WSF_OLD_GUIDANCE_COMPUTER¶
- processor WSF_OLD_GUIDANCE_COMPUTER¶
Note
This refers to the ‘old’ implementation and should be avoided for new work.
processor <name> WSF_OLD_GUIDANCE_COMPUTER ... base processor commands ... ... WSF_OLD_GUIDANCE_COMPUTER Commands ... end_processor
Overview¶
WSF_OLD_GUIDANCE_COMPUTER implements a processor that models homing guidance. It obtains the platform’s current target track (track) from the track_manager and computes desired vertical and lateral forces that will steer the platform to intercept the track. The desired side forces (possibly limited by max_commanded_g) are set into a WSF_OLD_GUIDED_MOVER, which ultimately determines the resulting trajectory. It is important to note that WSF_OLD_GUIDED_MOVER also internally consults a supplied aero definition, which will further apply certain practical constraints. For particular portions of the flight, often both at the beginning and near the intercept point, the resulting motion may differ significantly from the ideal due to physical limitations.
Velocity Pursuit = Turn to place the velocity vector along line-of-sight to the target. Since this guidance mode does not compensate for target velocity, air-to-air engagements against a high-speed target will tend to toward a tail chase into the target from behind.
Proportional Navigation = Turn to null the inertial line-of-sight rates to zero. Since this guidance mode compensates for target maneuver in real-time, it is a nearly ideal intercept scheme for air-to-air maneuvering engagements.
The profile is modifiable, but the default trajectory flown will be a short period of velocity-pursuit guidance. After the velocity vector is approximately directed at the target, a switch to proportional navigation will be made. If the guided platform velocity is higher than the target velocity, and if no constraints intervene, a target intercept will occur.
The processor contains a simple drift model to account for self-positional uncertainty. Target positional uncertainty depends upon the accuracy of the platform current target track. Default behavior is perfect self-positional awareness. See the time_between_GPS_fixes and IMU_drift_rate keywords.
Commands¶
- guide_to_truth <boolean-value>¶
Specifies if the perceived target location (as defined by the current target track) or the truth target location should be used in the guidance computations.
Default: false
Specifies the gain to apply to a perceived inertial line-of-sight rate, to null the rate to zero. The default value may be inadequate, as the appropriate value is usually a matter of experimentation. Reducing the value to 0.0 would result in no guidance to intercept, and increasing the gain by an order of magnitude (10.0) would likely saturate the control system into a “bang-bang” type of oscillation, and wash out any ability to simultaneously accomplish g-biasing.
Default: 1.0
- velocity_pursuit_gain <float-value>¶
Specifies the gain to apply to a perceived off-boresight target angle, to place the velocity vector back on the target. (The gain comments made for proportional_navigation_gain also apply here.)
Default: 1.0
- g_bias <float-value>¶
Specifies the degree to which the trajectory should be compensated for vertical acceleration due to gravity. The default value of unity results in a somewhat flat trajectory along the line of sight to the target. Some missiles may be intentionally over-biased to fly an arcing trajectory to the target, descending upon it from above (use > 1.0, but < 10.0). Some inexpensive infrared-guided missiles do not contain an attitude reference gyro, and cannot “know” which way is up. Because they cannot compensate for gravity, they tend to fly a “sagging” trajectory below the line-of-sight to the target (use 0.0). Extreme positive values of g-bias (> 10.0) may possibly vertically saturate the control system, and wash out any ability to accomplish target homing guidance in the lateral direction.
Default: 1.0
- continuous_g_bias¶
This flag requests that g bias be used throughout the whole flight profile. The default behavior is not to turn on g bias compensation until the horizontal target distance is large relative to the vertical drop to the target. If a ballistic falling trajectory to the target has the ability to intercept, then further g-bias is not needed, and would likely even increase the miss distance.
- max_commanded_g <acceleration-value>¶
This sets the maximum commanded lateral turn acceleration that the guidance computer will request of the guided mover. This models a guidance-imposed structural limit constraint to prevent airframe destruction.
Default: 25.0 g
- guidance_delay <time-interval-value>¶
This keyword specifies a time interval after launch when guidance request is clamped to null. This models a no-guidance period to clear the launch rail, or perhaps a time before uplink guidance has been established.
Default: 0.0
After the velocity-vector to target line-of-sight angle decreases below this value, the guidance algorithm will switch from velocity pursuit to proportional navigation. This value is not used if either omit_velocity_pursuit or omit_proportional_navigation is specified.
Default: 30.0 deg
- omit_velocity_pursuit¶
This flag forces the trajectory guidance to include only pure proportional navigation, from the very beginning of the flight, with no velocity pursuit segment.
This flag forces the trajectory guidance to include only velocity pursuit, with no switch to proportional navigation.
- time_between_GPS_fixes <time-duration>¶
For the drift model, this duration specifies how often a positional update is received. The value will not be used unless the drift rate (below) is input as non-zero.
Default: 5.0 minutes
- IMU_drift_rate <speed-value>¶
For the drift model, this specifies the rate of drift of positional perception.
Default: 0.0 - No drift
- pitchover_guidance¶
Specifies that the weapon will use a proportional navigation guidance scheme only that is modified for weapons that must perform a pitchover maneuver to point at the target. The proportional_navigation_gain is adjusted throughout the weapon’s flight based upon the weapon’s closure rate. (All indented commands below require that this option be enabled.)
- maximum_loft_time¶
Specifies the time the missile will loft using the specified loft angle.
Default: 0.0
- loft_angle¶
Specifies the angle the missile will initially climb at for the specified maximum_loft_time.
Default: 0.0
- frequency_cutoff¶
A method of specifying the doppler frequency that determines the method of computing the pitchover gain. The guidance computer assumes a frequency of 10 GHz to compute the closing doppler frequency (i.e., doppler shift = 10.0e9 / light_speed * doppler_range_rate).
Default: 0.0 Hz
The minimum pitchover gain that is used when the doppler shift is less than zero.
Default: 0.0
The maximum pitchover gain that is used when the doppler shift is equal to or greater than the frequency_cutoff.
Default: 0.0
A factor that is applied to the computed pitchover gain.
Default: 0.0
Used to compute the pitchover gain when the doppler shift is less than the specified frequency_cutoff. The pitchover gain is computed as follows: Pitchover_gain = nav_gain_slope_1 * doppler_shift + min_nav_gain.
Used to compute the pitchover gain when the doppler shift is greater than or equal to the specified frequency_cutoff. The pitchover gain is computed as follows, unless limited by the max_nav_gain: Pitchover_gain = nav_gain_slope_2 * (doppler_shift - frequency_cutoff) + 0.5.
