Only GPS

I always wondered, can I use the plane only with gps? I mean without the ins aligned. If not, why? Having a gps would allow me to do all stuff right? The ins is for back up.

Thanks

GPS is a 3-axis instrument while the INS is a 6-axis instrument.

The general performance of all systems requires this 6-axis information so GPS is insufficient. Further GPS as an externally-provided signal could not be guaranteed. And so tactical aircraft like the A-10 were designed intrinsically around INS. The later inclusion of GPS was applied as an optional correcting bias to INS which allowed the design to remain practically unchanged while receiving the benefits of GPS while smoothly reverting to sole INS reliance as necessary.

In practice it is INS which is irrevocably integrated and GPS which is a luxury. The nature of INS makes it an ideal central repository of information to which many correcting sources may be applied, GPS, radio altimetery, barometry, etc.

For precise weapons delivery of unguided munitions, only gyroscopic systems have the precise character required.

In cases of INS failure it is necessary that HARS is used for attitude to which the EGI GPS portion of the system may be used as an aid to positional navigation. If GPS-only is chosen without an INS failure, INS continues to provide attitude information.

It is possible to fly a navigation-only flight with GPS and HARS but it is not the primary operating configuration and extra features like CCIP/CCRP calculations will be not available.

Thanks man.

Does an INS like the Mirage's have some kind of map loaded or something? How does it know that n distance is equal to n Latitude/longitude displacement?

If you input the initial coordinates, why does it have to align? Cant it start using the gyros and accelerometers from that initial point?

These are some doubts i always had about ins. Thanks

Thanks man.

 

Does an INS like the Mirage's have some kind of map loaded or something? How does it know that n distance is equal to n Latitude/longitude displacement?

 

If you input the initial coordinates, why does it have to align? Cant it start using the gyros and accelerometers from that initial point?

 

 

These are some doubts i always had about ins. Thanks

AFAIK the alignment doesn't align the position. It aligns the gyroscopes and accelerometers that meassure rotation and acceleleration. They need to be aligned as close to "zero" (= no movement, meaning no acceleration and no rotation) as possible. This is why the aircraft has to stand still during alignment. This is for aircraft on a stable surface. It is oviously a bit trickier with aircraft on aircraft carriers. They are usally connected to the carrier by a special cable for the alignment process, so the carrier can feed its movements to the aircrafts INS which can then align based on that data. In flight alignments rely on the aircrafts own data, which is why you have to fly straight and with consistent speed during in flight alignment. It's still much less precise than the ground alignment of course.

Regarding your other question about coordinates: That's actually a good question I don't know either. Given that the distances between longtitudes differ depending on the distance to the poles there has to be something more to position calculation than just meassuring the distance?

AFAIK the alignment doesn't align the position. It aligns the gyroscopes and accelerometers that meassure rotation and acceleleration. They need to be aligned as close to "zero" (no movement) as possible. This is why the aircraft has to stand still during alignment. This is for aircraft on a stable surface. It is oviously a bit trickier with aircraft on aircraft carriers. They are usally connected to the carrier by a special cable for the alignment process, so the carrier can feed its movements to the aircrafts INS which can then align based on that data. In flight alignments rely on the aircrafts own data, which is why you have to fly straight and with consistent speed during in flight alignment. It's still much less precise than the ground alignment of course.

 

Regarding your other question about coordinates: That's actually a good question I don't know either. Given that the distances between longtitudes differ depending on the distance to the poles there has to be something more to position calculation than just meassuring the distance?

Hi qui gon.

So what does it really do the alignment process? What does it mean to align to zero movement? Cant they sense they are still?

An in flight aligment with gps like the a10c is not precise? Even with gps? I guess that has something to do with the fact it doesnt align position. But still dont get it.

So what does it really do the alignment process? What does it mean to align to zero movement? Cant they sense they are still?

I'm no expert in this, but as I understand it, they actually don't just know what "still" is and need to be adjusted everytime. IIRC the reason for this is the rotation of the earth. But unfortunately I have not enough knowledge about this subject to go any deeper than that.

An in flight aligment with gps like the a10c is not precise? Even with gps? I guess that has something to do with the fact it doesnt align position. But still dont get it.

Yes, AFAIK the GPS has nothing to do with the INS, except that it updates the position in regular intervalls to correct for INS drift. Without GPS you would have to do so called "position fixes" to update the INS position. This means you have to fly over a recognizable landmark which has it's coordinates stored in the INS. Once the aircraft is above the landmark, you hit a button and the INS corrects its position. This process is implemented in DCS with the M2000 and the Viggen. If the aircraft has GPS (like the A-10C), then the GPS can provide corrections as I just said.

Interesting read QuinGon :thumbup:

...This process is implemented in DCS with the M2000 and the Viggen

One slight comment is that, I didn't test it recently but the INS alignment wasn't working correctly in M2000 but the Viggen could be used as a good illustration.

Thanks, but careful, as I said, I'm no expert in this field. This is just my understanding and if it's not quiet right, someone pls correct me :)

Thanks guys!

The m2000 aligment works fine AFAIK, the INS update is what doesnt work.

I see, gravity and earth rotation must be taken into account so the ins knows the movement doesnt come from the plane!

I think i got it :)

The m2000 aligment works fine AFAIK, the INS update is what doesnt work.

A brief (and somewhat simplified) guide to inertial navigation:

IMU: Inertial Measurement Unit. A sensor package with accelerometers and gyroscopes.

INS: A computer system that uses the data from the IMU in order to determine your current position. It may be completely independent of external signals, requiring manual position updates (including your initial position) using known landmarks, or it may be integrated with GPS (typically using a kalman filter, which combines the short term accuracy of the IMU with the long term accuracy of GPS).

First of all, all the IMU does is to measure acceleration in the X, Y and Z axis. From the acceleration measurements, you get speed by integrating the acceleration, and distance traveled by integrating the speed (but this is typically done by the INS; the IMU is just the box with the sensors). In order to keep the accelerometers aligned in inertial space, you either place them on a gyro stabilized platform (gimballed IMU), or use complicated transformation matrices which require a lot of computing power (strap down IMU).

The A-10 uses a gimballed system, and during alignment, the platform is positioned in the orientation which gives the minimal initial value for the X and Y axis accelerometers (i.e. minimal gravitational acceleration in the sensor's axis) and the maximal value (1g) for the Z axis (-Z to be precise) accelerometer. The gyros will measure the earth's rotation, and thus a baseline rotational value (measured over a relatively long period of time) is needed in order to compensate for this in the INS.

Does an INS like the Mirage's have some kind of map loaded or something? How does it know that n distance is equal to n Latitude/longitude displacement?

 

If you input the initial coordinates, why does it have to align? Cant it start using the gyros and accelerometers from that initial point?

No map needed. Given two L/L points and a pencil and paper you can work out the great circle azimuth, distance, etc. It's just a math equation. Maybe an intimidating one but to a computer no sweat. One interesting thing is that great circle navigation (aka straight line) on a globe is that it's not a constant-heading solution and airplanes like the A-10 will provide such navigation solutions over long distances.

For the same reason if I gave your position and said "take ten steps, where are you now?" you couldn't say. Ten steps in what direction? Alignment is more fully called a "gyro-compass alignment."

So what does it really do the alignment process? What does it mean to align to zero movement? Cant they sense they are still?

 

An in flight aligment with gps like the a10c is not precise? Even with gps? I guess that has something to do with the fact it doesnt align position. But still dont get it.

Alignment is mostly a direction-seeking process. It wants to know that the nose is pointing 102.67° heading or whatever it is. Given that you're at some L/L and not moving relative to the surface the accelerations that the system sees in the few minutes must be due to the rotation of the Earth, there's only one particular orientation of the airplane that corresponds to that. At the same time there's a precise determination of what direction down is (can't assume the parking spot isn't sloped) and a calibration of what zero velocity is.

Position and orientation are the outputs of an INS. They are changed over time by velocity and angular rate which must be established externally at some instant somehow. Translational and rotational accelerations are the only things the INU can sense directly. To imagine the INU's job you have to imagine yourself in a windowless room with only weights on springs (accelerometers) as your only source of information. Accelerations are applied as modifications to the assumed velocity/angular-velocity and modified velocities effect how the position evolves from one moment to the next.

Calibrating zero velocity is an understandably important task and many INSs in airplanes will have a way of manually or automatically improving their understanding of what z-vel is. It might be a button. In the A-10C's case there is an automatic refinement of z-vel after you switch from align to nav while remaining parked (NARF). The manual even says to switch to nav as soon as it starts flashing to get the best z-vel calibration possible.

In-flight alignments are pretty crap. It's all based on assuming you're flying straight and level, constant speed, no slip, etc. which you simply can't do to the same precision as on the ground parked. You'll find the EAC and JDAMs won't play nice with an air align. With even a partial align you won't get EAC to stay on until a little while into the flight when the filters improve the INS solution to a better state. It's enough to fly home but not ideal going to combat.

Thanks guys.

Guys, I need to revive this thread :)

 

I had this https://forums.eagle.ru/showthread.php?t=161437

 

And wanted to ask about the GPS only possibilities on the a10c.

 

If I dont have the INS aligned, why wont the GPS be able to provide accurate position?

yes you can.

A-10C Flight Manual Page 186

FLIGHT DRIVER Status, Center 5. Displays status of current flight driver. This can be either:

o BLENDED: Combination of INS and GPS navigation input.

o INS: Only INS navigation input.

o GPS: Only GPS navigation input.

This can be selected from the NAV page. You will generally set this to BLENDED unless either the INS or GPS systems become inoperative. If inoperative, you would want to select only the operating system (INS or GPS).

[TABLE]NAV MODE | Attitude Source | POS source | Velocity Source

BLENDED | EGI INS | EGI | EGI

INS - only| EGI INS | EGI INS | EGI INS

GPS | If INS is valid, INS;otherwise HARS | EGI GPS | EGI GPS[/TABLE]

I know and did that, take a look at my post pls. There you can see my question.

 

Gps didnt provide any position.

 

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Do you check the GPS page. it takes abotu 2 minutes to complete GPS init, the pos data is available asap the init is completed. and you need cycle NAV mode to GPS on NAV page

Basically you can't force GPS-only. Airplane will still use INS for attitude if it's valid even if you choose GPS as your flight driver. Position should be GPS but attitude will be INS.

Of course you can always select HARS on NMSP for HARS attitude. There's no such thing as GPS attitude no matter what happens.

GPS is a 3-axis instrument while the INS is a 6-axis instrument.

What does this mean? How does a navigational instrument can give positional information with more than 3-axis? Does the "3-axis" mean something else in this context than the x,y,z in space?

What does this mean? How does a navigational instrument can give positional information with more than 3-axis?

Pitch, roll, yaw.

Oh, so the "axis" are not the coordinates of the aircraft but rather the variables in the calculation formula. Got it, thanks

Oh, so the "axis" are not the coordinates of the aircraft but rather the variables in the calculation formula. Got it, thanks

The directional labels are confusing as they describe the object's position relative to a camera, but it gets the point across.

Yeah, i thought of those "axis" as positional components (like x,y,z in R^3 space) but instead they are the variable components that are used by the calculation formula to determine the change in position.