readme.design

The OpenGPSRec receiver software runs on the real time operating system RTAI-Linux. It compiles with

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                    Design Concepts of OpenGPSRec
                    *****************************

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              >>> This document is work in progress! <<<

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Changes:

- 05.12.03  MULTISAT mode

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S/W design
----------

OpenGPSRec consists of the following parts:

1) real-time (RT) process (ogrt_proc.c)

- access GP2021 correlator registers every 0.505 msec
- check if DUMP event has occurred
- read NCO values (carrier and code phase) every TIC (100 msec)
- extract time information from subframe headers

2) user space process (ogr_main.c and friends)

- calculate navigation solution
- display data on screen
- write data to disk

3) user space process (gpsplot.c and friends)

- graphical visualization of in- and quad-phase data

h/w access to the PROTO-3 ports are restricted to the RT process.


Data exchange between ogrt_module and ogr_main:

- shared memory regions (struct Chan[])
- FIFOs (nav bits, I/Q data)


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Operating modes:
----------------


1) standard (navigation) mode: 
------------------------------
Track signals of all visible satellites and find navigation 
solution.


2) MONOSAT mode: 
-----------------

One master channels tracks the direct signal, the remaining 
11 slave channels follow the master channel in code and doppler 
space with predefined spacing. I.e. the tracking function 
controlling the master channels writes simultaneously all 
12 DCOs. (FIXME: How does this work, if the channels differ 
in doppler as well?)

In code space: phase difference is set during initialization,
code frequency changes are applied simultaneous. 

Since code phase cannot be set directly, we have to adjust
code frequency until the desired code phase offset appears.
That is accomplished in functions "ch_*_follow_TIC()".

Carrier and code DCO adjustements are set simultaneously
for all channels using the "all_carrier()" / "all_code()" 
calls.


3) MULTISAT mode: 
-----------------

*** TBI (to be implemented) ***

similar to MONOSAT mode, except there are N>1 master channels.
Each controls M slave channels in open-loop. (E.g. N=2, M=5; 
N=3, M=3; N=4, M=2. If N=4, receiver could determine navigation 
solution as well.)


4) Track rising/setting satellites: 
-----------------------------------

Sats within a predefined elevation angle range are scheduled
to be tracked in MONOSAT mode, i.e. all 12 correlators
track one PRN. Code phase changes are determined by taking 
into account 24 correlator outputs (12 prompt and 12 early 
values). Carrier phase changes are taken from the 
line-of-sight optical path length. I/Q are saved to data 
file 'output/rst-WWWW-SSSSSS-PRN.dat' every 20 ms.

Code tracking:

Code phase error of leading channel is calculated from all 
12 correlator outputs (weighted mean). Trailing channels are
kept at predefined code offset w.r.t. leading correlator.

Carrier tracking:

Carrier phase error of leading channel is calculated from all 
12 correlator outputs (weighted mean). Trailing channels are
kept in sync with leading correlator.

Problem: we get info on code/carrier phase only at TIC (100 ms).

Thus, update trailing code by 1/5 and carrier by 1/100 of actual 
phase error (synchronization).

(*** FIXME: currently only setting events are tracked ***)


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GP2021 correlator:
------------------

carrier DCO

Counter of carrier cycles, 2^27 corresponds to 1/1.405 MHz =  0.712 usec
(1.405 MHz is down-converted carrier frq, sampling frq is 5.714 MHz).
Nominally, counter is incremented by (nominally)

                2^27 * 1.405 MHz / 5.714 MHz = 33010105; 

frequency resolution: 1.405 MHz / 33010105 = 42.6 mHz

                    2222222211111111110000000000
                    7654321098765432109876543210 
                    ----------------------------
                    ABBBCCCCCCCCCCCCCCCCCCCCCCCC

BBB (bits 24-26) is index into sin/cos 8-phase look-up table

   sin[8] = [-1 +1 +2 +2 +1 -1 -2 -2]
   cos[8] = [+2 +2 +1 -1 -2 -2 -1 +1]

------------------------------------------------------------------------

code DCO

Counter of code cycles, 2^26 corresponds to 1/2.046 MHz =  0.488 usec.
Counter is incremented by (nominally)

                2^26 * 2.046 MHz / 5.714 MHz = 24028328; 

frq. resolution: 2.046 MHz / 24028328 = 85.15 mHz

          33333333222222222211111111110000000000
          76543210987654321098765432109876543210 
          --------------------------------------
          ABBBBBBBBBBBCCCCCCCCCCCCCCCCCCCCCCCCCC

BBBBBBBBBBB (bits 26-36) is index into 2046 halfchip table

------------------------------------------------------------------------

timing signals:

DUMP event occurs in each correlator channel after 1023 chips 
(2046 halfchips), about 1 msec. I.e. DUMP events are _not_ 
synchronized.

TIC event occurs every 0.0999999 sec (simultaneous for all 
12 channels)
 
At TIC current state of following registers are read:

  carr_cycle_l    : full carrier cycles (low 2 bytes)
  carr_cycle_h    : full carrier cycles (high 2 bytes)
  carr_dco_phase  : fractional carrier cycles 
                    (0,...,1023), resolution: 2pi/1024
  code_slew       : code slew after next DUMP (0,...,2047)
  code_phase      : code phase counter on TIC (0,...,2046)
  code_dco_phase  : fractional code phase, 
                    (0,...,1023), resolution: 2pi/1024
  epoch           : 1 msec (0,...,19) and 20 msec (0,...,49)
                    epoch counter

At each DUMP the following registers are read:

  i_dith   : in-phase, dither
  q_dith   : quad-phase, dither
  i_prompt : in-phase, prompt
  q_prompt : quad-phase, prompt

Technical data:

  sampling (clock) frequency            : 40/7 MHz = 5.714 MHz
  down-converted carrier frq. (nominal) : 1.405 MHz
  carrier DCO step (nominal)            : 0x1f7b1b9 = 33010105
  code DCO step (nominal)               : 0x16ea4a8 = 24028328
  unit step                             : 175 nsec (40/7 MHz)
  carrier DCO                           : 2^27 = 1 carrier cyc.
  code DCO                              : 2^26 = 1/2 chip


Carrier and code loops:
-----------------------

OpenSourceGPS default values:

  i_sum = i_prompt + i_early
  q_sum = q_prompt + q_early

  prompt_mag = rss( i_prompt_20ms, q_prompt_20ms);
  early_mag  = rss( i_early_20ms,   q_early_20ms);

carrier PLL:

  trk_carr_k = -9
  trk_carr_d = 21

  carfrq_inc = -trk_carr_k * 
      ( q_sum << 14) * sign( i_sum) / rss( i_sum, q_sum);

  dcarfrq = carfrq_inc + trk_carr_d * (carfrq_inc - carfrq_inc_prv)
          = (trk_carr_d+1) * carfrq_inc - trk_carr_d * carfrq_inc_prv

  carfrq = carfrq + dcarfrq >> 14;

note: carfrq given in units of 1.405 MHz / 33010105 = 42.574 mHz 
not Hz! 

code PLL:

  trk_code_k = 25
  trk_code_d =  3

  codfrq_inc = earlylate * trk_code_k * 
        (prompt_mag - 2 * early_mag);

  dcodfrq = codfrq_inc + trk_code_d * (codfrq_inc - codfrq_inc_prv);
          = (trk_code_d+1) * codfrq_inc - trk_code_d * codfrq_inc_prv;

  trk_div = 19643

  codfrq = codfrq + dcodfrq / trk_div;

Parameters used in rcvr_par.dat:
--------------------------------

pul_cod_dampratio        :       0.707
pul_cod_loopgain         :       0.05
pul_cod_bandwidth        :       2.0
pul_car_dampratio        :       0.707
pul_car_loopgain         :       1.25
pul_car_bandwidth        :      50.0
trk_cod_dampratio        :       0.707
trk_cod_loopgain         :       0.05
trk_cod_bandwidth        :       1.0
trk_car_dampratio        :       0.707 
trk_car_loopgain         :       1.25
trk_car_bandwidth        :      25.0

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Theory of 2nd order PLL:
------------------------

  Zeta      : damping ratio; Zeta = 0.707 is critically damped
  TUpd      : update period is 1 ms 
  wn        : natural frequency
  BandWidth : loop bandwidth
  Gain      : loop gain

  Zeta = 0.707
  wn   = 2 * BandWidth / (Zeta + 1/(4*Zeta));
       = 1.8856 * BandWidth
  wT   = wn * TUpd;

  C1 = 1/Gain * 8 * Zeta*wT / (4 + 4*Zeta*wT + wT^2);
  C2 = 1/Gain * 4 * wT^2    / (4 + 4*Zeta*wT + wT^2);

  DscCarPhs(i) = atan2( Qp(i), Ip(i)) / (2*pi)

  DeltaPhs  = (C1 + C2) * DscCarPhs(i) - C1 * DscCarPhs(i-1);
  CarFrqOfs = DeltaPhs / TUpd;

thus

  wT = 2 * 0.707 * C2 / C1
  wn = wT / TUpd

  BW = wn / 1.8856
     = 0.74989 * C2 / C1 / TUpd
  G  = 1 / (C1+C2) * (4*wT^2 + 8*Zeta*wT) / (4+4*Zeta*wT+wT^2)

with 

  carrier:   C1 = 21 * 1e-3
             C2 =  1 * 1e-3

  code:      C1 =  3 * 0.02
             C2 =  1 * 0.02

we obtain for the carrier PLL:

  BandWidth BW = 35.5 Hz
  Gain      G  = 4.3

and for the code PLL:

  BandWidth BW = 12.5 Hz
  Gain      G  = 0.0004

Tsui, Fundamentals of GPS receivers [2001] recommends:

  carrier:   BW = 20 Hz
             G  = 2*pi*0.2 = 1.2566

  code:      BW = 1 Hz
             G  = 0.05

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Fixed point arithmetics:

Represent numbers between -2^16 and 2^16 (= 65536) with resolution 
of 2^-14 as long

  0x00000001L <=> 2^(-14)
  0x00004000L <=> 1
  0x40000000L <=> 2^(+16)

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