RVSM performance monitoring - Why, Where and How
Almost 40 years ago, the initial discussions at ICAO level took place to implement Reduced Vertical Separation Minima (RVSM) in some regions to increase capacity . Meanwhile, the RVSM system has been established on a large scale and related procedures are well-known. In this article we look at the methods to ensure acceptable performance of the altimetry system. These tend to evolve and depending on the operating region and aircraft equipment, you will have different options available.
Why? - RVSM performance monitoring
Are design standards and maintenance practices not sufficient to ensure adequate performance of the altimetry system? Let us check the Altimetry System Error (ASE) plot of a particular airframe over time. Figure 2 represents samples of the ASE of a specific airframe taken over time by remote monitoring stations.
The above figure represents a “bad case” aircraft but makes it obvious that such performance degradations may occur over time during RVSM operation despite regular maintenance checks.
The above ASE was measured in flight, while no significant trend was observed during maintenance checks.
As it turns out, the ASE drift is hard to monitor by maintenance testing, as the sensors are subject to pressure variations that occur only in flight and cannot be reproduced in a hangar. Most of the variations are Mach number related and are significantly affected by the condition of the aircraft skin . Very common causes for such errors are paint degradation and hail damage. Figure 3 depicts a typical example.
Manufacturers usually provide guidance on how much skin degradation can be accepted and there are special tools to assess such conditions . Additionally, air data computers can suffer from sensor degradation over time, but these effects are generally easier to monitor with maintenance checks.
It is critical to understand that ACAS or STCA systems will not protect against an ASE-induced collision hazard, as they rely on reported barometric altitude .
In order to get a better idea of the vertical plane navigation we shall look at the related terms in the following paragraph.
Total Vertical Error (TVE)
Similar to the lateral domain, the vertical navigation error can be split into different components. Figure 4 below depicts this context. The difference between assigned altitude and actual altitude is known as Total Vertical Error (TVE). It is comprised of the Altimeter System Error (ASE) and the Flight Technical Error (FTE). The difference between the reported altitude and the assigned altitude is referred to as Assigned Altitude Deviation (AAD). The following should be remembered:
- The flight crew can see the FTE on their PFD.
- The flight crew cannot see the ASE.
- ATC can see any AAD.
Where? - Which regions and aircraft are affected?
ICAO standards require continuous monitoring of RVSM operations to ensure the target level of safety is maintained. Regional Monitoring Agencies (RMA's) have been established for this purpose . When it comes to selecting the aircraft to monitor, there is a regime known as RVSM fleet monitoring. For each aircraft type and fleet, a representative fraction of aircraft is selected and only those will need to be monitored .
How? - The monitoring methods
The tricky question is how to monitor the altitude-keeping performance of aircraft while in-flight, knowing that the frame of reference is barometric altitude. This adds another complication when other “geometric altitude frames” are used for comparison. Clearly, meteorological information, such as pressure and temperature patterns need to be known  . Over the years, several methods of RVSM performance monitoring have been developed. There are some regional differences as to what methods are currently accepted .
The Height Measuring Unit (HMU)
This is the “old-school” method. A fixed monitoring station with quite a limited coverage is used to measure the geometric height of an overflying aircraft. Different technologies are in use, such as primary radars in a dual-axis arrangement (historically) or multilateration stations (more recently) .
The Time-Difference-Of-Arrival (TDOA) of the mode S data is used to calculate the physical location of the aircraft. In Europe, the stations are located near Strumble (UK), Geneva (Switzerland), Linz (Austria) and Nattenheim (Germany) .
Aircraft Geometric Height Measuring Elements (AGHME)
This is essentially the FAA-version of the HMU. AGHME use multilateration as a technology and are located across the US and Canada . Both, HMU and AGHME require the aircraft to maintain wings-level steady flight for a certain period to achieve good quality results .
GPS Monitoring Unit (GMU)
Another option is the somewhat cumbersome installation of a mobile “RVSM monitoring kit” onboard an aircraft. The kit samples GPS pseudo-range data during the flight and post-flight processing permits accurate parameter estimation .
Due to the significant extra cost of this method, it is rarely used these days. As more and more aircraft are equipped with sophisticated GPS receivers and the GPS parameters are even transmitted via ADS-B, the obvious evolution is described in the next paragraph…
ADS-B Height Monitoring System (AHMS)
The idea to use ADS-B OUT data to check altitude keeping accuracy is definitely a smart and cost-effective solution. From an operator’s perspective, there is virtually nothing to do here. There are some pitfalls, which have to be kept in mind when using this method (on behalf of the monitoring agency) . The reasons lie in the manner that GNSS height is calculated onboard the aircraft. GNSS position is calculated in Earth-Centered-Earth-Fixed (ECEF) frame (x/y/z). It is then converted to WGS-84 (LAT/LONG/height). The ADS-B OUT data can be Height Above Ellipsoid (HAE) or Height MSL.
Some installations use HAE, some MSL. This depends on the RTCA DO-260 version .
To further complicate things, the standard to convert HAE to MSL can be NATO STANAG 4294 Navstar GPS System Characteristics Appendix 6 or EGM-96 .
HAE to MSL conversion is not the same for all receivers.
The monitoring agency has to take care of this in order to avoid unduly “flagging” of aircraft . Currently, the FAA permits US operators to use the US domestic RVSM airspace without additional approval, if they are equipped with a qualified ADS-B OUT system . In Europe, this is not possible as of today.
A glimpse into flight testing: The Orbis matching method
What to do, if an aircraft does not have a civil mode S XPDR and ADS-B OUT? Dedicated GMU flights might be too expensive. Think of a military fighter jet operator aiming to achieve RVSM monitoring in an efficient manner…
First of all: What is Orbis matching?
The Orbis matching method was developed by Lawless  and uses GPS-derived velocity components to estimate True Airspeed (TAS). The test aircraft performs a constant altitude, constant airspeed turn. The parameters that are required are at minimum:
- Total Air Temperature (TAT)
- Total pressure (uncalibrated)
- Static pressure (uncalibrated)
- GPS velocity data
The instruments record the relevant data during the maneuver and a locus of the east/west and north/south velocity components is plotted, the so-called “Orbis”.
The first thing to note about the Orbis plot is that it represents the lateral velocity components, not the ground track of the aircraft.
Without noise, turbulence and flight technical error, it would result in a circle. This is true, even if there is wind aloft . In the Figure 7 below, the blue locus represents GPS-velocities, the red locus represents uncalibrated TAS. The GPS locus (blue) is offset by the wind vector.
What might not be immediately obvious is the fact, that the residual error in the right side of Figure 7 is almost exclusively caused by the static error . This can be traced to the relationship between the total pressure error, temperature error and static pressure error. . The static error is the main contributor to the overall TAS error and therefore this approximation is valid in this context .
Results show that the method is sufficiently accurate to monitor the ASE in the context of RVSM operation .
 ICAO, Manual on a 300 m (1 000 ft) Vertical Separation Minimum Between FL 290 and FL 410, ICAO, 3rd edition, 2012
 Sultana, Continued Monitoring of RVSM, Eurocontrol, presentation at ASE workshop Atlantic city, 2017
 Warburton & Magnan, Improving RVSM-critical area inspection, Creaform/AMETEK, presentation at ASE workshop Atlantic city, 2017
 FAA, Altimetry System Error (ASE) Data Introduction to Discussions, FAA, presentation at ASE workshop Atlantic city, 2017
 Ten Have, Precision Aircraft Height Estimation with Multiple Radars, Journal of the Institute of Navigation, vol. 40, no. 2, 1993
 Middle East Regional Monitoring Agency, RVSM Height Keeping Performance webinar, 2020
 Tranchet & Bordenave, GNSS height on Airbus A/C, AIRBUS, presentation at ASE workshop Atlantic city, 2017
 FAA, Use of Automatic Dependent Surveillance—Broadcast (ADS–B) Out in Support of Reduced Vertical Separation Minimum (RVSM) Operations, US Federal Register, Vol. 83, No. 245, 2018
 Lawless & Jones, What is Orbis?, Flight Test News, Issue 15-02, SFTE
 Morton & Senter, APPLICATION OF ORBIS METHOD FOR STATIC SOURCE ERROR TESTING, US Navy, presentation at ASE workshop Atlantic city, 2017
 FAA, RVSM MONITORING GROUPS AND MINIMUM MONITORING REQUIREMENTS, August 2020