When to use different radar modes

[hollowman8904]

When is it appropriate to use the different radar modes (high/interleaved/low)? The manual states the different ranges of each mode, but does not discuss why you would want to use anything but high.

[nomdeplume]

Short version: Low-PRF is potentially useful when you're in a "look-up" situation trying to find contacts with a low closure rate, e.g. slow-flying helicopters or planes flying perpendicular to you.

 

Why: High-PRF uses the Doppler shift from the radar returns to work out the contact's closure rate. This provides "look-down" capability by allowing the radar to eliminate returns from the ground by simply ignoring anything that is "stationary" i.e. approaching the aircraft at the aircraft's own ground speed (+/- a bit to also filter out vehicles travelling on roads, so maybe around 130 kph). This also means that it will ignore any aircraft with a closure rate similar to your ground speed.

 

In practice, that means an opponent can easily cause your radar to lose lock by turning perpendicular to your radar beam (i.e. putting you "on the beam", at their 3 or 9 o'clock) while flying lower than you (so your radar is in look-down mode). This practice is referred to as "notching" as it is deliberately causing the enemy radar to disregard you because you're in the "doppler notch", i.e. the range of closure rate that will cause your return to be ignored.

 

Low-PRF mode is not subject to the Doppler speed filtering (I think the rate of radar returns is not high enough to work out the closure rate), so it can detect slow-flying targets in the above situation. However, because it's not filtering out returns, it will be completely swamped if it's actually looking at terrain. If a contact is much closer to you than it is to the ground, its radar return may be strong enough for the radar to distinguish it from the background. But for something like a helicopter that's below you and near the ground, low-PRF won't find it. It also won't find a fighter closing at high-speed if it's below you; the radar will just see a wall of returns it can do nothing useful with. This also applies to co-altitude contacts if there's e.g. a mountain behind them.

 

In DCS, helicopters are also often hidden from your radar due to this effect, since they nearly always fly lower than you, and fly relatively slowly. I'm not sure this is realistic since the rotors should be very visible, fast-moving objects on radar, but it is useful for game balance purposes. Basically: a hovering helicopter will be invisible to your radar in high PRF look-down mode. It's also unlikely to be visible to you in low-PRF mode unless you're somehow at a lower altitude than the helicopter, and it has no terrain behind it (from your perspective).

 

The notion of "look-down mode" itself is potentially a bit nuanced, and I don't know how it's implemented in DCS or the M-2000C module. I think there's basically two methods for determining if the radar needs to consider itself in "look-down" mode: a) by dynamically measuring the volume of returns and using that to enable/disable the filter as required or b) enabling it whenever any part of the radar cone is below a predefined altitude. Option b is the simplest and requires configuring the aircraft's avionics with the highest altitude of any terrain in the area it's expected to be operating in. This means your radar can be in "look-down" mode even if there is no terrain behind the target from the perspective of your aircraft, and even if the target is at a higher altitude than you.

 

So: low-PRF is only useful when you're searching for slow- targets like helicopters, or other fighters trying to hide in your doppler notch, so long as they don't have terrain behind them. However its drawbacks are that it has a reduced detection range, and you can't hard-lock (PIC/DTT) or guide S-530 missiles (since the RDI radar needs high PRF for both). If you try to launch an S-530 in PID/TWS mode with low/med-PRF, the radar will try to transition to high-PRF and go to PIC, but may fail - and even if it succeeds, the lock may drop anyway.

 

Medium- (or Interleaved-) PRF tries to strike a balance between the two modes, by switching between the two for each scan (interleaving). In theory this means you get the benefits of both modes while mitigating the drawbacks of both. But since it's not using its full output power for either mode, you don't get the same range as in pure High-PRF. Guidance and PIC aren't available with Medium-PRF either.

[VTJS17_Fire]

Short version: Low-PRF is potentially useful when you're in a "look-up" situation trying to find contacts with a low closure rate, e.g. slow-flying helicopters or planes flying perpendicular to you.

Good explanation of the pulse repetition frequency (PRF).

 

But why should the low-PRF good for "look-up" situations? The clutter filter in high-PRF only filters out clutter below your altitude (notch gate). Above you, only clouds are filtered out by the system. Other contacts are shown on your screen.

[nomdeplume]

But why should the low-PRF good for "look-up" situations? The clutter filter in high-PRF only filters out clutter below your altitude (notch gate). Above you, only clouds are filtered out by the system. Other contacts are shown on your screen.

True. But I guess if it's activating the clutter filter when the radar thinks it's in "look-down" mode, then it can occur that the filter is active even when it's not needed.

 

So perhaps low-PRF is really only useful in that very specific regime: where you are trying to maintain contact with something that is not actually in front of terrain, but the high-PRF notch gate would be active anyway.

 

But this can be a pretty big area. For example if you're on the deck, on the Caucasus map there's peaks around 15,000 feet at least - so even if your radar is most definitely pointing "up" it'll still be in "look-down" mode because of the possibility of terrain behind it. You'll only truly be out of "look-down" mode if you're above the highest terrain and your entire search cone is pointing up. This is of course assuming the simpler "option b" above is in effect, i.e. the "look-down" is activated based on a predetermined altitude.

Edited September 21, 2016 by nomdeplume

[jojo]

As a matter of fact, currently in DCS, target's notching doesn't work when you're in look up.

This is why it's useful to dive harder than your target after the shot, to put it and keep it in look up, so he can't break your radar lock (HFR).

Edited September 21, 2016 by jojo