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 offer 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.

Satellite Based Augmentation Systems (SBAS) including the FAA’s Wide Area Augmentation System (WAAS) and other Global Navigation Satellite Systems (GNSS) can be used.  These systems use the existing antenna and hardware of many GPS receivers to receive corrections from above-equator, geo-synchronous satellites. A number of low cost SBAS-enabled GPS receivers are available.  The operator should enable SBAS corrections unless another correction source is connected and is preferred for use.

Reliable differential corrections remaining available from Coast Guard differential beacon stations can be used 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 MTK II have improved satellite reception and satellite-acquisition times.  This particular chip set is being built into several commercial GPS receivers.  For land surveys, or for highly portable configurations, the miniature, display-less, QSTARZ BT-Q818X is available at low cost and offers both bluetooth and USB interfaces.  Select the switch position for one reading per second.  If using the BT-Q818X with a hand-held magnetometer, a Body-Glove case can be acquired to clip the GPS unit to the top of a ball cap or to a pole extending above the head.  Information is available at www.qstarz.com, and units can be purchased on eBay or Amazon.com.  Note that some GPS units tend to go into a power-saving standby mode and repeat stale positions if the operator does not remain in motion throughout the survey.

If using a bluetooth GPS receiver, follow the manufacturer's and Microsoft's setup information to link it to the PC -- the appropriate software driver will usually be installed automatically by your Microsoft operating system.  If your PC does not have bluetooth capability, a small, inexpensive bluetooth wireless adapter can be purchased to plug into a USB port.  The bluetooth units also emulate a standard serial port, and you will need to find and configure the comport and baud rate, as below.

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 (if you use a second one for the magnetometer -- use different models of USB/RS232 adapters).  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.