2. Air Operations Branch Director Course
U.S. AIR FORCE
AUXILIARY
Air Operations Branch Director Course
Planning Air-to-Ground and High-Bird Communications Operations
Calculating Line of Sight

3. The Problem VHF Communication is limited to line of sight
An aircraft at altitude has a great advantage over ground-based radio stations, but it is limited

4. Caveat VHF is basically line-of-sight limited, but…
VHF Radio signals do actually bend slightly towards the Earth surface, so actual reception range is about 15% better than geometric line of sight

5. General Factors Impacting Range
Terrain
Signal strength / radio condition
Weather

6. Related Problem ELT Airborne Reports
When prosecuting an ELT search, we may get reports of aircraft that did or did not hear the ELT
We can take into account the altitude of the reporting aircraft and draw circles on a map representing line of sight

7. Related Problem ELT Airborne Reports

8. ELT Airborne Reception Range
Altitude AGL (1000 ft)
Line of sight (NM) 1 36 2 52 3 63 4 72 5 81 10
114 15
140 20
162 25
180 30
198 35
214
Variables
Signal strength (recall that, at best, ELTs transmit at only 300 mA)
Weather
Terrain
Condition of the beacon and antenna
Ref: AFRCC and Canadian SAR calculations

9. Our LOS Estimation Table
We’ll introduce our own estimation table
We’ll estimate slightly longer distances than the RCC table
The RCC table seems conservative (perhaps because ELT signals aren’t particularly strong)
Other references seem to indicate that 15% over geometric line of sight would be reasonable

10. LOS Estimation Table Altitude AGL (ft) RCC Line of sight estimate (NM)
Geometric Line of sight (NM)
Geometric Line of sight + 15% (NM)
1,000 36 34 39
2,000 52 48 55
3,000 63 59 68
4,000 72 78
5,000 81 76 87
6,000 83 95
7,000 90
104
8,000 96
110
9,000
102
117
10,000
114
107
123

11. Sample Problem
Several aircraft on a search mission are assigned to an altitude of 1000 ft AGL
How far away from the mission base can the aircraft go and still be in communications range?
Assume terrain is no factor
Assume no repeaters are in use
Answer: 39 NM

12. Sample Problem
An airborne repeater is being sent to orbit a station at an altitude of 5000 ft AGL
Assume flat terrain for entire service area
What is the radius of the repeater’s service area on the ground?
Answer: 87 NM

13. What about air-to-air range?
Can two aircraft talk to each other if they can both “see” the same point on the horizon?
Yes, even if they are at different altitudes
Think of the point as defining a plane-surface touching the Earth’s sphere; anyone on that surface will see the point as being on the horizon
Anyone on or above the plane will have an unobstructed line of sight to anyone else on or above the plane

14. Computing Air-to-Air Range
The horizon point represents the line of sight limit for each aircraft
So, we can add together the line of sight distances for the two aircraft – i.e. they can talk to each other
Overhead view

15. Sample Problem
An airborne repeater is being sent to orbit a station at 8000 ft AGL
Search aircraft are assigned 1000 ft AGL sorties
How far away can the search aircraft operate and still be in-range of the airborne repeater?
Assume terrain is no factor
Answer: 39 NM NM = 149 NM

16. What about terrain? Determine MSL altitudes for terrain
Note altitude of terrain between aircraft radio stations
So long as aircraft are well above terrain, ignore anything near to aircraft
Find the highest point in middle area (roughly the middle third between the aircraft)
Use the altitude of that highest point as the basis from which to calculate the effective height of the aircraft for the purpose of determining line of sight

17. What about terrain? Example: high terrain between aircraft
Use the altitude of the highest ground between as the “imaginary surface” from which we’ll compute altitude
Aircraft must fly high to stay in sight of each other

18. What about terrain?
Another example: lower altitude between aircraft

19. Sample Problem
Aircraft 1 (high-bird) is orbiting at 4000 ft MSL over a point with a surface elevation of 1000 ft MSL
Aircraft 2 is searching at 1000 ft AGL over terrain at 2000 ft MSL
Terrain in-between is no higher than 1500 ft MSL
If the aircraft are 100NM apart, can they communicate? 1 2
3000 MSL
4000 MSL

20. Sample Problem (cont.) Prevailing terrain is 1500ft MSL
Aircraft 1 is at 4000ft MSL or 2500ft over terrain
Line of sight for aircraft 1 is 62 NM
Aircraft 2 is at 3000ft MSL or 1500ft over terrain
Line of sight for aircraft 2 is 47 NM
Total line of sight is 109 NM

21. Terrain in Minnesota
Line of sight throughout most of Minnesota can be estimated reasonably with a flat terrain model
Watch out, however, for some areas like river valleys

22. Aircraft Platform-Specific Factors
Aircraft Attitude
Due to the placement of the VHF antenna on the bottom of the aircraft, aircraft attitude will be important.
Straight and level flight usually will give good results.
A banking aircraft will block the signal in the direction of the bank.
Climbing and descending will also influence propagation.
A climbing aircraft will block the signal with the tail
A descending aircraft will block the signal with the nose.

23. Aircraft Platform-Specific Factors
Propagation Pattern
Propagation is best to either side of the aircraft.
Propagation off the nose is impacted due to the antenna placement and the interference of the nose and engine.

24. Flight Planning
Remember aircrew should be given MSL altitude for station
Plan timing of high-bird sortie relative to other operations (we want it there when it is needed)
Remember a C172 sortie is limited to 3 hours; aircraft will need to periodically land and refuel

25. Flight Planning - IFR
As an alternative to executing a published holding pattern, the aircraft can request ATC for a clearance to fly a block of airspace corresponding to an elongated holding pattern (with a 2 minute in-bound leg).
The advantage of flying an elongated holding pattern is two-fold:
Reduces crew fatigue
Reduces the amount of time with the aircraft in a bank impacting radio signal propagation

26. High-Bird Types Airborne repeater
Advantages: automatic and rapid, everyone hears
Limitations: only one aircraft in fleet equipped; cannot offer continuous service
Manual message relay (stations call “high-bird” and request message relay)
Advantages: can take advantage of ground-based repeaters to further extend effective range
Limitations: slow and awkward, traffic cleared slowly, stations step on each other trying to call high-bird
Combined – one aircraft provides both services

27. High-Bird Crew Planning
An airborne repeater requires minimal crew (pilot plus perhaps one to monitor equipment)
A high-bird providing manual message relay should have a crew of three (pilot, radio-operator, and radio-operator/scribe)
Manual message relay is a heavy workload
Lots of time logging and writing down messages
Heavy frequency congestion from high-bird vantage point
Other limiting factors:
Weight and balance limits for aircraft

28. What about ground-based repeaters?
Remember that they are often on high-ground and on a tall building
We can treat them like low aircraft
Use estimate of about 25NM line of sight

29. Distances Across Minnesota
PKD
MN/NW
DLH
MN/SW
STP
MN/NE
MN/SE
MN/N
MN/WC
114NM
130NM
150NM
152NM
145NM
115NM
107NM
MKT
60NM
119NM
125NM
INL
122NM

30. Sample Problem
We have a mission base at STP and we want to communicate with an aircraft at 1000 ft AGL in the northwest corner of the state
If we put a high-bird over PKD at 11,000 ft MSL, will that provide the communications that we need?

31. Sample Problem (cont) Distances: PKD to MN/NW: 152 NM
PKD to STP: 145 NM

32. Sample Problem (cont) Terrain: At PKD: 1443 MSL At MN/NW: 795 MSL
At STP 705: MSL (in river valley)
Between PKD and MN/NW: ~1200 MSL (we’ll round that to 1000 ft)
Between PKD and STP: ~1200 MSL (we’ll round that to 1000 ft)

33. Sample Problem (cont)
Air-to-air communication between PKD and MN/NW should be no problem
High-bird is at about 10,000 ft above terrain, with a line of sight of 123 NM
Search aircraft is 1000 ft above terrain with a line of sight of 39 NM
Combined line of sight is 162 NM which is less than distance of 152 NM

34. Sample Problem (cont)
Direct air-to-ground communications with STP has problems
High-bird is at about 10,000 ft above terrain, with a line of sight of 123 NM, which is less than distance of 145 NM
The location of STP in a valley wouldn’t help either
There is, however, a repeater about 2 miles southeast of STP
Its line of sight can be estimated at 25NM and its distance to PKD just a couple miles further
Combining the line of sight of the aircraft with that of the repeater, we have 123 NM + 25 NM = 148 NM, which is just about exactly distance from the repeater to PKD

35. Sample Problem (cont)
Determination: We can establish the required communications
Our solution uses a ground-based repeater
This requires that the high-bird provide manual message relay (i.e. we are not using the airborne repeater)

36. Final Notes These line of sight estimates are inexact
Atmospheric conditions can impact results actually seen
Some reports indicate we can sometimes do better than the numbers in our estimation table
Its difficult to fully account for terrain without running a computer program
This material should, however, help you plan operations that require us to stretch our lines of communications

37. Questions?