I decided to split this articles in different parts due to its length. This article covers chapter I and II.
As I mentioned already, this series will be reviewed later when features such as the TWS Auto and the new WCS will be released.
I went through most of these topics in this live chat with some fellow F-14 Pilots and RIO (use the timestamps in the description to navigate).
I. PRF mention and P/PD/CW differences
Pulse Repetition Frequency: The pulse repetition frequency (PRF) is the number of pulses of a repeating signal in a specific time unit, normally measured in pulses per second and is expressed in hertz (Hz).
PRF limits depend on the detection range of the radar and the relative velocity between the target and the F-14.
The AN/AWG-9, despite being technology from the ‘60s, has one of the biggest antennas ever mounted on a fighter aircraft and sports a powerful transmitter. Its detection range is greater than many other more modern fighters.
Low PRF (the range buttons above the DDD affect the PRF frequency). Pulse is used in “all-aspects”, its reliability vs clutter makes PSTT the strongest “grip” on the target.
Pulse is a DDD-only mode, it has no representation on the TID. It can be used for basic group mapping and has a detection range of ~50/60 nm for a fighter-size aircraft. Pulse uses range gates to track the target.
Pulse modes available are Pulse Search and P Single Target Track.
High PRF. Higher detection range, drawbacks are the presence of two detection gaps (see AVI-02-R) and decreased look-down performance.
The Pulse Doppler uses the relative difference of velocity (Doppler Effect). A target moving perpendicularly to the radar falls into the same group of clutter filtered by the radar as they are considered unwanted returns (Mainlobe Clutter Filter). This explains the common “notching” manoeuvre used by defending targets.
Depending on the radar mode (RWS, TWS), an additional FM ranging is employed, reducing the detection range of the PD radar.
CW is a now obsolete antenna but in the ‘60 was still used to guide the AIM-7 Sparrow. The main difference is that a Pulse radar transmits short pulses and uses the time delay to calculate the distance from a target. A Continuous Wave radar use a stream of modulated signals and uses the difference between the reflected and the emitted signals frequencies.
CW has no minimum range, has reduced acquiring range compared to Pulse radars and its detected and jammed more easily.
II. Zero Doppler Filter
The Zero Doppler Filter is a low pass filter and has the effect of creating a blind spot when:
ΔV = VF-14 – VTGT = ±100kts
Being hardware limitation, it cannot be bypassed manually. The options to defeat a target hidden in the ZDF are:
- use Pulse radar;
- work on the geometry.
Since a target filtered by the ZDF has relative speed close to the F-14’s, this issue must be taken into account as the target aspect varies from Hot to Cold (e.g. when the F-14 is chasing a target).
A target close to enter the ZDF blind spot appears close to the MLC trace on the DDD, for the DDD shows AZM vs Closure Rate in PD mode.
The TID in Aircraft Stabilized mode can be used to determine when there is a risk of ZDF.
The example in Plate 2.1 illustrates three different scenarios. In each scenario, the Blue vector represents the F-14, the green vector the Target and the dotted line, the angle of the Velocity vector. ΔV is indicated by the internal dimension line. The vectors are in scale.
Rules of thumb:
- If the target aspect is “hot”, “flanking” or “beaming”, there is no risk of ZDF.
- If ΔV > 100, there is no risk of ZDF.
- If VTGT > VF-14 the outcome depends on the aspect of the target.
Therefore, if a “cold” target is shown via DL on the TID but the AWG-9 is not picking it up, ZDF can be the cause.
The two main ways to counter a target entering ZDF (willingly or by chance) are:
- Switching to Pulse STT;
- Changing the geometry of the engagement.
The first solution is covered in Chapter IV.
The second solution is intuitive: considering the second example in Plate 2.1, if the F-14 RIO commands to Pilot to turn away from the target, the velocity difference (ΔV) will avoid prevent the target to be filtered by the Zero Doppler.
The Plate 2.2 illustrates the example: ΔHDG = 30°; ΔV = 90kts, the target is filtered.
By manoeuvring, ΔHDG = 45°; ΔV = 170kts, the target is not filtered anymore.
A missile launch is such conditions is unfavourable but, if the missile has been launched already, this manoeuvre allows the RIO to reacquire the target, hence having the AWG-9 WCS resuming the missile guidance.
DDD and ZDF
The following example (Plate 2.3) shows a target entering and leaving the Zero Doppler filter.
A target is spawned parallel to the F-14 (V=600kts, Active Pause activated), same latitude (6500ft). The target starts at 400kts, then accelerates to 700kts, then decelerates to 200kts.
- The first DDD shows the target flying at a constant speed of 400kts. Since the closure rate is constant, the mark is not moving.
- The target reaches the first waypoint and accelerates to 700kts. The closure rate is decreasing and the speed is changing. This behaviour is clearly indicated on the DDD.
- ΔV is now < 100 kts. The target is being filtered by the ZDF. The mark is fading and barely visible.
- As the target decelerates, it leaves the filtered speed and reappears on the DDD.
- The target is set to decelerate from 700kts to 200kts, the trace of such trend is clearly visible.
I shown a very similar behaviour in the video linked above.
I hope you have found this interesting, feel free to share feedback and suggestions!