The first Earth orbiting satellite to carry an 
imaging radar system will be SEASAT, scheduled for 
launch in 1978. Its main purpose will be to image 
the oceans, polar ice and coastal areas of North 
America. Unlike LANDSAT it will not be aimed at 
imaging the entire world. The image data will be 
received through modified LANDSAT - ground stations. 
The additional equipment to do so will require size- 
able expenditures (severalMill. U.S. $ ). However, 
the U.S. National Aeronautics and Space Administra- 
tion (NASA) might be in a position to have portab- 
le versions of the SEASAT radar receiving equip- 
ment made available temporarily. 
Table 1 
Radargrammetrically relevant parameters of the SEASAT- 
satellite imaging radar system 
  
    
       
     
  
   
    
    
    
   
   
  
  
     
     
   
   
Launch 
Orbit altitude 
Orbit inclination 
Orbit period 
May 1978 (scheduled) 
790 - 820 km 
108? retrograde 
100 minutes 
Radar vavelength 25 cm (L-band) 
Elevation angles of 
line of sight 16.99 — 23.19 
Swath width 100 km 
x 
Resolution along track 7 m (25 m) 
Resolution across track 
in slant range 8 m 
in ground range 25 m 
Dynamic range 50 db 
Transmitted radar image data received by modified 
LANDSAT stations 
Recording of received data probably on magnetic 
tape 
Map film generation by optical or digital correla- 
tion 
  
  
    
  
  
*The 7 m resolution will in most radar presentations be 
artificially degraded to 25 m to achieve equal resolu- 
tion along and across track and to smooth the graininess 
of the original data (multi-look mode of presentation) 
  
  
   
  
    
   
   
  
     
     
   
  
  
   
    
    
  
    
    
   
  
    
Other satellite imaging radar projects which 
presently are only in a planning stage concern the 
exploration of the planet Venus (Rose and Friedman, 
1974) and multifrequency radar images of the Earth 
to be produced on board the Space Shuttle (Cohen 
et al., 1975). 
Table 1 summarizes those parameters of the 
SEASAT imaging radar that are of radargrammetric 
relevance. Differences between airborne and satel- 
lite imaging radar do not concern the principle of 
operation of the sensor, nor are there differences 
in resolution, swath width or scale. Essential 
differences relate only to the "look-angles" or 
elevation angles of the line of sight, considera- 
tion of the planetary curvature and various effects 
of orbit parameters. With airborne radar, eleva- 
tion angles are normally rather large and vary 
greatly within a swath, e.g., from 400 (near range) 
to 80° (far range). In a satellite radar at a 
great orbital altitude, the horizon and energy re- 
quirements limit the elevation angles to compara- 
tively small values, and the variation of the ang- 
les from near to far range is rather small (see for 
example SEASAT, Table 1). This permits image ac- 
quisition under almost constant look angles as 
opposed to airborne radar, and might enhance the 
value of the resulting imagery for geoscience 
applications. 
4. REVIEW OF PROJECTION EQUATIONS FOR 
REAL- AND SYNTHETIC- APERTURE RADAR 
The basic radar imaging and projection equa- 
tions have been derived on many occasions in the 
past (for example: Konecny and Derenyi,1966; Ako- 
wetzki, 1968; Derenyi,1970; Gracie, et al., 1970; 
Hockeborn, 1971; Leberl, 1970; Norvelle,1972; 
Greve and Cooney, 1974; DBA-Systems, 1974). They 
were well-established prior to 1972. However, it 
might be of value to specifically review the dif- 
ferences which exist in the projection equations 
for radar with real and with synthetic aperture; 
synthetic aperture radars have become of growing 
importance during the last few years and are the 
only system to be made use of from satellites. 
Irrespective of the type of side-looking radar 
imagery, the basic measurement to work with is the 
slant range r, between the antenna and point P, and 
time of imaging, t These entities are obtained 
in a simple process from measurements in the photo- 
graphic radar record (see for example, Konecny and 
Derenyi,1966; Rydstrom, 1968; Leberl, 1972a). The 
basic radargrammetric difference between real and 
synthetic aperture radar is the following: for real 
apertures, all points imaged at time t lie on a 
surface whose orientation is defined by the atti- 
tude of the real antenna; for synthetic apertures, 
the orientation of the imaging surface is deter- 
mined by the velocity vector of the real antenna. 
The position of the radarsensor is denoted by a 
vector s(t)- (x, (t) , yg (t) zg(t)); the sensorposition 
is a function of time t. The velocity vector of 
the antenna is the first derivative of s(t) with 
respect to time t and denoted by $(t)-(Xg(t),ys(t), 
2; 03. The antenna attitude, finally, is deter- 
mined by the three classical orientation angles of 
photogrammetry: é(t), w(t), k(t). In the following, 
the time dependency will not be explicitely indi- 
cated, so that s=s(t),s=5(t), etc. 
A formulation for the projection equation of 
side-looking radar is now (Figure 2a,b): 
  
p*str 
(1) 
r = Upu + Vor t s 
where p is the position vector of an imaged point 
in object space; u, v and w are unit vectors de- 
fining the rectangular antenna coordinate system; 
r is the range-vector from the antenna to the ob- 
ject point; and the auxiliary vector P=(up,vp,w ) 
describes the location of the object point in the 
u, v, w antenna coordinate system (see Figure 2b): 
u. = r sin: 
P 
L 
Yo = (sin = Sin )?r (2) 
w = -cosQr 
P 
where ris the "cone-complement angle."  Hockeborn 
(1971) calls r "squint"; it is also explained by 
Graham (1975), Leberl (1972a,b; 1975a,b) and dis- 
cussed in considerable detail by Leberl (1976b). 
Equation (1) can be rewritten in a more familiar 
form as: 
  
# Vectors are denoted by underligned lower case 
letters. The dot product is denoted by (:), and 
the crossproduct by (x). Matrices are under- 
lined upper-case letters.