Forum: F-16 Design & Construction

Forum Veteran Joined: Nov 07, 2003 - 09:12 PM Posts: 901

Hi all,

Am I right to state that the Viper is equipped with the CARA (Combined Altitude Radar/Radio Altimeter) that provides the AGL (Above Ground Level) readings?

Just curious, what is the 'combined' actually made up of? A combination of more than one type of altitudes?

cheers, Very Happy

F-16.net Sponsor

There is indeed a radar altimeter on board although I'm not sure if it's a CARA or not. The plane has a "ground clobber" warning so therefore it needs a HAGL input device, hence the RadALT.

CARA is Combined Altitude Radar Altimeter aka AN/APN-232. Two antennas under the nose, in the fwd equipment bay, provide altitude measurement from ground level up to 30,000 feet. CARA provides inputs for altitude low warning functions, the ALT LOW warning light on the left glareshield, the RADAR ALT caution light on the avionics caution panel, RDR ALT on the sensor power panel, the WARN and AL mnemonics in the HUD as well as the radar altitude scales in the HUD.

Salute!

We finally got the radar altimeter, but too late to help a few of my buddies.

First thing we SLUF drivers noticed was NO RADAR ALTIMETER! So two troops pranged when they could have had a warning they were getting too low above the terrain in poor weather/night. See SLUF thread on other forum.

Such a simple thing, but the initial Vipers lacked several avionics systems that we old A-7D drivers had gotten used to. Smaller plane, limited space, etc.

later,

Gums and Habu2...Stemming off of your posts, is my question. Do the ALT LOW and Radar ALT caution lights come on at a specific AGL altitude, or a combination of low AGL altitude and aircraft attitude? I know "Bitchin' Betty" will come on invariably at lower altitudes, but of course, her warnings do not always mean impending danger.

TC-

Blk-40's no longer have the ALT LOW light on the left eyebrow row (I don't think Blk-50's do either but not sure). The Blk-30 did have the eyebrow light but I never flew one so I don't know how it was mech'd. The Radar ALT light on the caution panel only comes on if the system has detected a fault with the RALT itself, nothing to do with altitude.

The pilot can set two altitudes that Betty will start squawking about - ALOW and LIS (line in the sky). ALOW runs strictly off of the RALT and is not predictive. For instance, most dudes set 500' in the ALOW when flying low levels to give them a warning if they descend below 500' AGL (the lowest we can fly a low level). Betty will squawk "Altitude, Altitude" continuously until you get back above 500 feet. Again, it's reading strictly height above the terrain directly below the jet. The current ALOW setting is visible in the HUD directly below the RALT window (in this example it would read 'AL 500') and also flashes when you descend below the ALOW setting.

The LIS runs strictly off of MSL altitude (height above sea level) and Betty will say "Altitude, Altitude" only once when you descend below the LIS setting. I used it mostly for BFM engagements where I would set the LIS 1,500' above the "floor" to give me a warning that I was approaching the bottom of the MOA.

If you asked 1,000 viper drivers how they set up ALOW/LIS you would get 1,000 different answers. I think the important thing is that you have a good reason for whatever it is you set and recognize when Betty starts Bitchin'. Not as easy as it sounds when you're looking over your shoulder trying to keep one of your bro's from gunning you.

Speaking only to Blk-40 again (don't know how the other blocks are set up), a recent upgrade introduced GCAS (Ground collision avoidance system) that uses digital terrain elevation data in combination with the aircraft's current position and flight parameters to predict where the jet is going to be in a few seconds. Betty will squawk "Pull Up, Pull Up" and assumes you will take a few seconds to recognize what is going on and recover to a wings-level 5g pull (I think) to avoid hitting the dirt. You preset the lowest altitude you want the recovery to happen (50' for instance) and if you execute the recovery according to the assumptions you will bottom out at 50' above the ground.

Again, I'm sure there are minor differences between the blocks, but that's basically how they work. Clear as mud?

Push it UP!
Burn

Forum Veteran Joined: Nov 07, 2003 - 09:12 PM Posts: 901

Thanks for the response guys,

habu2, it is the various display, visual and audio advices that the CARA is providing, gives it the name 'Combined' altitude? It is not like, it 'combine' the altitude information from GPS or a combination of digital and analog information? Shocked

Burn, if my knowledge is correct, the radar altimeter is tied to the NVP, Terrain Following Radar too? If we lose the CARA, the Very Low capability of the TFR is out too?

Thanks, Very Happy

Pumpkin,

Not sure how the CARA ties in to the NVP, never flew TFR.

Burn

Burn-breath!

You telling me we back to the 500 foot AGL low-levels we had in the 60's and early 70's??????

I thot we had 200 feet if we had all the squares filled.

Hell I wish I had the Red Flag debrief tape of Cylon flight back in 1983 or so. Wingie and I evaded a few SA-6 sites, and would fly behind a knoll just as the Sam site was about to fire. They used our tape to show others how to do it.

Oh well, with JDAM why get low? OTOH, it's nice to sneak in NOE and then pop and launch that suker before they know you are close.

later,

Burn, I was paraphrasing a 25/30/32 Avionics System Manual so later blocks may have been (and probably were) diffierent.

TC, as Burn stated, pilot sets the limits, IIRC thru the UFC/DED interface.

Pumpkin, I don't know where they came up with the Combined term. Possibly because CARA is a pair of antenna, one TX the other RCV.

Burn, I don't think CARA and NVG are 'connected' at all. DTE gives you 'virtual' information i.e. what the DTE elevation database thinks zero AGL is for where your GPS thinks it is. CARA is based on 'real' radar returns from 'real' zero AGL (or AWL I guess too).

Speaking of 'real' I think CARA is +/-2% below 5000 ft AGL, 1% above that (to 30K)

Gums - Yeah, 500' is our min in training. Yet more proof that we're shifting away from the Fulda Gap mentality. Why expose yourself to every farmer with an AK and get shot to hell (ala Tornado / F-15E in DS I) when you can take down the SAMs and have near impunity above 10k'? That's over-simplified, I know, but that seems to be the general philosophy.
I'd love to see your Flag footage though!

Quote:

I don't think CARA and NVG are 'connected' at all

H-Deuce I assume you meant CARA/NVP? Again, I never used the NVP for anything other than FLIR, so I don't know how the TFR worked.

I may not have been clear about the GCAS system. It works totally off of a 'digital picture' like you mentioned, and has nothing to do with RALT data. GCAS goes off of the nav 'solution' (or where the jet THINKS it is), and compares that to it's terrain database in the DTC to tell you when it thinks your going to hit the ground. Just like anything digital the 'garbage in - garbage out' law applies. I.E. If you are flying down a narrow canyon and the jet thinks it is a 1/2 mile to the left of where it acutally is, the GCAS will probably catch on fire and Betty will bail out because GCAS thinks you are flying underground Shocked

!

Push it up!
Burn

Forum Veteran Joined: Nov 07, 2003 - 09:12 PM Posts: 901

Thanks anyway Burn. Wink

habu2, that is alright. Thanks. I just can't stop thinking why they chose to use the word 'combined'. Maybe there isn't a good reason at all. Shocked

For the TFR, I guess I have not put it correctly. I should have said the FLCS is receiving information from both the CARA and the TFR. The FLCS will pull the Viper out of the terrain following flight, should the CARA went off line. How does this sounds to you?

Thanks, Very Happy

Quote:

H-Deuce I assume you meant CARA/NVP?

Yep - fat-fingered that one... Embarassed

Forum Veteran Joined: Nov 07, 2003 - 09:12 PM Posts: 901

I found a saved copy of the article. Unforunately, the site is a broken link now. The CARA is employed as part of the FLCS-SWIM (automatic terrain following and system-wide integrity management). SWIM was only briefly mentioned here in an article to the AFTI.

cheers, Very Happy