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Horizontal Situation Indicator

The horizontal 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.

Three switches 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.

The magnetic 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.

The course 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.

Course deviation 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.

Glide slope 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).

The "desired 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.

The primary 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 data.

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.

For example, 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.

During ascent 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.

The course 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.

The primary 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.

The horizontal 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.


Curator: Kim Dismukes | Responsible NASA Official: John Ira Petty | Updated: 04/07/2002
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