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Characteristics
Boresight mode is initiated by the WSO using the Radar Mode knob. Once selected, the radar enters a status similar to holding half-action. The antenna is moved to a fixed position along the boresight line at 0° azimuth and -2° elevation below the FRL, or Fuselage Reference Line. The optical sight is also adjusted and depressed 35 mils, matching the antenna’s position. The target is illuminated as long as the target is close to the sight.
Target lock from Boresight mode.
When the pilot manoeuvres to superimpose the optical sight to a target, its echo is displayed on the radar scope, and the WSO can attempt a lockon. If successful, the pilot can now manoeuvre, and the avionics will still track the target. The crew can, therefore, adjust the geometry and engage as required. From this perspective, Boresight is a means of speeding up the acquisition of a radar lock after a Tally, so the WSO does not have to fiddle with the antenna elevation or azimuth since the pilot is pointing the Phantom as if it were a finger straight at the target.
Unavailable radar lock.
In case a radar lock cannot be obtained, the crew can still engage a target feeding a simulated Doppler information to the missile via the familiar Aspect Switch.
If the Phantom’s crew opts to employ an AIM-7 without radar lockon, they must, in primis, select the appropriate speedgate. A chart in the kneeboard helps the WSO to select the appropriate value. Each position of the Aspect Switch corresponds to a speed interval. The labels provide a good idea of such intervals.
If everything goes according to plan, the AIM-7 Sparrow should look for reflections from the target in a narrow cone around the gun pipper. The speedgate selected will help it identify the correct target rather than fall for clutter.
However, many factors can lead to a thrashed missile even when the parameters are apparently correctly set.
In the scenario demonstrated in the video linked above, a Tu-22M is flying hot and a few thousand feet lower than the Phantom. Attempting to lock the Tupolev will alert its crew and cause it to activate its ECM, breaking the radar lock. Given the good weather conditions and the abovementioned issues, the crew decides to engage using Boresight mode.
Before watching how the action unfolds, let’s consider a few potential issues.
Issue I: Lookdown
When looking up or feet-wet, aeroplanes probably represent the most prominent targets. There is no “competition” for the attention of the Sparrow’s seeker, and a quick test shows how the missile tracks correctly almost with any speedgate selected, Wide included. Look-down is different, and the first recommendation is to use boresight to highlight the target and then proceed to lock it. If this is not possible, the chances of hitting the target decrease dramatically.
Issue II: Closure rate
In this case, the distance covered in 5 wide sweeps is about a quarter of a grid, which, for a scale of 50 nm, results in circa 1000 kts. Now, checking the speedgates chart, it looks like “Forward” covers it quite well. “Nose” also seems close, but since I am in pure pursuit and the target is drifting, the closure rate can only decrease at this point, so I prefer to be conservative.
This kneeboard page will be discussed later in a dedicated article.
Back to the example, the ATA can help the Pilot put the target on the nose. Then, it is a matter of spotting it and placing the sight over it. Radar-wise, if the clutter becomes a problem, Gain can be reduced to somewhat declutter the radar screen, although it depends on scenarios, antenna elevation angle and circumstances.
Once Boresight mode is selected, the radar scope shows the returns from the sidelobe, the terrain and the contact. In this case, the main lobe intercepts the terrain after illuminating the target, thus returning a clear target echo.
As mentioned, the speedgate chosen for the engagement is Forward. Then, at a range of circa 13 nautical miles, FOX-1.
It seems that the AIM-7F is tracking correctly, and eventually, the target is splashed.
At this range, bombers’ ECM is often still an issue, but we have avoided the problem by using Boresight mode. At least in this scenario.
To demonstrate the criticality of selecting the correct speedgate, I have recorded the same scenario, but Wide is selected. As you can see, the Sparrow completely ignores the target.
After experiencing these examples, two more potential issues come to mind.
Issue III: No lead
Note, however, that the radar can stop illuminating the target briefly and then illuminate it again. The Sparrow should resume as it was, but this can affect the missile’s kinematics performance.
More information about Lead Collisions can be found in this article.
Issue IV: Manoeuvring Targets
The crew should consider how the engagement will present circa 2 seconds after pressing the trigger and select the speedgate option accordingly. In fact, 1.5 seconds are necessary for the missile launch, and then half a second passes before the initiation of the speedgate sweeping.
Conclusions
The Boresight mode is a very handy and flexible tool capable of expediting radar lockon acquisition and missile engagement in particular situations.
Employing an AIM-7 in boresight mode requires effort from both crew members. The challenges are not limited there, as each employment requires careful selection of the appropriate speed gate by the WSO and target tracking by the Pilot. On the other hand, this mode allows the Phantom to engage hostile aircraft even in conditions it normally would not be able to.
Lastly, it is essential to note that Boresight is not a “Stealth” mode. The radar emits radio waves to guide the missile or illuminate a contact, thus potentially triggering its onboard warnings.
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