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

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

4 Baseline L2C Schemes Message Type 1 Long Frame (1.0 min)       Message Type 2Message Type 3Message Type 4Message Type 5 Message Type 1Message Type 2Message Type 3Message Type 4Message Type 5 Message Type 1Message Type 2Message Type 3Message Type 4Message Type 5 Long Frame (1.0 min) Message Type 1 Medium Frame (0.8 min)      Message Type 2Message Type 3 Message Type 1Message Type 2Message Type 4Message Type 5 Message Type 1Message Type 2Message Type 3 Medium Frame (0.8 min) Message Type 4 Message Type 1 Short Frame (0.6 min) Message Type 2Message Type 3 Message Type 1Message Type 2Message Type 4 Message Type 1Message Type 2Message Type 5 Short Frame (0.6 min)     Message Type 1Message Type 2Message Type 4 Short Frame (0.6 min) Message Types: 1 = Clock/Eph Part 1 2 = Clock/Eph Part 2 3 = Iono, Bias, Health 4 = Almanac 5 = Text (NANUs) 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 Very Small Don't Care Assume Circular Orbit Combine if a Circular Orbit Very Small Simplify Small, Only ±2 Degrees Relative to the Nominal Small Relative to a Nominal A Scale Factor of 2 -6 Will Give a Resolution of ±1.4 Degrees

13 Compressing Almanacs - IVe

14 Compressing Almanacs - V Have 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 (WN a ) + 8 bits (t oa ) 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 Proposed "Mini-Almanac" Packet 31 BITS PRN a 6 BITS DELTA_A 8 BITS OMEGA_0 7 BITS ARGUMENT OF LATITUDE 7 BITS L1 HEALTH L2 HEALTH L5 HEALTH Reference Values: e = 0  i = SC (i = 55 deg) OMEGA_DOT = -2.5x10-9 SC/sec A ref = 26,559,710 m M 0 +  = Argument of Latitude Satellite clock terms not transmitted

17 Proposed Message Type 6 Format with Mini-Almanacs

18 Example L2C Schemes Message Type 1 Medium Frame (0.8 min)      Message Type 2Message Type 3 Message Type 1Message Type 2Message Type 5 Message Type 1Message Type 2Message Type 3 Medium Frame (0.8 min) Message Type 6 Message Type 1 Short Frame (0.6 min) Message Type 2Message Type 3 Message Type 1Message Type 2 Message Type 1Message Type 2Message Type 5 Short Frame (0.6 min)     Message Type 1Message Type 2 Short Frame (0.6 min) 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 Or Some Other Flexible Mix of Long, Medium, and/or Short Frames Message Type 6 Message Type 1Message Type 2Message Type 5 Medium Frame (0.8 min) Message Type 6 Elapsed Time: 3.2 min (192 sec) Message Type 6 Message Type 1 Short Frame (0.6 min) Message Type 2Message Type 3 Message Type 1Message Type 2 Message Type 1Message Type 2Message Type 5 Short Frame (0.6 min) Message Type 1Message Type 2 Short Frame (0.6 min) Message Type 6 Elapsed Time: 4.8 min (288 sec)

19 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 Follow-Up Work

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 Message Type 1 Medium Frame (0.8 min)      Message Type 2Message Type 3 Message Type 1Message Type 2Message Type 5 Message Type 1Message Type 2Message Type 3 Medium Frame (0.8 min) Message Type 6-1 Message Type 1 Short Frame (0.6 min) Message Type 2Message Type 3 Message Type 1Message Type 2 Message Type 1Message Type 2Message Type 5 Short Frame (0.6 min)     Message Type 1Message Type 2 Short Frame (0.6 min) 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 Or Some Other Flexible Mix of Long, Medium, and/or Short Frames Message Type 6-2 Message Type 6-3 Message Type 1Message Type 2Message Type 5 Medium Frame (0.8 min) Message Type 6-4 Elapsed Time: 3.2 min (192 sec) Message Type 1 Short Frame (0.6 min) Message Type 2Message Type 3 Message Type 1Message Type 2 Message Type 1Message Type 2Message Type 5 Short Frame (0.6 min) Message Type 1Message Type 2 Short Frame (0.6 min) Elapsed Time: 4.8 min (288 sec) Message Type 6-1 Message Type 6-2 Message Type 6-3 Message Type 6-4

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