Restricted Laser Tracking of Satellites

Werner Gurtner
May 2, 2005

1. Introduction

There are satellites that must (or can) only be tracked by laser ranging under certain restrictions or conditions:

• The corner cubes may not be visible under certain geometric conditions.
  
  An example is the Gravity Probe B. Its corner cubes are mounted on the back plane of the satellite. As the satellite is actively kept at a constant orientation in space the corner cubes are only visible from a specific station during part of the possible passes.
  
  Tracking outside the effective pass interval does not harm the satellite but no returns are possible.
  
• Some satellites are equipped with optical sensors that may be damaged by the SLR laser beam if the station is within the field of view of this optical sensor. Depending on the way the sensors are used (fixed nadir orientation, swept left and right to the satellite orbit, programmed off-nadir pointing) the pass restrictions can be more or less complicated. We need to be within the operating range of the corner cubes, yet out of the vulnerable range of the detectors.

• Fixed nadir pointing: The "forbidden" zone for laser tracking is symmetric around the station's zenith with a maximum elevation depending on the field of view of the sensor and an appropriate safety allowance;
• Off nadir pointing: Some options here include
  1. sweeping motions perpendicular to the satellite orbit which can lead to one short forbidden time interval when the station is within the sweeping band of the sensor;
  2. fixed or programmed off-nadir pointing which may lead to forbidden time intervals in any part of a satellite pass
     
• In case of multiple sensors there may be more than one forbidden time interval per pass

With fixed nadir pointing, the satellite can be protected by imposing a maximum allowable elevation for the station to operate. The elevation would be pass dependent and all ranging to the satellite must cease above this level.

For off-nadir pointing, operating restrictions at each station will depend upon station position, spacecraft position and orientation, and the field of view of any vulnerable on-board detectors. In general this information will not be available at the stations, and the respective satellite mission control center must provide the tracking constraints to each of the participating stations in advance.

In cases where satellites can be repositioned or re-oriented to a non-nominal direction (actively or because of an attitude control system failure) it may be necessary to update these tracking or viewing constraints in a very short timeframe. In some cases it may not be practical or prudent to issue long-term viewing constraints which may inadvertently place the satellite in jeopardy.

In order to be able to track the satellites under such restrictions, we need to:

• Set up procedures to prevent a station from inadvertently damaging the vulnerable satellite equipment;
• Define an acceptance procedure for stations to pass before any laser tracking on the relevant satellite can begin;
• Relieve the accepted stations from any legal reliability or financial consequences in case of unintentional damage

2. Procedures

2.1 Fixed nadir pointing

The mission control center for the relevant satellite defines the maximum elevation (including a safety factor) up to which laser ranging can be performed. For the time interval during which the satellite is above this maximum elevation, the tracking system has to shut down / block the laser automatically. An additional level of safety can be added by splitting the pass into two independent segments, so that the system will not track the pass segment above the maximum elevation, at all.

The defined maximum elevation can include a maximum off-nadir pointing angle within which the satellite can operate. If this angle is small it may be more effective to decrease the maximum elevation accordingly to avoid having to compute individual pointing-dependent pass segments.

Example: ICESsat: Maximum elevation set to 70 degrees

2.2 Off nadir pointing (pass- and station-dependent forbidden zones)

In cases where corner cubes or vulnerable detectors are pointing to off-nadir positions satellite passes may have to be divided into more complicated pass segments. The mission control center will generate a station-dependent pass segment list or viewing table and distribute it to the stations in advance.

As stations may use different minimum elevations for different satellites or weather conditions or depending on their actual horizon mask, the pass segment lists will be based on a low minimum elevation angle, e.g. 5 degrees. Stations will set their own minimum elevation angle as required.

The pass segment list (see below) will contain all pass segments for a time period to be selected by the mission control. The list will include the station code, the satellite name, the start and end dates/times for all pass segments, the maximum elevation for each pass segment, and the segment length. The following example defines the contents and format of the list.

Example:

Satellite         :  GP-B
Generation Date   :  2004-07-23 19:07:00 [UTC]
Generated by      :  GP-B Mission Operations / Stanford University
Minimum Elevation :  5 deg
-----------------------------------------------------------------------------
                               Start Date/Time      End Date/Time  MaxEl Dur
ID      SAT     COSPAR  SIC         [UTC]               [UTC]      [deg][min]
---- ---------- ------- ---- ------------------- ------------------- -- ----- 
1824 GP-B       0401401 8603 2004-07-24 00:46:57 2004-07-24 00:53:51 80   6.9
1824 GP-B       0401401 8603 2004-07-24 02:23:59 2004-07-24 02:28:26 10   4.4
1824 GP-B       0401401 8603 2004-07-24 11:51:43 2004-07-24 11:55:45 27   4.0
1824 GP-B       0401401 8603 2004-07-24 13:29:21 2004-07-24 13:33:19 27   4.0
----------------------------------------------------------------------------- 
                               Start Date/Time      End Date/Time  MaxEl Dur
ID      SAT     COSPAR  SIC         [UTC]               [UTC]      [deg][min]
---- ---------- ------- ---- ------------------- ------------------- -- ----- 
7810 GP-B       0401401 8603 2004-07-24 00:47:15 2004-07-24 00:52:53 13   5.6
7810 GP-B       0401401 8603 2004-07-24 02:25:23 2004-07-24 02:32:23 67   7.0
7810 GP-B       0401401 8603 2004-07-24 04:02:14 2004-07-24 04:05:52  7   3.6
7810 GP-B       0401401 8603 2004-07-24 13:28:30 2004-07-24 13:32:27 27   3.9

Descriptive file header: 4 lines, contents start after position 20
Table headers: 4 lines each
Read Format (data record): I4,1X,A10,1X,I7,1X,I4,1X,2(I4,1X,5(I2,1X)),I2,1X,F5.1

This version is a slight modification of the original format, decided by the Format and Procedure Working Group on April 26, 2005, to be used after June 1st 2005.

A file may contain pass segments for more than one station, see the example. A station can easily extract its records from the pass segment list (e.g., using the UNIX grep utility).

The station will "fold" these pass segments onto the locally computed pass start and end times to generate the valid pass definition.

By following the prescribed schedule, the tracking system will range to the satellite only within the accepted pass segments, switching off the laser beam during the forbidden time intervals.

In case of mere geometrical blockage, like e.g., for GP-B, the existence of a pass segment list is not mandatory for tracking.

2.3 Additional safety measures

Additional safety measures can be in force:

• Stations can only be allowed to track passes that are included in the pass segment list. If the pass is not on the list or if the station does not receive the list, then the station must not track.
• Stations are only allowed to use the prediction sets provided by or designated by the responsible mission control. ILRS will not allow other centers to generate and distribute predictions for such satellites.
• Some of the vulnerable satellites will be maneuvered or reoriented with little notice. Others may have immediate maintenance or attitude control lapses: To prevent stations from tracking during abnormal conditions a special "go - nogo key" file may be maintained by mission control. Stations have to access this key (e.g, ftp) less than 15 minutes before tracking and, till the end of the pass, in intervals defined in the file. Tracking is not allowed if the key is set to "nogo" or if the key cannot be accessed.
  
  The file (one line) contains the satellite name, the 7-digit Cospar number , the 4-digit SIC of the satellite, the requested control interval (minutes, zero if not used) and the go / nogo key (go or nogo).
  
  Read format: A10,1X,I7,1X,I4,1X,I2,1X,A4
  
  Filename: 'satellitename'.gng (satellite name without blanks, hyphens or underscores)
  
  Examples:
  
  ICESAT     0300201 8201  5 go
  
  This version is a slight modification of the original format, decided by the Format and Procedure Working Group on April 26, 2005, to be used after June 1st 2005.

3. Acceptance procedures

The precise acceptance procedures will be defined by the mission sponsor as a subset of the procedures described below.

3.1 Description of the tracking system procedures

Each SLR tracking station must prepare a detailed description of its procedure to handle the restricted tracking of vulnerable satellites, e.g.:

• Incoming mail processing: Interval, software used
• Computation of start- and end-times of passes
• Procedure to compute actual pass segment start and end times, i.e. including restrictions
• Handling of pass segments: As individual passes or as one pass with laser beam blockages during forbidden zones
• Start of tracking
• Degree of automation, manual interaction
• Laser control / interruption
• Verification of non-operation in case of missing pass segment definitions or predictions of the current day
• Assessment of possible failures of procedures

3.2 Test campaign

For each candidate station, the mission control center will prepare a test campaign with a suitable satellite by sending an appropriate pass segment list under the same restrictions/conditions as the satellite in question.

The candidate station will track the test satellite under the restricted rules for at least five successful passes. The station will send a report of the tracked passes to the mission control center, together with a list of the effective pass segment start and end times.

Stations capable of pass interleaving should track about half of the test passes without and half with pass interleaving.

The mission control center will also verify that the forbidden zones were properly omitted from tracking by e.g.,  using

• the submitted normal point data.
• full-rate data of the test passes to do the verification on a more detailed level
• the epochs of noise returns of the saturated receiver (set range gate window large enough to get a noise return for every laser firing epoch)

3.3 "Dry run" on the vulnerable satellite

After successful restricted tracking of the test satellite the station can be asked to successfully track some passes of the vulnerable satellite without laser ranging and submit a report about this "dry run" tracking to the mission control center.

Finally the mission control center will send the candidate station a written authorization to include the satellite into its routine tracking with a written waiver of any legal liability.

Copies of all reports and authorizations have to be sent to the ILRS Central Bureau.

3.4 Verification of actual passes

The mission control center can request full-rate data of the first few actual passes and occasionally later during the mission life time to do a more detailed verification of the proper handling of the restricted pass segments. KHz-Stations will decimate the full-rate test data to 10 Hz before submission.

4. Liability in case of unintentional damage

The mission control center will issue a written document relieving the accepted tracking station of any liability or financial consequence in case a component of the satellite is unintentionally damaged by the laser beam.

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Last modified date: Tuesday, May 03, 2005
Author: Werner Gurtner
Maintained by: Carey Noll