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Document Title: 16
User’s Manual for the GPS Orion-S/-HD Receiver
Document No. Issue 1.0
GTN-MAN-0110 June 22, 2003
DLR/GSOCNo part of this document shall be reproduced in any form or disclosed to third parties without prior authorization.
Aided operation of the Orion-S receiver is supported by a variety of dedicated commands:
• For configuring the orbital elements of the user satellite, the LO (Load Orbit) com-
mand is used. The first and second line of the elements set are each embedded into
a separate LO command and consecutively transmitted to the receiver.
• Using the TO (Transmit Orbit) command the currently loaded mean orbital elements
can always be dumped in the form of a single F52 output message.
• Aiding is activated (or deactivated) using the AM (Aiding Mode) command.
Upon commanding, a new element set is always stored in non-volatile memory. The informa-
tion is thus preserved and made available again after a reboot of the receiver. This allows a
power saving, intermitted operation, in which the receiver is powered up for only some parts
of each orbit.
Use of the aiding mode provides a particularly simple way to initialize the receiver on a LEO
satellite, since a single element set is good for initialization at multiple epochs. Typically, the
twoline elements are accurate enough to allow aiding for a period of at least one week follow-
ing their validity epoch. If continuous tracking can be ensured by an appropriate antenna
orientation and elevation mask, the receiver may be commanded to unaided mode after suc-
cessful acquisition of a 3D navigation fix. It will henceforth use the latest navigation solution
to forecast the instantaneous visibility conditions and expected Doppler shifts of the GPS
satellites.
3.2.5 Relative Navigation
The Orion-S receiver operates a DGPS task providing simple relative navigation of two host
vehicles via the exchange of raw navigation solutions. To operate the relative navigation fea-
ture, the secondary I/O ports (port B) of two receivers must be connected via a bi-directional
serial radio link with a 19.2 kB data rate. On the B port, each receiver outputs an F40 naviga-
tion message as well as an F42 raw data message once per second. Vice versa, it decodes
F42 messages on input and uses them to compute a differential navigation solution. The
remote measurements are differenced against the receiver’s own raw data thus eliminating
common errors like GPS clock errors, broadcasts ephemeris and to a fair degree ionospheric
errors. A differential position is then computed after carrier smoothing of the differenced
pseudorange measurements. Likewise, velocity is obtained from differential range rate
measurements that are derived from a second order polynomial approximating the differen-
tial carrier phases. Further details of the employed algorithms and concepts are provided in
[15]. The achieved accuracy amounts to typically 0.5 m in position and 0.5 cm/s velocity.
For orbital applications the resulting relative navigation solution can conveniently be output in
a reference frame aligned with the radial, along-track and cross-track direction, but a stan-
dard WGS-84 representation is also available. In either case it is necessary to activate the
respective output message (F45 or F46) using the DR (Data Rate command). Note that the
relative navigation solution is always one second late compared to the standard navigation
output to accommodate the required time for exchanging raw measurements via the auxiliary
port.