Operational and calibration faqs

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Frequently Asked Questions
Date: January 9, 2003
Product: ADU 5
Version: 1.0

Operational and calibration FAQs

Q: If I am upgrading to ADU5 from previous ADU receivers (ADU2 or ADU3), what differences can I expect in terms of integration and interfacing?

A: The differences are very few; The ADU5 is identical to previous ADU receivers in terms of form and fit. All connectors (power, antenna and I/O) remain the same, as does the sensor housing itself. In terms of functionality there are several significant improvements, which result in some additional serial interface commands and responses, primarily those concerning onboard SBAS and Beacon DGPS capabilities.
Q: What is the advantage of using ADU5 over previous models of ADU?

A: Performance and functionality have been enhanced in all operational categories, including position and attitude accuracy, reliability and solution availability. Maximum data output rate has been increased from 2 Hz to 5 Hz. Angular rate information has been added to the output data structures. The calibration procedure has been greatly simplified.
Q: How long has this type of technology been available? Is it proven technology?

A: Thales Navigation (then Ashtech) introduced the first commercially available GPS attitude system in 1991. Many of the original Ashtech 3DF receivers are still in operation today. The ADU5 is now a fifth-generation attitude determination product, and performance enhancement has been the main focus for each generation of these products. The ADU5 receiver represents a mature and stable technology based on years of experience and development.
Q: For heading computation purposes, must the ADU5 antenna array be aligned with the centerline of the vehicle or platform it is mounted on?

A: No. The vector between antennas 1 and 2 typically defines the heading vector. If there is an offset or skew angle between this vector and the true centerline, it can be “dialed out” either in the calibration routine or by means of an attitude setup configuration command ($PASHS,3DF,OFS…..). The heading offset angle can be determined by a variety of independent methods, depending on the application.
Q: Are there any restrictions in terms of antenna mounting configurations?

A: Very few, as long as the antennas have good skyward visibility and can maintain a rigid fixed relationship to each other. For instance, it is not necessary for all antennas be on the same horizontal plane. The geometrical shape of the antenna array can take almost any form and be of any size, depending largely on cable run limitations. The only restriction is that the radial vectors formed from Antenna 1 to the other three antennas should not be co-linear i.e. they should not closely overlay each other. Certain antenna configurations can be used to maximize angular accuracies for any specific axis of rotation. Antenna mounting and configuration are key factors for ADU5 performance and this is thoroughly discussed in the user manual. Thales Navigation tech support personnel can work closely with the user to help design the best possible configuration for your application.
Q: What types of antennas are used with ADU5?

A: Depending on the application, Thales Navigation typically recommends either the “Marine IV” L1 GPS antenna (p/n 103661) or the L1 GPS aircraft antenna (p/n 103062).

In order to use the Beacon DGPS capability of ADU5, the dual purpose L1/Beacon antenna (p/n 110438 – Marine or 110902 - ACFT) must be used for the primary (#1) antenna.
Q: It’s been said that GPS attitude systems are subject to occasional computation outages due to poor satellite geometry. Is this also true of ADU5?

A: Any GPS receiver can suffer from accuracy degradation or outages especially when antennas are not situated in optimum locations. Solution availability is one of the most important considerations for GPS navigation. ADU5 attitude availability is generally specified at >99.9% but extensive testing has shown that given proper antenna installation, it is common for ADU5 to go many continuous days without suffering any outage events at all.
Q: Are all four antennas needed for the attitude computation? Can I install just two antennas and compute only the heading?

A: Yes, all four antennas are needed in order to do any attitude computation. Even though theoretically it only takes 3 points (i.e. 3 antennas) to define a plane, the fourth antenna is essential in order to provide redundant measurement data and increase the speed of the attitude initialization. The ADU5 will not compute any component of attitude without all 4 antennas tracking satellites. Other Thales Navigation products are available to those users who are only concerned with heading and position information. Even if the user only wants heading information, the ADU5 has the advantage of being able to increase its angular accuracy by means spreading the antennas further apart.
Q: Do the antenna coax cables all have to be the same length in order to compensate for cable delay of the signal?

A: No. Cable length delay does not need to be considered for ADU5 operation. The ADU5 attitude computation algorithm uses carrier-phase differencing techniques to remove the effect of any delay caused by using different coax cable lengths.
Q: Can ADU5 be installed in a helicopter?

A: Yes. The revolution of the rotors does not cause any signal interruption or loss-of-lock. However, care must be taken in the location of antennas in order to minimize multipath caused by the rotor mast. ADU equipment has been successfully installed and operated in helicopters on many occasions.
Q: Does ADU5 support use of local RTCM corrections from an external source (not Beacon or SBAS)?

A: Yes, the use of local RTCM corrections for DGPS positioning is fully supported.

Q: What are the most important things to consider for ADU5 antenna installations?

A: Location, location, location and rigidity. The site that offers the best possible skyward visibility is always the most desirable. Rigidity of the antenna array is also extremely important. Once the array has been calibrated, the spatial relationship between the antennas must be maintained within millimetric tolerances.
Q: What relationship does antenna separation have on attitude accuracy?

A: Angular accuracy is directly related to the separation (vector length) between the antennas. The ADU5 data sheet contains information about the accuracies to be expected given various antenna separations.
Q: Are there any theoretical or practical limits to antenna separation?

A: Yes, although the answer to the preceding question is true, there are some practical limitations to consider; Extending the vector length between the antennas can reach a point of diminishing returns if the antennas are so far apart that they occupy completely different multipath environments. This introduces “differential multipath” which is more difficult to deal with than multipath common to all antennas. Hence, a point can be reached where the benefit of longer vector length is negated by the more complex multipath problem. It is also harder to maintain rigidity in the antenna array as the separation increases, as in the case of a seagoing vessel which flexes and bends to a lesser or greater degree depending on the sea state. Length of cable runs is also a practical consideration.
Q: Are there any vehicle dynamics issues, which might affect system performance?

A: High rates of change in vehicle attitude should not affect ADU5 performance unless such maneuvers impact satellite tracking; An example would be an airplane in steep climb or a steeply banked turn, when skyward visibility can be blocked by wings or fuselage and cause loss of satellite lock. If in fact they exist, the upper limits for roll, pitch and turn rates have not been encountered by current ADU customers. The ADU5 is subject to government restrictions on operating speed and altitude (1,000 knots and 60,000 feet respectively) but these restrictions can be lifted with valid export license and special firmware.
Q: Does the ADU5 calibration need to be performed every time the system is used?

A: No. Providing that the antenna array stays unchanged and does not suffer any deformation, the calibration procedure only takes place once, on initial installation. However it is always good practice to run more than one calibration during the installation process in order to verify that the final result is of good quality.
Q: What are the primary factors that adversely affect ADU5 performance?

A: These are four in number: 1.) High multipath environment- the existence of reflective surfaces near the antennas, which cause signal multipath. This is probably the single most detrimental influence. 2.) Satellite geometry, specifically PDOP >4. 3.) Poor quality calibration. 4.) Actual deformation of the antenna array after it has been calibrated.

Q: How can I be sure that the ADU5 is providing accurate attitude information?

A: There are several QC indicators provided to the user in the data output structures; the most important of these is the baseline RMS (BRMS) indicator. BRMS is the residual of the differences between the actual vector lengths (which are defined exactly by the calibration process) and the instantaneous vector measurements between the four antennas. A typical clean BRMS value will be in the range of 1-2 centimeters. If the BRMS value exceeds 4 centimeters, the attitude solution can be compromised or outages can occur. There are four factors that adversely impact the BRMS threshold- they are the same four factors outlined in the preceding question.
Q: Does the ADU antenna array need to be motionless during the calibration procedure?

A: No. The calibration software can accommodate a dynamic platform and based on the user selection of either static or dynamic conditions, will use an optimized algorithm for the calibration processing.
Q: Does the vehicle or platform need to be in a level state during the calibration process?

A: No. If the platform is not level during the calibration the user can indicate this to the software and the resulting calibration vectors will be automatically rotated to a level state.

If this is done, pitch and roll offset angles are also computed. The user has the choice to use the unadjusted (or “non-rotated”) vectors plus the pitch and roll offsets, or use the adjusted (rotated) vectors with no offsets.
Q: The calibration procedure for the earlier generation attitude receivers was somewhat confusing and unfriendly. Is this method still used for calibration of ADU5?

A: No. The earlier method of calibration required a high degree of user interaction and was rigorous in its data acquisition requirements. New calibration software is now available that is highly automated and user-friendly. It performs all the steps of calibration, including data acquisition, vector processing and upload of parameters to the receiver with very little need of interaction by the user. It is also greatly improved in reliability and speed.

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