Air Operations Branch Director Course

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U.S. AIR FORCE AUXILIARY. Air Operations Branch Director Course. Planning Air-to-Ground and High-Bird Communications Operations Calculating Line of Sight. The Problem. VHF Communication is limited to line of sight

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U.S. AIR FORCE

AUXILIARY

### Air OperationsBranch Director Course

Planning Air-to-Ground and High-Bird Communications Operations

Calculating Line of Sight

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
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
General Factors Impacting Range
• Terrain
• Signal strength / radio condition
• Weather
Related ProblemELT 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
ELT Airborne Reception Range
• 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

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
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

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?

• 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

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
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 + 110 NM = 149 NM

• 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
• 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
• Another example: lower altitude between aircraft
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

3000 MSL

2

4000 MSL

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
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
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.
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.
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
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
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
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
• 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

150NM

114NM

152NM

130NM

122NM

107NM

145NM

114NM

MN/NW

MN/NE

MN/N

INL

MKT

DLH

MN/SW

MN/SE

STP

MN/WC

PKD

60NM

152NM

115NM

119NM

125NM

Distances Across Minnesota
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?
Sample Problem (cont)
• Distances:
• PKD to MN/NW: 152 NM
• PKD to STP: 145 NM
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)
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
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
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)
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