# Presentation: 487882

## GPS SKY

**What is the Sky Doing Today?**

## Objective

## Summary of Topics

**-  Importance of Planning**
**-  Review how GPS Calculates Positions**
**-  Check Out The Sky**
**-  Planning Ahead**

## Planning 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**

## JTRACK

**Tracking a GPS Satellite Vehicle (SV)**
**One of 29**
**12,000 miles up**
**6000 mph**

**JTRACK**

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

## How to Calculate a Position

- Measure the Distance to the Satellites

- Review

**Notes:**
Review why it’s so important to keep track of the number of satellites and the PDOP. 

First, # of satellites. The question is what is the minimum number of satellites required for the GPS receiver to calculate its location in terms of latitude, longitude and elevation.  

Remember how the receiver determines its location - it measures the distance to a number of satellites.  There’s only one place we can be based on all of these measurements.   

## How to Calculate Distance

**How to Calculate Distance**

- Speed of Light
- _x   Travel Time_
- Distance

**Time signal left satellite**

**Current time**

- Review

**Notes:**
However, the distance is not measured directly, it’s also calculated.  That calculation is based on measuring time.  

Clocks inside the GPS receiver are very inaccurate compared to the clocks inside the satellite.  So essentially we have another unknown.  

All together we have 4 unknowns:  time, latitude, longitude and elevation.  
In 1 microsecond the signal travels about 300 meters.  

## Minimum # of Satellites Required

_**4 Unknowns Require 4 Satellites**_

**time  .....................****	****(1)****	****satellite**
**latitude  ................****	****(1) ****	****satellite**
**longitude ..............****	****(1) ****	****satellite**
**elevation ...............****	****(1) ****	****satellite**

**4 Satellites = 3D ****Good**

**3 Satellites = 2D ****BAD**

- A Garmin will allow 2D positioning!  Watch Out!

**Notes:**
Now, think of solving for all of these unknowns as an algebra problem.  In other words we need one equation for solve for one unknown, 2 equations for 2 unknowns, etc.  So if you think of each satellite as providing an equation, then that means we need a minimum of 4 satellites to calculate the latitude, longitude and elevation of our position.  

Please note that I use 1st, 2nd, 3rd as a way of counting satellites.  I do not mean that if you had only 2 satellites you would at least have determined your latitude.  

## PDOPPosition Dilution of Precision

- -	Want index as low as possible

**PDOP****Position Dilution of Precision**

**Notes:**
Low as possible:  Your Garmins estimate DOP.  Not PDOP

## Garmins View of Mapping

- Typical Mapping Grade GPS Settings

**- PDOP = 99 **
**- SNR = 2**
**- Mask Angle = 5 **o

- PDOP = 6
- SNR = 6
- Mask Angle = 15o

- Garmin Says...
- GO FOR IT!

**Notes:**
Low as possible:  Your Garmins estimate DOP.  Not PDOP

## PDOP / DOPPosition Dilution of Precision

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

**PDOP / DOP****Position Dilution of Precision**

- Garmin only Shows DOP
- That’s 1/2 of PDOP

- Garmin III Plus
- Screen

**Notes:**
Low as possible:  Your Garmins estimate DOP.  Not PDOP

**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 Graphs

- _[Instructor Runs Trimble Planning Software for Today](file://AKROGIS/jjcusick/04GPSTraining/GPS4Ologists/Presentations/Shortcut%20to%20Planning.lnk)_

## PDOP and Visibility Graphs

## It’s Your Decision

**Plenty of satellites**
**In a hurry to go**
**Limited control**
**Must collect now**

**Reasons to plan**

**Reasons not to plan**

**More efficient**
**More economical**
**Forewarned **

**Notes:**
Validate that it’s going to be nearly impossible to find a time that Garmins won’t be able to collect.  That doesn’t mean it will never happen and this may help you to identify when you’re already in a marginal situation so that any additional obstructions will mean that you can’t collect.  

## Plan Ahead

**External Antenna**
**Wait till Good Sky Returns**
**Offsets**
**Map Using other Means?**

**Steps **_**You**_** Can Take**

**Notes:**
Plan Ahead  

## Coming Up
GPS Sky
Garmin Setup 
Get Outside!

**GPS Sky**
_**[Garmin Setup ](file://AKROGIS/jjcusick/04GPSTraining/GPS4Ologists/Presentations/03_NavSetup.ppt)**_
**Get Outside!**

**Break**

**Notes:**
break