Integration of GPS with Other Sensors and Network Assistance - Understanding GPS: Principles and Applications - page 546

spondingly narrower Doppler size due to the sinc function, and so more Doppler
bins (and more correlators) will be required to cover the same uncertainty range.
The over-the-air protocols for A-GPS have provisioned methods of sending the
entire navigation data message to the handset so it has a priori knowledge of each bit
and can subsequently wipeoff the data if needed for additional signal processing
gain. The handset has to assemble the total bit sequence through a number of differ-
ent messages. For example, in the GSM protocol, most of the bits for each satellite
from subframes 1-3 (words 3 through 10) are delivered via the “Navigation Model”
assist data message. Most of the bits for subframes 4 and 5 (words 3 through 10) are
delivered via the “Almanac” assist data message. The bits contained in each word's
6-bit parity field are not sent, as these are computable after the handset has the data
elements. Each subframe has a constant preamble that does not need to be sent, and
the 17-bit HOW word contained in each subframe (word 2) is predictable with time.
Thus, the remaining missing bits in the navigation message, primarily the TLM Mes-
sage (14 bits), the antispoof flag (1 bit), the alert flag (1 bit) and the TLM-reserved
bits (2 bits) have been accumulated into one additional garbage collection message
and appended at the end of the “Reference Time” network-to-handset message.
At least two alternatives exist to sending and receiving navigation data bits over
the network that achieve most of the benefit: predicting the navigation data bits, as
discussed earlier, and guessing the navigation data bits. Estimated bits [75] can sub-
stantially increase the required number of correlators for longer coherent integra-
tions. In guessing the navigation data bits, a hypothesis corresponding to each
possible bit transition is formulated, and parallel integrations are performed, with
the integration resulting in the largest signal correlation peak determined to be the
correct bit sequence. For a sequence of n data bits, 2 n parallel integrations are
required, corresponding to each hypothesized bit sequence. This increases the num-
ber of correlators dedicated to each satellite for which bits are guessed by 2 n . In [75],
a practical limit of 5 estimated bits is imposed, corresponding to 32 parallel integra-
tions.
Modernized future signals such as the L2C signal will offer a dataless compo-
nent to the signal, which allows for long coherent integration periods without regard
to data bit modulation interference, eliminating the need to transmit the bits to the
mobile user through a cellular network.
The possible forms for navigation assistance include the following:
• DGPS correction data;
• Approximate altitude of the mobile;
• Approximate mobile location;
• Real-time satellite integrity information;
• Fine GPS timing information;
• Satellite clock correction coefficients;
• GPS satellite ephemeris information.
Although the improvements derived through the application of DGPS correc-
tions to each measured pseudorange have diminished since the presidential directive
to deactivate SA, the benefit may still be worthy of consideration in the E-911 envi-