# Presentation: 388515

- Bill Feess, Aerospace
- Karl Kovach, ARINC

- 2 May 2001

**Proposed Satellite Mini-Almanac for L2C Message Type 6**

## 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 1⁄2-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

## ICD-GPS-200 Design

- Subframe 1

- Subframe 2

- Subframe 3

- Subframe 4

- Subframe 5

- Subframe 1

- Subframe 2

- Subframe 3

- Subframe 4

- Subframe 5

- Subframe 1

- Subframe 2

- Subframe 3

- Subframe 4

- Subframe 5

- Subframe 1

- Subframe 2

- Subframe 3

- Subframe 4

- Subframe 5

- Subframe 1

- Subframe 2

- Subframe 3

- Subframe 4

- Subframe 5

- Subframe 1

- Subframe 2

- Subframe 3

- Subframe 4

- Subframe 5

- Subframe 1

- Subframe 2

- Subframe 3

- Subframe 4

- Subframe 5

- Subframe 1

- Subframe 2

- Subframe 3

- Subframe 4

- Subframe 5

- Subframe 1

- Subframe 2

- Subframe 3

- Subframe 4

- Subframe 5

- Subframe 1

- Subframe 2

- Subframe 3

- Subframe 4

- Subframe 5

- Subframe 1

- Subframe 2

- Subframe 3

- Subframe 4

- Subframe 5

- Subframe 1

- Subframe 2

- Subframe 3

- Subframe 4

- Subframe 5

- Subframe 1

- Subframe 2

- Subframe 3

- Subframe 4

- Subframe 5

- Subframe 1

- Subframe 2

- Subframe 3

- Subframe 4

- Subframe 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)

**     ********                         ********                 ********                 ********                  ********                 ******** **

**     ********                         ********                 ********                 ********                  ********                 ******** **

## Baseline L2C Schemes

- Message Type 1

- Long Frame (1.0 min)

**         ********                               ********                        ********                       ********                        ********                        ******** **

- Message Type 2

- Message Type 3

- Message Type 4

- Message Type 5

- Message Type 1

- Message Type 2

- Message Type 3

- Message Type 4

- Message Type 5

- Message Type 1

- Message Type 2

- Message Type 3

- Message Type 4

- Message Type 5

- Long Frame (1.0 min)

- Long Frame (1.0 min)

- Message Type 1

- Medium Frame (0.8 min)

**         ********                               ********                        ********                       ********                        ********                    **

- Message Type 2

- Message Type 3

- Message Type 1

- Message Type 2

- Message Type 4

- Message Type 5

- Message Type 1

- Message Type 2

- Message Type 3

- Medium Frame (0.8 min)

- Medium Frame (0.8 min)

- Message Type 4

- Message Type 4

- Message Type 1

- Short Frame (0.6 min)

- Message Type 2

- Message Type 3

- Message Type 1

- Message Type 2

- Message Type 4

- Message Type 1

- Message Type 2

- Message Type 5

- Short Frame (0.6 min)

- Short Frame (0.6 min)

**         ********                               ********                        ********                       ********                                     **

- Message Type 1

- Message Type 2

- Message 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

## 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?

## 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)

## 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

## 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

## Compressing Almanacs - IVa

## Compressing Almanacs - IVb

## Compressing Almanacs - IVc

## 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

## Compressing Almanacs - IVe

## 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)

## 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

## Proposed "Mini-Almanac" Packet

## Proposed Message Type 6 Format with Mini-Almanacs

## Example L2C Schemes

- Message Type 1

- Medium Frame (0.8 min)

**         ********                               ********                        ********                       ********                        ********                    **

- Message Type 2

- Message Type 3

- Message Type 1

- Message Type 2

- Message Type 5

- Message Type 1

- Message Type 2

- Message Type 3

- Medium Frame (0.8 min)

- Medium Frame (0.8 min)

- Message Type 6

- Message Type 1

- Short Frame (0.6 min)

- Message Type 2

- Message Type 3

- Message Type 1

- Message Type 2

- Message Type 1

- Message Type 2

- Message Type 5

- Short Frame (0.6 min)

- Short Frame (0.6 min)

**         ********                               ********                        ********                       ********                                     **

- Message Type 1

- Message 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 6

- Message Type 1

- Message Type 2

- Message Type 5

- Medium Frame (0.8 min)

- Message Type 6

**Elapsed Time:**
**3.2 min (192 sec)**

- Message Type 6

- Message Type 6

- Message Type 1

- Short Frame (0.6 min)

- Message Type 2

- Message Type 3

- Message Type 1

- Message Type 2

- Message Type 1

- Message Type 2

- Message Type 5

- Short Frame (0.6 min)

- Short Frame (0.6 min)

- Message Type 1

- Message Type 2

- Short Frame (0.6 min)

- Message Type 6

- Message Type 6

**Elapsed Time:**
**4.8 min (288 sec)**

## 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

- 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**

- Back-up Slides

## 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

## 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

## Previous Almanac Schemes

- Message Type 1

- Medium Frame (0.8 min)

**         ********                               ********                        ********                       ********                        ********                    **

- Message Type 2

- Message Type 3

- Message Type 1

- Message Type 2

- Message Type 5

- Message Type 1

- Message Type 2

- Message Type 3

- Medium Frame (0.8 min)

- Medium Frame (0.8 min)

- Message Type 6-1

- Message Type 1

- Short Frame (0.6 min)

- Message Type 2

- Message Type 3

- Message Type 1

- Message Type 2

- Message Type 1

- Message Type 2

- Message Type 5

- Short Frame (0.6 min)

- Short Frame (0.6 min)

**         ********                               ********                        ********                       ********                                     **

- Message Type 1

- Message 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 1

- Message Type 2

- Message 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 2

- Message Type 3

- Message Type 1

- Message Type 2

- Message Type 1

- Message Type 2

- Message Type 5

- Short Frame (0.6 min)

- Short Frame (0.6 min)

- Message Type 1

- Message 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

## Staggered Almanac Scheme

**Staggered Almanac Scheme**

- Message Type 1

- Long Frame (1.0 min)

**         ********                               ********                        ********                       ********                        ********                      ******

- Message Type 2

- Message Type 3

- Message Type 1

- Message Type 2

- Message 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 2

- Message Type 5

- Message Type 1

- Message Type 2

- Message 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 2

- Message Type 3

- Message Type 1

- Message Type 2

- Message 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 6-3

- Message Type 1

- Long Frame (1.0 min)

- Message Type 2

- Message Type 5

- Message Type 1

- Message Type 2

- Message Type 3

- Long Frame (1.0 min)

- Message Type 6-2

- Message Type 6-4

- Message Type 6-1