Thesis - Leigh Moody.pdf - Bad Request - Cranfield University

Chapter 3 / Sensors / Radar Altimeter
_ _
within prescribed limits. A “wide” beam sensor is modelled that can be
characterised as either a continuous wave or a pulsed device whose first
return represents the geodetic ground height. Digital LandMass Survey
(DLMS) map data is used to obtain an estimate of the ground height. The
true ground height is obtained by removing statistical map triangulation and
interpolation height errors from the Digital Terrain Elevation Data (DTED).
The reference height is the difference between the geodetic height above the
earth’s surface and the corrected map height.
REFERENCE
INPUT
1
PZ_GFM
RADALT
ERROR MODEL
3.7-2
M_PZ_GFM I_PZ_GFM
RADAR ALTIMETER
1553 I/F
GUI
1
2
E_PZ_GDM
Figure 3-37 : Radar Altimeter Model
The radar altimeter model does not support narrow beam devices that
require DLMS data at the point of ground impact. When the DTED is deactivated
the geodetic height is referred to the WGS 84 ellipsoid and is thus
comparable with barometric altimeter data.
3.7.2 Reference Height Above the Ground
Since the DTED contains map errors the reference ground height is,
ZG
f , m
G
E
E E G
ZG ZG
( ) P P
~
kˆ P − P • − +
P : = T ⋅
∆
m
d
d,
f
d,
f
Equation 3.7-1
This data is provided at a reference frequency (RAfR) of 400 Hz. The
transform from the Earth to LGA frame, and the Earth radius at point (d)
directly beneath the missile, are defined in §17.10 and §18.3 respectively.
Assuming that the first energy returns represent the geodetic vertical,
~
P : = P − P + ∆P
ZG
f , m
ZG
d,
m
ZG
d,
f
ZG
d,
f
Equation 3.7-2
The measured height of the ground below the missile along the geodetic
vertical is obtained by 3 rd order bi-cubic interpolation between a 4x4
ordinate height patch centred at the missile position, a process described in
§22.6. Whilst transitions between map squares is free from discontinuities,
the mapping and interpolation processes used in generating paper maps and
their digitisation, result in four major errors:
• A zero mean Gaussian height error (∆M1) in metres