Resolution Constraints

Resolution Constraints for sensors allow you to limit access based on sensor characteristics and the quality of the inview. The Ground Sample Distance is the smallest size of an object on the ground that can be detected by the sensor. This constraint can be placed on a sensor object or on an object (other than AreaTarget) for which access from a sensor is being computed (a sensor must be involved in the access computation for this constraint to apply). The Ground Sample Distance is based upon the access geometry and the physical attributes of the sensor. Enter the minimum and maximum Ground Sample Distance (GSD) in the units of measurement specified at the scenario level.

The most common application of GSD is from an elevated sensor (aircraft or spacecraft) looking at an object on the ground. The sensor is modeled as capturing an array of pixels, where each pixel has the same sized square shape when projected in front of the sensor. The sensor is also assumed to be pointed directly at the target. When the sensor is looking straight down on the target, the size of the pixel on the ground is simply the size of the square projected to the distance of the ground. As a single pixel is the smallest element of the image, this distance which is denoted as the GSD, represents the smallest discernible feature size in the image.

The Ground Sample Distance computation is a function of the elevation angle at the location of the target object for object classes which are considered to be located at or near the ground. When the sensor is not looking straight down, the footprint of the pixel is not square on the ground. Instead, it is rectangular with elongation along the direction from the parent object sub-point to the sensor target. In this case, the GSD is defined as the longer dimension of the rectangle. The group of ground-oriented objects contains all fixed location objects and all vehicle objects with the exception of LaunchVehicles and Satellites.

For LaunchVehicles and Satellites, the expectation that the direction of interest for computing resolution is in the local horizontal plane is dropped, thus removing the dependence on the elevation angle. In this case, the GSD is always computed as if the sensor were viewing the target from an elevation angle of 90 degrees. This decision in the GSD model is made to reflect the ability of the sensor to detect features on the side of the target object facing the sensor.

When an access involves a child object (i.e., sensor, transmitter, receiver, antenna, or radar), use the child object's parent or grandparent in the table below to determine whether the constraint can be used for access to that child object. For example, if a constraint can be used for accesses to all vehicles, then that constraint can also be used for any child object of a vehicle.

In the following table, abbreviations in the Constraints used for accesses to... column are:

F = facility	Pl = place	T = target	V = all vehicles	S = satellite	M = missile	L = launch vehicle	G = ground vehicle
A = aircraft	Sh = ship	Sn = sensor	AT = area target	LT = line target	P = planet	St = star

Constraint	Constraint used for accesses to...
Ground Sample Distance	F, T, P, V, AT