GPS SKYGPS SKY

What is the Sky Doing

Today?

ObjectiveObjective

Summary of TopicsSummary of Topics

- Importance of Planning

- Review how GPS Calculates Positions

- Check Out The Sky

- Planning Ahead

Planning the GPS DayPlanning the GPS Day

•Recreational GPS is very forgiving

•It’s for fun! Not for worrying about

PDOP,SNR,Mask Angle,blah blah…

• Designed to Work inside your Pocket!

•Still important - as will be demonstrated

Tracking a GPS Satellite Vehicle (SV)

One of 2912,000 miles up6000 mph

JTRACK

http://liftoff.msfc.nasa.gov/RealTime/JTrack/3D/JTrack3D.html

How to Calculate a PositionHow to Calculate a Position

Measure the Distance to the Satellites

How to Calculate DistanceHow to Calculate Distance

Speed of Light x Travel Time

Distance

Time signal left satelliteTime signal left satellite

Current timeCurrent time

Minimum # of Satellites RequiredMinimum # of Satellites Required4 Unknowns Require 4 Satellites

time ...………………(1) satellitelatitude ……………. (1) satellitelongitude .…………. (1) satelliteelevation …..…….…(1) satellite

4 Satellites = 3D Good3 Satellites = 2D BAD A Garmin will A Garmin will

allow 2D allow 2D positioning! positioning! Watch Out!Watch Out!

- Want tetrahedron as large as possible- Want index as low as possible

PDOPPosition Dilution of Precision

Garmins View of Mapping

Typical Mapping Typical Mapping Grade GPS Grade GPS

SettingsSettings

- PDOP = 99

- SNR = 2

- Mask Angle = 5 ºº

PDOP = 6PDOP = 6

SNR = 6SNR = 6

Mask Angle = Mask Angle = 15º15º

Rules of Thumb

PDOP of 6 or Less - GoodDOP of 3 or Less - Good

PDOP / DOPPosition Dilution of Precision

Garmin only Garmin only Shows Shows DOP DOP

That’s 1/2 That’s 1/2 of PDOPof PDOP

Tools for Predicting - Tools for Predicting -

See PDF on How To:CD</software/TrimblePlanSoftware

Almanac File::

Software:

From CD </software/TrimblePlanSoftware or http://www.trimble.com/planningsoftware_ts.asp

PDOP and Visibility GraphsPDOP and Visibility Graphs

Instructor Runs Trimble Planning Software for Today

PDOP and Visibility GraphsPDOP and Visibility Graphs

It’s Your DecisionIt’s Your Decision

• Plenty of satellites

• In a hurry to go

• Limited control

• Must collect now

Reasons to planReasons not to plan

• More efficient

• More economical

• Forewarned

Plan AheadPlan Ahead

• External Antenna

• Wait till Good Sky Returns

• Offsets

• Map Using other Means?

Steps You Can Take

17

Coming Up– GPS Sky– Garmin Setup – Get Outside!

Break