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 | | | |============================================================================| |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