Garmin G1000 Manual

							190-00709-04  Rev. AGarmin G1000 Pilot’s Guide for the Socata TBM 850377
HAZARD AVOIDANCE
NEXRAD AND AIRBORNE WEATHER  RADAR 
Both	Airborne	 Weather	Radar	and	NEXRAD	 measure	weather	reflectivity	 in	decibels	 (dB).		A	decibel	 is	a	
logarithmic	 expression	of	the	 ratio	 of	two	 quantities.		 Airborne	Weather	Radar	measures	 the	ratio	 of	power	
against	 the	gain	 of	the	 antenna,	 while	NEXRAD	 measures	the	energy	 reflected	 back	to	the	 radar,	 or	the	 radar	
reflectivity ratio.
Both	 systems	 use	colors	 to	identify	 the	different	 echo	intensities,	 but	the	colors	 are	not	 interchangeable.		
Airborne	 color	radar	 values	 used	by	Garmin	 Airborne	 Color	Weather	 Radar	should	 not	be	confused	 with	
NEXRAD	radar	values.
ANTENNA BEAM ILLUMINATION
The	radar	 beam	 is	much	 like	the	beam	 of	a	spotlight.		 The	further	 the	beam	 travels,	 the	wider	 it	becomes.		
The	 radar	 is	only	 capable	 of	seeing	 what	is	inside	 the	boundaries	 of	the	 beam.	 	 The	figure	 below	 depicts	
a	radar	 beam’s	 characteristics.	 	 The	figure	 illustrates	 vertical	dimensions	 of	the	 radar	 beam,	 although	 the	
same	holds	 true	for	the	 horizontal	 dimensions.		 In	other	 words,	 the	beam	 is	as	 wide	 as	it	is	 tall.		 Note	 that	
it	is	 possible	 to	miss	 areas	 of	precipitation	 on	the	 radar	 display	 because	 of	the	 antenna	 tilt	setting.		 With	the	
antenna	tilt	set	to	zero	in	this	illustration,	the	beam	overshoots	the	precipitation	at	15	nautical	miles.
Figure 6-94  Radar Beam from a 10 inch Antenna
80 
Altitude (x1000 ft.) 
Range (nautical miles) 
Antenna at Zero 
Tilt 
Half Power at Beam Sidelobes  
18,000 ft. 
18,000 ft. Max Power at Beam Center 10°
0 30 
0  45 60  75 90 15 
The	curvature	 of	the	 earth	 can	also	 be	a	factor	 in	missing	 areas	of	precipitation,	 especially	at	range	 settings	
of	150	nautical	miles	or	more.		Here	the	beam	overshoots	the	precipitation	at	less	than	320	nautical	miles.
320 nm
Figure 6-95  Radar Beam in Relation to the  Curvature of the Earth  
						

							Garmin G1000 Pilot’s Guide for the Socata TBM 850190-00709-04  Rev. A378
HAZARD AVOIDANCE
RADAR SIGNAL ATTENUATION
The	phenomenon	 of	radar	 signal	 attenuation	 affects	the	operation	 of	weather	 radar.		When	the	radar	 signal	
is	transmitted,	 it	is	 progressively	 absorbed	and	scattered,	 making	the	signal	 weaker.	 	 This	weakening,	 or	
attenuation,	is	caused	by	two	primary	sources,	distance	and	precipitation.
Attenuation	 because	of	distance	 is	due	 to	the	 fact	 that	 the	radar	 energy	 leaving	 the	antenna	 is	inversely	
proportional	 to	the	 square	 of	the	 distance.		 The	reflected	 radar	energy	 from	a	target	 40	miles	 away	that	fills	
the	radar	 beam	 is	one	 fourth	 the	energy	 reflected	 from	an	equivalent	 target	20	miles	 away.		 This	would	 appear	
to	the	 operator	 that	the	storm	 is	gaining	 intensity	 as	the	 aircraft	 gets	closer.		 Internal	 signal	processing	 within	
the	GWX	68	system	compensates	for	much	of	this	distance	attenuation.
Attenuation	 due	to	precipitation	 is	not	 as	predictable	 as	distance	 attenuation.		 It	is	 also	 more	 intense.		 As	the	
radar	 signal	 passes	 through	 moisture,	 a	portion	 of	the	 radar	 energy	 is	reflected	 back	to	the	 antenna.		 However,	
much	of	the	 energy	 is	absorbed.	 	 If	precipitation	 is	very	 heavy,	 or	covers	 a	large	 area,	the	signal	 may	not	
reach	 completely	 through	the	area	 of	precipitation.		 The	weather	 radar	system	 cannot	 distinguish	 between	an	
attenuated	 signal	and	an	area	 of	no	 precipitation.		 If	the	 signal	 has	been	 fully	attenuated,	 the	radar	 displays	
a	radar	 shadow.		 This	appears	 as	an	 end	 to	the	 precipitation	 when,	in	fact,	 the	heavy	 rain	may	 extend	 much	
further.		 A	cell	 containing	 heavy	precipitation	 may	block	 another	 cell	located	 behind	the	first,	 preventing	 it	
from	being	 displayed	 on	the	 radar.		 Never	fly	into	 these	 shadowed	 areas	and	never	 assume	 that	all	of	the	 heavy	
precipitation	 is	being	 displayed	 unless	another	 cell	or	a	ground	 target	can	be	seen	 beyond	 the	heavy	 cell.		The	
WATCH®	feature	 of	the	 GWX	 68	Weather	 Radar	system	 can	help	 in	identifying	 these	shadowed	 areas.		Areas	
in	question	 appear	as	shadowed	 or	gray	 on	the	 radar	 display.		 Proper	use	of	the	 antenna	 tilt	control	 can	also	
help detect radar shadows.
Attenuation	 can	also	 be	due	 to	poor	 maintenance	 or	degradation	 of	the	 radome.		 Even	the	smallest	 amount	
of wear and scratching, pitting, and pinholes on the radome surface can cause damage and system inefficiency.
RADAR SIGNAL REFLECTIVITY
PReciPitatiOn
Precipitation	 or	objects	 more	dense	 than	water,	 such	as	the	 surface	 of	the	 earth	 or	solid	 structures,	 are	
detected	by	the	 weather	 radar.	 	The	weather	 radar	does	not	detect	 clouds,	 thunderstorms,	 or	turbulence	
directly.	 	 It	detects	 precipitation	 associated	with	clouds,	 thunderstorms,	 and	turbulence.	 	 The	best	 radar	
signal	 reflectors	 are	raindrops,	 wet	snow,	 or	wet	 hail.		 The	larger	 the	raindrop,	 the	better	 the	reflectivity.		 The	
size	of	the	 precipitation	 droplet	is	the	 most	 important	 factor	in	radar	 reflectivity.		 Because	large	drops	 in	a	
small concentrated area are characteristic of a severe thunderstorm, the radar displays the storm as a strong 
return.		 Ice	crystals,	 dry	snow,	 and	dry	hail	 have	 low	levels	 of	reflectivity	 as	shown	 in	the	 illustration,	 and	
often	is	not	 displayed	 by	the	 radar.		 Additionally,	 a	cloud	 that	contains	 only	small	 raindrops,	 such	as	fog	 or	
drizzle,	does	not	reflect	enough	radar	energy	to	produce	a	measurable	target	return.  
						

							190-00709-04  Rev. AGarmin G1000 Pilot’s Guide for the Socata TBM 850379
HAZARD AVOIDANCE
Figure 6-96  Precipitation Type and Reflectivity
gROUnD RetURns
The	intensity	 of	ground	 target	returns	 depends	 upon	the	angle	 at	which	 the	radar	 beam	 strikes	 the	
ground	 target	(Angle	 of	Incidence)	 and	the	reflective	 properties	 of	that	 target.		 The	gain	 can	be	adjusted	 so	
shorelines,	 rivers,	lakes,	and	cities	 are	well-defined.		 Increasing	the	gain	 too	much	 causes	 the	display	 to	fill	
in	between	targets,	thus	obscuring	some	landmarks.
Cities	 normally	 provide	a	strong	 return	 signal.		 While	large	buildings	 and	structures	 provide	good	returns,	
small	 buildings	 can	be	shadowed	 from	the	radar	 beam	 by	the	 taller	 buildings.		 As	the	 aircraft	 approaches	
and	shorter	 ranges	are	selected,	 details	become	 more	noticeable	 as	the	 highly	 reflective	 regular	lines	and	
edges	of	the	city	become	more	defined.
Bodies	 of	water	 such	as	lakes,	 rivers,	 and	oceans	 are	not	 good	 reflectors	 and	normally	 do	not	 provide	 good	
returns.	 	 The	energy	 is	reflected	 in	a	forward	 scatter	angle	with	inadequate	 energy	being	returned.	 	 They	
can	appear	 as	dark	 areas	 on	the	 display.		 However,	 rough	or	choppy	 water	is	a	better	 reflector	 and	provides	
stronger returns from the downwind sides of the waves.
Mountains	 also	provide	 strong	return	 signals	 to	the	 antenna,	 but	also	 block	 the	areas	 behind.		 However,	
over	mountainous	 terrain,	the	radar	 beam	 can	be	reflected	 back	and	forth	 in	the	 mountain	 passes	or	off	
canyon	 walls,	using	up	all	or	most	 of	the	 radar	 energy.		 In	this	 case,	 no	return	 signal	is	received	 from	this	
area,	causing	the	display	to	show	a	dark	spot	which	could	indicate	a	pass	where	no	pass	exists.
angle OF inciDence
The	angle	 at	which	 the	radar	 beam	 strikes	 the	target	 is	called	 the	Angle	 of	Incidence.		 The	figure	 illustrates	
the	incident	 angle	(‘A’).		This	directly	 affects	the	detectable	 range,	the	area	 of	illumination,	 and	the	intensity	
of	the	 displayed	 target	returns.		 A	large	 incident	 angle	gives	the	radar	 system	 a	smaller	 detectable	 range	and	
lower display intensity due to minimized reflection of the radar energy.  
						

							Garmin G1000 Pilot’s Guide for the Socata TBM 850190-00709-04  Rev. A380
HAZARD AVOIDANCE
Figure 6-97  Angle of Incidence
A	smaller	 incident	 angle	gives	the	radar	 a	larger	 detectable	 range	of	operation	 and	the	target	 display	 shows	
a	higher	 intensity.		 Since	more	radar	 energy	 is	reflected	 back	to	the	 antenna	 with	a	low	 incident	 angle,	the	
resulting	detectable	range	is	increased	for	mountainous	terrain.
SAFE OPERATING DISTANCE
The	following	 information	 establishes	a	minimum	 safe	distance	 from	the	antenna	 for	personnel	 near	
operating	weather	radar.		The	minimum	 safe	distance	 is	based	 on	the	 FCC’s	 exposure	 limit	at	9.3	 to	9.5	 GHz	 for	
general	 population/uncontrolled	 environments,	which	is	1	mW/cm2.		 See	Advisory	 Circular	20-68B	for	more	
information on safe distance determination.
MAXIMUM PERMISSIBLE EXPOSURE LEVEL (MPEL)
The	 zone	 in	which	 the	radiation	 level	exceeds	 the	US	Government	 standard	of	1	mW/cm2	 is	the	 semicircular	
area	 of	at	least	 9.16	 feet	from	 the	10-inch	 antenna.		 All	personnel	 must	remain	 outside	 of	this	 zone.		 With	a	
scanning	or	rotating	beam,	the	averaged	power	density	at	the	MPEL	boundary	is	significantly	reduced.  
						

							190-00709-04  Rev. AGarmin G1000 Pilot’s Guide for the Socata TBM 850381
HAZARD AVOIDANCE
 
MPEL
B ou ndary
9.16 f t. for 1 0”  
antenna
Figure 6-98  MPEL Boundary
BASIC ANTENNA TILT SETUP
The following discussion is a simple method for setting up the weather r\
adar antenna tilt for most situations.  
It	 is	 not	 to	be	 considered	 an	all	encompassing	 setup	that	works	 in	all	 situations,	 but	this	 method	 does	provide	
good	 overall	 parameters	 for	the	 monitoring	 of	threats.		 Ultimately,	 it	is	 desired	 to	have	 the	antenna	 tilted	so	that	
the	 bottom	 of	the	 radar	 beam	 is	four	 degrees	 below	parallel	 with	the	ground.		 The	following	 example	explains	
one way of achieving this.
With	the	aircraft	 flying	level,	adjust	 the	antenna	 tilt	so	ground	 returns	are	displayed	 at	a	distance	 that	equals	
the	aircraft’s	 current	altitude	 (AGL)	divided	 by	1,000.		 For	example,	 if	the	 aircraft	 is	at	 14,000	 feet,	adjust	 the	
tilt	so	the	 front	 edge	of	ground	 returns	are	displayed	 at	14	 nautical	 miles.		Note	this	antenna	 tilt	angle	 setting.		
Now,	raise	the	antenna	 tilt	6	degrees	 above	this	setting.		 The	bottom	 of	the	 radar	 beam	 is	now	 angled	 down	4º	
from parallel with the ground.  
						

							Garmin G1000 Pilot’s Guide for the Socata TBM 850190-00709-04  Rev. A382
HAZARD AVOIDANCE
PRACTICAL APPLICATION  USING THE  BASIC TILT  SETUP
With	the	antenna	 tilt	set	as	previously	 described,	any	displayed	 target	return	 should	 be	scrutinized	 when	
flying	at	altitudes	 between	2,000	and	30,000	 feet	AGL.	 	 If	the	 displayed	 target	advances	 on	the	 screen	 to	
within	5	nautical	 miles	of	the	 aircraft,	 avoid	it.		This	 may	be	either	 weather	 or	ground	 returns	that	are	2,000	
feet	 or	less	 below	 the	aircraft.	 	 Raising	the	antenna	 tilt	4	degrees	 can	help	 separate	 ground	returns	from	
weather	 returns	in	relatively	 flat	terrain.		 This	aligns	 the	bottom	 of	the	 radar	 beam	 parallel	 with	the	ground.		
Return	the	antenna	tilt	to	the	previous	setting	after	a	few	sweeps.
If	the	 aircraft	 is	above	 29,000	 feet,	be	cautious	 of	any	 target	 return	 that	gets	to	within	 30	nautical	 miles.		
This	is	likely	a	thunderstorm	that	has	a	top	high	enough	that	the	aircraft	cannot	fly	over	it	safely.
If	the	aircraft	altitude	is	15,000	feet	or	lower,	setting	the	displayed	range	to	60	miles	may	be	more	helpful.		
Closely	monitor	anything	that	enters	the	display.
Also,	after	setting	 up	the	 antenna	 tilt	angle	 as	described	 previously,	 ground	returns	can	be	monitored	 for	
possible	threats.		The	relationship	 between	antenna	tilt	angle,	 altitude,	 and	distance	 is	one	 degree	 of	tilt	 equals	
100 feet of altitude for every one nautical mile.
Vertical Change of Radar Beam (feet)
Change in Antenna Tilt
10 nm 0
1000
2000
3000
4000 1000 2000 3000 4000
-1° 0°
-2°
-3°
-4° +1°
+2°
+3°
+4°
Figure 6-99  Vertical Change in Radar Beam per Nautical Mile
Therefore,	
with	the	antenna	 tilt	set	so	that	 the	bottom	 of	the	 beam	 is	four	 degrees	 below	parallel	 with	
the	ground,	 a	target	 return	 at	10	 nm	 is	approximately	 4,000	feet	below	 the	aircraft;	 at	20	 nm,	 8,000	 feet;	
at	50	 nm,	 20,000	 feet.		In	other	 words,	 at	this	 tilt	setting,	 a	ground	 return	(such	as	a	mountain	 peak)	being	
displayed	 at	10	 nm	 would	 have	a	maximum	 distance	below	the	aircraft	 of	4,000	 feet.	 	When	 that	ground	
target	return	moves	to	5	nm,	the	maximum	distance	below	the	aircraft	is	2,000	feet.
This	 setup	 provides	 a	good	 starting	 point	for	practical	 use	of	the	 GWX	 68.		There	 are	many	 other	factors	 to	
consider	in	order	to	become	proficient	at	using	weather	radar	in	all	situations.  
						

							190-00709-04  Rev. AGarmin G1000 Pilot’s Guide for the Socata TBM 850383
HAZARD AVOIDANCE
WEATHER MAPPING AND INTERPRETATION
WEATHER DISPLAY INTERPRETATION
When	evaluating	 various	target	returns	 on	the	 weather	 radar	display,	 the	colors	 denote	 precipitation	
intensity	and	rates	shown	in	the	table.
Weather Mode ColorIntensity
Approximate
Precipitation Rate 
(in/hr.)
Black< 23 dBZ< .01.
Green23 dBZ  to < 32 dBZ.01 - 0.1.
Yellow32 dBZ to < 41 dBZ0.1 - 0.5
Red41 dBZ to < 50 dBZ0.5 - 2
Magenta50 dBZ and greater> 2
Table 6-10  Precipitation Intensity Levels
thUnDeRst ORms
Updrafts	and	downdrafts	 in	thunderstorms	 carry	water	 through	 the	cloud.		 The	more	 severe	 the	drafts,	 the	
greater	 the	number	 and	size	of	the	 precipitation	 droplets.		With	this	in	mind,	 the	following	 interpretations	
can	be	made	from	what	is	displayed	on	the	weather	radar.		Avoid	these	areas	by	an	extra	wide	margin.
•	 In	areas	 where	 the	displayed	 target	intensity	 is	red	 or	magenta	 (indicating	 large	amounts	 of	precipitation),	
the	turbulence	is	considered	severe.	
•	 Areas	 that	show	 steep	color	gradients	 (intense	color	changes)	 over	thin	bands	 or	short	 distances	 suggest	
irregular	rainfall	rate	and	strong	turbulence.
•		Areas	that	show	 red	or	magenta	 are	associated	 with	hail	or	turbulence,	 as	well	 as	heavy	 precipitation.		
Vertical	 scanning	 and	antenna	 tilt	management	 may	be	necessary	 to	identify	 areas	of	maximum	 intensity.  
						

							Garmin G1000 Pilot’s Guide for the Socata TBM 850190-00709-04  Rev. A384
HAZARD AVOIDANCE
Along	squall	lines	(multiple	 cells	or	clusters	 of	cells	 in	a	line)	 individual	 cells	may	be	in	different	 stages	
of	development.		 Areas	between	 closely	spaced,	 intense	targets	may	contain	 developing	 clouds	not	having	
enough	 moisture	 to	produce	 a	return.	 	 However,	 these	areas	could	 have	strong	 updrafts	 or	downdrafts.		
Targets	showing	wide	areas	of	green	are	generally	precipitation	without	severe	turbulence.		
Irregularities	 in	the	 target	 return	 may	also	indicate	 turbulence,	 appearing	as	hooks,	 fingers,	 or	scalloped	
edges.		 These	irregularities	 may	be	present	 in	green	 areas	with	no	yellow,	 red,	or	magenta	 areas	and	should	
be	treated	as	highly	dangerous	areas.		Avoid	these	areas	as	if	they	are	red	or	magenta.
Figure 6-100  Cell Irregularities
Steep Gradient
Squall Line
Hook or FingerScalloped Edge
Thunderstorm	 development	is	rapid.		 A	course	 may	become	 blocked	 within	a	short	 time.		 When	 displaying	
shorter	 ranges,	periodically	 select	a	longer	 range	to	see	 if	problems	 are	developing	 further	out.	 	That	can	
help	prevent	getting	trapped	in	a	blind	alley	or	an	area	that	is	closed	at	one	end	by	convective	weather.
Figure 6-101 The Blind Alley - Horizontal Scan  
						

							190-00709-04  Rev. AGarmin G1000 Pilot’s Guide for the Socata TBM 850385
HAZARD AVOIDANCE
In	areas	 of	multiple	 heavy	cells,	use	the	Vertical	 Scan	feature	 along	with	antenna	 tilt	management	 to	
examine	the	areas.		Remember	to	avoid	shadowed	areas	behind	targets.
Figure 6-102  The Blind Alley
The Blind Alley at Close Range The Large Storm Behind
tORnaDOes
There	are	no	conclusive	 radar	target	 return	 characteristics	 which	identify	 a	tornado.		 However,	 tornadoes	
may	be	present	if	the	following	characteristics	are	observed:
•	 A	narrow,	finger-like	portion	extends	and	in	a	short	time	curls	into	a	hook	and	closes	on	itself.
•	 A	hook,	 which	 may	be	in	the	 general	 shape	of	the	 numeral	 6	(or	 9	in	 the	 southern	 hemisphere),	 especially	
if	bright	 and	projecting	 from	the	southwest	 quadrant	(northeast	 quadrant	in	the	 southern	 hemisphere)	 of	
a	major	thunderstorm.
•	 V-shaped	notches.
•	 Doughnut	shapes.
These shapes do not always indicate tornadoes, and tornado returns are not limited to these characteristics. 
Confirmed	radar	observations	 of	tornadoes	 most	often	 have	not	shown	 shapes	 different	 from	those	 of	a	
normal thunderstorm display.
hail
Hail	 results	 from	updrafts	 carrying	water	high	enough	 to	freeze.	 	 Therefore,	 the	higher	 the	top	 of	a	
thunderstorm,	 the	greater	 the	probability	 that	it	contains	 hail.	 	Vertically	 scanning	the	target	 return	 can	
give	the	radar	 top	of	a	thunderstorm	 that	contains	 hail.		Radar	 top	is	the	 top	 of	a	storm	 cell	as	detected	 by	
radar.		It	is	 not	 the	actual	 top,	or	true	 top	of	the	 storm.		 The	actual	 top	of	a	storm	 cell	is	seen	 with	 the	eyes	
in	clear	 air	and	 may	 be	much	 higher	 than	the	radar	 top.	 	The	actual	 top	does	 not	indicate	 the	top	 of	the	
hazardous area.
Hail	 can	fall	below	 the	minimum	 reflectivity	 threshold	for	radar	 detection.		 It	can	 have	 a	film	 of	water	 on	
its	surface,	making	its	reflective	characteristics	similar	to	a	very	large	water	droplet.		Because	of	this	film	of	
water,	 and	because	 hail	stones	 usually	 are	larger	 than	water	 droplets,	 thunderstorms	 with	large	 amounts	
of	wet	 hail	 return	 stronger	 signals	than	those	 with	rain.		 Some	 hail	shafts	 are	extremely	 narrow	(100	yards	
or	less)	 and	make	 poor	radar	 targets.		 In	the	 upper	 regions	 of	a	cell	 where	 ice	particles	 are	dry	 (no	 liquid	
coating),	target	returns	are	less	intense.  
						

							Garmin G1000 Pilot’s Guide for the Socata TBM 850190-00709-04  Rev. A386
HAZARD AVOIDANCE
Hail	shafts	 are	associated	 with	the	same	 radar	 target	 return	 characteristics	 as	tornados.		 U-shaped	cloud	
edges	three	to	seven	 miles	across	 can	also	 indicate	 hail.		These	 target	returns	 appear	quite	suddenly	 along	
any	edge	 of	the	 cell	 outline.		 They	also	change	 in	intensity	 and	shape	 in	a	matter	 of	seconds,	 making	vigilant	
monitoring essential.
OPERATION IN WEATHER MODE
 WARNING:  Begin transmitting only when it is safe to do so.  When transmitting while the aircraft is on the 
ground, no personnel or objects should be within 9.16 feet of the antenna.
 CAUTION:  In Standby mode, the antenna is parked at the center line.  It is always a good idea to put the 
radar in Standby mode before taxiing the aircraft to prevent the antenna\
 from bouncing on the bottom stop 
and possibly causing damage to the radar assembly.
When	 the	weather	 radar	system	 is	in	 the	 Weather	 or	Ground	 Map	mode,	 the	system	 automatically	 switches	
to	Standby	mode	on	landing.
In	Reversionary	 mode,	the	weather	 radar	system	 automatically	 switches	to	Standby	 mode.	The	system	
remains	 in	Standby	 mode	until	both	displays	 are	restored.		 In	Reversionary	 mode,	the	weather	 radar	system	
cannot	be	controlled.
Figure 6-103  Horizontal Scan Display
Radar ModeScan LineAntenna Stabilization Status