situation indicator for the commander and pilot displays a pictorial
view of the vehicle's position with respect to various navigation
points and shows a visual perspective of certain guidance, navigation
and control parameters, such as directions, distances and course/glide
path deviation. The flight crew uses this information to control
or monitor vehicle performance. The HSIs are active during the entry
and landing and ascent/RTLS phases.
Each HSI provides
an independent source to compare with ascent and entry guidance,
a means of assessing the health of individual navigation aids during
entry and information needed by the flight crew to fly manual ascent,
RTLS and entry.
are associated with each horizontal situation indicator. The commander's
select switches are on panel F6 and the pilot's are on panel F8.
The HSI select mode switch selects the mode-entry, TACAN or approach.
The HSI select source switch selects TACAN, navigation or microwave
scan beam landing system; its 1, 2, 3 switch selects the data source.
When positioned to nav, the HSI is supplied with data from the navigation
attitude processor and the 1, 2, 3 switch is not used. In TACAN,
the HSI is supplied with data derived from the 1, 2, 3 switch, thus
TACAN 1, 2 or 3. In MLS , the HSI is supplied with data derived
from the 1, 2, 3 switch, thus MLS 1, 2 or 3.
Each HSI displays
magnetic heading (compass card), selected course, runway magnetic
course, course deviation, glide slope deviation, primary and secondary
bearing, primary and secondary range, and flags to indicate validity.
Each HSI consists
of a case-enclosed compass card measuring zero to 360 degrees. At
the center of the compass card is an aircraft symbol, fixed with
respect to the case and about which the compass card rotates.
heading (the angle between magnetic north and vehicle direction
measured clockwise from magnetic north) is displayed by the compass
card and read under the lubber line located at the top of the indicator
dial. (A lubber line is a fixed line on a compass aligned to the
longitudinal axis of the craft.) The compass card is positioned
at zero degrees (north) when the heading input is zero. When the
heading point is increased, the compass card rotates counterclockwise.
pointer is driven with respect to the HSI case rather than the compass
card. Therefore, a course input (from the DDU) of zero positions
the pointer at the top lubber line, regardless of compass card position.
To position the course pointer correctly with respect to the compass
card scale, the software must subtract the vehicle magnetic heading
from the runway azimuth angle (corrected to magnetic north). As
this subtraction is done continuously, the course pointer appears
to rotate with the compass card, remaining at the same scale position.
An increase in the angle defining runway course results in a clockwise
rotation of the course pointer.
is an angular measurement of vehicle displacement from the extended
runway centerline. On the HSI, course deviation is represented by
the deflection of the deviation bar from the course pointer line.
Full scale on the course deviation scale is plus or minus 10 degrees
in terminal area energy management and plus or minus 2.5 degrees
during approach and landing. The course deviation indicator is driven
to zero during entry. When the course deviation input is zero, the
deviation bar is aligned with the end of the course pointer. With
the pointer in the top half of the compass card, an increase in
course deviation to the left (right) causes the bar to deflect the
right (left). Therefore, the course deviation indicator is a fly-to
indicator for flying the vehicle to the extended runway centerline.
Software processing also ensures that the CDI remains fly to, even
when the orbiter is heading away from the runway.
In the TAEM
example, at a range of 9 nautical miles (10 statute miles), the
CDI would read about 7.5 degrees, with the extended runway centerline
to the right of the orbiter. In course deviation geometry, if the
orbiter is to the left of the runway, it must fly right (or if the
orbiter is to the right of the runway, it must fly left) to reach
the extended runway centerline. The corresponding course deviation
bar would deflect to the right (or to the left in the latter case).
The reference point at the end of the runway is the microwave landing
system station. The sense of the CDI deflection is a function of
vehicle position rather than vehicle heading.
deviation, the distance of the vehicle above or below the desired
glide slope, is indicated by the deflection of the glide slope pointer
on the right side of the HSI. An increase in glide slope deviation
above (below) the desired slope deflects the pointer downward (upward);
the pointer is a fly-to indicator. In the HSI example, the pointer
shows the vehicle to be below the desired glide slope by about 4,000
feet (in TAEM, each dot represents 2,500 feet).
glide slope" is actually only a conceptual term in HSI processing.
At any instant, glide slope deviation is really the difference between
the orbiter altitude and a reference altitude computed in the same
fashion as the guidance reference altitude. Also included in the
reference altitude equation are factors for a "heavy orbiter" and for high winds.
The GSI computation
is not made during entry or below 1,500 feet during approach and
landing; therefore, the pointer is stowed and the GSI flag is displayed
during those intervals.
and secondary bearing pointers display bearings relative to the
compass card. These bearings are angles between the direction to
true or magnetic north and to various reference points as viewed
from the orbiter. For the bearing pointers to be valid, the compass
card must be positioned in accordance with vehicle heading input
When the bearing
inputs are zero, the pointers are at the top lubber line, regardless
of compass card position. Like the course pointer, the bearing pointer
drive commands are developed by subtracting the vehicle heading
from the calculated bearing values. This allows the pointers to
be driven with respect to the HSI case but still be at the correct
index point on the compass card scale. When the bearing inputs are
increased, the pointers rotate clockwise about the compass card.
The pointer does not reverse when it passes through 360 degrees
in either direction.
if the primary bearing is 190 degrees and the secondary bearing
is 245 degrees, the bearing reciprocals are always 180 degrees from
(opposite) the pointers. The definition of primary and secondary
bearing varies with the flight regime.
The HSI is
capable of displaying two four-digit values in the upper left and
right side of its face. These numbers are called primary and secondary
range, respectively. Each display ranges from zero to 3,999 nautical
miles (4,602 statute miles). While their meaning depends on the
flight regime, both numbers represent range in nautical miles from
the vehicle to various points relative to the primary and secondary
runways. In the HSI example, the primary range is 9 nautical miles
(10 statute miles); the barberpole in the secondary range slot is
an invalid data indication.
The HSI has
four flags- off, brg (bearing), GS (glide slope) and CDI-and two
barberpole indications that can respond to separate DDU commands,
identifying invalid data. Off indicates that the entire HSI display
is invalid because of insufficient power. Brg indicates invalid
course, primary bearing, and/or secondary bearing data. GS indicates
invalid glide slope deviation. CDI indicates invalid course deviation
data. Barberpole in the range slots indicates invalid primary or
secondary range data.
When the HSI
source switch is in nav , the entire HSI display is driven by navigation-derived
data from the orbiter state vector. This makes the HSI display dependent
on the same sources as the navigation software (IMU, selected air
data, selected navigational aids), but the display is independent
of guidance targeting parameters. As stated previously, when the
TACAN/nav/MLS switch is in the nav position, the source 1, 2, 3
switch is not processed.
The TACAN or
MLS position of the source switch should be used only when TACAN
or MLS data are available. TACAN data can be acquired in Earth orbit
but would be unavailable during blackout; therefore, TACAN is generally
not selected until acquisition after blackout. MLS has a range of
20 nautical miles (23 statute miles) and is normally selected after
the orbiter is on the heading alignment cylinder.
The glide slope
deviation pointer is stowed when the entry mode is selected and
the flag is displayed. The GSI in TAEM indicates deviation from
guidance reference attitude in plus or minus 5,000 feet. The GSI
in approach indicates guidance reference altitude for approach and
landing in plus or minus 1,000 feet; it is not computed below 1,500
feet and the flag deploys.
In the entry
mode, the compass card heading indicates the magnetic heading of
the vehicle's relative velocity vector. In TAEM and approach, the
compass card indicates magnetic heading of the body X axis.
In the entry
mode, the course deviation indicator is a valid software zero with
no flag. In TAEM, the CDI indicates the deviation from the extended
runway centerline, plus or minus 10 degrees. In approach, the CDI
indicates the deviation from the extended runway centerline, plus
or minus 2.5 degrees.
In the entry
mode, the primary bearing indicates the spherical bearing to way
point 1 for the nominal entry point at the primary landing runway.
The secondary bearing indicates the spherical bearing to WP-1 for
the NEP to the secondary landing runway. In TAEM, the primary bearing
indicates the bearing to WP-1 on selected HAC for the primary runway.
The secondary bearing indicates the bearing to the center of the
selected HAC for the primary runway. In approach, the primary and
secondary bearings indicate the bearing to WP-2 at the primary runway.
In the entry
mode, the primary range indicates the spherical surface range to
WP-2 on the primary runway via WP-1 for NEP. The secondary range
indicates the spherical surface range to WP-2 on the secondary runway
via WP-1 for NEP. In TAEM, the primary range indicates the horizontal
distance to WP-2 on the primary runway via WP-1. The secondary range
indicates the horizontal distance to the center of the selected
HAC for the primary runway. In approach, the primary and secondary
ranges indicate the horizontal distance to WP-2 on the primary runway.
major modes 102 and 103 (first and second stage) and RTLS, the horizontal
situation indicator provides information about the target insertion
orbit. The compass card displays heading with respect to TIO, and
north on the compass card points along the TIO plane. The heading
of the body plus X axis with respect to the target insertion orbit
is read at the lubber line.
pointer provides the heading of the Earth-relative velocity vector
with respect to the TIO plane. The CDI deflection indicates the
estimated sideslip angle, the angle between the body X axis and
the relative velocity vector.
bearing pointer during major modes 102 and 103 is fixed on the compass
card at a predetermined value to provide a turnaround heading in
the event of an RTLS abort. During RTLS major mode 601, the pointer
indicates the heading to the landing site runway. The secondary
bearing provides the heading of the inertial velocity vector with
respect to the TIO plane.
situation CRT display allows the flight crew to configure the software
for nominal winds or high head winds. The software item entry determines
the distance from the runway threshold to the intersection of the
glide slope with the runway centerline. The high-wind entry pushes
the intercept point close to the threshold. The distance selected
is factored into the computation of reference altitude from which
the GSI is derived.
The HSI contractor
is Rockwell International, Collins Radio Group, Cedar Rapids, Iowa.