Each - Draper Laboratory

The chip units are defined for P code (which is used in
PNS) by 96.146 ft/chip or 9.775 x 10 -8 s/chip.
The filter performs the GPS DI updates plus Kalman
updates for the other sensors. A set of corrections for
the navigation system and clock model are computed
and then fed back to the navigation algorithm for a
reset of the full system state.
The algorithm-embedded software is coded in three rate
groups: high (100 Hz), medium (50 Hz), and low (10
Hz). High rate performs IMU compensation, attitude
integration, and incremental transition matrix calculations.
Medium rate performs navigation position and
velocity integration, bookkeeping of the receiver clock
error estimate and satellite atmospheric delay estimates,
and all GPS receiver interfacing (described below).
Both high and medium rate perform resets based on
corrections supplied by the nonlinear filter. Low rate
performs all filter updates and sends corrections to high
and medium rate.
Deep Integration GPs
Background of Draper’s Deep Integration
DI was developed to extend GPS tracking to poor
GPS signal-to-noise conditions, especially intentional
jamming environments. Deep integration requires
a custom receiver configured so that the navigation
software can issue the numerically controlled oscillator
(NCO) commands (overriding the internal tracking
loops) and also receive integrated correlator outputs.
For previous results with DI, see Ref. [2].
Prior to PNS, Draper DI was used successfully in artillery
shells with high dynamics and short duration, where
the instrumentation was limited to inertial sensors and
the receiver.
For the personal navigator, Draper extended the use
of DI in significant ways. First, mission duration in
the tests was stretched from minutes to one half hour.
There is no inherent mission duration limitation here.
Second, the capability of the nonlinear algorithm was
extended to perform both the nonlinear GPS updates and
conventional Kalman updates (from the Doppler radar
and altimeter). In contrast to the fixed set of satellites in
view for a short time-of-flight missile, the ground navigation
system described here needed to adapt to satellite
configuration changes. Finally, of course, this was
all done with hardware compressed to a point practical
for use by a foot soldier.
A key advantage of DI for the ground navigation application
is the ability to recover satellite track after signal
36 A Deep Integration Estimator for Urban Ground Navigation
is temporarily lost, perhaps due to masking from a
landscape fixture. A second advantage is that deep integration,
by design, is able to track a satellite when its
power is weaker, due to factors such as forest canopy or
indoor attenuation.
Summary and Technical Overview
In conventional operation, the GPS receiver is based
on internal tracking loops, in which tracking loops are
maintained for GPS code and carrier signals, based on
correlator outputs and NCO commands, both of which
are invisible to the end user. The user is supplied with
pseudo and delta range information tapped from these
loops, or final position and velocity. Conventional GPS
is covered in numerous sources, among which Ref. [3]
may be cited.
In deep integration, the correlator outputs are issued to
the navigation processor, along with a code phase (or
equivalently, pseudorange) for the replica signal. The
navigation software sends rate commands to the receiver
NCOs, which the receiver uses to generate the replica
signal. This operation replaces the internal loops.
In practice, there is an alternation between modes in
PNS. Sometimes (initially and during extended signal
loss), the receiver maintains control of tracking loops.
Whenever possible, internal loops are replaced by the
DI process. These modes are referred to as “receiver
control” (internal loops) and “host control” (deep
integration).
Description of PNs Deep Integration
A compressed technical summary of deep integration
can be given by reference to the main interfaces in PNS,
shown in Figure 2. First, the code and carrier NCO
commands issued to the receiver are discussed in detail.
Then the receiver outputs sent to navigation and their
transformation into filter observations are discussed.
Finally, the filter corrections applied to the navigator
are discussed.
GPS
Receiver
I, Q, r*
50 Hz
. .
tcode , tcarr Navigation
Medium
Rate
10 Hz
Corrections
Figure 2. Deep integration interfaces in PNS.
dz
PNS Processor
Navigation
Low
Rate