**[INSTALLATION AND FLIGHT INSPECTION OF HEIGHT MONITORING STATIONS
(HMU) IN EUROPE]{.underline}**

**(ABSTRACT PREPARED BY M. YVES GARRIGUES)**

**1 - BACKGROUND**

The inaccuracy of traditional altitude measuring chains for aircraft
increases with the altitude.

Up until the 1960s, the performance of commercial airliners in service
did not allow them to reach very high altitudes in flight, and a
conventional vertical separation of 1000ft minimum was fixed. It was
entirely compatible with the altimetry errors observed. All this was put
into question with the appearance of the first high-performance jets
and/or turboprop aircraft. It is for this reason that, as from 1966,
ICAO adopted the following rule :

\- From the transition altitude up to flight level FL 290: the vertical
separation minimum remains at 1000 ft. Above level FL 290 and up to FL
410: the vertical separation minimum is fixed at 2000 ft.

\- This allows levels FL 310, 330, 350, 370, 390 and 410 to be used.
These six levels are reduced to three available levels with application
of the quadrant rule. Flight levels above FL 410, which are only
accessible to Concorde flights and a few other rare high-performance
aircraft, also have a vertical separation minimum of 2000 ft.

The idea of reducing the vertical separation minimum to 1000 ft (RVSM:
Reduced Vertical Separation Minimum) instead of 2000 ft between FL 290
and FL 410 appeared towards the late 80s. The reasons were both to
optimise aircraft fuel consumption of course, and also to improve the
capacity of the airspace. Allied to this factor is an improvement in the
accuracy of altimetry measurements on the new generations of aircraft.

The vertical separation minimum in effect since 1966 generally only
provides three possible levels between FL 290 and FL 410 for a given
route. It appears that doubling the levels available provides the
possibility of giving the aircraft a choice of flight levels to allow
them to choose the best option in terms of altitude and route followed,
which varies according to the influence of the wind.

At certain peak periods the flight levels are saturated in the European
upper airspace. The implementation of RVSM will certainly not absorb the
currently increasing traffic on its own, (approximately 6% per year
before 11 september 2001), however it will contribute to a marked
improvement.

There should be a resulting improvement in the capacity of the airspace:
a total of six additional flight levels will be available (300, 320,
340, 360, 380 and 400). However, this does not mean that the quantity of
air traffic could double in the volume under consideration!

On 27 March 1997, ICAO authorised limited implementation of RVSM between
FL 330 and FL 370 in the area of the North Atlantic Minimum Navigation
Performance Specifications (NAT MNPS). This authorisation was extended
between FL 310 and FL 340 as from 8 October 1998.

**Further to this implementation, the following improvements were
observed:**

1\. 50% of the fuel penalties attributed to the NAT system itself were
eliminated (these penalties are based on the routes, weather forecasts,
flight levels and congestion of the system).

2\. It was possible to publish 25% fewer compulsory routes, which means
more flexibility for airlines in the choice of other routes.

3\. There was a 5% increase in flights authorised to use both the
altitude and route desired

4\. In view of these benefits, as from 24 February 2000 ICAO authorised
RVSM to be used in the Ocean Pacific area. Extension of RVSM to the West
Atlantic area of the New York FIR portion (WATRS) was planned as from 1
November 2001, and finally to the Continental European area as from 24
January 2002.

In the oceanic areas these applications were fairly limited, firstly in
view of the small volume of traffic, and secondly because of the
specialised nature of the type of aircraft (long-haul flights only) and
airlines. On the other hand, in the Continental European area this
operation affects a large population and is decisive for future
applications (North America, the Far East, etc.) moving towards a
possible world-wide generalisation.

The operational implementation of RVSM in Europe, as done previously in
the oceanic areas, has required a large number of conditions to be met.
Among the most important ones, two major ones can be identified:

\- operational approval of aircraft,

\- setting up an altitude hold monitoring system.

**2 - OPERATIONAL APPROVAL OF AIRCRAFT**

On this point, which is not the main subject of this article, we would
recall that operational approval, which is subject to the sovereignty of
the States of registration, requires the aircraft to be certified, and
for the operators to set up a training programm for crews, as well as a
system for updating the various manuals concerned (Operation Manual,
Maintenance Manual, etc). Certification is not a difficulty for recent
generation aircraft such as the A320, A330, A340, B777, etc. On the
other hand, it can require major technical modifications to be carried
out for certain types of aircraft, even as far as installing an Air Data
Computer system (ADC) on some of the older generations. However, the
North Atlantic experience has shown that the average operational gains
allowed a rapid return on investment for the operators (not more than
one year).

For groups of aircraft, of which the design and manufacture are
nominally the same in all aspects that may affect the accuracy of
altitude hold performances, ICAO recommends that the Minimum Aviation
System Performance Standards (MASPS), verified during the certification
process be as follows :

\- the Total Vertical Error (TVE) does not exceed an average of 80 ft
(25 m) ;

\- the standard deviation is less than (92-0.004z2) for 0\<z\<80, z
being the average TVE, in feet, or (28-0.013z2) for 0\<z\<25, when z is
expressed in metres ;

\- and the components of the Total Vertical Error have very specific
characteristics.

For aircraft not belonging to a group, i.e. those with unique altimetry
system characteristics, the values are slightly different, particularly
the aircraft ASE, which must not exceed 200 ft (61 m) in all flight
conditions.

These ICAO technical recommendations have been transcribed in Europe in
the airworthiness regulations (document JAA TGL 6), which is the
applicable document for certification.

Normally, aircraft that do not satisfy these requirements are not
authorised to fly in an airspace where a reduced vertical separation
minimum is applied.

**3 - SETTING UP OF MONITORING SYSTEMS**

One of the difficulties of RVSM is the level of confidence that can be
given to the aircraft certification by the States of registration, as
well as maintaining the certified characteristics over time.

For this reason, ICAO recommended setting up a monitoring system in
order to determine to what degree aircraft measure up permanently to the
altitude hold performances.

**This monitoring system has two purposes:**

1\. In the period preceding the effective implementation of RVSM, i.e.
as long as flights have a vertical separation of 2000 ft, the monitoring
systems check the altitude hold accuracy of aircraft that are already
certified. These inspections contribute to the safety case. It had to be
shown that spacing at 1000 ft in the upper airspace does not deteriorate
the current safety level of flights in Europe. The final decision to
implement RVSM on 24 January 2002, or to postpone it was dependant on
the results of this study.

2\. In the period following implementation of RVSM, the monitoring
systems have to ensure that there is no degradation in performances of
altitude hold accuracy for aircraft authorised to fly in the European
Continental RVSM airspace, through a lack of maintenance or for any
other reason. It was felt that this monitoring phase should last for
seven years, meaning until January 2009. At the end of this period, the
monitoring systems should, in principle, be withdrawn from service.

For the European Continental RVSM, Eurocontrol was requested by the
European Civil Aviation Conference (ECAC), an emanation of ICAO, to set
up this system.

The monitoring system is composed of stations fixed to the ground, named
HMU (Height Monitoring Unit). However, as it is illusory to think one
can succeed in monitoring 100% of the fleet, the HMU is completed by an
individualised monitoring system named GPU (GPS Based Monitoring Unit).
This is installed on board certain carefully selected aircraft, which
are representative of the categories not well monitored by the ground
monitoring system. This system was developed by ARINC for the NAT RVSM.
The principle of a GMU is simple. It consists of a digital recording of
the GPS altitude of the equipped aircraft, expressed of course in the
WGS84 reference system. From these recordings, an analysis is carried
out a posteriori on the ground to determine the altitude hold
deviations.

**4 - PRINCIPLES OF MEASUREMENT**

**BY TRANSPONDER MULTILATERATION**

The principle of the HMU is to measure the geometric height of each
aircraft and to provide this height, together with the aircraft
identification and its assigned flight level, to a computer named TMU
(Total Vertical Error Monitoring Unit), which sends the data it has to a
unit located in Brétigny/Orge (Eurocontrol Monitoring Cell).

An HMU station is therefore a unit comprising :

\- a sub-assembly composed of a central HME computer which determines
the x, y, z position of the aircraft, the TMU computer, and systems for
communication with the Monitoring Cell;

\- several sub-assemblies (at least four) which are the measurement
stations sited in such a way as to satisfy the above requirements.

The measurement station is a transponder multilateration sensor that
detects mode A, mode C and mode S replies from the aircraft situated in
its detection area. The arrival times of each reply are dated, and using
an algorithm, the HME computer calculates a 3D position without any
co-operation required from the aircraft or its crew (the transponder
works anyway).

When an aircraft enters the coverage area of an HMU, the closest ground
receiver (measurement station) starts to detect and extract the reply
codes from the transponder (mode A, mode C and short squitter messages
giving the mode S address if it is equipped). Normally this occurs at a
distance of 70 NM, well beyond the limits of the radioelectric horizon
at the flight levels concerned.

The same thing happens for the other measurement stations when the
aircraft enters their coverage area.

At the measurement station, the transponder replies received go through
a complex process including, in particular, rejection of the effects due
to multipaths or to garbling, and extraction by statistical methods (not
all replies are needed to elaborate a position).

Time stamping of the pulses deemed to be correct is carried out by each
measurement station, using the GPS signal. Since this includes
inaccuracies as the time information can have a difference of a few
nanoseconds on a cycle of one second on average, a Cesium atomic clock
is used which will provide the required accuracy in the short term.

After that, this information is sent to the central computing station.
The TMU computer needs valid data from at least four stations in order
to elaborate an x, y, z position.

It is to be noted that there is a difference in design between the
Nattenheim-Geneva system, built by the British firm Roke Manor Research,
and the one at Linz built by the Czech firm ERA. At Linz the pulses are
not time stamped on the measurement site, but sent directly to the
central computing site, which does the time stamping.

The code values statistically retained are sent to the central computer
at the rate of 1 Hz and this is what decides which transponder replies
are received by at least 4 measurement stations and can be used to
calculate a position from the algorithm elaborated by the manufacturer.

In order to measure the Total Vertical Error, the altimetry system error
and the assigned altitude deviation, the ground computer needs to know
the true geometric altitude, the geometric height of its assigned
altitude, the altitude sent by the transponder and the altitude
indicated in the cockpit.

In the case of the HMU, the hypothesis is made that the correspondence
error is null, consequently the technical flight error is equal to the
assigned altitude deviation. In other words, the altitude sent by the
transponder is equal to the altitude indicated in the cockpit.

The geometric height of the assigned altitude is a parameter that the
computer in the central station elaborates from meteorological data
supplied by the World Met Office located in the United Kingdom. From a
network of observations made over the whole of Europe, they transmit the
geometric heights that have been forecasted according to a previously
defined grid, which includes the HMU coverage areas. These are valid for
24 hours and updated every 4 hours. In actual fact, they are sent, after
automatic analysis, to the HMU stations by the Eurocontrol Centre in
Brétigny every 6 hours.

The processing chain of the HMU central station functions in the
following way :

\- the HME computer supplies the true altitude of the aircraft from data
received from the measurement stations ;

\- the TMU computer (Total vertical error Monitoring Unit) evaluates the
errors (Total Vertical Error and deviation from the assigned altitude),
using the previous data, the flight plan data and meteorological data.

The monitoring cell in Brétigny studies these results to prepare the
safety case requested by ICAO (International Civil Aviation
Organization). They carry out the necessary investigations with the
users of aircraft that pose a problem.

The aim of the altitude hold monitoring system is, of course, to cover
the largest possible number of aircraft using the airspace situated
between FL 290 and FL 410.

For the NAT airspace, two HMU measurement stations have been installed
at the entrance points of the airspace concerned (Strumble in Wales and
Gander in Newfoundland). Taking into account the specific character of
the airspace and the existence of a transition area, which is
unavoidable, it was not difficult to determine the location of these
sites.

For the Europe area, the choice was rather more complicated. Studies
started in 1996.

The number and position of the HMU stations to be installed depends
directly on the proportion of traffic to be monitored.

A study was carried out on the air traffic density established from the
flight plan information available at the CFMU.

It has to be noticed a correct correlation between the density of air
traffic areas and the European \'economic banana\'.

The Frankfurt region shows the highest density (30840 flights). The
calculations show that if one single HMU were installed in this region,
it would monitor practically half of the total fleet. The addition of a
second station in the Geneva region would reach 60%. A third station
further East, in Austria, would take this figure to 65% by introducing a
wider diversity in the types of aircraft and operators (flights towards
the Middle East and Far East).

On the other hand, the siting of additional stations would not increase
the percentage affected to any large extent, and to cover all of the
fleet a very large number of stations would be needed (approximately a
hundred), which is economically unrealistic.

The number of three stations installed in the regions mentioned above
therefore seemed to be a good compromise. However instead of Frankfurt,
which is a congested region from a radioelectric point of view, it was
the Nattenheim region, located about a hundred kilometres to the West
which was finally chosen.

The specifications for each of the stations were fixed as follows :

\- Coverage area: circle with a radius of 45 NM.

\- Availability: 24 hours a day.

\- The sections of air routes monitored by the station must have a
minimum length of 25 NM.

**5 - DESCRIPTION OF THE HMU STATIONS**

**ARCHITECTURE IN GENEVA**

Once the number and location of the HMU stations had been decided (i.e.
the sites of Linz, Nattenheim and Geneva), the next decision was to
establish the optimum architecture and particularly the geographical
layout, of each of the measurement stations. Here we will only examine
the Geneva station on which STNA worked directly.

The Lémanique basin surrounding the city of Geneva is a plain which
extends towards the North-East and which is edged by a chain of
mountains to the North (Jura), to the South (Chablais) and to the West
(Revermont and Bugey).

The area allows very easy installation of a central system (HME and TMU
computers) in the Sky Guide \' NAG \' building at Geneva Cointrin
airport. However, the measurement stations necessarily have to be
installed in the neighbouring terrain, which is mountainous.

Several criteria need to be taken into account in the choice of sites
for the measurement stations:

**a) **High points, which meet the required coverage specifications, but
are nevertheless easily accessible for installation and maintenance.

**b) **Sites already having appropriate installations if possible with,
in particular, an electric power supply and adequate security.

**c) **Sites with no radio disturbance, particularly in the ATC
transponder and GPS receiver radio-frequency ranges.

**d) **Possibility of transmitting data from the measurement station to
the central station. The dedicated telephone line solution was quickly
abandoned in favour of the solution of microwave transmission. This
decision heavily influenced the choice of sites since it requires either
line-of-site ranges between the stations and the NAG, or the
installation of relays.

After much long and difficult preliminary work, five sites were chosen
for the measurement stations. Other sites initially considered were
eliminated since they did not comply with the above requirements. The
sites selected are described below and are shown on the map :

1\. Le Cunay (Switzerland) altitude 1609m: this is a site on which there
had previously been a Sky Guide remote VHF transmission centre. This
site already had a microwave data-link with NAG. It is accessible in
winter after removal of snow.

2\. Septmoncel (Jura department, France) altitude 1157 m: this is a
power transmission pole operated by TDF. This site corresponds well to
the specifications. It was however necessary to take down a neighbouring
power transmission pole which interfered with the microwave data-link.
The site is accessible practically all year round.

3\. Chevreaux (Ain department, France) altitude 637 m: this is a
communications tower operated by TDF. This site corresponds well to the
specifications and there was only one difficulty: that was the question
of heating the HME cabinet installed in very large, rather cold,
premises at the bottom of the tower. The problem has now been solved by
installing appropriate partitions.

4\. Le Grand Colombier (Ain department, France) altitude 1443 m: this is
the site where the DGAC\'s CBY VOR is already located. This site, which
is interesting from a geographical point of view, required quite a lot
of civil engineering work to set up the HMU measurement station
(installation of a shelter, electric power supply to the shelter,
installation of two power transmission poles -- one for the HMU and one
for the microwave data-link). There are several drawbacks to this site:
access is difficult in winter (in the worst case the only solution is by
helicopter); risk of disturbance of aircraft transponder replies due to
the DME station sharing the site with the VOR; and the low temperatures
which could be reached in winter if there is a power cut, which may
affect the atomic clock.

5\. La Pointe des Brasses (Haute Savoie department, France) altitude
1502 m: this is at the top of a medium mountain ski resort. On the site
there was previously a private telecommunications relay belonging to a
private local area network (ambulance service). The site needed repair
of the underground electric power supply, replacement of the old power
transmission pole with a larger one, a great deal of thermal insulation
of the existing premises by exterior siding, and particularly meticulous
lightning protection. The problem of access in winter was solved by
using snow scooters rented from a local firm.

The microwave data-link between the Pointe des Brasses, Chambéry and the
NAG building was made by using a relay located in a TDF Communications
Tower in Mont Salève.

The link between Septmoncel, Chevreaux and the NAG building was made by
using a relay site on the Dole radar site in Switzerland. In fact, the
parabolic data-link antennae were installed in an old faired direction
finder antenna tower.

**6 - FEATURES OF EQUIPMENT AND FACILITIES**

**a) **Measurement stations

The measurement stations include a rack installed in premises which have
a heat setting and a 220V electrical power supply, including

\- the transponder receiver

\- the Ethernet link system

\- the frequency reference system

\- the atomic clock

\- the GPS receiver

\- the telecommunication equipment (microwave data-link transceiver)

Regarding the antennae, the system has a DME type antenna for reception
of transponder replies, a GPS antenna and a lightning grounding
dissipator.

**b)** Microwave data-links

The microwave data-links use transceivers made by Ericsson with Andrew
antennae.

The data-link has a capacity of 2 Mbps, it operates on a frequency of 7
GHZ, with a power of 2W. The transborder link required special
negotiations between the French and Swiss authorities. On the French
side, the negotiation was carried out by the BMNF (the French military
frequency office) because of the transborder aspect.

**7 - RESULTS**

At the end of the year 2000, all the work was finished on the three HMU
sites and flight inspections could be carried out in order to validate
the measurements.

These tests provided evidence of the fact that the system had the
required level of accuracy within the coverage volume.

A few areas show less accuracy, either because of the geometrical
configuration of the measurement stations or because of antenna problems
that were revealed and are presently being resolved. In particular, in
the case of the Geneva HMU, the altitude differences in the measurement
stations can explain certain areas with less accurate results (more
abundant multipaths).

The Geneva, Linz and Nattenheim stations respectively were monitoring
approximately 600, 450 and 650 aircraft per day at mid 2001.

Note : It can be noted that the TMU provides Radar type information,
which is therefore not so rich in detail. Furthermore ERA, the firm that
developed the Linz system, is considering supplying the same sort of
system to be used for radar to one of the countries in the Mediterranean
area.

There is an unquestionable economic advantage, the cost of this type of
system being approximately one tenth of that of a radar. It does,
however, require overflying aircraft to be 100% equipped with mode S.

**[8 - FLIGHT INSPECTIONS OF THE HME STATIONS]{.underline}**

**8 - 1. The Challenge**

After deployment of the HME stations at the three sites of Nattenheim,
Linz and Geneva, Eurocontrol wished to check that the performances of
these stations met the requirements of the specifications given to the
manufacturers. This was also necessary to certify the system.

A European call for tender was put out.

STNA made a bid, calling upon its partner, SEFA, for the \'aircraft
operation\' aspect.

**The specifications can be summed up as follows:**

\- The HME system measures the height of the entering aircraft
overflying the coverage area, using the WGS 84 reference system,

\- The measurement error is to be evaluated with sub-metric accuracy in
the service area: a circle of 45 NM centred on each station for flight
levels between FL 290 and FL 350,

\- The analyses of the results and conclusions are to be carried out by
Eurocontrol.

The Aircraft used for the flight inspection is capable of flying at FL
350 with an autonomy of 5 h 30 min, while delivering a trajectory
reference based on the VP-DGPS system, which is more than sufficiently
accurate. The system is integrated in the CARNAC 21 calibration bench,
developed by the company SAGEM.

STNA, having made a bid using this aircraft at a competitive cost, won
the contract.

**8 - 2. Mission**

Eurocontrol defined the flight profiles, as the measurements had to be
made at night (after midnight local time) in order to minimize ATC
constraints.

Three flights were planned over the Nattenheim and Linz stations at FL
290, 330, and 350. After the first flight the lower levels, which were
not of great interest, were abandoned in favour of the higher levels. In
addition, it was decided also to inspect the Linz station, which had
been already calibrated by another competitor.

Finally, further to some tuning and operations problems at the
Nattenheim and Geneva stations, some additional flights were carried
out.

The calendar was scheduled as follows: (for a total of 56 h 38min of
flight):

<table>
<tbody>
<tr class="odd">
<td><p>Nattenheim</p></td>
<td><p>21/22 September 2000</p></td>
<td><p>2000 FL 290 and</p>
<p>FL 350</p></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="even">
<td><p>Genève</p></td>
<td><p>12/13 October 2000</p></td>
<td><p>FL 350</p></td>
<td><p>Linz</p></td>
<td><p>16/17 October 2000</p></td>
<td><p>FL 350</p></td>
<td><p>Nattenheim</p></td>
<td><p>25/26 October 2000</p></td>
<td><p>FL 350</p></td>
<td><p>Genève</p></td>
<td><p>30/31 October 2000</p></td>
<td><p>FL 350</p></td>
</tr>
<tr class="odd">
<td><p>Genève</p></td>
<td><p>04/05 January 2001</p></td>
<td><p>FL 350</p></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
</tr>
</tbody>
</table>

The aircrew was composed of two pilots, the STNA mission leader and an
observer from Eurocontrol.

There was also an engineer from STNA on the ground taking care of the
implementation and monitoring of the VP-DGPS beacon.

The trajectory reference system (GPS accuracy differential) was obtained
in the WGS 84 reference system both in real time and also in deferred
time by using a \'backward and forward\' post-processing system. The
average accuracy readings obtained were respectively 30/40 cm and 8/12
cm.

This measurement campaign demonstrated the capabilities of the VP-DGPS
trajectory reference system in terms of range.

**8-3. Results**

The results were presented in the form of files and graphs using
\'Excel\'. Reports and CD-ROMs assembling the raw data, processing
information and tabulation were provided.

The aim was to calculate and show the errors at each station for each
flight segment.

As the HME system is based on a multi-distance calculation of the
aircraft position, an accuracy degradation factor according to the
distance from the centre of the system had to be taken into
consideration. A graph of error according to distance was therefore
added.

The interpretation of the results and conclusions was left in the care
of Eurocontrol.

**GLOSSARY**

  ------------ ------------------------------ --------- ------------------------ --------- ------------------------------ --------- ----------------------- ---------- ------------------------------------------- ---------- ------------------------------------- --------- -------------------------------------- ------------- ------------------------------------------------------
  **ADC**      Air Data Computer                                                                                                                                                                                                                                                                                                   
  **CFMU**     Central Flow Management Unit   **HMU**   Height Monitoring Unit   **HME**   Height Measurement Equipment   **NAT**   North Atlantic region   **ICAO**   International Civil Aviation Organization   **RVSM**   Reduced Vertical Separation Minimum   **TMU**   Total vertical error Monitoring Unit   **VP-DGPS**   Very Precise -Differential Global Positioning System
  **WGS-84**   World Geodetic System 84                                                                                                                                                                                                                                                                                            
  ------------ ------------------------------ --------- ------------------------ --------- ------------------------------ --------- ----------------------- ---------- ------------------------------------------- ---------- ------------------------------------- --------- -------------------------------------- ------------- ------------------------------------------------------