Differential GPS Req.

MagSurvey Software Differential GPS Requirements

GPS -- The GPS receiver must be of good quality with GGA and VTG output sentences available. If VTG is not available, RMC may be used in its place. If configurable, limit the updates to include only GGA and VTG (or RMC) sentences. Most GPS receivers automatically output updated sentence groups once per second, which is adequate for most surveys. If your GPS is capable of updates faster than once per second, it is advised to limit the updates to a maximum of four times per second.

If configurable, it is recommended that the NMEA 0183 communications rate be set at the standard 4800 baud, N,8,1. A PC’s RS-232 port will usually work well with the NMEA-0183 RS-422 signal levels, if true RS-232 levels are not available from the GPS. See the notes below regarding use of USB GPS units, or USB-to-Serial adapters.

Use of full time differential corrections is strongly recommended in order to prevent plotting data that might become displayed and stored some distance from its proper position. Accurate GPS positions can also indicate "holes" in the scan pattern. MagSurvey can now be set to require, or to not-require, differential corrections. It is common to use GPS receivers having built-in WAAS differential corrections -- including Garmin ( www.garmin.com ) , and Magellan ( www.ashtech.com ). Where radio beacon differential will suffice, Leica offers the compact apMK31 "smart antenna" ( www.leica-geosystems.com ). Beware of Garmin receivers that have only "Garmin proprietary" sentences. MagSurvey is not configured to use these sentences, as they do not contain all of the information required to calculate sensor lay-back for boat-towed operations.

Differential Correction Source (DGPS) -- If not already built into the GPS receiver, the use of an external differential correction receiver can be used.

	The FAA’s Wide Area Augmentation System (WAAS) is widely used. This system (http://gps.faa.gov) uses the existing antenna and hardware of many GPS receivers to receive corrections from one of two above-equator, geo-synchronous satellites. A number of low cost WAAS-enabled GPS receivers are available. The operator should enable WAAS corrections unless another correction source is connected and is preferred for use. WAAS coverage is available for the United States. Other compatible systems are available in Europe (EGNOS), Canada (CWAAS), and Japan (MSAS).
	Reliable differential corrections are available in many areas from Coast Guard differential beacon stations and require a suitable Differential Beacon Receiver.
	For higher accuracy, satellite-based corrections are available in some regions through subscription from Rockwell’s Racal and with the StarFix or SeaStar systems, through the John E. Chance, Inc. division of Fugro, at a price premium. The OmniSTAR division of Fugro also provides satellite-based corrections by subscription, but restricts use to land-based operations.
	A dedicated GPS base station transmitting real-time differential corrections over radio frequencies might also be implemented where surveys of exceptionally high positional accuracy are required.

Recent Advances in GPS Receiver Technology -- Recent GPS chip sets such as the SIRF Star III have improved satellite reception and satellite-acquisition times. This particular chip set is being built into several marine and hand-held GPS receivers. For land surveys or for highly portable configurations, the miniature, display-less, GlobalSat BU-353 (USB version) is available at low cost. Be sure to acquire a model that has WAAS enabled by default -- likely requiring firmware versions 3.1.1, and above. If using the BU-353 with a hand-held magnetometer, the magnetic mounting pad should be peeled-off and removed, and it is recommended that the unit be positioned on a pole above the head, or attached to the top of a hat. Information is available at www.usglobalsat.com, and units can be purchased on eBay or Amazon.com.

If your computer has only USB inputs available and your GPS outputs in Serial RS-232 or NMEA RS-422, you will need to use at least one USB-to-serial adapter (use different models, if two are used). This requires installing the adapter's driver software, and then determining the emulated comport and baud rate from the manufacturer's literature -- or by checking the computer's Control Panel -> System -> Hardware -> Device Manager -> Ports (Com & LPT). It is important to install the driver software according to the developer's instructions, and this usually involves installing the driver before plugging-in the equipment.

Perception Systems, Incorporated

8117A Northboro Ct, West Palm Beach, FL 33406 USA

Tel: 561-433-0033 Email: info@magsurvey.com