GPS/MET Instrument Configuration

List of Figures

• Figure 1. Occultation Zone Diagram
• Figure 2. Occultation Azimuths Diagram
• Figure 3. Difference between Sphere and Ellipsoid

Introduction

This is easy since the receiver is already required to internally maintain its position, velocity, and clock to a few kilometers, meters per second, and microseconds respectively (usually at least an order of magnitude better than this) in order to stay in tracking operation. This internal knowledge, along with the states of the GPS satellites themselves (a required subset of the navigation calculations), is sufficient to schedule high, medium, and normal rate tracking (50, 1, and 0.1 Hz respectively) according to a few simple rules, given some operator selectable boundary parameters.

Even when the receiver is not tracking, it has a moderately accurate propagator which allows it to keep its state vector within kilometers over periods of hours or days. Of course, unmodeled clock drift is a part of the position that is difficult to propagate over large periods of time.

Parameters

Parameter Description

Azimuth and elevation of zero causes the highest priority point for normal tracking to be straight back. An azimuth of 90 degrees causes highest priority to be to the "left" of the flight path ("left" refers to an astronaut with his feet toward the earth facing forward along the velocity vector). An azimuth of 180 degrees causes highest priority to be along the velocity vector.

An elevation value of 90 degrees causes highest priority to be straight up (plus or minus a few tenths of a degree for flight path angle). An elevation value of -27 degrees will place the highest priority at the horizon as viewed from 780 km.

Note that the priority referred to here is only for normal rate tracking attempts and does not affect high rate track selection.

The occlazs parameter is used to determine which satellites in the occultation zone (see occlbott and occltop next) are eligible for scheduling. It defines the width of azimuths centered on the currently entered boresite (determined by the azimuth parameter). A value of 0 prevents any occultations from being scheduled. A value of 360 degrees allows all occultations to be scheduled. A value of 180 degrees allows all occultations 90 degrees on either side of boresite to be scheduled. The default value is 90 degrees (45 degrees either side of boresite).

The occlbott and occltop parameters are used to determine what satellites are thought to be in view and where to attempt atmospheric occultation high rate tracks.

A satellite whose raypath to GPS-MET passes nearer to the surface of the earth (at closest approach) than occlbott is considered down and unavailable.

A satellite whose raypath to GPS-MET passes further from the surface of the earth than occlbott but nearer than occltop and which is within half of the occlazs arc from the antenna boresite defined by azimuth is a candidate for a high rate tracking attempt.

The occlded parameter is used to set a zone in which double difference (reference) satellites are discouraged. Satellites whose angles to zenith are greater than that indicated by occlded are significantly de-prioritized.

Satellites which are not needed for high rate data but which are up and below the level set by occlmed are set to "medium rate" (1 Hz) for ionosphere data collection.

In ground test mode, elevation and azimuth define the boresite of the antenna in local true coordinates. The occl* zones are defined at a "pseudo-horizon" distance of about 1 kilometer, see below.

Algorithm

First, the "zoning line" is established as a line running from the center of the earth through the surface at the limb (horizon) as viewed from GPS-MET. See Figure 1. The various tracking zones are then determined as angles from GPS-MET zenith measured as distances from earth surface on this line. (Earth surface, for these purposes, is taken as a sphere with radius equal to the distance from earth center to the surface at the GPS-MET subsatellite point (see Figures 1. and 3.). This gives nearly exact results for occultations occurring near the same latitude as GPS-MET and a reasonable mean for those off-latitude).

Next, a first pass is made through all GPS PRN numbers to determine "existence," "up" and "down," and "occultation zone." Satellites for which there is no broadcast almanac in the receiver are declared "non-existent." Satellites whose angles from zenith are greater than that indicated by occlbott are declared "down" and are not processed further. Remaining satellites whose angles to zenith are greater than that indicated by occltop are eligible for high rate occultation tracking. Any of these satellites which are within the azimuth zone defined by the azimuth and occlazs parameters (see Figure 2.) and which are above the horizon mask for one of the ground sites in the onboard table are then set as "occultation satellites" for high rate tracking. All others are returned to the "up" pool as "undecided."

A second pass is then made through all GPS PRN numbers to find a "double difference" or "reference" satellite for each valid "occultation satellite." All remaining satellites are scanned (i.e., a satellite cannot double difference itself) and the one with the best combination of visibility to the selected ground site and the boresite on GPS-MET is picked. Satellites whose angles to zenith are greater than that indicated by occlded are significantly de-prioritized in this process, though they remain eligible. If no "reference" satellite is available for a particular occultation, the event is cancelled an the occultation satellite is returned to the "up" pool.

At this point, a report is generated for transmission to the ground as ASCII log data. Here are some lines from a simulated occultation and a descriptino of their meaning. They are for determinations made ten seconds apart.

1: 15 dd: 31 gnd: 3 at -18.583 186.046 161.1
1: 15 dd: 31 gnd: 3 at -18.622 186.161 134.2
1: 15 dd: 31 gnd: 3 at -18.663 186.278 106.8
1: 15 dd: 31 gnd: 3 at -18.704 186.396 79.0
1: 15 dd: 31 gnd: 3 at -18.745 186.515 50.7
1: 15 dd: 31 gnd: 3 at -18.787 186.636 22.0
1: 15 dd: 31 gnd: 3 at -18.830 186.758 -7.2
1: 15 dd: 31 gnd: 3 at -18.873 186.881 -36.8
1: 15 dd: 31 gnd: 3 at -18.917 187.006 -66.8
1: 15 dd: 31 gnd: 3 at -18.962 187.133 -97.2

The fields are as follows:

1	Occultation number (instantaneous, always 1 in this case).
15	PRN number of occultation satellite (high rate).
dd: 31	PRN number of reference (double difference) satellite.
gnd: 3	Ground site number three in the receiver internal table, initialized automatically and configurable from the ground. (In this case refers to Fairbanks).
at -18.583	Latitude North (- South) where the ray path comes closest to earth center.
186.046	Longitude East ditto
161.1	Height in kilometers above the ellipsoid at that point.

After this, a count is made of the number of high rate track requests about to be made. If that number is greater than four (a compiled constant), occultations are eliminated from the sides (furthest from velocity vector and anti-velocity vector) until the high rates number is four or less. Any such eliminations are also reported in the ASCII log data.

Finally, all the non-high-rate satellites in the "up" pool are prioritized. Those whose angles to zenith are greater than that indicated by the occlmed parameter (which is drawn in Figure 1. above occlded but which is independent and can be placed anywhere) are set to "medium" or one Hz rate. All the rest are prioritized geometrically from boresite highest (anti-velocity vector for azimuth and elevation zero) to anti-boresite lowest (velocity vector).

Finally, a table of all of the currently requested occultation and corresponding reference satellites is constructed so that in the next evaluation the same reference satellite will continue to be assigned to a continuing occultation track even though another satellite may have slightly exceeded the merit of the initial reference satellite through geometry changes. Changing reference satellites in the middle of an occultation will make it useless.

Note that it is possible for more than one occultation satellite to share a ground site and a reference satellite, thereby obtaining two occultations for three high rate tracks. It is also possible to have two occultations, two reference satellites, and two ground sites in play at once.

Standard TurboRogue scheduling code then takes these determinations and updates channel assignments to the various satellites and tracking rates as appropriate.

Suggest Ground Parameters

Note that in either case, the operator must switch to ORBITING mode briefly (but after occl enable is set) to start all of the appropriate "autorog process" timers, then switch back to the appropriate non-ORBITING mode.

Currently, only the test mode (not the actual ground occultation mode) is supported. If there is sufficient demand (like another Mauna Kea demonstration), I can make the needed modifications.

The difference is this. In test mode, the "zoning line" is always picked to be about one kilometer away from the calculated receiver position. This makes it relatively easy to pick zone parameters from elevations (for example, a zone limit of plus 1000 meters will give a local elevation angle of plus 45 degrees). In "actual" occultation mode, the same calculations made in orbit would be employed to automatically select events from current receiver conditions.

Recommended Ground Parameters for Test Mode

Disable

To disable medium rate tracks, set occlmed to -100000 meters or lower.