GPS Software Receivers - Fundamentals of Global Positioning System Receivers: A Software Approach - page 193

7. IODC: The issue of data, clock has 10 bits. Bits 83 - 84 are the most
significant bits (MSB) and bits 211 - 218 are the least significant bits (LSB).
As discussed in Section 5.13, the LSB of the IODC will be compared with
the issue of data, ephemeris (IODE) of subframes 2 and 3. Whenever these
three data sets are not matched, a data set cutover has occurred and new
data must be collected.
8. TOW: The time of the week has 17 bits from 31 - 47 in binary form. These
data are converted into decimal form and the time resolution is 6 seconds
as shown in Figures 5 - 7 and 5 - 8. In order to convert to seconds the data
are multiplied by 6. Another important factor is that the TOW is the next
subframe, not the present subframe as discussed in Section 5.10. In order
to obtain the time of the present subframe 6 seconds must be subtracted
from the results.
9.6 OBTAINING EPHEMERIS DATA FROM SUBFRAME 2
The data from subframe 2 can be obtained and converted into decimal form in
a similar manner as discussed in the previous section. Referencing Figure 5.9b,
the following sets of navigation data can be obtained from subframe 2:
1. IODE: The issue of data, ephemeris has 8 bits from 61 - 68. This bit pattern
is compared with the 8 least significant bits (LSB) of the issue of data, clock
(IODC) in subframe 1, and the IODE in subframe 3. If they are different, a
data set cutover has occurred and these data cannot be used and new data
should be collected.
2. C rs : The amplitude of the sine harmonic correction term to the orbit radius
has 16 bits from 69 - 84 in two's complementary form. These data are
converted into decimal form.
3. n : The mean motion difference from computed value has 16 bits from
91 - 106 in two's complementary form. These data are converted into dec-
imal form. The unit is in semicircles/sec, thus, the data are multiplied by
π to change to radians.
4. M o : The mean anomaly at reference time has 32 bits in two's comple-
mentary form. These data are divided into two parts, the 8-bit MSB from
107 - 114 and 24-bit LSB from 121 - 144, and are converted into decimal
form. The unit is in semicircles, thus, the data are multiplied by π to change
to radians.
5. C uc : The amplitude of the cosine harmonic correction term to the argument
of latitude has 16 bits from 151 - 166 in two's complementary form. These
data are converted into decimal form.
6. e s : The eccentricity of satellite orbit has 32 bits in binary form. These data
are divided into two parts, the 8-bit MSB from 167 - 174 and 24-bit LSB
from 181 - 204, and converted into decimal form.