Forum: F-16 Procedures

Well, if you do a sustained dive from low altitude, you will definitely get an INS vertical velocity error....about the time you hit the ground.

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It's been a long time since I worked on any INS platform but from what I do recall, there's several factors involved. The fact that you mentioned 'sustained' is an indicator of some of the error-position handling algorithms. (At low speeds, near-earth velocities.)

On the older ASN-92 platforms, F4 pilots, frustrated that their dead-reconing skills were more accurate than the INS (In some, but not all flights) would pull into a vertical climb, roll 360 degrees and dump the INS, resulting in the entire system (5 boxes) needing to be pulled to investigate.

I don't know why, but bootstrap systems are more prone to VV errors, perhaps because the vertical aggregate was derived from gyro precession values and X/Y/Z Accel rates which essentially point back to the software.

It may also be attributed to quality of parts used although I doubt this. Detection of small changes at small velocities is more challenging than detecting big changes at big velocities may be at play here as well.

Or... Smile

Calibration at I-Level or Depot level might be suspect....

When in doubt, blame the developer Smile

Point of INS alignment was to accurately determine true north....as the jet moves from an initial location through measurement of accelerations in all 3 axis (x,y,z...horizontal and vertical) measurement from previous position to current position occurs (inertial navigation) so initial current position in regards to earths surface is critical but also in relation to latt/long reference system (NAD 1927, WGS84, etc.) all of which modeled earths surface in different ways. Position errors could occur from errors in all these data points (starting position, database system errors, errors in acceleration measurements, errors in true north determination, etc.)
Alignment still uses measurement of accelerations to find earth's rotational velocity at entered latitude and takes time to make the appropriate determination (true north is 90 degrees from earths rotational velocity at a given latitude) whether you are using an EGI or spinning mass INS. GPS can help with present position and refine velocities but the good old 8 minutes still gives a better refinement, spinning mass INS or EGI. But an EGI is way better at 4 mins than a spinning mass at 8 mins any day and better than standard RLG+GPS. Integration of GPS and INS is better in EGI than in blk 40 with separate GPS and RLG where INS data and GPS only mingle in kahlman filter... but measured in minor differences.

In the end depends on what type of accuracy you needed for how long. Air to Air didn't need much so scramble and go ASAP, especially now with inflight alignment, you can get going quick. Want to drop IAMs? better get a better platform.

This phenomenon seems to only be present in some INS systems and the solution is a period of level flight to allow the INS to correct itself. I guess this is just a function of older INS/RLG systems.

Forum: F-16 Procedures

INS alignment time and time to takeoff from cold start