb.unavco.org/category/gnss-and-related-equipment/meteorological-systems/85/
Power system	http://kb.unavco.org/category/gnss-and-related-equipment/power/93/
Enclosure	http://kb.unavco.org/category/gnss-and-related-equipment/equipment-enclosures/17/
Data communication	http://kb.unavco.org/category/gnss-and-related-equipment/comms-and-networking/55/

Calibration of electronic distance measuring (EDM) instruments and GNSS antennas may still be required for the most demanding users. While EDM calibration baselines and GNSS calibration basenets are only being maintained by a few CGRSC agencies, historical information remains available in some jurisdictions. Table 3 provides links to online information available in Alberta, British Columbia and Quebec.

Table 3 EDM Calibration Baselines and GNSS Calibration Basenets

Calibration	Province	Locations	Reference
EDM	AB	Calgary, Edmonton, Grande Prairie, Lethbridge	Electronic Distance Measurement (EDM) calibration baseline lengths
	BC	Cranbrook, Prince George, Surrey, Vernon	Cranbrook Calibration Baseline 1996 Adopted Values Prince George Calibration Baseline 1999 Adopted Values Surrey Calibration Baseline 1999 Provisional Values Vernon Calibration Baseline 1997 Adopted Values
	QC	Chambly, Saguenay, Mont-Joli, Neuville, Port-Cartier, Terrebonne, Trois-Rivières et Val-d'Or	Bases d'étalonnage pour télémètre électronique (French only)
GNSS	AB	Edmonton	Edmonton GPS Validation Network
	AB	Calgary	Calgary GPS Validation Network
	BC	Greater Vancouver	Greater Vancouver GPS Validation Network
	BC	Okanagan	Okanagan GPS Validation Network

To achieve sub-centimetre accuracy with GNSS technology, antenna calibrations continue to be required. Absolute antenna calibrations are usually estimated using robots that rotate the antenna to measure phase center offsets (PCOs), as well as the phase center variations (PCVs) as a function of azimuth and elevation of the incoming signal. A few specialized centers such as Geo++ and NGS have played a leadership role in the calibration of geodetic quality antennas used in the global IGS network. The ANTEX format has been developed to disseminate calibration values to end-users in a standard format.