Simultaneous collocated observations from two different orbits lead to several advantages. These advantages are: (i) During times of collocation two sensors can be cross-calibrated at various look angles within the commonly imaged scenes. The solar zenith angle will also usually vary from encounter to encounter thus providing a wide range of radiance conditions, (ii) When the two satellites are not collocated they can provide a wide range of sensor look angles for a given scene which they both image prior to and after passing through the encounter position. These data are useful for determining the bidirectional reflectance distribution function (BRDF). The problem of determining the number of times per year (on the average) that the subsatellite points of two satellites come within D km of each other is addressed in this paper. For the space station (altitude: 500km, inclination: 28°) and a Sun-synchronous satellite (altitude: 705 km, inclination: 98-21°) the answers are 16, 41 and 82 times per year for encounter distancesDof 20, 50 and 100 km, respectively. The relationship between encounters per year and distance D is linear. The answers were obtained in two ways: (i) a closed-form statistical approach which led to a simple algebraic expression and (ii) a 'Monte Carlo’ type computer solution. The largest difference between the two solutions was less than 12 per cent.