Module2 Part3 Satellite Alatimetry (1)

8/9/2019 Module2 Part3 Satellite Alatimetry (1)

1/16

Satellite Radar Altimetry Over 70% of the earths

surface (Ocean and large

lakes at MSL) the arthssurface corres!onds to thegeoid"

A s!ace#$orne nadir !ointingradar can $e used todetermine the satellite to

surface distance $y sim!lymeasuring the to ay traveltime of a radar !ulse"

&oot!rint si'e is #*km+de!ending on surfaceroughness+ s!atial sam!lingde!ends on or$it"

,hen com$ined ith accurateor$it !ositioning candetermine sea surface heightand hence geoid veryaccurately"


2/16

Altimetry -rinci!le

h = ct2

Altitude -ulse Length

/ransmit &reuency

O!erating Mode

Antenna Si'e

-ointing Reuirements Transmitter Power

Pulse repetitionFrequency

On-Board Processing


3/16

AL/1/23

(a)Errors in orbit determination

Atmos!heric 3rag

4igh order com!onents of arth5s gravity 6eld

(reuires h 800km)

(b) Signal to Noise Requirements

/ransmitter -oer is limited

9eed reasona$le Signal#to 9oise (S9R) ratio

:urrent s!ace#$orne altimeters

/O-;


4/16

Range Resolution 9eed a short duration !ulse in order to>

(a) Measure range !recisely

($) Resolve surface roughness

&or !ulse duration >

Se!aration of echoes>

1n order to resolve se!arate echoes from surfaces A and ? e reuire>

Smallest ave (roughness) of interest is @0"8m

4ence reuire>

t=2h

c

2h

c

2x0.5

3x108 3ns


5/16

Measurement !recision 1n general can achieve measurement !recision of 0"#0" B instrument

resolution $y 6tting a knon instrument res!onse to the data>

-ulse idth related to the &reuency ?andidth of the radar (?)>

MaBimum Bfor arth O$servation limited $y international &reuency

allocations" /his limits the choice of transmit freuencies"

9eed a centre freuency (f) high enough to su!!ort this $andidth

B =1

for = 3ns require B330 MHz


6/16

/ransmit &reuency

Other factors aCecting transmitfreuency>

Atmos!heric /ransmission /echnology

Antenna Si'e

Atmos!heric Refraction

On#$oard conDicts

/o date all satellite altimeters

have used a freuency fof

f@ E F4' (Gu $and)

RS#> f = E"* H0"8 F4'


7/16

O!erating Mode

:once!tuallysim!lest is ?eam

Limited O!eration" Reuire that

aveform curvatureto $e small enoughithin $eam limitedfoot!rint that all of

the surface isilluminatedsimultaneously"

4ence reuire thatrange diCerence due

to $eam s!read (R)is less than the

surface roughness s

to $e resolved"


8/16

?eam curvature reuirements

R =R h =R(1 cos)

=R 1 (1

2

2+ ....)

? R2

2h2

2

for R


9/16

Antenna ?eamidth

&or RS# > h=7*0km+ ="cm+ s=0"8m

4ence reuire 3 I"7m JJ

Antenna very large and eB!ensive ,ould also reuire very accurate !ointing

&or h K Ecm and h = 7*0 then reuire >

Antenna Beamwidth 2=

D

= Wavelength

D = Antenna Diameter

For : 2s

h

Need : D

2

h

2s

2h

h 0.016


10/16

O!eration Mode


11/16

-ulse Limited O!eration Alternative method of

o!eration

2se $road $eam # henceAntenna small (:hea!+

&easi$le+ less critical !ointingreuirement")

Reuirement on 3R is that itshould $e greater than the

eBtreme (Es) of the largest

roughness likely to $eencountered"

/hus reuire >

4ence >

&or Ocean aves s(maB) @

8m" 4ence 0"7 and 3 0"Im

6s(max)

h

D

2

h

6s(max)


12/16

-ulse Limited Feometry

B!andings!herical ca! of

microave energyintersects ground"Area illuminatedeB!ands as diskuntil rear of the!ulse arrives #

then eB!ands asannulus"


13/16


14/16

-ulse Limited &oot!rint# Let A(t) $e the area of the surface !erceived $y the altimeter to $e

illuminated $y the altimeter at a given instant of time"

A(t) gros linearly ith time until the disk changes to an annulus+after hich A remains constant"

/hus the area of the -ulse Limited &oot!rint (&or a Dat surface) is >

and the -L& diameter is >

&or RS#+ 3-L& = "7 km

APLF

= hc

DPLF= 2 hc


15/16

-ulse ?lurring

As the surface $ecomes more rough the leading edge tends to$ecome more $lurred


16/16

Module2 Part3 Satellite Alatimetry (1)

Documents

Transcript of Module2 Part3 Satellite Alatimetry (1)