2. Air Operations Branch Director Course
U.S. AIR FORCE
AUXILIARY
Air Operations Branch Director Course
Navigation Fundamentals & Understanding GPS for Sortie Planning

3. Goals for understanding
Prepare mission staff to be able to plan and brief missions that can be executed efficiently and conveniently with our equipment
Navigation fundamentals
Capabilities and limitations of GPS
Capabilities of Apollo GX55 GPS units in Minnesota Wing aircraft
What information is needed to program the GPS unit for search patterns
What can and cannot be programmed in the GX55
What can be “improvised”
Learn about the “gotchas” using GPS

4. PART 1 Navigation fundamentals
Latitude and longitude
Describing directions

5. Latitude Parallels Measure How Far North or South of Equator
Zero Degrees is Equator
“90 Degrees North” is the North Pole
“90 Degrees South” is the South Pole
North Latitudes
Equator
South Latitudes

6. Longitude Longitude Half Great Circles Intersecting at the Poles
West Longitude
Prime Meridian
East Longitude
Longitude
Half Great Circles Intersecting at the Poles
Measure How Far East or West of England
Zero Degrees is Prime Meridian (England)
Numbers between 0 and 180 are either East or West Longitude
180 Degrees is opposite side of globe from England
near international date line in Pacific Ocean

7. Latitude and Longitude
Latitude is Based on Earth’s motion
Axis of rotation defines poles and Equator
Longitude is Arbitrary
Greenwich, England was chosen for ‘prime meridian’.

8. Where’s Minnesota? Twin Cities The Northwest corner of the state:
Northern Minneapolis is at 45 Degrees North (half way between the Equator and North Pole!)
93 Degrees West is roughly the Eastern edge of St. Paul.
The Northwest corner of the state:
Exactly 49 Degrees North
Roughly 97 Degrees West

9. How Big is One Degree?
Degrees of Latitude are always the same distance apart, about 60 Nautical Miles
Degrees of longitude vary in distance – near the poles the lengths are quite small.
In Minnesota, a degree of longitude is about 40 to 44 Nautical Miles across

10. Dividing Degrees into Smaller Units
A ‘Minute’: 1/60th of a Degree
roughly a mile in size
Minutes are usually broken down into tenths of minutes
Alternatively, a ‘Second’ is 1/60th of a minute

11. Expressing Latitude and Longitude in Degrees and Minutes
Small high circle after number denotes degrees
Apostrophe after number denotes minutes
Example: Minneapolis Flying Cloud Airport
44o 49.63’ N 93o 27.43’ W read as… 44 degrees minutes North 93 degrees minutes West

12. Expressing Latitude and Longitude in Degrees, Minutes, and Seconds
A double-quote after a number denotes seconds
Example: Minneapolis Flying Cloud Airport
44o 49’ 37.8” N 93o 27’ 25.8” W read as… 44 degrees 49 minutes 37.8 seconds North 93 degrees 27 minutes 25.8 seconds West

13. Determining Coordinates from a Chart

14. Expressing Direction: The Compass Rose
360
330 30 N
300 60
270 W E 90
240
120
Directions are expressed as a number from 001 to 360 S
210
150
180

15. Magnetic Variation
The Magnetic North Pole is located in Canada, not at the True North Pole.
The difference in direction between the two poles is measured and referred to as magnetic variation

16. Magnetic Variation in the US
-15º
+15º
+20º
-10º
-5º 0º
+5º
+10º
Easterly Variation
Westerly Variation
Note: These lines move over the years because the magnetic north pole is in motion
Agonic Line

17. PART 2 - GPS Overview of GPS for SAR
Motivation
Potential uses for SAR
How it works
Accuracy
Limitations and Gotchas
Terminology
Describing search patterns for the GX-55
Improvising

18. Motivation GPS is a powerful tool for search and rescue
Allows very precise search patterns
Makes manageable what would otherwise be very difficult patterns
Expanding square at any angle
Creeping line along a course
Offset route searches
Grid searches over indistinct terrain
GPS should be our primary tool for Search and Rescue navigation
GPS is of course also very handy for general navigation

19. Potential uses of GPS for Air and Ground SAR
Getting to/from a search area
Going to a location designated on a map
Going to a site identified by another SRU
A ground team going to a site previously identified by an aircraft
Navigation while conducting a search pattern
Clue logging (and re-finding)

20. GPS for SAR: Potential advantages
More flexible search area partitioning
More accurate logging
More accurate search lines
Easier and more accurate communication of location information

21. Other GPS SAR Uses Electronic distress signals
PLBs – Personal locator beacons
ELTs – Emergency locator transmitters
EPIRBs – Emergency Position Indicating Radio Beacons
New 406 MHz digital beacons sometimes transmit GPS coordinates
GPS then becomes a tool for both the rescuer and the rescued

22. How it Works – The Basics
Spaced-based system (unlike Loran or VOR)
‘Constellation’ of 24 satellites in six orbital planes
21 active satellites plus 3 operating spares
In “High” orbit of about 12,000 miles
Each circles the Earth about every 12 hours

23. How it Works – The Basics
GPS satellites transmit information
“Pseudo-random” code with time information
Satellite orbital position data
“Almanac” data
“Ephemeris” data
Updated atmospheric models
GPS receiver uses this data to figure out what time it is and what time the signals were sent

24. How it Works – The Basics
GPS receiver measures distance to satellites by determining the amount of time that the radio signal takes to travel from each satellite
Each distance measurement effectively defines a sphere around a satellite
Multiple satellites must be used to determine a position
Given two satellites, two sphere intersect to determine a circle
Given three satellites, a sphere and a circle intersect to determine two points
A fourth satellite can determine a positive 3D position

25. Accuracy A complex question The short story
DOD has a 66 page document describing the performance of GPS Standard Positioning Service (SPS)
The short story
Garmin states that their GPS receivers “are accurate to within 15 meters on average”
Typically about 6 to 12 Meters accuracy can be seen

26. Accuracy
Accuracy and reliability is actually a complex subject. There are many factors that can impact system.
Receiver errors
Atmospheric (ionosphere) errors
Solar activity (sun spots and solar storms)
Location of receiver
(some parts of the Globe get better coverage than others)
Orbital errors (inaccuracies in the reported orbital position)
Poor satellite geometry (satellites lined up or bunched up)
Limited number of satellites in view
Satellite malfunctions (or satellites taken out of service)
“Multi-path” errors (radio signal reflections)
Results vary hour by hour, day by day

27. Accuracy
There is a substantial difference between typical accuracy and worst-case accuracy
Described as a statistic: x% had an error of y meters or less
Typical 95% horizontal error results for a typical day:
Global average
7.8 – 8.3 meters
Average for worst site (on globe)
meters
Errors worse than 30 meters are possible given the potential for various atmospheric conditions and receiver faults and the possibility that satellites can be taken out of service

28. Accuracy and “Selective Availability”
Past feature of GPS SPS that purposefully degraded accuracy of position determination for non US-military use
Civilian accuracy was typically about 100 meters under Selective Availability
SA was discontinued May 1, 2000
Has not been used since
It's been replaced by “selective deniability,” which allows the US military to geographically designate areas in which to degrade GPS quality.

29. Comparing Accuracy with and without SA – A sample
The plots show that SA causes 95% of the points to fall within a radius of 60.7 yards. Without SA, 95% of the points fall within a radius of 7.9 yards.

30. Comparing Accuracy with and without SA – Recap
Before, with 100 meter typical accuracy allowed you to identify what stadium you were in
Now with 6-12 meter typical accuracy, you can tell about which yard line you are on

31. Accuracy – WAAS Wide-Area Augmentation System
Designed specifically for aviation
Commissioned by FAA in 2003
Uses ground stations and satellites
25 ground reference stations cover the entire US and parts of Canada and Mexico
Augments GPS Standard Positioning Service
Provides better integrity and accuracy
Typical accuracy of 3-5 meters horizontal, 3-7 meters vertical
Can be used for precision approaches

32. Accuracy – WAAS How it works:
Two master stations, located on either coast, collect data from the 25 reference stations and create a GPS correction message.
This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere.
The corrected differential message is then broadcast through one of two geostationary satellites, and is then received by a WAAS-capable GPS receiver.

33. Accuracy – Differential GPS
Provides high accuracy for a small area
Uses a local ground station transmitter
Accuracy can be better than one inch

34. Limitations Requires good line-of-sight to satellites
May occasionally have difficulty using GPS in vehicles
Generally unusable inside a building (or cave)

35. Other concerns
GPS can be subject to accidental and intentional interference
Easily jammed using strategically placed low-power transmitters
WAAS is also at risk
Selective Availability (SA) or SCATANA could be instituted during a national emergency (but this is unlikely)
Current plan calls for possible use of “Selective Denial” where GPS is degraded or denied to specific geographic areas

36. Gotchas
True vs. Magnetic directions (a configuration option in some units)
The Apollo GX 55 will always use magnetic directions
Batteries!
The need for training and practice
Expressing coordinates
Seconds vs. Decimal Minutes

37. Terminology
Describing places
Describing directions

38. Terminology: Waypoint
A specific named location either defined by the user or defined in the instrument’s database
Waypoints sometimes come in flavors:
User defined waypoints
Built-in database waypoints (example: an airport)

39. Terminology: Defining Different Directions
Desired Track / Course
Bearing
Track
Heading
** In general, you should take note whether your GPS is giving you directions as True or Magnetic directions
** The GX-55 always gives Magnetic directions

40. Describing Locations A Choice to Understand
We describe latitude and longitude normally using degrees and minutes
When dealing with fractions of minutes there is, however, a choice
There are essentially two options:
One can use seconds (of which there are 60 in one minute)
One can use decimal-minutes (i.e. tenths and hundredths of a minute)
Many GPS units can be configured to display one way or the other

41. Describing Locations Yet another option
Sometimes, latitude and longitude are expressed in degrees only.
Thus the following are equivalent expressions of longitude:
93o 20’ 00” o

42. Describing Locations CAP Standard Method
The standard we will use in CAP is degrees and decimal minutes
Example:
45 degrees 35.4 minutes North
93 degrees 42.2 minutes West
This is the standard way the Air Force provides coordinates to us for search and rescue
This is also the way our GX-55 normally displays position information
In CAP we will not usually use “seconds” unless working with another agency that wishes to do so.

43. Describing Locations: Communicating with Others
The seconds vs. decimal minutes question is a big source of confusion even within single organizations
Some people erroneously say “seconds” when they mean “hundredths of a minute”
Some people say “point” or “decimal” when they should have said “minutes” and “seconds”
Take nothing for granted when getting information
Be accurate and clear when giving information
Especially when working with other-agencies, triple-check all coordinates to make sure we’re all speaking the same language
Some organizations normally uses Degrees-Minutes-Seconds as their standard way of describing positions, but they will use the word “decimal” or “point” to separate the three parts of the coordinate

44. Describing Locations Yet Another Approach to be Aware of
Some GPS units also offer the option to display position information using Universal Transverse Mercator” (UTM)
An alternative to using degrees and minutes
Beyond the scope of this course

45. GX-55 Search patterns Describing the patterns Parallel Line (Grid)
Creeping Line
Expanding Square
Route and offset route

46. Parallel Line Search Pattern a.k.a. “Grid Search”
“US Grids” are areas 15 minute to a side, serially numbered for each sectional chart (MSP means “Twin Cities” sectional)
7.5 minute quarter grids are named A, B, C, and D – in reading order, left to right, top to bottom
’ W o
’ W
’ N
’ N ’ ’
414
445
446
MSP
413 A B C D A

47. Parallel Line Search Pattern a.k.a. “Grid Search”
The GPS unit labels the four corners (and also four search pattern entry points) 1, 2, 3, and 4 – clockwise starting in northwest.
’ W o
’ W
’ N
’ N 1 2
MSP
413 A 4 3

48. Describing a Parallel Line Pattern
Grid identifier and starting waypoint (i.e. corner of quarter-grid)
Track spacing
Direction of tracks – either E/W or N/S
Note: Search area for a parallel line pattern in the GX55 is always a quarter-grid

49. Parallel Line Pattern Example
Quarter Grid MSP 413A1
Northwest corner of grid MSP 413A
Track spacing 1 NM
Tracks running east to west
MSP
413 A
Track spacing

50. Creeping Line Search Pattern
Shaped much like a parallel line pattern, but with legs aligned perpendicular to a route (rather than by ordinal directions)
Descriptive parts:
Starting point (any waypoint)
Direction
Starting left or right side
Track spacing
Leg length
Number of legs
Direction
Track spacing
Leg length
Starting Waypoint
Starting on left side

51. Expanding Square Descriptive parts
Starting waypoint (at center of pattern)
Initial direction
Track spacing
Number of legs 3s s s 2s s 2s 3s
Initial Direction

52. Expanding Square Descriptive parts
Starting waypoint (at center of pattern)
Initial direction
Track spacing
Number of legs 3s s s 2s s 2s 3s
Initial Direction

53. Offset Track Line Search a.k.a. Route Search
For a returning route search, 2 passes are made, each ½ track spacing offset from base course
A non-returning route search will have a leg on the base course
TO Waypoint
Base Course
FROM Waypoint

54. Offset Track Line Search a.k.a. Route Search
For the purpose of using the GPS, we can program only one leg at a time
A route leg will either be…
On the course line (in which case a simple flight plan will suffice)
Offset from the course line
Described as a distance offset and a direction offset (either right or left of course)

55. Improvising
As a last resort, any GPS can display running latitude and longitude
The crew fly the pattern watching the numbers
This can be used to guide an arbitrary grid search
Example usage: we need to fly a grid which has non-standard boundaries
This generally requires additional preflight preparation (to predetermine the lines of latitude and longitude)