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Bill Feess, Aerospace Karl Kovach, ARINC

Proposed Satellite Mini-Almanac for L2C Message Type 6. Bill Feess, Aerospace Karl Kovach, ARINC. 2 May 2001. Problem. Long time to transmit a full set of constellation almanacs ICD-GPS-200 design takes 12.5 min to transmit full set of almanacs

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Bill Feess, Aerospace Karl Kovach, ARINC

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  1. Proposed Satellite Mini-Almanac for L2C Message Type 6 Bill Feess, Aerospace Karl Kovach, ARINC 2 May 2001

  2. Problem • Long time to transmit a full set of constellation almanacs • ICD-GPS-200 design takes 12.5 min to transmit full set of almanacs • L2C design requires 12 sec to transmit 1 message (= 1 subframe) • Half as fast as ICD-GPS-200 design due to ½-rate FEC encoding • Baseline L2C design uses 1 message to transmit 1 SV almanac • Same as ICD-GPS-200 design (1 message = 1 subframe), 300 bits total • When 5 different message types are being transmitted, it would take 60 sec between each transmission of an SV almanac • To transmit a full set of almanacs, up to 24 - 28 minutes would be required, depending on constellation size (24 to 28 SVs) • This long time poses an operational problem to some users

  3. ICD-GPS-200 Design Frame 1 (0.5 min) Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5 Frame 2 (0.5 min) Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5 Frame 3 (0.5 min) Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5 Frame 4 (0.5 min) Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5 Frame 5 (0.5 min) Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5 Frame 6 (0.5 min) Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5 Frame 7 (0.5 min) Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5 Master Frame (12.5 minutes) Frame 8 (0.5 min) Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5 Frame 9 (0.5 min) Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5 Frame 10 (0.5 min) Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5 Frame 11 (0.5 min) Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5 Frame 12 (0.5 min) Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5             Frame 24 (0.5 min) Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5 Frame 25 (0.5 min) Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5

  4. Baseline L2C Schemes Long Frame (1.0 min) Message Type 1 Message Type 2 Message Type 3 Message Type 4 Message Type 5 Long Frame (1.0 min) Message Type 1 Message Type 2 Message Type 3 Message Type 4 Message Type 5 Long Frame (1.0 min) Message Type 1 Message Type 2 Message Type 3 Message Type 4 Message Type 5       Medium Frame (0.8 min) Message Type 1 Message Type 2 Message Type 3 Message Type 4 Medium Frame (0.8 min) Message Type 1 Message Type 2 Message Type 4 Message Type 5 Medium Frame (0.8 min) Message Type 1 Message Type 2 Message Type 3 Message Type 4      Message Types: 1 = Clock/Eph Part 1 2 = Clock/Eph Part 2 3 = Iono, Bias, Health 4 = Almanac 5 = Text (NANUs) Short Frame (0.6 min) Message Type 1 Message Type 2 Message Type 3 Short Frame (0.6 min) Message Type 1 Message Type 2 Message Type 4 Short Frame (0.6 min) Message Type 1 Message Type 2 Message Type 5 Short Frame (0.6 min) Message Type 1 Message Type 2 Message Type 4     Or Some Other Flexible Mix of Long, Medium, and/or Short Frames

  5. Solutions to Problem • Do Nothing (simply tell users "tough luck") • Half-hour to collect a full set of almanacs isn't that long • Not for many types of receivers that are ON for hours at a time • However, handheld receivers are a much different story • Find a Way to Transmit the Almanacs Quicker • Limited on maximum available data rate -- not feasible • Stagger almanac transmissions between SVs -- feasible • e.g., A/C/E-plane SVs transmit almanacs for B/D/F-plane SVs • Compress the almanac data to make it smaller -- feasible • But how much compression is possible?

  6. Compressing Almanacs - I • Almanacs are used by a GPS receiver for: • SV visibility determination • What SVs are visible or will soon become visible • Geometry-based SV selection • GDOP, PDOP, HDOP, etc. • SV signal acquisition aid • Approximate Doppler offset and code delay • Almanacs are NOT used by a GPS receiver for: • Positioning, timing, or navigation • Not enough precision (this is what clock/ephemeris data is for)

  7. Compressing Almanacs - II • Driver on almanac precision is code delay accuracy • Needed for direct P(Y)-code signal acquisition • Acquire P(Y)-code signal without help from C/A-code • Direct P(Y) acquisition needs fairly good accuracy • Roughly about the GPS receiver's time uncertainty • 10 sec  3,000 m 2-week old almanac • L2C almanacs not used for direct P(Y) acquisition • L2C receivers don't need to do direct P(Y) acquisition • They'll do the normal C/A-code or Moderate-code acquisition • Therefore, no major accuracy driver on L2C almanac precision • Direct P(Y) receivers will still use ICD-GPS-200 almanacs • Collected from the P(Y)-code signal on either L1 or L2

  8. Compressing Almanacs - III • Really 2 main drivers on L2C almanac precision • Angular accuracy for visibility and geometry • Wag a sensitivity threshold of a "couple of degrees" • Receivers that do visibility/geometry computations every 5 min • Doppler accuracy for signal acquisition • Wag a sensitivity threshold of a "couple of hundred Hertz" • Receivers that have 32 Doppler search bins • See what can do by editing ICD-GPS-200 almanacs • Using the above wags as guidelines

  9. Compressing Almanacs - IVa

  10. Compressing Almanacs - IVb

  11. Compressing Almanacs - IVc

  12. Compressing Almanacs - IVd Small, Only ±2 Degrees Relative to the Nominal Assume Circular Orbit Very Small Simplify Small Relative to a Nominal A Scale Factor of 2-6 Will Give a Resolution of ±1.4 Degrees Combine if a Circular Orbit Don't Care Very Small

  13. Compressing Almanacs - IVe

  14. Compressing Almanacs - V • Have 3+8+7+7 unique bits per Almanac (= 25 bits) • Two ways to pack the almanac data bits • Can pack into a number of fixed message types • Type X always has almanacs for PRNs 1 to n • Type Y always has almanacs for PRNs n+1 to 2n • Etc. • Can pack into a single message type • Must include PRN number with each almanac • Trade-off considering 236 usable bits per message type • Less 10 bits (WNa) + 8 bits (toa) common across all almanacs • Turns out to be a wash for up to 28 SVs (always 4 messages)

  15. Compressed Almanac Proposal • Proposal here is to compress to 7 SVs per almanac message • 31 bits total per almanac (6 bits for PRN + 25 bits for orbit/health) • With this compression, a complete set of almanacs for a 28 SV constellation could be sent in 4 min or less • Factor of 7 savings • Result is thus 7 times shorter almanac collect time • Less drain on handheld receiver battery • Eliminate any need for periodic "almanac download" actions

  16. 31 BITS 1 15 22 29 30 31 7 ARGUMENT OF LATITUDE 7 BITS PRNa 6 BITS DELTA_A 8 BITS OMEGA_0 7 BITS L1 HEALTH L2 HEALTH L5 HEALTH Reference Values: e = 0 i = +0.0056 SC (i = 55 deg) OMEGA_DOT = -2.5x10-9 SC/sec Aref= 26,559,710 m M0+ = Argument of Latitude Satellite clock terms not transmitted Proposed "Mini-Almanac" Packet

  17. Proposed Message Type 6 Format with Mini-Almanacs

  18. Example L2C Schemes Medium Frame (0.8 min) Message Type 1 Message Type 2 Message Type 3 Message Type 6 Medium Frame (0.8 min) Message Type 1 Message Type 2 Message Type 6 Message Type 5 Medium Frame (0.8 min) Message Type 1 Message Type 2 Message Type 3 Message Type 6 Elapsed Time: 3.2 min (192 sec) Medium Frame (0.8 min) Message Type 1 Message Type 2 Message Type 6 Message Type 5      Message Types: 1 = Clock/Eph Part 1 2 = Clock/Eph Part 2 3 = Iono, Bias, Health 4 = Long Almanac 5 = Text (NANUs) 6 = Mini-Almanacs Short Frame (0.6 min) Message Type 1 Message Type 2 Message Type 3 Short Frame (0.6 min) Message Type 1 Message Type 2 Message Type 6 Short Frame (0.6 min) Message Type 1 Message Type 2 Message Type 5 Short Frame (0.6 min) Message Type 1 Message Type 2 Message Type 6 Short Frame (0.6 min) Message Type 1 Message Type 2 Message Type 3 Short Frame (0.6 min) Message Type 1 Message Type 2 Message Type 6 Short Frame (0.6 min) Message Type 1 Message Type 2 Message Type 5 Elapsed Time: 4.8 min (288 sec) Short Frame (0.6 min) Message Type 1 Message Type 2 Message Type 6     Or Some Other Flexible Mix of Long, Medium, and/or Short Frames

  19. Follow-Up Work • Canvass GPS receiver manufacturers on concept • e.g., Brief concept at the L2C Industry Day • Verify requirements for mini-almanac accuracy • R_Dot (i.e., Doppler offset  350 Hz OK?) • Elevation Angle (i.e., visibility computation  2 degrees OK?) • Others (e.g., consensus on no direct L2C acquisition?) • Validate accuracies with proposed message structure and bits • Modify the draft L2C signal PIRN to document new design • Consider same change for the L5 signal data design

  20. Back-up Slides

  21. Type 4 vs Type 6 Messages • No conflict between Type 4 and Type 6 messages • Type 6 doesn't necessarily replace Type 4 • L2C "flexible protocol" allows either or both (or even neither) • The L2C PIRN leaves the decisions up to the operator/users • Some benefit to having both Type 6 and Type 4 • Supports all conceivable receiver designs and user needs • Some benefit to having just Type 6 (or just Type 4) • Minimize throughput load on a very low data rate channel • Don't repeat redundant information unless benefit gained

  22. Staggered Almanac Messages • L2C "flexible protocol" is really very flexible • Any message type in any 12-sec slot from any SV • Within reason due to SV memory and operator workload • Enables many schemes to transmit common data • Message Type 1 and Type 2 contain unique data • Data which is unique to the transmitting SV • Message Types 6, 5, and 4 contain common data • Data which is the same no matter which SV transmits it • Staggered almanac messages is one such scheme

  23. Previous Almanac Schemes Medium Frame (0.8 min) Message Type 1 Message Type 2 Message Type 3 Message Type 6-1 Medium Frame (0.8 min) Message Type 1 Message Type 2 Message Type 6-2 Message Type 5 Medium Frame (0.8 min) Message Type 1 Message Type 2 Message Type 3 Message Type 6-3 Elapsed Time: 3.2 min (192 sec) Medium Frame (0.8 min) Message Type 1 Message Type 2 Message Type 6-4 Message Type 5      Message Types: 1 = Clock/Eph Part 1 2 = Clock/Eph Part 2 3 = Iono, Bias, Health 4 = Long Almanac 5 = Text (NANUs) 6 = Mini-Almanacs Short Frame (0.6 min) Message Type 1 Message Type 2 Message Type 3 Short Frame (0.6 min) Message Type 1 Message Type 2 Message Type 6-1 Short Frame (0.6 min) Message Type 1 Message Type 2 Message Type 5 Short Frame (0.6 min) Message Type 1 Message Type 2 Message Type 6-2 Short Frame (0.6 min) Message Type 1 Message Type 2 Message Type 3 Short Frame (0.6 min) Message Type 1 Message Type 2 Message Type 6-3 Short Frame (0.6 min) Message Type 1 Message Type 2 Message Type 5 Elapsed Time: 4.8 min (288 sec) Short Frame (0.6 min) Message Type 1 Message Type 2 Message Type 6-4     Or Some Other Flexible Mix of Long, Medium, and/or Short Frames

  24. Staggered Almanac Scheme A/C/E-Plane SVs Long Frame (1.0 min) Message Type 1 Message Type 2 Message Type 3 Message Type 6-1 Message Type 6-2 Long Frame (1.0 min) Message Type 1 Message Type 2 Message Type 5 Message Type 6-3 Message Type 6-4 Long Frame (1.0 min) Message Type 1 Message Type 2 Message Type 3 Message Type 6-1 Message Type 6-2 Long Frame (1.0 min) Message Type 1 Message Type 2 Message Type 5 Message Type 6-3 Message Type 6-4       B/F/D-Plane SVs Long Frame (1.0 min) Message Type 1 Message Type 2 Message Type 5 Message Type 6-3 Message Type 6-4 Long Frame (1.0 min) Message Type 1 Message Type 2 Message Type 3 Message Type 6-1 Message Type 6-2 Long Frame (1.0 min) Message Type 1 Message Type 2 Message Type 5 Message Type 6-3 Message Type 6-4 Long Frame (1.0 min) Message Type 1 Message Type 2 Message Type 3 Message Type 6-1 Message Type 6-2       Total Elapsed Time: 1.0 min (60 sec) Compare this 1.0 minute elapsed time against the "traditional scheme" 24 to 28 minute elapsed time

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