How to Simulate Delta-DOR Measurements in ODTK

Introduction

Delta Differential One-way Range (Delta DOR) is a type of derived measurement that has become common in deep-space mission planning. Delta DOR is especially useful as a spacecraft approaches a planetary object. The measurement is valuable to the navigation process because it provides plane-of-sky information, which is complementary to typical line-of-sight measurements such as range and Doppler.

A Delta DOR measurement is (conceptually) constructed by forming the difference of two Differential One-way Range (DOR) measurements. An individual DOR measurement is the difference in range between a target object and two ground stations. In a Delta DOR combination, one DOR measurement involves the spacecraft and the other involves a quasar near the line of sight to the spacecraft. We say that Delta DOR is only "conceptually" constructed since the spacecraft and quasar DOR measurements are not taken at the same time. Thus, we must interpolate the quasar DOR information to the time of the spacecraft DOR measurement.

The individual DOR measurements are taken using two widely separated ground stations. The ground stations alternate between simultaneously observing the spacecraft and the quasar in a predefined cadence. The typical DOR sequence follows the pattern of Q-S-Q-S-Q-S-Q, where Q is the quasar and S is the spacecraft. The antennas at the ground stations slew between the two targets during the collection process. The ground station pair forms a baseline.

An operational usage of Delta DOR measurements typically uses two or more baselines. The Delta DOR observations are constructed during estimation.

Terminology

We will use the following terminology when describing Delta DOR data generation:

ODTK documentation

The Simulator Scope Functions topic in the ODTK help system describes the Delta DOR simulation function.

Simulation paradigm

Delta DOR measurements are simulated outside of the normal ODTK data simulation process. Since Delta DOR is a derived measurement type, it is not simulated directly. Instead, the spacecraft and quasar DOR measurements are simulated in a manner that mimics an operational collection. The Delta DOR simulation workflow works around a couple of limitations of the ODTK simulator: For these reasons, a simulator run first generates all the desired tracking data. These data includes typical sequential range and total count phase observations. Ephemeris generated from this simulation is then used to produce the DOR measurements.

Note: You can generate DOR measurements using any reference ephemeris, but the ephemeris must be consistent with the other simulated measurements.

DOR measurements are generated using an ODTK attribute function on the simulator for one baseline at a time. This simulator function was a new feature with the release of ODTK 6.5.2. You can run the function two or more times to construct measurements on different baselines or on the same baseline. Specify a time frame over which you want to generate measurements. ODTK looks for windows within the time frame when the required dual ground station visibility exists. When ODTK identifies a window of availability, it generates a single measurement sequence. The next measurement sequence is not generated until ODTK finds another window of availability. Typically, there is one window of availability for each baseline in a 24-hour period. You can generate DOR measurement sequences over specific time intervals by specifying a narrow time window.

Non-DOR simulation

The zip folder attached to this article contains an ODTK scenario (MarsSat_Delta_DOR.sco). This scenario emulates a transfer trajectory between the Earth and Mars. During non-DOR data simulation, the simulator will generate sequential range and TCP measurements over the span of the trajectory.

The scenario has an associated VBScript (MarsSat_Delta_DOR.vbs). Load the script with the scenario and confirm that it appears in the Scripting Tool. You can run the non-DOR part of the measurement simulation exercise individually or as part of the script, followed by the simulation of the DOR observations.

If you are running from the script:

1. Set thr RunRangeTCPSim variable to true.
2. Click Execute.

Otherwise, run Simulator_RangeTCP. This generates non-DOR measurements and establishes the reference ephemeris for DOR simulation.

DOR Simulation

Use the MarsSat_Delta_DOR.vbs script to generate the spacecraft and quasar DOR measurements. You need to update the file path at the top of the script before running it in the Scripting Tool. The script also shows the calling sequence for the simulator attribute function. You can look at the MeasurementStatistics of the ground stations to verify that two of them have DOR measurements defined. Only one member of a baseline needs to have the DOR measurement statistics defined.

Note that:
Update the Scenario properties to add the generated DOR observation files for use in estimation.

DOR processing

Your main interest is processing Delta DOR measurements, so use the Filter MeasTypes list to restrict data processing to DSN Delta DOR. The DOR measurements generated during data simulation should provide one sequence per baseline each day. You will estimate the orbit three time times: After each estimation run, archive the Filter and Smoother produced ephemeris files. You will use them later in the visualization exercise. To archive the files:

1. Create three directories for the ephemerides.
2. Copy the files into them after each run.

All Delta DOR

1. Verify that the CustomDataEditing schedule in the filter is not enabled and then run the Filter and the Smoother.
2. Update the Data Product List in the static product builder to point to the new output files.
3. Generate all the products.
4. If this is your first time running through the exercise and the file paths are correct, you can reload the data product list to refresh it.
5. Verify that the filter was able to track to the spacecraft using only Delta DOR and that the processing appears to be reasonable. You should expect position uncertainties near 1000 km.

Limited Delta DOR

1. Open the CustomDataEditing schedule and verify that there are two entries:

2. Both entries should reduce the processing of measurements to contain seven-day gaps in the data.
3. Enable the CustomDataEditing schedule.
4. Rerun the filter and smoother. This run will still use only Delta DOR measurements, due to the contents of the Filter MeasTypes list.
5. Use the residual plots to verify that data processing was limited as expected.
6. Verify that the rest of the processing again appears to be reasonable.

Limited Delta DOR Plus Range and TCP

1. Remove DSN Delta DOR from the Filter MeasTypes list to enable all measurement types to be processed.
2. Leave the CustomDataEditing schedule enabled.
3. Rerun the filter and smoother.
4. Use the residual plots to verify that data processing was limited as expected.
5. Verify that the rest of the processing appears to be reasonable again.

Delta DOR measurement processing visualization

You can use STK to see the effect of Delta DOR processing on orbit uncertainty. There is a subfolder called "STK" in the zipped folder attached to this article.

1. Open the STK scenario (ToMarsIn2020.sc) in that folder.
2. Set the satellite in the STK scenario to use the Filter ephemeris from the limited DeltaDOR run.
3. In the 3D graphics window, you should see a triad of three baseline vectors attached to the satellite and a vector pointed toward the Earth. The baseline vectors show the directions of the three DOR baselines.
4. Animate the scenario.

When STK processes a Delta DOR measurement, you will see the position uncertainty collapse along the baseline vector that produced the measurement.