The capabilities of multibeam altimetry are discussed and an interferometric multibeam technique for doing precision altimetry is described. The antenna feed horn arrangement and the resulting footprint lube pattern are illustrated. Plans for a shuttle multibeam altimetry mission are also discussed

Beck, F. B.

Mcgoogan, J. T.

Parsons, C. L.

English

NASA Technical Reports Server

A P P L I C A T I O N  OF PUSHBROOM ALTIMETRY 
FROM S P A C E  U S I N G  LARGE S P A C E  ANTENNAS 
C. L. Parsons and J. T. McGoogan 
NASA Goddard Space F l i g h t  Center 
Wallops I s l a n d ,  VA 23337 
and 
F. B. Beck 
NASA Langley Research Center 
Hampton, V A  23665 
Large Space Antenna Systems Technology - 1984 
December 4-6, 1984 
63 
https://ntrs.nasa.gov/search.jsp?R=19850015506 2020-03-22T19:03:42+00:00Z
OCEAN DYNAMICS FEATURES 
The E a r t h ' s  oceans are now known t o  con ta in  dynamical f e a t u r e s ,  such as 
c u r r e n t s  and edd ie s ,  which have a d i r e c t  correspondence t o  a tmospheric  f e a t u r e s .  
Eddies and t h e  meanderings of t h e  major c u r r e n t s  are mesoscale f l u c t u a t i o n s  which 
have analogues i n  t h e  cyclones,  an t i cyc lones  and f r o n t s  of our  weather.  Hence, 
it $s important  t o  develop the  means t o  monitor t h e i r  development, movement, and 
d i s s i p a t i o n  i f  w e  are t o  understand t h e  dynamics of t h e  oceans.  
t o  u se  pas s ive  i n f r a r e d  radiometry t o  map the  sea s u r f a c e  temperature  v a r i a t i o n s  
r e s u l t i n g  from ocean dynamics. Figure 1 shows such an  image produced by t h e  
Advanced Very High Resolu t ion  Radiometer (AVHRR); i t  c o n t a i n s  t h e  Gulf Stream 
flowing a c r o s s  t h e  bottom of t h e  scene and a prominent w a r m  co re  r i n g  above i t .  
One technique is  
P 
Figure 1 
64  
CONVENTIONAL ALTIMETRIC SIGNATURE OF THE GULF STREAM 
In addition to sea surface temperature, the ocean can be monitored using the 
visible wavelengths (as in the case of the Coastal Zone Color Scanner) or micro- 
waves. 
because they are unaffected by the atmosphere. The GEOS-3 radar altimeter pro- 
duced the topographic record shown in Figure 2 as it traversed the Gulf Stream. 
Because this current is nearly geostrophic, the one-meter dynamic height differ- 
ential between its eastern and western walls could be related directly to current 
velocity if the orientation of the satellite's groundtrack with respect to the 
current's flow axis was known. 
such as SKYLAB, GEOS-3, and SEASAT this information is not available. 
Active microwave instruments offer the potential of all-weather monitoring 
With conventional nadir-tracking altimeters 
GULF STRERM RNRLYSIS GEOS-3 REV 1909 
C R O S S I N G  A N G L E :  I S  D B G R E E S  
o 80.0 90.0 1bo.0 1ia.a 120.0 1 h . a  i i a . 0  i50.0 I 
:E - Kfl . I o '  
Figure 2 
65 
CAPABILITIES OF MULTIBEAM ALTIMETRY 
The top of Figure 3 shows the results of a simple simulation of a five-beam 
altimeter raking across an eddy which has a radius o f  30 km and a maximum height 
signature of 45 cm. 
is assumed to have a height resolution of only 10 cm; still, the shape of the 
eddy is clearly detected. 
sured topographic heights can easily be used to compute velocity. 
The interbeam spacing at the ground is 10 km. The altimetei- 
Using a numerical differentiation formula, these mea- 
The bottom of Figure 3 shows the computed velocity pattern. 
veloc-ity is seen at a distance of 10 km from the eddy center. 
the pattern that should be observed based on the eddy model used in the simula- 
tion, 
the circulation of the eddy can be calculated. This "curvature" measurement is 
independent of the attitude of the sensor. 
highly sophisticated advancements in technology but only on the addition of off- 
nadir beams, which seems to be within reach based upon NASA Langley's 15-m antenna 
progress. 
A clear maximum 
This is exactly 
By using a numerical computation of the Laplacian of the height field, 
These results are not dependent on 
10 km 
0 2 4 6 8 10 sec 
I I I I I I I I I I I 
0 2 0  40 60 80 km 
Figure 3 
I 66 
INTERFEROMETRIC MULTIBEAM TECHNIQUE 
To do precision altimetry, a range tracker needs a sharp waveform returned 
from the ocean surface. Off-nadir, the waveform tends to smear and stretch. 
Therefore, either a large real aperture system is needed to narrow the beamwidth 
and hence the duration of the returned waveform, or an interferometric technique 
schematically shown in Figure 4 can be used. 
and separated by distance D are fed from the same rf source to produce multi- 
lobed beams where the lobe width is set by D and the overall envelope is a func- 
tion of d. In this example, each antenna has two offset feeds so that two off- 
nadir beams are simultaneously generated. 
peaked response as a result of the interferometer lobe intersections on the 
surface. 
Two antenna dishes of diameter d 
The signal returned will show a multi- 
D = Boom length 
d = Dish diameter 
A = Beam offset 
Antenna 
element 
footprint 
Total swath 
/ 
\ 
Constant 
range cone 
I 
Interferometer 
maximum 
Echo1 
Track 
point 
h 
Figure 4 
67 
USE OF THE 15-METER HOOP/COLUMN ANTENNA 
A cooperative study with Langley Research Center personnel has been con- 
ducted to determine the suitability of the 15-mhoop/column antenna for multi- 
beam altimetry. Figure 5 shows an artist's view of its deployment on Shuttle. 
The antenna has the feature that its surface can be used in quadrants with each 
quadrant separately illuminated by microwave feed horns located on plates mounted 
to the center post. 
feed horns on each of the two plates associated with opposite quadrants of the 
15-m antenna, orienting them properly, and illuminating approximately 3-m-diam- 
eter spots within the two quadrants, it is possible to produce beam footprints 
centered at nadir ?r 1.5', and * 3.0' off-nadir. 
This is ideally suited for interferometry. By placing five 
Figure 5 
68 
FEED HORN ARRANGEMENT 
The arrangement of t h e  f i v e  p a i r s  of feed  horns on t h e  mounting p l a t e s  i s  
shown i n  Figure 6. 
fe rence  lobes.  
This  produces f i v e  f o o t p r i n t s  each of which con ta ins  20 i n t e r -  
APERTURE #I APERTURE #3 
NADIR BEAM 
FEED PANELS WITH ACROSS-TRACK BEAM FOOTPRINTS 
Figure  6 
69 
MULTIBEAM ALTIMETER FOOTPRINT LOBE PATTERN 
The envelope of the antenna pattern resulting for the -1.5" angle footprint 
is shown in Figure 7. 
the appropriate pair of feed horns trained on this footprint is contained within 
the envelope. For this antenna configuration, the lobe angular spacing is .146", 
which is equivalent to a real-aperture dish diameter of 8.65 m. The gain at the 
center lobe of each envelope is on the order of 52 dB, which is ample for real- 
aperture altimetry at Shuttle altitudes. 
The lobe structure resulting from the interference between 
ERTQ/CBCHRELL .SJiL18TH 15-METER P I E 13.3 GHZ 31 JUL 81 
WF= 3.7863 IF= 09.1409 FF = 0. FRED BECK HHRRflW EEHli TEST WESH = 2.2 
14DB EDGE THPER 3 5  GHF HOP.14 PHTTERN CBFlPUTED FRO14 SRK '5; S?IBF;PlJTIFIE DUHL 
FIE-SHAPED PARHBOLBIO ALPHA = 22.50 DEG FUCHL LIG = 366.85 
PRII*ICIFAL PLRbIE OF C!rT IS THETH = 917. DEL  
l l l l l l l l l J l I 1 I I  ;-*yr 1: '\ I I;+-- ---... I I I 1 I 1 1 r - T - r - r -  
{ { !  52.4 DB { i 1 ' 1  
' I  
I 
I '. /. i 1 
'i I I 
i 
F... 
i 
-200 -0.5 -1,s 
PH I 
Figure 7 
I 70 
DATA PROCESSING TECHNIQUES 
Because satellite attitude uncertainties directly translate into height 
measurement errors, it is crucial that pointing be known as precisely as possible. 
However, for a Shuttle multibeam altimetry mission that is concerned mainly with 
the monitoring of eddies and currents, advantage can be made of crossing arc 
analyses. 
orbit produce 25 intersections for a 5-beam multibeam system (fig. 8). 
measured height differences at the 25 points are minimized, then the tilt will be 
effectively removed. 
altimeter with the 15-mantenna there are no known impediments in the way of 
successfully measuring mesoscale ocean dynamic features for the first time. 
Over a particular area of interest, an ascending orbit and a descending 
If the 
For a Shuttle mission experiment incorporating a multibeam 
Figure 8 
71 


Application of pushbroom altimetry from space using large space antennas

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server