X-Virus-Scanned: clean according to Sophos on Logan.com X-SpamCatcher-Score: 15 [X] Return-Path: Sender: To: lml@lancaironline.net Date: Tue, 09 Jan 2007 09:42:53 -0500 Message-ID: X-Original-Return-Path: Received: from pop-scotia.atl.sa.earthlink.net ([207.69.195.65] verified) by logan.com (CommuniGate Pro SMTP 5.1.4) with ESMTP id 1745946 for lml@lancaironline.net; Tue, 09 Jan 2007 03:43:12 -0500 Received-SPF: none receiver=logan.com; client-ip=207.69.195.65; envelope-from=pbricker@earthlink.net Received: from user-vcaupfp.dsl.mindspring.com ([216.175.101.249] helo=paulrzm5oth3zt) by pop-scotia.atl.sa.earthlink.net with esmtp (Exim 3.36 #1) id 1H4CYr-0004Mt-00 for lml@lancaironline.net; Tue, 09 Jan 2007 03:42:25 -0500 From: "Paul Bricker" X-Original-To: "'Lancair Mailing List'" Subject: RE: [LML] Re: AHRS X-Original-Date: Tue, 9 Jan 2007 00:42:17 -0800 X-Original-Message-ID: <015001c733ca$14453a00$6401a8c0@paulrzm5oth3zt> MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0151_01C73387.0621FA00" X-Mailer: Microsoft Office Outlook 11 thread-index: Acczl4ZS7tCizhBfSCehAZQcNcukTQALKMMg X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.3028 In-Reply-To: This is a multi-part message in MIME format. ------=_NextPart_000_0151_01C73387.0621FA00 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Brent posted a question concerning what happens to the solution out of the Xbow when GPS is lost, and doesn't the presence of the GPS imply that it is needed. Having managed the development of a GPS/Mems IMU navigator for an unnamed military airborne platform I have some background in the subject. Just having some background does not provide all the answers, as the "graceful degradation" of the solution when GPS is lost is dependent on the implementation. Xbow is your best source for definitive information, but I don't know how willing they are to share this information, as I'm sure they spend big bucks developing and refining their implementation. GPS and IMU (inertial measurement units - fancy name for a package of gyros and accelerometers) have different and complementary strengths and weaknesses. GPS primarily measure range to the satellites, with relatively small "bias errors". Their weakness is a lag in response to dynamics (turns are the primary issue) and relatively high random errors. IMU respond quickly to dynamics, but have many error sources, most of which are slowly varying biases (I know-that's an oxymoron) Almost all integrated navigators combine the measurements in a Kalman filter. This is a software implemented data filter that estimates the best fit solution (State Vector (SV) in the parlance of the industry) to the measurements is receives. The SV out of the Xbow includes attitude (pitch/roll/heading) and velocity (speed, VSI and drift) and time If the measurements are perfect then the solution is very good, but the measurements are never perfect. Kalman filters do a good job to correcting random errors, but bias errors lead to errors in the SV. To counter this most Kalman Filters also have estimate several bias errors "states", mostly attributed to the IMU. These can include accelerometer biases, gyro drift rates, turn-on-to-turn-on errors, ... The list is much longer than I can remember. The GPS measurements allow the filter to estimate these error states. That is one reason we wait before moving the aircraft, and it take some time and software to be able to reinitialize in flight. The instruments have to stabilize, and the filter has to eat a sufficient number to measurement updates to allow the "error states" to stabilize. If the GPS goes away in flight many of these estimates continue to correct the IMU measurements, allowing for a relatively good solution for some time. (You should get a GPS failure warning to tell you to transition to the back-up instrumentation.) Just how good, and for how long, is the implementation dependent variable. Brent's suggestion to disconnect the antenna in VFR conditions and watch what happens is a good one. Be away that IMUs exhibit "g sensitivity", and acceleration due to turns and speed changes will affect the IMU measurements and therefore the SV. Perhaps Mike at Crossbow can provide some data on performance after GPS failure in flight. I don't know if this is any help, or just the ramblings of a retired engineer. As Grayhawk says."use at your own risk" Paul Bricker N63PB Trying to get into primer ------=_NextPart_000_0151_01C73387.0621FA00 Content-Type: text/html; charset="us-ascii" Content-Transfer-Encoding: quoted-printable

Brent posted a question concerning = what happens to the solution out of the Xbow when GPS is lost, and = doesn’t the presence of the GPS imply that it is needed. =

 

Having managed the development of a GPS/Mems IMU navigator for an unnamed military airborne platform I have = some background in the subject. Just having some background does not provide = all the answers, as the “graceful degradation” of the solution when = GPS is lost is dependent on the implementation. Xbow is your best source for definitive information, but I don’t know how willing they are to = share this information, as I’m sure they spend big bucks developing and refining their implementation.

 

GPS and IMU (inertial measurement = units – fancy name for a package of gyros and accelerometers) have different and complementary strengths and weaknesses. GPS primarily measure range to = the satellites, with relatively small “bias errors”. Their = weakness is a lag in response to dynamics (turns are the primary issue) and = relatively high random errors. IMU respond quickly to dynamics, but have many error = sources, most of which are slowly varying biases (I know-that’s an = oxymoron)

 

Almost all integrated navigators = combine the measurements in a Kalman filter. This is a software implemented data = filter that estimates the best fit solution (State Vector (SV) in the parlance = of the industry) to the measurements is receives. The SV out of the Xbow includes = attitude (pitch/roll/heading) and velocity (speed, VSI and drift) and = time

 

If the measurements are perfect = then the solution is very good, but the measurements are never perfect. Kalman = filters do a good job to correcting random errors, but bias errors lead to = errors in the SV. To counter this most Kalman Filters also have estimate several = bias errors “states”, mostly attributed to the IMU. These can = include accelerometer biases, gyro drift rates, turn-on-to-turn-on errors, ….. The list = is much longer than I can remember. The GPS measurements allow the filter to = estimate these error states. That is one reason we wait before moving the = aircraft, and it take some time and software to be able to reinitialize in flight. The instruments have to stabilize, and the filter has to eat a sufficient = number to measurement updates to allow the “error states” to = stabilize.

 

 If the GPS goes away in = flight many of these estimates continue to correct the IMU measurements, allowing = for a relatively good solution for some time. (You should get a GPS failure = warning to tell you to transition to the back-up instrumentation.) Just how good, = and for how long, is the implementation dependent variable. =

 

Brent’s suggestion to = disconnect the antenna in VFR conditions and watch what happens is a good one. Be away = that IMUs exhibit “g sensitivity”, and acceleration due to turns = and speed changes will affect the IMU measurements and therefore the = SV.

 

Perhaps Mike at Crossbow can = provide some data on performance after GPS failure in = flight.

 

I don’t know if this is any = help, or just the ramblings of a retired engineer. As Grayhawk = says…”use at your own risk”

 

Paul = Bricker

N63PB

Trying to get into = primer

 

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