Sponsored Links
This presentation is the property of its rightful owner.
1 / 37

# Air Operations Branch Director Course PowerPoint PPT Presentation

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

### Download Presentation

Air Operations Branch Director Course

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

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

Answer: 39 NM

### 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?

• 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

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 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?

• 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 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 terrain?

• 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

### 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

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

### 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