Radio Science Receiver (RSR)
Downlink Frequency (DLF) Prediction File
Software Interface Specification (SIS)
Author:
Richard A. Simpson
Radio Science Advisor
NASA Planetary Data System
July 21, 2017
|============================================================================|
| |
| Change Log |
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|============================================================================|
|Revision|Issue Date| Sections | Change Summary |
| | | Affected | |
|========|==========|==========|=============================================|
| 1.0 |2017-07-21| All |Adapted from an ad hoc reversion of |
| | | |0159-SCIENCE, Rev. B, which was drafted by |
| | | |D.S. Kahan on 18 May 2009. This version is a|
| | | |distillation of of the 2009 document to the |
| | | |essentials needed to understand the DLF. |
| | | |Readers should refer to the original |
| | | |0159-SCIENCE for background information. |
|============================================================================|
Contents
Section
1 Introduction
1.1 References
2 Functional Overview
2.1 Radio Science Receiver (RSR) Operation
2.2 Numerically Controlled Oscillator (NCO) Phase and Frequency
2.3 Analyzing RSR Data
3 Detailed Interface Description
3.1 DLF File Header
3.2 DLF Tracking Mode Header
3.3 DLF Tracking Mode Data Table
3.4 DLF File Trailer
Figures
3-0 High-level Layout of DLF File Structure
3-1 Example DLF File Header
3-2 Example DLF Tracking Mode Header
3-3 Example DLF Tracking Mode Data Table
3-4 Example DLF File Trailer
1 Introduction
This Software Interface Specification (SIS) describes the format and content of
the Downlink Frequency (DLF) Prediction File generated by the Radio Science Systems
Group (RSSG) at the NASA Jet Propulsion Laboratory (JPL). The file consists of
coefficients which may be used to evaluate Everett polynomials to compute the
expected frequency of a downlink signal from a spacecraft as a function of time.
Factors incorporated into the coefficients include (as appropriate):
location, motion, and frequency of the uplink signal source;
spacecraft position, motion, turn-around ratio, and/or onboard oscillator
frequency;
receiving antenna location and motion;
tracking mode;
Earth ionospheric and tropospheric effects; and
relativistic effects
When applied to radio occultations, the difference between the predicted and
observed downlink frequencies can be attributed to phase perturbations along the ray
path introduced by an ionosphere or neutral atmosphere.
1.1 References
The predicted frequencies in the DLF are used to tune the Radio Science Receiver;
its operation is described in document [1].
[1] 820-013 Deep Space Mission System (DSMS) External Interface Description,
JPL D-16765, 0159-Science, Radio Science Receiver Standard Formatted Data
Unit (SFDU), Revision B, February 29, 2008.
[2] H. and B. S. Jeffreys, Methods of Mathematical Physics, Third Edition.
The Syndics of the Cambridge University Press, 1962. p. 269.
2 Functional Description
2.1 Radio Science Receiver (RSR) Operation
The RSR is a computer controlled open loop receiver that digitally records a
spacecraft signal through the use of an analog to digital converter (ADC) and up to
four digitally filtered sub-channels. The digital samples from each sub-channel are
stored to disk in one second records in real time. In near real time the one second
records are partitioned and formatted into a sequence of RSR SFDUs that are
transmitted to JPL's Advanced Multi-Mission Operations System (AMMOS). Included in
each RSR SFDU is the ancillary information needed to reconstruct the signal
represented by the recorded data samples in that SFDU. Analysis of variations in
the amplitude, phase, and frequency of the recorded signals provides information on
the ring structure, atmospheric density, magnetic field, and charged-particle
environment of planets which occult the spacecraft. Variations in the recorded
signal may also be used for gravity wave detection.
After a spacecraft transmits a radio frequency (RF) signal, it is received on
Earth one light time later. The RF signal is down converted to an intermediate
frequency (IF) of about 300 MHz and then fed via an IF distribution network into one
input of an IF Selector Switch (IFS). The IFS allows any of several RSRs to select
from any of the IF signals feeding the IFS. From the IFS the signal then goes to an
RSR Digitizer (DIG) and a series of digital down converters and filters. It is in
these down conversions that the predicted frequency in the DLF is used. For a more
detailed description of RSR operation, see [1].
2.2 Numerically Controlled Oscillator (NCO) Phase and Frequency
At the beginning of each pass, Downlink Frequency (DLF) predictions (in the form
of a text file) are loaded into the RSR to tune the numerically controlled
oscillator (NCO) to the expected frequency of the spacecraft signal once each
millisecond. In the DLF, predicted frequencies are provided at time intervals up to
several minutes apart at time precisions of milliseconds. A set of four coefficients
is provided on each line, and the RSR interpolates between the given frequencies
using these coefficients according to formulas for an Everett Polynomial (see
Section 2.3). Millisecond time precision was chosen to minimize frequency residuals
when uplink transmissions, using linear frequency ramps and ramp rate changes on
integral seconds, were echoed back to Earth by the spacecraft transponder and
arrived in the middle of a DLF prediction interval.
In conventional RSR operation [1], tuning intervals are specified with one second
precision; the RSR calculates frequency and phase polynomial coefficients for each
second and the NCO values are calculated from the coefficients at each millisecond.
The coefficients are stored in the RSR secondary header CHDO and can be used to
recover the sky frequency during later processing.
In a variation of RSR operation developed by RSSG for use with Mars Reconnaissance
Orbiter (MRO), the coefficients change irregularly. This is because the MRO
spacecraft frequency is controlled by an uplink from an Earth-based transmitter.
The uplink frequency is piecewise linearly continuous, and the break points where
the ramp rate changes are at integer seconds. Because the two-way light time drifts
during an RSR observation, the transmitter and receiver ramps are not synchronized.
As a consequence, only the first of the three RF frequency points and subchannel
frequency points stored in the RSR file remain valid. In addition, only the zero-
order terms of the sub-channel frequency polynomial and sub-channel phase polynomial
fields remain valid. Remaining entries in these fields are "NaN" ("not a number,"
or "7fffffffffffffff" in hexadecimal). Consequently, processing tools that read the
RSR file according to [1] will be unable to extract the original frequency
prediction normally stored in the RSR data. To make it possible for the user to
retrieve the original frequency predictions, the original downlink frequency (DLF)
files are included in the MRO archive for RSR files which need them. Details on how
to use these files are given in section 2.3.
2.3 Analyzing RSR Data
In the following equation Resid_Freq is the residual frequency measured after
spectral
analysis of RSR data; it is the 'observable' -- the difference between the
predicted frequency and the actual frequency.
Sky Freq = Pred_Freq + Resid_Freq
The DLF which accompanies an RSR file contains rows of data in the following format:
TIME FREQUENCY (HZ) D2N D2N+1 D4N D4N+1
t0 f0 d20 d21 d40 d41
t1 f1 etc
where f is the predicted frequency at time t and D2N, D2N+1, D4N, and D4N+1
are the coefficients of an Everett polynomial (d20, d21, d40, and d41, respectively;
see [2]) at each time step. Frequency at time t can be computed according to the
following equation:
Pred_Freq = (1-p)*f0 + g2(1-p)*d20 + g4(1-p)*d40
+ p*f1 +g2(p)*d21 + g4(p)*d41
where p = (t-t0)/(t1-t0), and g2(x) and g4(x) are Everett polynomials:
g2(x) = x*(x*x - 1.0)/6.0, and
g4(x) = x*(x*x - 1.0) * (x*x - 4)/120.0
When adding values derived for Resid_Freq from the RSR data to values derived for
Pred_Freq from the DLF, care should be taken to make sure that the time tags match
precisely.
3 Detailed Interface Description
The physical layout of the DLF is shown in Figure 3-0. The structure is divided
into four sections: the file header, the tracking mode header, the tracking mode
data table, and the file trailer. The tracking mode header and tracking mode data
table occur in pairs. There may be up to three pairs in a single DLF -- one for
each possible tracking mode (one-way, two-way, and three-way). Original DLF records
contain up to 82 bytes with an ASCII carriage-return line-feed pair (ASCII 13 and
ASCII 10) in last two positions. Archival versions of the DLF for the MAVEN mission
have all records padded to exactly 82 bytes with the record delimiter pair in
positions 81-82.
|==========================|
| FILE HEADER |
|--------------------------|
| TRACKING MODE HEADER |
|--------------------------|
| TRACKING MODE DATA TABLE |
|--------------------------|
| FILE TRAILER |
|==========================|
Figure 3-0. High-level Layout of DLF File Structure
3.1 DLF File Header
Figure 3-1 contains an example DLF File Header, where byte positions are shown
across the top. In the first record the spacecraft identifier is in bytes 19-22,
the DSN receiving antenna number is in bytes 28-29, the UTC start year and day are
in bytes 38-43, the UTC start time is in bytes 45-52, and the UTC end time is in
bytes 59-66. Record 2 gives the file creation date and time, Record 3 gives the
pass number, Record 4 gives the downlink frequency band, and record 5 gives the
uplink frequency band. In the case of no uplink, record 5 may be omitted.
|================================================================================|
| 1 2 3 4 5 6 7 8|
|12345678901234567890123456789012345678901234567890123456789012345678901234567890|
|================================================================================|
|** RMT_PRDX S/C=0202, DSS26, START=17/055 16:50:39, END=04:36:15, Test |
|*1 CREATED=17/054 19:47:27, MOD_NSS=17/054 19:47:27, NO_DSS_MODIFICATION |
|*2 PASS=55 |
|*3 DOWNLINK_BAND=X, TFREQ= 8445767500.0000 |
|*3 UPLINK_BAND=X |
|*@ END OF HEADER |
|================================================================================|
Figure 3-1. Example DLF File Header
3.2 DLF Tracking Mode Header
Figure 3-2 contains an example DLF Tracking Mode Header, where byte positions are
shown across the top. In the second record the frequency band is given in bytes 4-
9, the tracking mode is given in bytes 11-15, and the start and end times are
repeated. Records 3-4 provide column headings for fixed width displays of the date
table which follows.
|================================================================================|
| 1 2 3 4 5 6 7 8|
|12345678901234567890123456789012345678901234567890123456789012345678901234567890|
|================================================================================|
|# |
|*F X-BAND 1-WAY START=17/055 16:50:39 END=17/056 04:36:15 |
|# TIME FREQUENCY(HZ) D2N D2N+1 D4N D4N+1 |
|# |
|================================================================================|
Figure 3-2. Example DLF Tracking Mode Header
3.3 DLF Tracking Mode Data Table
Figure 3-3 contains part of an example DLF Tracking Mode Data Table, where byte
positions are shown across the top. Bytes 1-12 contain the UTC Earth Receive Time
(ERT) in hh:mm:ss.sss format. Bytes 13-30 contain the predicted received frequency
to 100 microhertz precision. Bytes 31-43, 44-56, 57-68, and 69-80 contain the
Everett polynomial coefficients d20, d21, d40, and d41, respectively. Bytes 81-82
contain the ASCII carriage-return line-feed pair record delimiter.
|================================================================================|
| 1 2 3 4 5 6 7 8|
|12345678901234567890123456789012345678901234567890123456789012345678901234567890|
|================================================================================|
|16:50:39.064 8445435617.2148 521.778 512.163 56.62 73.06|
|17:04:17.583 8445430870.7205 425.293 475.023 50.06 77.37|
|17:16:44.249 8445426986.7520 374.208 463.911 47.76 79.60|
|17:27:47.953 8445423930.1833 368.704 489.553 50.03 85.07|
|17:37:40.546 8445421589.0428 377.834 517.563 50.59 84.73|
|# ... |
|================================================================================|
Figure 3-3. Example DLF Tracking Mode Data Table
3.4 File Trailer
Figure 3-4 contains an example DLF File Trailer, where byte positions are shown
across the top. In the original file, the record contains only nine bytes plus the
ASCII carriage-return line-feed record delimiter. In the archival format, the
record is padded to 82 bytes with the carriage-return line-feed record delimiter in
positions 81-82.
|================================================================================|
| 1 2 3 4 5 6 7 8|
|12345678901234567890123456789012345678901234567890123456789012345678901234567890|
|================================================================================|
|*= END =* |
|================================================================================|
Figure 3-4. Example DLF File Trailer
A Abbreviations
Abbreviations and acronyms used in this document are defined where they first
occur in the text. A complete list is provided here for the convenience of the
reader.
ADC Analog to Digital Conversion
AMMOS Advanced Multi-Mission Operations System
ASCII American Standard Code for Information Interchange
CHDO Compressed Header Data Object
DIG Digitizer subassembly
DLF Downlink Frequency (prediction)
DSN Deep Space Network
DSS Deep Space Station
ERT Earth Receive Time
HZ Hertz
IF Intermediate Frequency
IFS IF Switch
JPL Jet Propulsion Laboratory
LO Local Oscilator
MAVEN Mars Atmosphere and Volatile EvolutioN (mission)
MRO Mars Reconnaissance Orbiter (mission)
NASA National Aeronautics and Space Administration
NCO Numerically Controlled Oscillator
1PPS 1 Pulse Per Second
RF Radio frequency
RSR Radio Science Receiver
RSSG Radio Science Systems Group
SFDU Standard Formatted Data Unit
UTC Coordinated Universal Time
|