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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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Air Operations Branch Director Course

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Air operations branch director course



Air OperationsBranch Director Course

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

Calculating Line of Sight

The problem

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



  • 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

General Factors Impacting Range

  • Terrain

  • Signal strength / radio condition

  • Weather

Related problem elt airborne reports

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

Related problem elt airborne reports1

Related ProblemELT Airborne Reports

Elt airborne reception range

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

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

Los estimation table

LOS Estimation Table

Sample problem

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

Sample problem1

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

What about air to air range

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

Computing air to air range

Overhead view

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 problem2

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

What about terrain

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

What about terrain1

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

What about terrain2

What about terrain?

  • Another example: lower altitude between aircraft

Sample problem3

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?


3000 MSL


4000 MSL

Sample problem cont

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

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 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 factors1

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

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

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

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

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

What about ground based repeaters

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

Distances across minnesota

























Distances Across Minnesota

Sample problem4

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 cont1

Sample Problem (cont)

  • Distances:

    • PKD to MN/NW: 152 NM

    • PKD to STP: 145 NM

Sample problem cont2

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 cont3

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 cont4

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 cont5

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

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



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