1. A projectile tracking device comprising: detector apparatus for converting into electronic form images in at least two infrared wavebands; optics for projecting onto the detector apparatus an image in the at least two infrared wavebands of a scene across which a projectile passes; first logic operative to obtain from the images in electronic form apparent brightnesses of the projectile at the optics in at least two infrared wavebands; second logic operative to estimate the speed of the projectile from the ratio of the two measured apparent brightnesses; third logic operative to obtain from the images in electronic form an azimuth of the projectile from the optics at successive times; and fourth logic operative to estimate the direction of the trajectory of the projectile and the distance to the trajectory in an azimuth plane including a location of the detector apparatus from the measured azimuths in combination with the ratio between the measured apparent brightnesses; wherein at least some of at least one of said first, second, third, and fourth logic comprises a computer connected to receive image data from the detector apparatus and programmed to estimate parameters of the projectile trajectory using at least two of: a method comprising measuring the azimuth of the projectile from the optics at successive times, and estimating the direction of the trajectory and the distance to the trajectory of the projectile from the measured azimuths in combination with the ratios between the measured apparent brightnesses; a method wherein a perpendicular from the optics to the projectile trajectory and hence the direction of the projectile trajectory are estimated by locating the point with zero value of the second derivative with respect to time of the direction to the projectile, further combining the estimated projectile speed with the projectile trajectory direction for calculating distances from the optics to the projectile; and a method further comprising measuring the direction of the projectile from the optics at successive times, and estimating the trajectory of the projectile and the distance to the trajectory from the measured directions in combination with the estimated speed; and to provide a final estimate using a comparison of estimates from at least two said methods of estimating parameters.