Overview of the INS

[Larzei]

This is an overview of the INS system in the F-18C as the manual is outdated. I hope this information can be useful for some of you. 

 

Preliminary condition for alignment: 

Parking brake ON

Left engine >60% N1

 

Normal alignment time 8 minutes

Stored heading alignment 90 seconds. This do not any require pre-alignment and shutdown beforehand or special settings in mission editor, and therefore will be available on first startup. 

Alignment process is shown in a quality number on the HSI:

- NO ATT = INS warm up and leveling phase

- 99 = starting of course alignment. The number is decreasing as alignment improves and go into fine alignment

- 0,5 OK = alignment finnished. Turn INS knob to NAV (without GPS available)  or IFA for GPS assisted INS (AINS) before flight. 

 

Ground/carrier alignment:

- CV CBL - Alignment on ship using external cable from ship to aircraft for initial position. Selected automatically with INS knob to ''CV'' 

- CV RF - Alignment on ship using datalink. Not implemented.

- CV MAN - Pilot mus manually insert initial position. Not implemented.

- GND - used for alignment on ground. Waypoint 0 is the initial reference position for alignment and automatically set to your position on spawn. Waypoint 0 coordinates will show on HSI while aligning.  

Note:

*During CV alginment, NO WYPTS will show below ''qual'' and ''time'' on HSI as initial position is slaved to carrier using either CBL or RF. 

*Stored heading is possible in both CV and GND alignment.

 

If the INS looses alignment in flight due to a system failure, perform the following:

In flight alignment without GPS available:

- set INS knob to IFA and IFA RDR will show on HSI. This uses positional velocity updates from radar (PVU) to align INS. 

In flight alignment with GPS available:

- set INS knob to IFA and IFA GPS will show on HSI.

Note: Approx time for IFA RDR/GPS is 10 minutes. 

 

Other:

On the INS panel you can also select

- TEST and  GB - Gyro Bias calibration is not implemented. 

-GYRO - This is using the INS system as a attitude and heading reference system only and it will not give any position outputs. 

 

NON GPS Environment - set mission date before 1986. If playing redfor after 1986, set ''unrestricted sat/nav'' to OFF in mission editor. GPS is available in missions set after 1986, but redfor planes need to have ''unrestricted sat/nav'' in ME set to ON . 

(picture from ED F-18C manual)

Drift rate on the INS without the GPS correction is exessive and require updates regularly. 

How to update the INS position during flight:

- Tacan update: tune into an tacan station available in the tacan database (HSI->DATA->TCN). Press UPDT on HSI and select TCN. You can then choose to select ACPT or RJT, when clicking accept the INS position is updated. Approx 30 feet accuracy. 

- Waypoint fix: target designate your waypoint and fly over it's position on the map. It must be easy to recognize from the airplane, for instance the end of a runway, a special building etc. Your hud or HMD will most likeley show the waypoint has drifted off the real position. Press DSG before you fly over the point and ACPT the moment you are right above it. INS is now updated accordingly. 

- AUTO and MAP function is not implemented

Note: when updating the position the top middle part of HSI is supposed show a correction value before you accept or reject it, but this is not implemented. 

Edited September 27, 2023 by Larzei

[Tholozor]

52 minutes ago, Larzei said:

So a couple of question guys, I would appreciate if someone could help me out here. 

1) Is it possible to enter CB RF and CV MAN alignment? 

2) What does UPDT-> AUTO and UPDT-> MAP do? 

3) Is there or will there be a possibility to slave a sensor to a known location and update via this method? 

4) How do IFA RDR work? 

1) Not at this time.

2) Currently nothing, the UPDT functions are mostly unimplemented.

3) This would be part of the UPDT function.

4) Uses the radar to obtain precision velocity updates (PVU) for INS alignment (does not provide positional information).

[Larzei]

11 minutes ago, Tholozor said:

4) Uses the radar to obtain precision velocity updates (PVU) for INS alignment (does not provide positional information).

Thank you Tholozor. I see the IFA RDR works independent on radar settings (even when OFF) so I gues it is not fully implemented either, but rather just thrown in there to make it possible to re-align without GPS if  needed? 

[QuiGon]

On 9/4/2023 at 1:58 PM, Larzei said:

NON GPS Environment - set mission date before 1986. If playing redfor after 1986, set ''unrestricted sat/nav'' to OFF in mission editor. GPS is available in missions set after 1986, but redfor planes need to have ''unrestricted sat/nav'' in ME set to ON .

I'm pretty sure GPS in DCS becomes available in 1994, not 1986.

  

14 hours ago, Larzei said:

Thank you Tholozor. I see the IFA RDR works independent on radar settings (even when OFF) so I gues it is not fully implemented either, but rather just thrown in there to make it possible to re-align without GPS if  needed? 

A precision velocity update (PVU), which afaik is what the Hornet should do when selecting IFA RDR, would only fix the drift rate of the INS, but not the actual drift.
But as you have said yourself: these finer aspect of the Hornet's INS aren't really implemented yet.

There's also a nasty side effect of the Hornet's insufficent INS implementation: When flying without GPS the CCIP bomb mode will not work properly. It will accumulate an offset over time (especially in missions where wind is present), causing the CCIP pipper to be completely off target. That happens even when using AGR.

Edited September 8, 2023 by QuiGon

[Larzei]

2 hours ago, QuiGon said:

I'm pretty sure GPS in DCS becomes available in 1994, not 1986.

I will test later today and update the original post. 

2 hours ago, QuiGon said:

A precision velocity update (PVU), which afaik is what the Hornet should do when selecting IFA RDR, would only fix the drift rate of the INS, but not the actual drift.
But as you have said yourself: these finer aspect of the Hornet's INS aren't really implemented yet.

If it is an alignment it should only orient itself to where it is on the earth (lat/long) and then calculate true north based on earth rotation, I don't think it can fix the drift rate (unless it also performs a calibration in this process). I will only guess the PVU works to put ground speed into the equation which tell the airplane how it's moving while sensing the gravity and earth rotation. I would also guess it would require a specific flight profile with an optimal speed, altitude, bank/pitch limitations and so on the be working correctly. 

 

3 hours ago, QuiGon said:

There's also a nasty side effect of the Hornet's insufficent INS implementation: When flying without GPS the CCIP bomb mode will not work properly. It will accumulate an offset over time (especially in missions where wind is present), causing the CCIP pipper to be completely off target. That happens even when using AGR.

Have not tried CCIP yet, but a INS fix 6-8 miles before a target point in CCRP works really well. It is great fun to play around without the GPS a little also to see how it worked in the ´´good old days.´´

[QuiGon]

47 minutes ago, Larzei said:

If it is an alignment it should only orient itself to where it is on the earth (lat/long) and then calculate true north based on earth rotation, I don't think it can fix the drift rate (unless it also performs a calibration in this process). I will only guess the PVU works to put ground speed into the equation which tell the airplane how it's moving while sensing the gravity and earth rotation. I would also guess it would require a specific flight profile with an optimal speed, altitude, bank/pitch limitations and so on the be working correctly.

 

I don't think the PVU alignment takes position into account, at least not in the F-15E where this is already implemented. AFAIK it will just meassure the aircrafts movement over ground (as you said) and thus neutralize (or minimize) the drift rate of the INS, regardless of what the actual aircraft position is.

See this comment on PVU alignment on the F-15E from KlarSnow who is a former F-15E WSO afaik:

  

47 minutes ago, Larzei said:

Have not tried CCIP yet, but a INS fix 6-8 miles before a target point in CCRP works really well. It is great fun to play around without the GPS a little also to see how it worked in the ´´good old days.´´

AFAIK the CCIP issue has nothing to do with position drift, but with the INS drift rate as a result of wrong velocity meassurements. Hence a position fix will probably not fix the CCIP issue, but I haven't tried that yet tbh.

The bug is described throughout this thread, including various tests:

 

Edited September 8, 2023 by QuiGon

[norman99]

Hopefully, with realistic INS drift/updates already being implemented in the F-15E, it’ll help highlight what’s missing in the current Hornet module, and motivate ED to implement these features in the near future.

[Ignition]

On 12/2/2023 at 4:58 AM, norman99 said:

Hopefully, with realistic INS drift/updates already being implemented in the F-15E, it’ll help highlight what’s missing in the current Hornet module, and motivate ED to implement these features in the near future.

In the F-15E it's still very green, there are some things that are missing and/or working incorrectly, but it is in a very nice starting point since it has been out from a few months now.

I hope after so many many years of development of the F-18C ED finally add a proper INS simulation to this aircraft.