Resolution in the along-track direction will be strongly dependent on two main factors:
Horizontal beam width and
Tow speed.
These factors are not much of a problem at operations of short range scales and high ping rates, but they can become a concern for the operator when he begins to operate at longer ranges with slower ping rates and lower frequencies such as operations with a 50 kHz towfish.
Horizontal beam width
As the narrow horizontal sonar beam travels out in distance from the towfish the beam begins to spread or widen (the sonar's horizontal beamwidth). Again, In discussing resolution we are concerned with distinguishing two distinct targets on the sea floor, separated in the direction of the tow. If the two objects are separated by a distance which is less than the spread of the sonar beam at that range, then during the entire time that the objects are being passed, one or the other of the objects will be in the beam and returning an echo to the sonar. In such a case the returns would run together and the two targets would appear as one larger target. At closer ranges the beam is narrower and the along- track resolution will be better.
Above image shows two sets of two targets spaced equally. One set close to the towfish the other set distant in across track range. This demonstrates what happens to targets when the beam spreads. The targets close to the fish get imaged separately and you see two targets, the targets out in range get imaged together and you see one elongated target. Below shows examples of beam spreading in the real world. The following image below is a .2 degree 500 kHz towfish imaging small targets. with the narrow .2 degree beam the small targets are all imaged separately. The image below the 500 kHz images is a 1 degree 100 kHz image of the same exact bottom. Notice how the image smears as you go out in the across track range. The targets out in range are either imaged twice by two consecutive beams or two targets are imaged by one beam. The trade off here is range for resolution.
Tow speed
Along-track resolution will also
depend on the tow speed and the interval between pulses, This becomes a factor
when operating at longer range scales. The operator must tow slow enough to obtain total coverage or
an object may be missed. Remember the more pings on a target the better the resolution.
