Avoiding the Common GPS Pitfalls: “GPS Best Practices” Jay Riester Mapping Tech Support

Avoiding the Common GPS Pitfalls:

“GPS Best Practices”

Jay Riester

Mapping Tech Support

Equipment Setup

• Current OS and GPS Firmware installed.

• Current Date and Time on the data collector.

• Data collector is fully charged.

• Turn off WiFi and Bluetooth to conserve battery.

• Automobile charger and serial clip for charging.

Current 2005 Geo OS and GPS FWStart/Settings/System/System Information

Current 2008 WM 6.0Geo OS and GPS FW

Start/Settings/System/System Information

Current 2008 WM6.1Geo OS and GPS FW

Start/Settings/System/System Information

Logging Settings

• 1 second interval is default.• Auto Pause Count for point/vertex.• Horizontal/Vertical and In the Field/Post

Processed accuracy settings.• Post Processing Distance from Base.• Accuracy based logging.• Estimated Accuracy icon on the status bar (click

on it to display).

Open conditions: Sub-foot to Decimeter

Multipath

• Occasionally multipath errors still occur.• Estimated error doesn’t always include multipath.• Zephyr Antenna has better multipath rejection (Stealth technology).

Best Practices: In the field

Minimizing multipath:– Multipath is like TV ghosting, caused by reflected

signals adding errors to the distance measured to the satellite.

– Multipath is worst when you collect GPS measurements in only a single location (point features).

– Multipath tends to cancel out when you collect GPS measurements while moving.

Best Practices: In the field

Collect good GPS measurements between features

• Maintain the GPS unit in an upright position and don’t drop the GPS unit to your in side in between collecting data features.

• Keep the GeoExplorer unit “high”.

• Face antenna to the Southern Sky when using unit without external antenna.

• Hold the antenna horizontal while walking.

• Don’t drop the antenna to your side (or it will pick up multipath).

Best Practices: In the fieldCollect good GPS measurements between features:

When you collect good carrier measurements before and after each point feature. Sub foot processing can use all the information to minimize the effect of multipath at the point feature.

Subfoot uses all these measurements to improve accuracy.

Best Practices: In the field (cont’)

Collect positions for at least 30 seconds at each point feature:– In theory, you don’t have to collect too many positions

in a point feature, once the accuracy indicator readings are good enough.

– But if you collect for about 30 seconds, you increase the chance of acquiring more satellites or cancelling out short-term multipath errors.

– If the accuracy indicator readings are poor, you should log more positions.

– This will typically be necessary in a hostile GPS environment.

Best Practices: In the field (con’t)

Antenna position– Can you really hold

the antenna still directly over the target?

– Use a pole, bracket, and plum line or bubble.

Log Now or Later

• Log Now – start a feature and simultaneously start collecting GPS position.

• Log Later – start a feature, enter attributes and start collecting GPS positions later. Use this for

lines and areas too. Also best for points if not using an external antenna.

Best Practices: In the field

Pause logging before entering attribute information:

– Allows you to concentrate on positioning:• Holding the antenna over the location to be

measured.• Holding the antenna horizontal with a good view of

the sky.

– It is not easy to do this while concentrating on entering attribute information.

Best Practices: In the field

– Avoid blocking satellites with your own body by using an external antenna and pole.

– Monitor your “In The Field Accuracy”.– The majority of satellites are between you

and the equator.– Be aware of surrounding Obstacles.

Denver, USA Frankfurt, Germany Christchurch, New Zealand

Planning Sky plot View future satellite

constellation, including trajectories and trails.

Use time in the field more efficiently

Increase productivity

In Field Planning

Best Practices: In the field

Buildings or other obstructions– Wait for stable carrier lock. – Avoid blocking satellites.– In difficult locations, move the antenna slowly.– Use the Planning software to see if better DOP will be

available later.– Use offset measurements or Laser Rangefinder.

Under trees: Not always subfootDepends on: • Number of satellites tracking carrier (5 plus).• Carrier lock duration (2 minutes).• Distance to base station (50 miles).

Best Practices: In the field

Trees and canopy

– Use an external antenna on a pole.– Wait for a stable lock before entering canopy

environments.– Collect data before and after entering canopy

environments (carrier data that is free of cycle slips).

– Use the Planning software to see if better DOP will be available later.

– Use offset measurements or Laser Rangefinder.

Light Canopy

Heavy Canopy

Best Practices: In the field

Difficult environments– Losing lock often

Sometimes it may not be possible to get a carrier solution

– In this case, the code solution will give better results than the carrier solution when you process your data.

– Use “Automatic” carrier/code processing (Office).– Turn off H-Star in the field (eliminates cycle slips).

• Real-time Subfoot requires carrier correction– Beacon & WAAS are code corrections (not sub-foot).

• Real-time Subfoot requires L1/L2– Hurricane antenna is L1, so not sub-foot in real time.– Hurricane antenna is sub-foot after post-processing.

• For GeoXH 2008, real-time Subfoot processing is done in the application, not in the receiver– So NMEA & TSIP are not sub-foot.– Need to use Trimble software:

TerraSync, GPScorrect, Pathfinder Tools and connection to a VRS via a cell phone w/data plan or a MiFi pod.

Real-time Subfoot operation

Thank You

Jay Riester

Mapping Tech Support

[email protected]

http://solutions.seilerinst.com

Questions?