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Home GPS systems (like your car navigator) is able to locate you in like a few seconds, but 737NG needs to align itself for 6-8 minutes.

Why do navigation systems in aircrafts take so long to align themselves?

Full name: INS = Inertial Navigation System

IRS = Inertial Reference System

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    $\begingroup$ I agree; I wish the IRS would hurry up and align itself with my position that I should not have to pay taxes. $\endgroup$ – Carlo Felicione Oct 28 '16 at 16:26
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    $\begingroup$ @CarloFelicione IRS would only hurry up and make you pay taxes faster and pay more. $\endgroup$ – vasin1987 Oct 28 '16 at 16:43
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    $\begingroup$ The AN/WSN-7 RLG used on Navy ships takes days to properly align. Regardless of the align method selected, a previously calibrated system requires between 16 and 20 hours to reach specified full navigational accuracy. A system that has not previously completed calibration requires between 68 and 72 hours to reach specified full navigational accuracy. $\endgroup$ – Steve Oct 28 '16 at 16:44
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    $\begingroup$ Further: The actual time required for the system to settle to within specification accuracy is determined by several factors. These include: geographic position, heading and speed of the ship, time of entry and accuracy of first position reset, the alignment method selected, and whether or not the navigation system has been previously calibrated. $\endgroup$ – Steve Oct 28 '16 at 16:44
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    $\begingroup$ @SpehroPefhany: that is exactly what happens. Getting a GPS fix "out of the cold" can take up to 30 minutes. This can significantly be sped up if you a) know the time, b) know where you are, c) have a current almanac, d) have current ephemeris data. Which is why mobile phones use so-called aGPS (accelerated / assisted GPS), where the phone gets a) the current time from its internal clock, b) a rough position from the cell carrier (via cell towers) and Google/Apple/Microsoft (via nearby WiFi networks whose location is known), c) and d) downloads the data via the cellular network instead of … $\endgroup$ – Jörg W Mittag Oct 28 '16 at 23:13
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squareIn short


The longer time required to align an inertial unit is the price to pay for its independency on any external equipment or signal, but Earth itself.

  • A GPS receiver is faster, but is only one small piece in a huge system. The GPS ground segment (control and monitoring stations) and space segment (satellites) are necessary for the GPS receivers (user segment in the jargon) to work. They have multiple possibilities to fail, this critical dependency cannot be afforded by an aviation navigation system, GPS must be backed up, and this is done using inertial (can be INS, IRS, IMU or combined with air data units like ADIRU).

During inertial alignment the unit accurately senses local vertical and true north (actually Earth polar axis). The problem is that sensing true north requires knowing latitude, and determining latitude requires knowing true north. This is solved using successive approximations of both data --fortunately this iteration is convergent-- until the desired precision has been achieved.

It's slow because Earth rotation produces tiny effects at small time scale, and we haven't found more practical sensors yet. The effects depends on the distance from the rotation axis, larger at the equator than at the poles.

  • While GPS relies also on very weak and seemingly random signals, they can be easily retrieved by digital signal processing as soon as the random generator algorithm and seed are known.

An IRS provides the position like a GPS receiver. IRS also provides the horizontal plane (attitude reference) and the north direction which are inaccessible to the GPS alone. This is another reason inertial systems are still used.


squareDetails


Gimballed vs strapdown

Gimballed inertial systems principle is simple: Accelerometers are maintained oriented with Earth using a gimballed platform, and all measurements are done in the Earth reference system.

In strapdown inertial systems, accelerometers and (laser/optical) rate gyros are aligned in the aircraft reference system, and measurements are done related to this system. Except this difference, the principles for alignment are quite the same.

enter image description here
Overall principle of a strapdown inertial system, source

Old gimballed inertial are longer to align because gimballed gyroscopes are less precise than fixed laser gyros. For the explanations, let's assume a gimballed stable platform.

The alignment consists of getting the Earth orientation and speed, and inertial unit initial position. After alignment, current speed, attitude and heading will be determined by integrating accelerations and rotation rates and adding the result to the initial values.

Alignment time vs accuracy obtained

Alignment aims to determine two axis: Vertical of the location (direction of gravity) and true north azimuth (parallel to Earth rotation axis). For this phase, both accelerometers and gyroscopes are used. While vertical is relatively easy to determine with accelerometers, true north direction direct determination is not possible:

  • True north direction cannot be sensed without knowing local latitude, and latitude cannot be determined without knowing the direction of true north.

The practical way to proceed is: Set an initial value for latitude (it can be assumed), determine approximate north direction. Knowing approximate north, determine approximate latitude. From approximate latitude, refine north direction, etc.

Accuracy of true north bearing increases with alignment time:

enter image description here
Source: An Improved Alignment Method for the Strapdown Inertial Navigation System

Alignment requires the unit to be motionless, hence any erratic motion decreases inertial signal/noise ratio which slows this process (passengers boarding, wind acting on the tail surfaces, etc).

Inertial doesn't need external equipment or signal

Inertial is a stand alone device, completely independent of any external equipment or man-made signal. It relies only on Earth rotation speed and gravity. That's why it's still used for navigation in spite of existing radio aids and GPS (GPS is actually another radio aid).


squareToday, inertial is mostly used to backup GNSS


One may wonder why inertial systems are still used at the GPS era, in spite of their limited accuracy and long alignment procedure. The main reason is that you cannot blindly rely on GPS constellation availability. Let's list some of the possible failures.

GPS is the US solution for satellite positioning or GNSS. While defense organizations can obtain credits for different reasons, the difficulty to set up a civilian GNSS has been striking with the numerous delays encountered building Galileo, the European GNSS. It needs to design, launch and maintain the satellite constellation (more than 20 satellites are required for accuracy and availability). The constellation must be permanently monitored and controlled from ground stations all over the World.

GNSS signal can be perturbed by natural phenomena: Sun activity, weather and volcanic ashes may partially or totally block radio signal, reducing accuracy and availability. If the GNSS constellation fails, no precise GNSS receiver can work:

  • For standard precision use (read: limited precision), GNSS (GPS, Glonass, Galileo, Compass, Navic) are/will be compatible and depending on their location, users can switch from one system to the other, and possibly can use the associated augmentation system (Waas, Egnos, SDCM, Gagan, etc).

  • Precise GNSS use actually requires deciphering keys for a given constellation, which are not shared between constellation operators for obvious commercial and defense reasons.

Dependency on the US DoD and military conflicts

The GPS is a US military system. Agreements have be signed between DoD and civilian organizations for a civilian exploitation of the GPS constellation. This gives worldwide aviation some acceptable guarantees about GPS signal access.

However GPS is still vulnerable to jamming by opponents. In theory it is also sufficient to destroy the four ground control antennas to make GPS receivers useless after a few weeks.

enter image description here
GPS control stations

GPS is not always accurate and available

Dilution of precision

GPS availability and accuracy is given by an indicator: dilution of precision or DOP. The larger the DOP, the less accurate the system. Accuracy is significantly degraded for a DOP larger than 3

It's worth remembering that, even without any of the problems previously listed, a maximum DOP value is not guaranteed, or even feasible, in polar regions, and is only guaranteed for 95% of the time in other regions. That was a design principle retained since the beginning, to limit the number of satellites in the space segment.

While high, 95% still means 72 minutes per day with unknown performance. Example of large DOP peak exceeding 3:

enter image description here
Accuracy degradation

Radio signal reception

Obviously radio signals are not received underground (tunnels), underwater (submarines), and can be significantly weakened by foliage, to the point GPS is difficult to use in tall and dense forests.

When GPS is only temporary unavailable, inertial solutions can be used to maintain some position determination capability until the GPS signal is reacquired. This is very often used in car navigation systems to deal with tunnels and tall buildings. The inertial unit is a small (and very low precision) MEMS contained in the GPS receiver case or the smartphone.

Orbital parameters

Finally, let's rethink about the statement that GPS fix is nearly immediate.

That's true as long as the receiver is already aware of where the satellites are located. The constellation periodically broadcast the orbital parameters (navigation message containing almanacs, ephemeris and time corrections) for each satellite. This takes 13 minutes to get the almanacs required for a coarse positioning, at the slow rate used.

If the receiver has not yet the almanacs, or they have been cleared, then 13 minutes are required before the first fix can be done (and 13 additional minutes if the reception is interrupted while receiving the almanacs).

To get the regular accuracy, the recent ephemeris of the satellites used for the fix are required, this may take an additional few seconds. Ephemeris are updated each 30 minutes if I remember well.

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    $\begingroup$ It should be mentioned that these days there is inertial reference system, which provides inertial navigation, but also feeds the attitude indicator, heading indicator, autopilot and, on aircraft that have them, the flight law controllers, all of which also benefit from the alignment. $\endgroup$ – Jan Hudec Oct 31 '16 at 18:58
  • $\begingroup$ @JanHudec: Related user manual of an ADAHRS where the synergy between data sources is visible (e.g. air data checked and validated by IMU). $\endgroup$ – mins Oct 31 '16 at 20:02
  • $\begingroup$ that is also important, yes. All in all, it makes the inertial reference an essential part of the avionics suite rather than just a navigation backup. $\endgroup$ – Jan Hudec Oct 31 '16 at 20:48
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Also to mention is GPS only really tells you where you "were" not where you are. GPS pulses are only received at the staggering rate of 1 Hz. So once a second you get a "position fix" on where you were one second ago, plus some dead reckoning on apps like Google Maps and such which attempt to guess where you are going between pulses. INS on the other hand computes almost continuously, with many aviation variant like the Honeywell 764 calculating position at about 240Hz.

Depending on the data interface connected, you won't get the position fixes that quickly (40hz) but that's still much faster than GPS. Plus an INS provides information to an airplane that's almost more important than position, and that's attitude, velocities, and accelerations. These are absolutely essential for systems like Autopilot, Heads up display, radar, etc. Plus with a closed loop Kalman filter, you can ensure that calculated errors are fed back into the system to give you extremely accurate position over a long period of time. (~0.1NM/he drift).

GPS is useless for autoland, autonavigation, and self contained approaches, which are a big deal for military applications. The Honeywell mentioned above is an EGI (enhances GPS/INS). It takes the GPS pulses and hacks the INS position every second. This really is the best of both worlds. When GPS is highly accurate (based on the position of the constellation overhead) then you can't beat a GPS position. But when GPS is bad, or denied, the INS will reject the hacks and get the job done.

GPS is convenient, but I don't think it'll ever replace a good old fashion INS.

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    $\begingroup$ Welcome to aviation.SE. This seems more a comment to another post rather than an answer to the original question. Please consider editing it to meet the standards of this site. Once you will have gained enough reputation, you will be able to comment under any post. $\endgroup$ – Federico Oct 17 '17 at 16:43
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    $\begingroup$ Welcome! That seems to be a (good) comparison between INS and GNSS, but this is not related to why INS alignment is slow. You should have answered this question: Do today's aircraft still have INS (inertial navigation system)? $\endgroup$ – mins Oct 17 '17 at 17:18

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