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 2018-05-08T16:38:19+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